WTT: Invidia N1 Street Catback for Cobb/similar

[rory_a]

Looking to quiet down the car. Looking to trade my Invidia N1 Steet Catback for a Cobb, ProDrive, Stromung, etc. something clean and quiestish. Might have some money on top for the right deal.

May also sell for the right price, just need to get something quieter. I don't know what that price is though, as I have been pricing them out, and I love the N1, most of the time.

[Jeikun]

So... What would be the right price?

[WRX06TR]

Hmmm...what do you think about the Blitz RX? Looks like the R but is much quieter. I don't have any sound clips of it tho.

[rory_a]

Jake, I don't know to be honest. Not super motivated to trade or sell, and really just looking to see if there's a quality quieter system out there..

Nick, I had the Blitz Nur Touring on the wagon and loved the sound, but not the coffee can tip. I'm thinking if I trade out it needs to be 3" all the way through as well, I don't think I'll have the vr39 forever.

[rory_a]

* I've been wanting to have an axle back made with a little bullet muffler and a straight pipe looking tip. Straight pipe without the noise. But I have a feeling the straight pipe would look out of place on the car as it sits now.

[rory_a]

Bumping this up because it's getting down to d-day... And one of the last things I need to do to make the car civil is quiet it down.

So, any Cobb, ProDrive, Stromung, MadDad, etc., exhausts out there?

[Snoshado]

Are you kidding? You had the sexiest sounding car out of them all this weekend! Your STI sounds like a real race car

[rory_a]

Priorities change. And it's no race car, 10x more into making it a better street car than making it fast. I'll change it's route one day, so currently looking for a compatible axleback or catback to buy. As it is, I put 40 miles on it a day, everyday, and only race it 12 or so times a year.


So, any ideas on compatibility of a ProDrive oval axle back or Cobb axle back to Invidia midpipe? Power effects of dropping to 2.5" at the axle back mating?

[cody]

Originally Posted by rory_a

Priorities change. And it's no race car, 10x more into making it a better street car than making it fast. I'll change it's route one day, so currently looking for a compatible axleback or catback to buy. As it is, I put 40 miles on it a day, everyday, and only race it 12 or so times a year.


So, any ideas on compatibility of a ProDrive oval axle back or Cobb axle back to Invidia midpipe? Power effects of dropping to 2.5" at the axle back mating?

I doubt that dropping to 2.5" (especially that far downstream) would make a lick of difference on the stock turbo, even at stage 2. When the gasses cool further toward the tailpipe, smaller diameters become less restrictive.

[Kevin M]

Originally Posted by cody

When the gasses cool further toward the tailpipe, smaller diameters become less restrictive.

This is pretty much pseudoscience bullshit. Is a 2.5" catback really that restrictive? No, if the muffler flows well. Is it because of pV=nRt? Absolutely not.

[cody]

Originally Posted by Kevin M

This is pretty much pseudoscience bullshit. Is a 2.5" catback really that restrictive? No, if the muffler flows well. Is it because of pV=nRt? Absolutely not.

Your mom's PV=nRt. I'm pretty sure it's well accepted that bends and smaller diameters are less likely to reduce power when they're closer to the tailpipe, but you're obviously the authority.

[Kevin M]

I'm simply saying that a generalized correlation seen among modified Subaru turbos- that a 3" catback mated to a good 2.5" catback or axleback is closer in performance to a full 3" than full 2.5"- is true, but it has been falsely attributed to the Ideal Gas Law, and to the idea that the exhaust flows better downstream because it is cooler. If you're ever around a dyno operator who is silly enough to let non-employees close enough, stick your hand right behind the tailpipe during a full boost, full throttle run and tell me that the exhaust has cooled down all that much. Or just look at my rear bumper sometime.

[cody]

Originally Posted by Kevin M

I'm simply saying that a generalized correlation seen among modified Subaru turbos- that a 3" catback mated to a good 2.5" catback or axleback is closer in performance to a full 3" than full 2.5"- is true, but it has been falsely attributed to the Ideal Gas Law, and to the idea that the exhaust flows better downstream because it is cooler. If you're ever around a dyno operator who is silly enough to let non-employees close enough, stick your hand right behind the tailpipe during a full boost, full throttle run and tell me that the exhaust has cooled down all that much. Or just look at my rear bumper sometime.

