High altitude PAX

[sperry]

So, I was reviewing John Evans' 3 part article from the Reno SCCA newsletter that he wrote about PAX issues due to high altitude, and I thought it might be an interesting discussion for the forum.

Here's the complete article, assembled from the last 3 Reno SCCA newsletters:

http://www.renoscca.com/newsletter/2...april%2006.pdf
http://www.renoscca.com/newsletter/2...20may%2006.pdf
http://www.renoscca.com/newsletter/2...0june%2006.pdf

PAX Ruminations, Part I
by John Evans

How much do you know about PAX? Most of us realize that PAX is a system of numerical handicaps that allow Solo drivers from different classes to compare their times against each other. For quite a few years, Reno Region has had annual PAX championships for Ladies and Open classes. These PAX championships have become such an accepted part of our Solo program that they’ve replaced what we used to call our Driver of the Year awards, which were subjective awards voted on by the Board of Directors. In this three-part series, John Evans helps us understand the strengths and weaknesses of the PAX system. – Ed.

Recently a proposal was made to drop the annual Reno Region PAX championships. The primary reason for this proposal was the perceived unfairness of the PAX handicaps of some Solo classes. While I agree the factors pose a disadvantage to some classes, especially at the altitude at which we run our events, I am ambivalent as to whether we should drop PAX. The PAX championship has its parameters as does any racing class. Just as there are models of cars within SCCA Solo classes that are underdogs and overdogs, there are also some PAX classes that I believe are overdogs and underdogs. If winning or placing high in PAX is important to the individual, they can choose, if they are wealthy enough, a car that is the top dog in the classes which have the most advantageous PAX handicap.

I was on a committee a couple of years ago that was formed to investigate the perceived unfairness of PAX. The gist of our conclusion was that it would take a redistribution of cars in the SCCA classes in order to correct PAX for our altitude along with modifying of the factors and then keeping them up-to-date which would be a very labor intensive and ongoing endeavor. Other regions have recognized some inequity in the factors. Northwest Region, for example, has created two separate classes, SPAX and SPAXL, for those who want to compete only in PAX.

None of what I am saying here is meant to criticize the efforts of Rick Ruth to maintain the PAX factors. It is a huge task. In discussions with Rick he has expressed disbelief that altitude can cause inequities in the PAX factors. I maintain that altitude does cause inequities and there are inequities in the factors even at sea level sites. I have looked through North American Pylon and other sources at events across the nation and seen some common disparities. All those sites are low altitude compared to here. One of the clearest examples was worsened by SCCA's placement of the Mini Cooper S into GS. After that, the GS PAX was out of kilter for a couple of years. Ruth may have fixed that with a PAX adjustment to GS of 0.75 seconds per 60-second course, although adjustments to other classes at the same time negate a portion of that, making it closer to 0.6 seconds. A clear example of the GS PAX being out of kilter is NWR's 2005 season. GS made up five of the top six PAX placements and seven of the top nine. Across the country five or six other classes show domination, though none so dramatic as GS. I do not believe all the best drivers in the country are in the same six or seven classes. The classes with the most drivers highly placed in PAX are generally the same ones region to region. Those same classes generally place higher here, only amplified. Why is that?

There is some science to the claim that PAX is unfair at this altitude, and I will get to that shortly. Besides altitude, there are other factors that cause a variance with PAX in our region. One, the effect of our typical course design of the past several years, can be explained in cold hard math, although I have hope our course design is evolving to lessen the effect of altitude and the disproportionate advantage of high power-toweight ratio cars. Another factor is the high humidity and cooler temperatures where the majority of sites are located that are used in PAX calculation. Yet another is our region's driver skill level. These last two are less easily provable, but I believe they are a factor and will explain later why I believe that.

In the last nine years I have run at least a full season in AS, BS, CS, DS, OSP(SU), Street Tire and CSP. I've driven in SS, ES, FS, GS, HS, CP, DP, ESP, ASP, EM and DM. I've placed everywhere from first in PAX to 50th in individual events and from 2nd to sad in the annual PAX championship. So I've paid close attention to PAX placements over the years and tried to analyze what, besides an on day or off day, is the reason a driver can run a car in one class and score high in PAX and run a seemingly equally prepared car of another class and score low, or how some drivers place high in PAX and some lower when so many of them are getting so much out of the particular car they are driving.

Of course driving ability is a huge factor in run times. How big? If one driver averages 40 mph through a 3200 foot course and another averages only 2 mph faster, the faster driver will beat the slower driver by 2.597 seconds. We don't often see that big a gap between experienced drivers sharing the same car, but we do see a 1-second difference. In that case the faster driver is averaging less than 1 mph faster. I've seen a skilled driver go several seconds faster than a rank novice in the same car, so yes, skill is an undeniable factor. But, when a single driver's PAX score can vary greatly from class to class, it is evidence that something in addition to driver skill separates the top ten or twenty drivers in PAX placement. So what is that factor?

For many years I thought the primary factor was the design or our courses. They were generally wider, faster and simpler than ones I ran at other regions, and visitors to our region sometimes commented on our faster courses. Over the last eight years I have experimented with setting up various course types, and I’ve seen how course design here can exacerbate the PAX inequities, but I am no longer convinced it is the primary cause -- and I don't want to run narrow, busy, slow courses anyway. I am convinced the unequal acceleration potential between cars in relation to their PAX handicap at our altitude is a greater factor than our region's course design.

