Turbo Exhaust Theory

[simpletool]

standard a R33 GTST makes nearly 250hp so on anything but a bone standard car 2.5inch is a waste of time. 3 inch (76mm) is the smallest exhaust you should be looking at for a 33 GTST. better even to go straight to 80mm which is a touch bigger.

+1

295 at the wheels is close to 350 at the engine ..Your exhaust flows engine hp not wheel hp..

+1

As for decent split dump systems. A good split dump would be great but ones that the pipes don't match up the turbine outlets (almost all of them) would create massive turbulence issues. A good split dump require the flange to be well adapted to the turbo outlet. The 2835 Pro S split dump is short but looks really well made, I'm sure that is part of the reason these turbos respond so well on the RB25.

I've changed to a bellmouth and I prefer the sound it makes - without the whooshing from the wastegate.

Edited June 28, 2011 by simpletool

[Kinks]

If a GTST can run 9psi in standard form, how is the standard exhuaust too restrictive?

engines are in fact air pumps, the more air you flow through a motor the more power you are making. volume counts, not boost level. you can boost up on a restrictive exhaust but you won't make much more power, diminishing returns set in because backpressure in the exhaust starts to balance the boost you're feeding in.

[Silvia GTR]

Seen this a couple of times now...

[Nee-san]

engines are in fact air pumps, the more air you flow through a motor the more power you are making. volume counts, not boost level. you can boost up on a restrictive exhaust but you won't make much more power, diminishing returns set in because backpressure in the exhaust starts to balance the boost you're feeding in.

Yes, your boost pressure is actually a measure of your engines inefficiency at processing air.

[sh@un]

engines are in fact air pumps, the more air you flow through a motor the more power you are making. volume counts, not boost level.

Not exactly. It's not airflow through the engine that matters, but rather the actual density (or oxygenization) of the air being drawn into the cylinder that determines how much fuel you can add, and thereby how much power you can make. that's why MAF sensors measure the Mass of the Air Flow, rather than the volume, and why there's a temp sensor in the intake for compensation. You could easily go up in CFM, but actually lose charge density per firing.

I agree unreservedly that boost level is largely irrelevant- only a muppet would think it's better to run 30psi for 600hp if the same engine could make the same power anyway on 25psi (and i know some people seem to think it's some kind of 'street cred' to have a huge boost number.

[Nee-san]

not exactly. it's not airflow through the engine that matters, but rather the actual density (or oxygenization) of the air being drawn into the cylinder that determines how much fuel you can add, and thereby how much power you can make. that's why MAF sensors measure the Mass of the Air Flow, and why there's a temp sensor in the intake for compensation.

I agree unreservedly that boost level is largely irrelevant- only a muppet would think it's better to run 30psi for 600hp if the same engine could make the same power anyway on 25psi (and i know some people seem to think it's some kind of 'street cred' to have a huge boost number)

Yeah, I don't think he was arguing against that in what he said, I guess the default assumption is "for a given intake temp"

and on the second point there is another large community where that is pretty prevalent Lol....

[sh@un]

we can all point and laugh when they're stuck at the side of the road re-attaching yet another blown off piece of induction hose i guess

[Harey]

Geeze I am glad this article was posted up, seems as though the outcome most people have come to is to buy 2.5" exhausts??????????

You realise it has 3 paragraphs explaining why bigger is better. Only negatives can be sound, so get something thats not loud but still flows well.

[blk94r33]

Right......

So........

"Hi, is a 2.5" exhaust big enough for standard boost on a 33 GTS+T?"

"Nah you need a 3" exhaust"

"Um, ok, didn't GTST's come with 2.5" exhausts standard?"

"Nah you need a 3.5" exhaust"

"I don't understand the point of a 3" exhaust, I only want to run near standard boost, meaning near standard flow INTO the engine, equaling near standard flow OUT of the engine, about ~10psi, a stock GTST exhaust can flow sufficient exhaust gas created by the burning of the air-fuel mix that the engine induces running the flow generated into the engine at 9psi, so surely a mandrel bent 2.5" system with a good cat and mufflers should do the same if not better job, I can't run much more than that anyway being a DE+T"

"You'll need an 12" exhaust"

"That doesn't make sense to me"

According to the experts here, the best option would be to to run a 5" in-case i go to 500kw later?

QUOTE:

As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.�

To get my car to the 250hp mark, it needs alot more than a 3" exhaust. Such as a decent inter-cooler, engine management and probably a clutch, at the very least.

I'm not arguing that a larger exhaust on a turbo car is a bad thing, apart from the noise, and to a certain extent the loss of low-mid range power. I understand the principle that you want as much pressure in front of the turbo as practical, and as little as possible behind.

I'm not arguing that pressure and flow are un-related, pressure is flow vs. a restriction, currently the car drops 1.5psi towards red-line, primarily, I suspect due to still running the N/A center section of the exhaust, thus, i'm changing it. I want 2.5", for now, as it will suit my current goals and not be excessively loud and attract attention.

If it turns out that it is too restrictive, then i'm down $200, go to 3", job done. The mufflers and cat I now have will support this, with the addition of a different dump/front pipe.

If not, and it holds 5psi from 2500rpm to red line, then i'll be happy.

The guy doing the job makes a living out of building exhausts, doesn't have to advertise due to word-of mouth, and knows his stuff, if he believes that 2.5" is enough as he has convinced me, then i'm happy to give it a go.

If i'm proven wrong, then it's my mistake for not listening to the guys with the 4 figure post counts and a list of peak power outputs for certain turbos sitting next to their computer.