So what is it attributed to? I'm sure turbulence plays a roll too.

And I've always heard that exhaust temperature drops dramatically every inch after it exits the cylinders. But go ahead and mount your EGT probe in your tailpipe instead of the headers, by all means.

[Kevin M]

The coupling of thermodynamics and aerodynamics in an unclosed system is arguably the most difficult physics we know of above the quantum level. My point is that if someone without a pHD tried to explain why exhaust diameter downstream matters less than upstream, they're doing some combination of guessing, repeating something they read on the internet, or talking out of his or her ass. So no, I can't explain the phenomenon to you in precise detail. I can say with certainty thanks to more than a decade of relatively good empirical dyno results that, given good component design (meaning good flowing mufflers, if present) that a full 3" turboback makes the most power, a full 2.5" the least, and a 3" front, 2.5" back setup is in between, but favoring the full 3" performance. The reason it's not a 50% difference is that downpipe inlet design is a more important factor than diameter.

Sorry for pulling the de rigeur SECCS nanoargument on you. It's political season, which gets me all riled up about anything a person asserts when it's using false arguments, bad logic, hearsay, or "pretty well accepted" conventional wisdom. Yes, you are correct that a 2.5" aftermarket catback or axleback is a pretty minimal sacrifice in peak power on a stock turbo Subaru. But it is the claim that the colder exhaust needs less volume, which dates back to a guy from Turbo XS incorrectly applying the Ideal Gas Law to exhaust flow that annoys me. Largely that's attributable to me being an employee of Shiv Pathak at the time it happened, and people ignored Shiv correctly pointing out that the ideal gas law only aplies to closed systems like baloons filled with air and pressure cookers and the like, because he happened to be a huge douche about arguing with anything his competitors said on the internet, especially if it was wrong.

If you want my poorly educated guess, I would say it's just that the pipe diameter in the latter half of the turboback has become a second order factor or below. Turbulence, bend design, muffler baffling and design, all these are more important. The thing to remember about exhausts is that the single most restrictive part of it is nearly all of the power loss you get from that exhaust compared to having none at all. So, if the muffler baffling design is the most restrictive part of the catback, and it has a randomly assigned restriction value of 12, and 3" piping has a value of 6, and 2.5" has a value of 8, the total restriction is going to be close to 12 if you ignore other factors like bend shape and the transition from 3" to 2.5" regardless of which diameter you are using for the catback.

[cody]

Fair enough. I'm not married to the idea that it is wholly or even partially related to gas temps anyway. I'd imagine it has more to do with turbulence being more intense right after the turbo.

[Kevin M]

Originally Posted by cody

Fair enough. I'm not married to the idea that it is wholly or even partially related to gas temps anyway. I'd imagine it has more to do with turbulence being more intense right after the turbo.

Basically yeah. Probably* more than half the total restriction of the stock exhaust is the shitty flange design.

*according to pseudoscientific personal observation and tuning lore I've picked up over the last decade

[Kevin M]

Maybe next month JC will pop in and give a close approximation of what's actually the cause of this observed phenomenon. He's probably the only person I have actually met who is qualified to do so.

[sperry]

I will say this: the exhaust coming out the tailpipe is much much cooler than the exhaust in the headers. Probably on the order of 1000F-1500F cooler. Not that 400F at the tailpipe is a cool breeze, but Cody is right about the exhaust cooling dramatically.

As far as the pv=nrt argument, I actually remember that thread on I-club. What a nightmare of people making up science that was.

More to the point of the thread. I went from that Invidia to a catted turbo xs DP and prodrive muffler. It's not much quieter. Sounds a little less angry, but if you're really looking for something stealthy, figure out how to bolt a stock sti muffler on there or something.

[rory_a]

This turned into quite the wtb thread! I was not so much concerned about the piping size change, and considering the two axle backs in question were known to produce good power, I wasn't worried about them being a bottle neck. I found a thread stating dyno results from a ProDrive/Turbo XS stealth back system in comparison to a stock STI axle back with the TXS seeing similar numbers, negligible drop. My main concern was dropping a half inch at the flange with no transition.

However, swapping my axle back is no longer an option, I don't believe anything larger than the Invidia can muffled will work with my new setup.

[michio10]

I got a set of Borla Cat back. its very quiet even with straight pipe... its nice