The PAX factors, according to Ruth's list of regions' results used, are all calculated from low altitude sites except one. Beside the Denver National Tour event, the highest is around 1100' ASL (Above Sea Level) and the lowest are virtually sea level. So how much difference does acceleration make and why should high altitude throw the PAX index further out of kilter?

On an autocross course, every space between sharp turns can be used for acceleration. I am not talking only about purely straight stretches, but distances where a car's throttle can be on or near the floor. These "distances" may have slight turns or jinks through them but they can be taken without letting off the throttle, so I will call them acceleration distances. The average speed over these acceleration distances can be calculated, imperfectly but close enough for our use, by taking the average of the corner exit speed and the speed just before braking for a turn. So a car exiting a turn at 20 mph and accelerating for 200 feet to a speed of 30 mph before braking, averages 25 mph over that 200 feet, a relatively short straight at Stead, and covers that distance in 5.455 seconds. On the same section of course, a car exiting the turn at 20 and attaining 40 mph, averages 30 mph and takes 4.545 seconds, a 0.91-second difference. That is almost a second gained in only 200 feet. There are other factors not contained in this calculation that both raise and lower this time difference, but it is close enough for our demonstration. Easily half of a typical 3000-foot course at Stead can be made up of acceleration distances after subtracting turns and slaloms. Multiply that 0.91 seconds times the number of acceleration distances, say 7.5, and you get a 6.83 second difference. So a car that can average only 5 mph faster over a 200-foot stretch can run 6.83 seconds faster for no other reason than acceleration potential. The PAX factors do take into account that some classes of cars accelerate faster, but at this point I only want to show what proportion of run time reduction can be due solely to acceleration. If half or more of a course is usable for acceleration, then something that affects acceleration is a major factor.

Given equal drivers, tires and prep, there are other car characteristics that play a role in determining run time, but none so great as acceleration. Cornering is an obvious factor. A car going faster in a turn traverses that distance in less time, reducing run time. In addition and possibly more important, if two cars with identical acceleration go through the same turn, the one going faster coming out will have a higher average speed at the end of the straight and further reduced run time. But speeds through turns do not vary as greatly between cars of varying classes as do speeds at the end of straights. The cornering G's delta between most stock class cars, all else being equal, is not nearly as great as the delta between power-toweight ratios. According to Motor Trend magazine, an H Stock Mini Cooper is very close to a Super Stock C5 in cornering G's. Furthermore, there are cars in higher classes with less cornering ability than some in lower classes.

Transition capability, which is related to but not the same as cornering capability, is a factor, especially in courses with slaloms and jinks. The car that can transition quickly can simply go faster through slaloms and jinks, reducing the run time in that particular element, and if a straight follows that element, then the average speed at the end is higher. Braking capability makes a difference, but a much smaller one than cornering and transition. If one can brake later, speed is maintained over a longer distance but that distance is never much more than 20 feet on most turns on an autocross course. All of these car characteristics effect run time but acceleration remains the one to look at.

In the following exercises I am not picking on any particular type of car but using cars I have driven and whose results I have watched over the years. Now, we know that cars with good cornering can maintain a higher speed through turns, which contributes to higher average speeds between turns. That explains why a Mustang in FS, which can eat up a CS Miata in acceleration, is beaten by the Miata on a very tight autocross course. A well-prepared stock Mustang simply does not go through the turns quite as fast as a stock, well prepared Miata, all else being equal. However, when we widen the course and add longer straights, the acceleration advantage of the Mustang will overcome the cornering advantage of the Miata, yet the CS Miata is PAX factored higher than the Mustang.

A CS Miata is also factored higher than a DS WRX. The WRX transitions much slower than a Miata. Its center of gravity is much higher than a Miata’s, its suspension geometry does not keep the wheels at an ideal angle to the surface and its stock wheel size allows less rubber per pound of vehicle than a Miata, so generally it goes through corners slower than a Miata, and if the course is very tight, the WRX's superior acceleration cannot make up for its slower exit speed from the turns. But to us at 5000' ASL, it is very surprising that the CS Miata is factored higher than a DS WRX. I say surprising because unless the course is exceedingly tight, the WRX beats the Miata every time. In 2003 in the Reno region, DS beat CS every event, sometimes by 3 to 4 seconds, except for one day when a CS MR2 beat DS by .6 seconds. A DS WRX beat CSP several days in 2003, yet CSP is factored 2.5 seconds faster than DS in a 60 second course. The reason was acceleration, certainly not handling.

What does altitude have to do with this? The WRX example could be explained by open, fast courses or by the turbo. In the next installment I will explain how the Miata loses 20% of its effective power at Stead, how the WRX loses less, and how the SS C5, while it loses 20% of its power the same as the Miata, loses very little of its effective power.