If anyone has bothered to look at my original post I asked what size the factory dump/front pipes were..... Why does every thread have to turn into a shit-storm.....

[ARTZ]

2.5 is fine.. My car made close to 300 engine hp (185rwkw) with a 2.5 inch exhaust standard intercooler and turbo on 11psi..

But a 3 inch doesnt need to be loud and is just as good and if you do want more power later you wouldnt need to change it... But now you do..

I really dont care what you do though, u asked for advice and we gave it..

[ARTZ]

my car now has 400hp maybe i should put my 2.5 inch exhaust back on it

[BrOwnY GT]

... i recently got a 3inch done turbo back on my gts-t completely stock not boosted got close to a 20kw gain so... thats all i care about have you're massive debate about size's rofl...

[blk94r33]

2.5 is fine.. My car made close to 300 engine hp (185rwkw) with a 2.5 inch exhaust standard intercooler and turbo on 11psi..

But a 3 inch doesnt need to be loud and is just as good and if you do want more power later you wouldnt need to change it... But now you do..

I really dont care what you do though, u asked for advice and we gave it..

But I didn't ask for any advice, I asked what size the standard GTST dump/front pipes were.....

I'm not fussed if I do have to change it either way, as it's not the most expensive part of the project. Whatever happens, i'll deal with it.

[MrStabby]

I'm not arguing that a larger exhaust on a turbo car is a bad thing, apart from the noise, and to a certain extent the loss of low-mid range power.

What leads you to believe there will be a loss of midrange power?

If anyone has bothered to look at my original post I asked what size the factory dump/front pipes were..... Why does every thread have to turn into a shit-storm.....

This isnt a shitstorm. Relax bro.

Also, you could;

1. get the 2.5in exhaust

2. get dyno run with it on, then another with it dropped so see if there is any power difference

3. post the result for the good of the community

[BrOwnY GT]

"I'm not arguing that a larger exhaust on a turbo car is a bad thing, apart from the noise, and to a certain extent the loss of low-mid range power. "

short video stock 3'inch with dyno, just need a 2.5' one now lol

Edited June 28, 2011 by nismo07

[blk94r33]

What leads you to believe there will be a loss of midrange power?

This isnt a shitstorm. Relax bro.

Also, you could;

1. get the 2.5in exhaust

2. get dyno run with it on, then another with it dropped so see if there is any power difference

3. post the result for the good of the community

After searching around here and the net generally, *some* believe that there is a small loss in power across certain RPM with different size exhausts, even on turbo cars... I'll try find links.

I am relaxed, "I ain't even mad", I originally said "2.5" is good for 250-300hp" that's more than within my goals, 2.5" will suit and hopefully sound nice without causing problems, asked what size the factory dump/front were, then got molested by the 3" warriors....

And yes, the plan is to dyno the car once the exhaust is done and boost controller on, not only to see what it's making now, but how much can be extracted before detonation or leaning become a problem...

As i've said, repeatedly, i'm not fussed if the pipework needs changing after a dyno run, i'm happy to deal with the consequences....

Edited June 28, 2011 by blk94r33

[Harey]

250hp at the engine is only roughly 150rwkw. That is what a standard RB25DET makes.

A normal setup with a few mods is around 190rwkw so as said by Arthur thats around 300hp at the engine. So going by the quote of 250-300hp you will be right at the end of the range. The article goes into great detail about why having a larger turbo back exhaust can only improve things. So if you are getting a new exhaust made then by the logic of the article you would get a 3" exhaust. You can say you dont care about the article, but then why are you posting in this thread?

[Harey]

2.5 is fine.. My car made close to 300 engine hp (185rwkw) with a 2.5 inch exhaust standard intercooler and turbo on 11psi..

Doesnt mean you wouldnt have had good gains with a 3" exhaust instead

[blk94r33]

250hp at the engine is only roughly 150rwkw. That is what a standard RB25DET makes.

A normal setup with a few mods is around 190rwkw so as said by Arthur thats around 300hp at the engine. So going by the quote of 250-300hp you will be right at the end of the range. The article goes into great detail about why having a larger turbo back exhaust can only improve things. So if you are getting a new exhaust made then by the logic of the article you would get a 3" exhaust. You can say you dont care about the article, but then why are you posting in this thread?

I never said I don't care about the article.

I'm posting here as I wanted to know the size of a standard dump/front pipe, and it was the only exhaust thread getting replies...

[Lithium]

After searching around here and the net generally, *some* believe that there is a small loss in power across certain RPM with different size exhausts, even on turbo cars... I'll try find links.

As i've said, repeatedly, i'm not fussed if the pipework needs changing after a dyno run, i'm happy to deal with the consequences....

Good on you for striving to do your own research/experimentation to satisfy yourself that you've done it right - there IS a lot of BS and misunderstanding on the internet and its self propegating, I wish I worked out way earlier that even a lot of the more clued up seeming (and sometimes actually are mostly switched on) people have holes in their understanding of things, no one can have all the answers. A lot of the time given explanations for a rule of thumb which is correct are wrong in terms of reasons, and thats where things can go awry.

Having said that, all things being equal (no compromising bends, equivalent mufflers etc) I am absolutely sure that going to a 3" or bigger exhaust will cost you nothing anywhere versus a 2.5" system on your car. Any loss in torque anyone has experienced with a turbo car as a result of changing exhaust would more likely be tune related, or some other issue - especially if the airflow (in an airflow metered car) reaches levels which there is overly safe mapping for in that particular instance.

Bare in mind when changing a cars setup that there are always all sorts of variables at play, don't always assume that the most obvious thing (or the first thing you think of) is actually the cause of the final outcome.