Rb26 Aftermarket Exhaust

[discopotato03]

Well its your calls but from my experience doing exhausts in sections starting at the front isn't a bad idea .

I don't see any reasons other than packaging that you couldn't start out in the 3.5 to 4 inch from the turbo to the cat and progress from there .

Also other than the transition in tube size I never found any losses from being big up the front and not so much in the middle and rear sections . I think this is a good way to go because you can develop an exhaust around how much power/gas flow you need vs the sound level you can live with / get away with . In my experience all the muffling gear is in the second half or rear third of the system so this should be where if there has to be flow limitations they should be .

The exhaust gas pressure and temperature ie always highest in an exhaust directly behind the turbo so this is where easy power can be gained or lost . If you have any significant pressure (referenced against atmospheric pressure) here then the gas out of the turbo won't pass as easily as it could do if there wasn't .

Take it to the grave , exhaust temperature drops very quickly after the turbo and when it does its density decreases and all else being equal it will slow down or lose momentum so to speak .

So in my opinion make it BIG up the front and experiment with slightly smaller tubing along the way to keep the gas speed from falling too quickly .

Also being big at the front tends to increase turbo whistling and whooshing noises if that's your thing .

My opinions only , cheers A .

[juggernaut1]

Who's theory is that?

I'd argue it's equally as important the whole length. No point having a great front section if the back end is junk

Some of the WRX owners are fitting smaller diameter tailpipes to their aftermarket exhausts and the dyno's they've been posting up are showing reasonably more mid-range and nominal peak hp loss.

Gas speed is higher where they are at their hottest...immediately post turbo...and cool down from there...... resulting in less flow requirements as the exhaust progresses.

Also scroll down to the part: Can a free-flow exhaust reduce efficiency?

http://www.max-boost.co.uk/max-boost/exhaust/exhaust.htm

Edited April 28, 2010 by juggernaut1

[R31Nismoid]

^ The stuff further down the page by the Garrett Engineer does tend to disagree with some of what's on the page earlier which is interesting to read (for those that might not scroll further down)

[FineLine]

Who's theory is that?

I'd argue it's equally as important the whole length. No point having a great front section if the back end is junk

Agreed, I run 3 and 1/2 all the way and never found a restriction until I added a custom free flowing 3 inch rear muffler to try and quieten it down a tad. The resulting power loss saw it binned faster than you can blink.

I then tried a flapper valve just before the cannon and got excactly the same power loss. 3 and 1/2 all the way for me.

[juggernaut1]

^ The stuff further down the page by the Garrett Engineer does tend to disagree with some of what's on the page earlier which is interesting to read (for those that might not scroll further down)

Yes his comments also disagree with the real world on the dyno findings too in relation to WRX's which is what his comments were aimed at....although I suspect he's generalising and applying that to WRX's.

Edited April 29, 2010 by juggernaut1

[Harey]

Agreed, I run 3 and 1/2 all the way and never found a restriction until I added a custom free flowing 3 inch rear muffler to try and quieten it down a tad. The resulting power loss saw it binned faster than you can blink.

I then tried a flapper valve just before the cannon and got excactly the same power loss. 3 and 1/2 all the way for me.

I am not sure what power you are making so its hard to say. Obviously in your case 3" even at the end of the exhaust is too much of a restriction. In this case I would propose that maybe a 3.5 inch dump pipe at the front is only just flowing what you are giving it. Possibly a 4" dump would increase power and response. And yes you could leave the 3.5" cat back system as the exhaust gasses have cooled down by then.

[discopotato03]

Actually I had not opened that link but Jason Cavaganah knows what he's talking about and I gladly defer to him .

I don't know if he's still working at Garrett North America or some other part of the Honeywell group .

A .

[FineLine]

I am not sure what power you are making so its hard to say. Obviously in your case 3" even at the end of the exhaust is too much of a restriction. In this case I would propose that maybe a 3.5 inch dump pipe at the front is only just flowing what you are giving it. Possibly a 4" dump would increase power and response. And yes you could leave the 3.5" cat back system as the exhaust gasses have cooled down by then.

Agreed, I believe I was at the limit of what that system could flow.

[Ten Four]

Haven't read whole thread, but what would it cost approx for a full 3.5" turboback?

[R31Nismoid]

Mild steel or Stainless?

Off-shelf mufflers or custom made?

What size of CAT?

As you can see... costs can vary pretty easily

[N1GTR]

Dont be lazy.

Go to a local shop and ask.

[Ten Four]

Stainless, decat.

I only ask cause I had some ghey big arse twin muffler system and I removed it for a 3" custom stainless as a quick fix ($550...)

Prev owner ceramic coated exhaust manifolds, turbine housing, dump pipes and front pipes - don't know sizing...

I'm down on power for what my setup is and am trying to figure out why atm.

[T04GTR]

exacly why i think go as big as as you can from the get go.

[R32 TT]

Prev owner ceramic coated exhaust manifolds, turbine housing, dump pipes and front pipes - don't know sizing...

I'm down on power for what my setup is and am trying to figure out why atm.

Stock Cams in that car James. Could this be the issue perhaps? When you say "down on power" what are you making? Isn't it making 400+ awhp?

.............(the other possibilty is it has pink wheels.. easy losing 50hp and small dose of street-cred..)

Edited April 29, 2010 by R32 TT

[Ten Four]

Ha. Aftermarket cams, not dialed in. Andrew at hyperdrive assured me that there would be an improvement but nothing drastic...

Its making around 370rwhp at 19 to 20ish psi.

Wheels are black now lol

[N1GTR]

Stock cams aren't such terrible things.

I managed 476hp atw on stock cams, the ecu wouldn't run more boost but it wouldve made heaps more.

Also, RacePace do everything on stock cams, do a search for "RB29" and that will show you stockers aren't that bad

[Scooby]

Stevo was that with the 26 or 26/30?

[N1GTR]

30, it's using tomei cams now though

[BeEnAr]

What about running twin 2.75 inch off each turbo? Or would you still need 3 inch? That would make it interesting, I've seen the twin pipes (one off each turbo as it's own system) at Central Circuit Drags in Japan but wouldn't have a clue what size they were, I suppose that would be another avenue to go down.

For the record I run unknown front pipes into 3 inch merge then Unknown high flow cat and 3.5inch Kakimoto after that with 2 mufflers. The least amount of restriction in a turbo car the better I always thought, you don't need to worry about back pressures like you do in N/A.

On the WRX's with them running the smaller rear pipes off the mufflers, they do that in WRC too so must be a reason for it, it's not to make them quieter because they are still loud as f**k!

[discopotato03]

I believe in the World Rally Car class they reduce the latter part of the exhaust to keep the gas speed up as it cools which helps make more mid range torque .

With WRC they are limited by turbo air restrictors so they have to make as much torque as possible over a limited rev range . They're changing up at five and a bit thousand revs because the restrictors won't allow enough airflow for them to make usefull power any higher .

Between ALs and the restrictors they do make good torque but its very hard on the engines .

A .