For reference:
2006 Reno Region SCCA PAX/RTP
SS 0.840 ASP 0.852 STS 0.797 XP 0.868 AM 1.000
AS 0.831 BSP 0.843 STS2 0.802 BP 0.862 BM 0.942
BS 0.822 CSP 0.838 STX 0.804 CP 0.856 CM 0.908
CS 0.813 DSP 0.829 STU 0.820 DP 0.853 DM 0.879
DS 0.798 ESP 0.826 SM 0.845 EP 0.858 EM 0.886
ES 0.806 FSP 0.814 SM2 0.854 FP 0.863 FM 0.884
FS 0.805 SU 0.853 GP 0.844 FSAE 0.948
GS 0.794 F125 0.937
HS 0.780 FJ1 0.781 FJ2 0.796 FJ3B 0.830 FJ4A 0.837
T (Street Tire) 0.968 [In addition to class factor above]


PAX Ruminations, Part II
by John Evans

In the last installment I explained how a number of factors affect run time and how important acceleration is to reducing run time. I used an example of the DS WRX versus the CS Miatas in the Reno region. At the Nationals in Topeka, the Miata is the dominant car in CS, and the WRX is an underdog in DS. The CS PAX factor is higher than the DS PAX factor. So according to PAX, the Miata is supposed to easily beat the WRX, however in Reno Region a WRX beat the CS Miatas consistently. The reason was acceleration, certainly not handling. A very open course could explain this because the published 0-60 time of the WRX is significantly lower than the Miata’s, but not every course had long straights and few corners. The reason was that the turbo allows the WRX engine to retain a greater percentage of its sea level power, while the Miata loses about 20% here (Stead is listed as 5,046 feet above sea level). See the accompanying table and websites at the end of this installment to see how I came up with the 20% and other figures used later. The WRX loses some useful power at 5000 ft because it is for all intents normally aspirated until boost is achieved, and max boost is achieved slower as altitude increases. The turbo also has to spin faster to reach max boost and loses efficiency from heat and friction in doing so. On the other hand, as claimed by a turbo "guru" at Cobb Tuning, there is less back pressure in the exhaust at altitude, so there is a greater pressure delta between the entrance gases and the exit gases of a turbo which allows the turbo to accelerate quicker. However it is analyzed, it is difficult to calculate the loss of power in the WRX but the bottom line is that the front part of the torque curve of the WRX is lessened and the peak dropped slightly but the Miata's power curve is really flattened.

For a comparison of performance differences caused by altitude change that some local autocrossers are familiar with, I will use San Francisco Region's sites around the Bay Area and our own site at Stead. On a typical "good" day at a 75' ASL SFR event, the temperature is 70 degrees Fahrenheit, barometer at 29.68 and relative humidity at 75%. At those readings, most engines develop 100% of their rated power. See the automotive power calculator websites I have listed. On a typical day at Stead with a temperature of 85, same barometer reading (adjusted for altitude) and a humidity of 40%, a normally aspirated engine develops only 79.5% of its rated power. I have lowered the humidity for Stead, which actually lessens the power loss. Contrary to popular belief, high humidity lowers power. If I kept the Stead humidity at 75%, a muggy day here, power drops to 77.9% of rated. We have experienced much higher temperatures at Stead. Another ten degrees Fahrenheit drops the power another 1.7%. To simplify our calculations I will round the power loss at Stead to 20%. To further simplify our calculations, I will use horsepower ratings only. Although torque is a factor in acceleration, hp is close enough for us to get a good idea of what happens with altitude.

At the theoretical SFR event, a WRX (with 227 hp, weighing 3100 lbs plus a 170 lb driver) has a power-to-weight ratio of one hp to 14.41 pounds, the Miata, say a 2001 model (with 142 hp, at 2400 lbs and the same driver) has a one to 18.10 power-to-weight ratio, a differential of 3.69, so of course the Miata is slower in a straight line -- but according to PAX it is supposed to make up the difference in the turns and then some. I could not find a formula that I could understand or a table that estimates how much power loss the WRX will suffer at 5000 ft. I have nothing but an educated guess and some "seat dyno" experience as to the percent of power loss the WRX will experience at Stead. It is noticeable but not like that of a normally-aspirated engine. I will estimate 5%, and believe that is generous, which drops the WRX to one to 15.16. The Miata drops to one to 22.62, a difference of 7.46, a differential that is impossible to overcome with handling if the course is open and even moderately fast.

At this point I would like to clear up confusion arising between superchargers and turbochargers and what happens to engines at altitude using one or the other. The confusion likely arises from the fact that the superchargers used on some big aircraft engines in the 1940s and on, used either clutches or gearing changes to increase the RPM of the supercharger relative to the engine RPM, or a second supercharger was activated, so boost would keep up with dropping air pressure as the plane climbed. They then performed more like wastegate-equipped turbochargers designed for high altitude use. That type of altitude-compensating supercharger may exist for automobiles, but I do not know of any hooked up in this manner. All of the automobile superchargers that I know of are directly driven by gear or belt and never change RPM relative to the engine so they act just like normally-aspirated engines as far as altitude effect is concerned. A supercharger does, however, increase horsepower at a rate greater than the additional weight of the supercharger versus adding a larger engine to produce the same power. While there are a few automotive turbocharger applications made specifically to maintain performance at altitude, the vast majority of automotive turbochargers are designed primarily for increasing the relative output of an engine in relation to its weight, although much benefit toward the former purpose does result. Automotive turbochargers are usually not over-configured enough to provide maximum boost at say 15000 feet altitude, nor can they provide "cool" boost at that altitude. If they were, turbo lag for normal driving would be unacceptable. Rather, they are configured to perform acceptably at sea level, and would be spinning well beyond their efficiency RPM at 15000 ft.

