The problem with altitude is not really a problem until maybe 8-9000 feet with most modern turbo cars, although I would say from 6000 up the effects starting to show. The rally cars can also run JEMS or Pectel or other racing ecu that have provision for anti-lag and other racing tweaks not found on a road car. Yet it uses the same turbo, injectors, in short, a completely stock STI 2 liter engine with a 30mm intake restrictor. The Mass Flow sensor of the Subaru engine is pretty much going to give the same AFR at 15,000 feet that it will give in Death Valley. This is because it measures not the air flow, but the air mass flowing in. So it compensates for relative humidity, temperature and pressure at all times. Of course, how one programs an ECU to deal with that information is another subject!
The altitude issue concerns the very nature of turbocharging. The compressor has a map of flow/pressure ratio (note: not absolute boost, rather boost relative to 1 bar atmospheric) that shows the heat of compressing the air, that being known as inlet temperature in every day language. The ideal sizing of a turbocharger is related to the power expected (a mass flow issue) and the engine displacement. Each compressor has what is called the sweet spot, a largish , but skinny, 45 degree oval where the inlet temperature is as low as possible for the boost levels. We also have to size the compressor so as not to over speed the turbo, more on that in a minute. Anyway, an engine runs up and down the rev range in this lower inlet temperature area if sized properly . Put in technical language, the turbocharger is running at its maximum efficiency . Now you can imagine what just turning up the boost on a motor does. It moves the compressor load outside of the sweet spot and begins to (sometimes quickly) increase the air inlet temperature. A bigger intercooler is not quite the answer, although when running 20+ pounds of boost we need to go to large front mount (no where else to put it!). What is needed is a compressor housing that has its sweet spot better matched to the power, and in this case of a 2 liter displacement, the higher boost being needed to make that extra power for the given displacement. With 91-93 octane fuel and a proper timing and fuel curve we simply do not need a huge intercooler because we just do not take the boost level up that much. This is because we do not boost levels that will cause detonation, unless of course ,one backs the timing way off and dumps loads of fuel into it. That will give a nice reading on the dyno. The car has awful throttle response and tuning hard setup like that will kill a motor over time. For this reason, buying a re-flash based on maximum HP quoted is all to often a bit of a fool's paradise. What we want is a motor tuned for proper ignition and fuel with the boost curve where it comes out against the detonation sensor. This gives huge torque, excellent spool and throttle response, and what ever power number that the motor feels good making on a given day. This is how the Prodrive Stage 3 is setup. 10 years of rally car experience, 6000 units sold and no blow motors.
The problem with 14,000 feet is the air density is 50% that if sea level. The effect is to make the turbocharger work as if it was not making 14 psi, but as if it were trying to make 28 psi at sea level. Besides over speeding the turbine, this is will physically overheat the motor due to too high of an inlet temperature for which no intercooler will correct. So, we have to turn down the boost starting at ~8000 feet as we have reached the limit of the turbo speed and are already way outside the sweet spot. The driver also has to gradually lower the operating limit of the engine from 6000rpm down to 4500 from 8-9000 feet to 14,000 feet . By limiting engine speed, working as an air pump the compressor is now limited to pumping as much pressure RATIO from the thin air as it was pumping from thick air at sea level. We have to remap the engine for these extreme altitudes and very high full throttle duty cycle, only seen at Pike's Peak. The next highest rally is Mexico WRC run at 6000-7000 ft, but the real kicker at extreme altitude is the constant climb.
Pat' Richard (Rocket Rally/Easy Street) and Stieg Bloomqvist (Prodrive, David Sutton Cars, Ltd) were the only Group N (30mm restrictor) cars that had ANY power as they went past me at 13000 feet Saturday. The Rocket Rally crew worked on the ECU mapping Tuesday and even Wednesday during the first stages of the rally to get it right.. Andy, the Prodrive UK engineer for Bloomqvist worked even Friday night tweaking their setup. Andy joked it was the slowest rally car he had ever tuned!
Pat had ten seconds on Stieg as of Saturday morning with the full climb ahead of them. Stieg was his usual smooth as glass self, but Pat's car broke a rear drive axle putting the power down on the very last of the lower tarmac section.
So the problem became that Pat had to try to keep our position by abusing the motor with too high of RPM, using 6200rpm all the way up and outside of any hope for a reasonable inlet temperature. This caused the turbo compressor to pump heat into the engine, which was being taken out through the cylinder walls . Just at the 14,000 foot finish we stuck the pistons as they continued to expand right past their normal racing clearance. Stieg was able to gain 10.5 seconds and just pull off the win. They do say you make your own luck in racing. In this case, Stieg's 30 years of experience got him to the top without breaking anything on the final run after three days of abuse. So much for altitude stories at Pike's Peak.
Regards,
==m==
Mike Shields
SPD Tuning Service
www.spdusa.com