So what about a Super Stock C5 at 5000 ft? It is normallyaspirated just as the Miata and it loses the same percentage of power as the Miata. But this is a different case and deals with what I never had a name for until Jim Gandy called it "excess power." Even a non-Z06 C5 has excess power on an autocross course, especially at the low altitude sites where the events are located that are used in PAX calculations. The driver of a stock Miata can stand on the throttle early as he is unwinding coming out of any turn at sea level unless it is icy or raining. But it is different with a C5. As a general rule when a C5 driver mashes the throttle to the floor early coming out of a turn at low altitude, useless wheel spin occurs, even if the driver has the car under control directionally. The acceleration potential of a Miata at 5000 ft is diminished by the full amount represented by the 20% power loss. The C5's effective acceleration is not diminished by the full reduction in power because it was unable to use all it had to 100% effectiveness at sea level. In many cases the C5 cannot fully utilize all the power it has at 5000 ft. So the C5's effective acceleration at 5000 ft is diminished far less in proportion to cars that can use their full power at low altitude. How much difference can that make? I will try to answer that in the next installment.

Following is a small percentage of the references I consulted while researching this article:

[table omitted... see the linked PDF]

The above table is from an old aircraft manual. Note that at 5000’ ASL it shows power reduced to 80.3 percent of sea level. The table assumes the temperature stays at 60 degrees F. and the relative humidity stays the same. Note the table also disagrees markedly with a figure of 3% power loss per 1000 ft stated in the "turbochargernz" site below for normally aspirated engines. The three automotive calculator sites listed are close to this table. They will vary with this table because of propeller efficiency changes relating to aircraft.

http://www.smokemup.com/auto_math/index.php#hp A handy website for calculating many things related to your car's performance.

http://wahiduddin.net/calc/calc_hp_dp.htm A website using dew point instead of % humidity.

http://www.csgnetwork.com/relhumhpcalc.html The web site used for the normally aspirated horsepower calculations at the San Francisco and Reno Region autocross sites.

http://www.turbochargersnz.com/about_turbochargers.html A clear and very simple explanation of turbochargers. It states a very conservative estimate, 3% loss per 1000 ft of altitude effect on normally aspirated engines. Other sources state a value around 4% per 1000 ft.

http://www.cobbtuning.com/tech/exhaustdesign/ A discussion of exhaust design commenting on lessened backpressure at altitude.


PAX Ruminations Part III
by John Evans

In the earlier installments we identified many of the factors that affect run time and learned what a great impact acceleration has on run times. We also learned how altitude affects cars unequally so far as acceleration is concerned. Now we will try to get a perspective on how the unequal effect of altitude can affect PAX placement.

The delta between the top ten PAX placements varies depending upon many factors such as length of course, who is running, number of participants, cross section of cars and type of course. However, if one looks at typical Reno results over a season, the average difference between PAX places averages about 0.175 seconds for a 50-second course. So how many PAX places could the 20% loss of power between a San Francisco Region event and an identical course at Stead represent?

First I want to restate that course design can exacerbate or mitigate a car's acceleration advantage or disadvantage. For example, a course with a great number of transitions and very few clear acceleration distances greatly reduces the importance of acceleration. On that type of course, the car's ability to transition and the driver's skill at maintaining speed becomes of greater importance. On the other hand a course with simple turns and many straights places greater emphasis on a car's ability to accelerate, and the altitude factor becomes very important. In the following example I am using a hypothetical course which has seven acceleration distances of 200 feet each totaling 1400 feet. This would be one-half to one-third of a typical course. I also want to reiterate that my examples assume equal driver skill. If a non-turbo car without excess power at SFR comes out of the turns of this hypothetical course at 20 mph and reaches 40 mph before braking on seven acceleration distances totaling 1,400 feet, it will cover the 1400' in 31.818 seconds. The same car at Stead coming out of the turns at 20 mph will only reach 36 mph and will take 34.091 seconds to cover the 1400', a 2.273 second difference. If all else were equal, that 2.273 seconds would drop a top driver 13 places in PAX.

The following two sentences are the most important in this article. Cars with low power-to-weight ratio and whose primary strong points are cornering and transition and have achieved a high PAX as a result, are affected by altitude the greatest in relation to PAX. Cars with excess power, whether they handle well or not, are the least effected by altitude in relation to PAX. For example, powerful cars like those in FS are factored lower than AS, BS, CS and ES because their handling is generally not equal to cars in those classes, yet some FS cars have excess power and are therefore less affected than some (but not all!) cars in the aforementioned classes. Most cars that are turbocharged, and that do not have excess power, fall in between the powerful cars and the puny cars. If a supercharged car has excess power it falls in with the powerful cars, if not it suffers like the puny cars. Cars that have only moderate excess power at sea level and lose their excess power at say 2500' fall in the middle also.

There is another factor that effects PAX equity in Reno and I mentioned it in the first installment, and that is the region's general driver skill level. Heavy weight is given to the class winners of Divisionals, National Tour events and the Nationals when calculating PAX. So the average level of the drivers used for the calculations is higher than Reno's average level. Why does that make any difference? It is certainly not as clear as other factors, but I believe power can make up for a portion of driver imperfections. If the driver of a powerful car scrubs off a little more speed in a turn than he should have, he can quickly make up some of it. If he pinches a turn in too close, then he can simply give more throttle than he would have been able to were he drifting to the outside of the course. If the driver of a puny car makes the same mistakes, they are not as quickly compensated for and therefore have a greater impact on run time. I believe one learns throttle modulation easier than one learns the precision needed to enter every turn at the best speed, maintain that speed through smooth movement of the steering wheel, choose the perfect line every time and air-kiss every apex and slalom cone.

Another factor is surface traction. Most of the sites used in calculating PAX are cooler than here and have higher humidity,both of which contribute to reduced traction. While this would affect cars of all types fairly equally as far as cornering is concerned, it definitely reduces the advantage of high-powered cars because they cannot use their acceleration as well, and I believe their PAX factors reflect that. The other surface factor that effects very stiff suspension vehicles (karts are a good example), is the very broken surface where the Solo Nationals have been held the last several years. I think karts have not performed at the Nationals as well as they would on even surfaces and yet they are factored so they perform quite well PAX-wise at the Nationals. I believe this helps explains why Eric Gangloff, obviously a very good driver as he scored fifth among the kart drivers at the Nationals, can so dominate PAX here when he runs. A soft PAX and a very good driver spell doom for the disadvantaged classes. I predict the karts will be a PAX killer at the Nationals on the new surface in 2006, with the only caveat being whether it rains when the karts run.

So, what class is the softest and what class the most difficult if one wants to excel in PAX at Reno Region events? Well, it is not as easy as that. The factors that affect PAX equity do not always respect class boundaries. There are cars within classes that are more affected by altitude and course design than others in the same class. Super Stock and AS are good examples. The 05 Boxster S of SS does not have excess power, while the Z06 does, so the lovely-handling Porsche, which is far from dominant at sea level anyway, is not a good car for PAX here. The C4 Corvette in AS, and especially the ZR-1, is good for PAX here but the wonderfully precise handling S2000, which is the driver's choice for AS at the Nationals, can barely get out of its own way at 5000'. Then there is the ferocious STi in AS which apparently is lacking in the handling department at low altitude but is a terror here because of its acceleration.

I can't give a sure thing, but if our courses stay open (and thus stay fun in my opinion), pick the high-powered or turbocharged cars in SS, AS, ASP, BSP, ESP, STX, BP, CP, SM, SM2, AM, BM, DM, and EM. Do not pick the lower-powered cars of CS, ES, HS, STS, CSP or FSP. Well, STS did win PAX here one day, but it was a Nationals-prepared car driven by a multiple National championship driver. HS has some cars with power-to-weight ratios greater than CS and ES, and HS is factored very low so there might be some picks there. The Lancer Ralliart is light and has 165 hp. The little lightweight Alfa Romeo twin cam roadster might do well in HS if someone would dare race a stock one. CSP and FSP must have a very strong engine to overcome their PAX burden. It was only last year that CSP had to be faster than SS according to PAX. I'm not sure where to put XP, DP, FP, CM or FM. There are some one-of-a-kind cars like Fordahl's Porsche 914-6 in FP that are blindingly fast at low altitude, but I don't know whether they could be enough faster than an equally driven SS Z06 at 5000' to make up for the 1.38 second higher PAX of FP. I think these Prepared classes depend much on the degree to which the engine is built and how well the chassis is sorted.

GS, DS, BS, and DSP, have some moderately high power-toweight ratio cars, and there the key is in which car to pick within the class. I know the former Integra Type-R of Kevin McCormick's was faster out of the turns and faster accelerating than our WRX even though the Type-R is normally aspirated. It certainly handled better than the WRX. After a divisional I drove both cars back-to-back and beat the WRX times of Glen Hernandez, Patty and myself by 1.4 seconds on a 45-second course. The stock Type-R is 600 pounds lighter than the WRX and is rated at 195 hp, so it does have a significantly higher power-to-weight ratio at sea level. It is not understandable to me why a car that handles almost as well as a Miata and has a much superior power-to-weight ratio is factored lower than CS. In BS the RX-8 is showing promise in PAX locally. I think the 2006 350Z with 300 hp may overcome the nimbleness of the also fast RX-8 on an open course. Much depends on whether the viscous LSD in the 350Z can get the power to the ground. There are some powerful cars in GS, like the 255 hp Altima and the 197 hp Honda Civic Si that might challenge the Mini-S, which may not be a PAX overdog this year because of the big adjustment to the GS PAX factor.

I hope you learned as much as I did while researching the article. I also came up with more unanswered questions and some new insights that I may write about later. One that I have a great interest in is the accuracy or fairness of the 2- second handicap for street tires. Should it be more or less? For the first time we are closer to having the means to test it and use direct observation rather than extrapolation. As for this article, if you find a math or logic error or have questions about the calculations, give me a call or an email.

jeracer@nvbell.net

Yes, it's brutally long considering the attention span around here, but it's very well written, and very interesting (if you're interested in this sort of thing).

The jist of the article is this:

The PAX factors we use for our region are flawed due to the majority of our events being run at high altitude. Vehicles with normally aspirated motors lose about 20% of their power, while turbocharged vehicles lose only about 5% of their power, giving the turbo vehicles an advantage at altitude. In addition, vehicles with excessive power (i.e. a Corvette that spins the tires at both low and high altitudes) don't feel the altitude effect nearly as much as something like a Miata that needs to be drivin with the throttle on the floor all the time.

John cites lots of technical resources, and waxed philosophically about his experiences in many types of cars. His conclusions include a list of the classes he feels are PAX killers in our region.

For the most part, I agree with everything that John says in the article. However, I disagree with the extent to which altitude effects modern cars. His calculation of 20% loss may be theoretically correct for normally aspirated motors, but as far as I can tell, that's on a N/A motor without an adaptive ECU, or variable timing, or any of the other tricks that auto manufacturers have been using in their motors to boost performance throughout the power band.

IMO, a modern N/A motor that's been climbatized to the altitude can't possibly be making only 80% of its rated power. Granted, there *is* less oxygen up here, and granted, a N/A motor *will* lose out more than a turbo motor, but I certainly don't see the discrepancy being 15%. My SVX feels faster down at sea level, but the WRX also feels far more powerful in the Bay than around here. A modern motor should advance timing to the edge of detonation, then back down for safety, meaning the motor will get far more out of the available oxygen than a non-adaptive motor will at altitude.

And, I know it may not really be in the scope of John's article, but I think he plays down driver skill more than he should. IMO, the "average" driver in Reno is nowhere near the level of the nationally competative drivers the PAX factors are based on. I'm one of the drivers that regularly places in the top 10 in overall PAX, yet I'm also 1 to 2 seconds off the pace at the national level! In addition, any time we get national quality drivers up here at altitude, it doesn't matter what sort of car they're in, we get trounced! Kevin McCormick is a great example (though he drives an '89 Civic Si IIRC, not a Type-R). When Dean and I were running STX WRXs built to the limit of the rules, he came in w/ his STS car and whooped us in *raw* times. IMO, the driver is still the most important piece of the puzzle, *especially* considering the level of driver ability in Reno. When everyone in the region is only capable of getting 60 to 70% of the speed out of their car (just throwing out numbers), how much does it really matter if you've got a power disadvantage? If you want to win PAX, all you have to do is get 80% out of the car and you'll leap to the front of the pack.

Also, I think John glosses over the level of preperation in our region. To be nationally competative, even in stock classes, you need to drop thousands upon thousands of dollars on tires and suspension bits to get "there". John compares CS Miatas with DS WRXs. IMO, Dean is a far better driver than I, so you can use my placement above him and the CS Miata he's driving this season as evidence that the WRX's turbo is a huge advantage. However, Dean's Miata is on bone stock suspension, without even a performance alignment. Meanwhile, my WRX has more money in go-fast bits then the cost of the car new! I'm not beating Dean just w/ my turbo, it's also the 255 width tires, massive brakes, custom coilovers, programable DCCD controller, water injection, etc, etc, etc, that make my car closer to the "ideal" SM car than Dean's Miata is to the "ideal" CS car. If I'm getting 70% out of my car, and Dean's getting 80%, there's still a good chance I can make up the difference in mods alone. Now, toss a set of 4-way, remote resivoir, Penske shocks on that Miata, an ideal front swaybar, agressive brake pads, and some as-wide-as-possible V-710's... I bet Dean *ruins* me in PAX.

And finally, there's the whole "so what" element to John's article. If you're racing in Reno with an old non-adaptive ECU'd motor, in a light "momentum" style car... well perhaps it's time for you to join the 21st century where the disadvantages of those cars have been erased by things like turbo-chargers, advanced electronic controls, active differentials, etc. No offence to the Miata owers of the world, but as fun as they are to drive, those cars were the autocross champs of a different era. We're in a horsepower world these days, but unlike the 60's, the modern muscle car has the means to put that power to the ground, thus rendering the Miata a "vintage" racer, with a gloried past.

Anyway, those are just some of my thoughts on the articles. Anyone else have some opinions?

[doubleurx]

Ow my eyes!

Wait so I only loose 5%. But I live here so in fact does that mean I just gain 7.375% for going to low altitude?

[Dean]

I think this section is more "true" and possibly one of the largest factor in play. The other being AWD that unless I mamistaken, he mostly ignores. Both have similar "fixing" properties for mistakes.

I believe power can make up for a portion of driver imperfections. If the driver of a powerful car scrubs off a little more speed in a turn than he should have, he can quickly make up some of it. If he pinches a turn in too close, then he can simply give more throttle than he would have been able to were he drifting to the outside of the course. If the driver of a puny car makes the same mistakes, they are not as quickly compensated for and therefore have a greater impact on run time. I believe one learns throttle modulation easier than one learns the precision needed to enter every turn at the best speed, maintain that speed through smooth movement of the steering wheel, choose the perfect line every time and air-kiss every apex and slalom cone.

In fact, there is one more contributor, Street Tires... Street tires are much easier for the novice or anyone for that matter to drive near the limit than race tires. The failure characteristics are much more predictable, and they provide significantly more feedback to the driver than race tires.

These three things are probably the larger contributors than altitude IMHO.

[Dean]

I need a top 5 PAX in the Miata at Squaw to show John it's the nut behind the wheel.

[Kevin M]

Originally Posted by sperry

And finally, there's the whole "so what" element to John's article. If you're racing in Reno with an old non-adaptive ECU'd motor, in a light "momentum" style car... well perhaps it's time for you to join the 21st century where the disadvantages of those cars have been erased by things like turbo-chargers, advanced electronic controls, active differentials, etc. No offence to the Miata owers of the world, but as fun as they are to drive, those cars were the autocross champs of a different era. We're in a horsepower world these days, but unlike the 60's, the modern muscle car has the means to put that power to the ground, thus rendering the Miata a "vintage" racer, with a gloried past.

Anyway, those are just some of my thoughts on the articles. Anyone else have some opinions?

I think you nailed it here. Autocross is not just about driver skill, it's (un)equal parts skill, choice of car, and choice of mods/preparation. You must be able to do all three of things at a nationally competitive level to win there, not just drive. Or at least, have someone who can do those other things for you.

As for Miatas, again I agree. There's a reason Miatas dominate all of their classes; their classes are setup that way. Well, CS is probably not going to be that way anymore, it looks like a real toss-up between the new MX5 and the Sky/Solstice, plus the occasional MR2. Anyways.

I concur with your assessment of power losses. Comparing my Miata (OBD-II, but I don't think it has much adaptability, it doesn't have a knock sensor) to, say, Dean's STX WRX map, and while we both lose power at 5000', I probably don't have more than maybe a 5% or so relative loss to Dean. I think 5% is reasonable for a turbo car, but 20% is way overblown for NA (an ASP Z06 corvette losing 80 rwhp? Ummm, no.) I think the altitude loss is as difficult to nail down as a ratio as drivetrain loss is, because it probably hugely varies on a car-to-car basis. Gearing and course design have far more to do with whose car is faster than the relative power losses. Dean can keep 100% of his power, and he'd still get stomped by the STS civics on saturday's course because he can't get into boost anyway- not an issue for NA cars.

[Kevin M]

Originally Posted by Dean

I need a top 5 PAX in the Miata at Squaw to show John it's the nut behind the wheel.

I bet you could do it in Cory's car up there, easy.

[M3n2c3]

Here's the way I look at the subject of acceleration potential, based on the info presented and my admittedly limited knowledge of how these engines work:

At high altitude, my n/a engine will have less air available, thus lowering power. A WRX will not be affected as severely due to its forced induction. As altitude increases, the disparity between the two is going to widen and the n/a engine will lose more of its ability to accelerate, thus rendering PAX factoring inaccurate. This will undoubtedly affect other cars differently, but I think that the RS and WRX make a good example due to their similarities.

The subject of the ECU altering timing is a good point, but it's only advantageous if the car is driven regularly at the course's altitude. Going from Carson to Stead is no big deal. But driving up to the Tahoe area from Carson creates a noticeable decline in my car's power output. If I simply drive my car up to Squaw the morning of the August autox event, will my car have time to acclimatize itself to the altitude before I take my runs? Probably not. Normal aspiration FTL.

Of course, I ultimately fall into the "so what" category - I'm a noob to racing, so the fact that a WRX will retain a better percentage of its power output than my RS should not be my concern, nor should worrying about my PAX score or inequities in the factors. My focus is just working on my car and learning to drive faster.

And we've had this discussion before - we're autocrossing for fun, paying minimal entry fees, and there are no real repercussions or rewards for losing or winning (unless you're going National, I guess). Sure, genuine effort should be put in to making sure events run smoothly and safely, but there are too many people taking it too seriously. I'll bitch half-heartedly about Cody having a better car for high elevation, but I'm certainly not going to demand that we launch an investigation into "fixing" PAX factors. I'll save that for the crotchety old-timers.

And if it really mattered, I'd have a different car!

[doubleurx]

eh? what you say sonny?, I'm turnin up my boost!

[sperry]

And that reminds me... when I tow the WRX over the pass before the August event, I'm gonna stop in Truckee and tear around for a while, then put the car back on the trailer and not start it again until I'm in the Squaw parking lot.

[doubleurx]

Originally Posted by sperry

And that reminds me... when I tow the WRX over the pass before the August event, I'm gonna stop in Truckee and tear around for a while, then put the car back on the trailer and not start it again until I'm in the Squaw parking lot.

Tear it on up to my house and I'll force you to drink some beer!

[tysonK]

Originally Posted by John Pax Evans

Pax stuff,Pax stuff,Pax stuff,Pax stuff,Pax stuff,Pax stuff...

You will lose if you drive an S2000 up here

Pax stuff,Pax stuff,Pax stuff,Pax stuff,Pax stuff,Pax stuff,

Son of a bitch, Pat you read this?

[Kevin M]

Originally Posted by tysonK

Son of a bitch, Pat you read this?

Imagine if you were on the moon. Then teh STi would be inveencible!

[MattR]

Bump...It's that time of year again! Is PAX Fair? Is the Reno region PAX champion the "Best " driver for this year? Where are we on this discussion?

I've read John's articles front to back, and I'm still not convinced that the altitude power loss of a N/A car is as significant as stated. Especially taking into consideration that AWD cars have huge power loss to the drivetrain, and huge HP rear wheel drive cars are constantly fighting to gain traction during cool events, etc.

[Kevin M]

I'm quite curious as to which of the "old timers" around here is going to put forth a point of view which suggests Lucas wasn't the region's best driver this season, regardless of PAX methodology.

[sperry]

Originally Posted by MattR

Bump...It's that time of year again! Is PAX Fair? Is the Reno region PAX champion the "Best " driver for this year? Where are we on this discussion?

I've read John's articles front to back, and I'm still not convinced that the altitude power loss of a N/A car is as significant as stated. Especially taking into consideration that AWD cars have huge power loss to the drivetrain, and huge HP rear wheel drive cars are constantly fighting to gain traction during cool events, etc.

I think John's analysis is pretty "on". But I think it's a moot point. So what if the turbo cars have an advantage... none of our region's drivers are consistantly good enough for it to make a real difference. Winners in our region tend to win dominantly... the only exception this year is SM, and that's a class where everyone in the class would have been decimated if a decent driver in a fully prepped car showed up.

IMO, if you care enough about winning to have a big problem w/ using the "national" PAX factors, and truely believe that the altitude is what's keeping you from winning, you should go out and buy a new turbo car. John Evans is a great example of that... he really thought he was getting beaten because of PAX and altitude... so he got a WRX for a season. Now he drives a Z. Seems like the turbo didn't make that much of a difference. Whatever the car you're racing with, you have to be able to drive it 100% if you want to win.

And the street tire modifier has seemed to work *very* well. Since we allowed people to switch between street and race tires within their classes, it seems like people placed just about where they would normally regardless of the tires under them. In fact, if it was at all warm, it looked like the race tires are still the faster tires even though the street tires have gotten proportionally better and better over the years. The nice thing is that if you want to run on street tires, you're not totally outclassed... you just have to drive a little better than the race tire folks. IMO, that's right on...

[MattR]

Originally Posted by BAN SUVS

I'm quite curious as to which of the "old timers" around here is going to put forth a point of view which suggests Lucas wasn't the region's best driver this season, regardless of PAX methodology.

Haha, I hope no one goes there...There is no doubt in my mind that Lucas as #1 in PAX is indeed the best driver in the region this year. I don't think anyone can argue with that.

Now 2nd place, that could be a fluke

[MattR]

Originally Posted by sperry

... the only exception this year is SM, and that's a class where everyone in the class would have been decimated if a decent driver in a fully prepped car showed up.

True, cars like this one would have murdered all of us.

[MikeK]

Originally Posted by MattR

True, cars like this one would have murdered all of us.

Carrie in Jim's car all year would have murderised us as well

[tysonK]

true

[SFVette]

Why is it that those who have not won PAX believe that winning it makes you the best autocrosser? I have yet to find a place in of the Reno SCCA rules that states PAX champion as the "Best driver of the Year" PAX is used to give out more awards and reward those that ran consistently well. It was adopted to avoid voting for a Driver of the Year which would have more complaints that the street tire handicap. I think PAX is good for comparing your own improvement against those experienced racers who finish close to the same spot each year.
It seems that John and others know the exact way that PAX handicaps each car so why can they not tell us the exact number needed for each car to make it fair? My guess is that like the PAX factors them selves the math is just educated guesses since not all cars are prepared to the fullest and not driven by a the same person. Motorsports will never be fair but that’s what makes it fun beating the guy who should have a better prepared car.
Lastly to fix PAX every one under the age of 30 gets their time, from 30 to 55 you get one extra run and 1/2 second off your time, over 55 you get two extra runs and 1 second off your time. The only catch is you have to run a car that is as old as the age range you’re classed in.
Thanks for letting me post
Lucas

[Dean]

You are welcome any time Lucas, especially as a WRX owner.

I'll take the 1/2 a second and the extra run, but not so sure about the car vintage. My 2 ton '66 Mercury Park Lane even in HS is unlikely to be competitive.

[MattR]

Originally Posted by SFVette

Why is it that those who have not won PAX believe that winning it makes you the best autocrosser? I have yet to find a place in of the Reno SCCA rules that states PAX champion as the "Best driver of the Year" PAX is used to give out more awards and reward those that ran consistently well. It was adopted to avoid voting for a Driver of the Year which would have more complaints that the street tire handicap. Lucas

Agreed, I just don't see where the issue is coming from . There is no way to have a "Driver of the Year " award without some scale and rules, like PAX, so the PAX championship just replaces the Driver of the year in my opinion.

[AtomicLabMonkey]

He's carrying the inside front pretty bad there...

[Dean]

A number of RWD cars do that in tight autocross turns. Just like AWD and FWD lift the inner rear.

OK, I know it is suspension geometry not which wheels drive, but those are the symptoms.

[AtomicLabMonkey]

That doesn't indicate a geometry problem, it's probably stiff front wheel rates and too much load transfer on the front end. That or he doesn't have enough droop travel available in the strut and it's jerking the wheel off the ground. Either way, it's pretty easily correctable.