Paulyy

What would be interesting is if you dyno the car with your ideal spec exhaust (3.25") and SFR exhaust and see the different power readings, EGTs & exhaust back pressure in the X-over.


Just thinking hypothetically.. If you could measure the exhaust temp decrease from the turbo to the exhaust tip. Then work out how much the gas will expand from the cooling and make compensations in the exhaust pipe ID to keep the exhaust gases from expanding and keeping a consistent velocity to the tip of the exhaust.


If that makes sense.. I wonder if that would cause back pressure or actually help.

blade7

If I'm reading that right the exhaust gas would contract as it cooled hence a smaller dia would work further down the system ?.

Paulyy

Correct.

Don't know if its plausible or not but interesting thought.

333pg333

That's what it appears some people are suggesting. My assumption was that the less b/pressure the better therefore that's why we specc'd the SFR exhaust like that. I can't say with any surety if it was detrimental to the previous motor. That was basically totally stock except for bigger cam and GT3076. It seemed to take sometime to spool on the dyno, but that may just be reflective of how the operator loaded the engine up.

I'd like to do some back to back testing but we're running out of time and money.

blade7

Temporarily replace the last part of your SFR system with a piece of 2.5" pipe and see what happens ?.

blade7

Pretty much what I posted back in 2010 .

Paulyy

I know exactly what you're talking about with that SFR exhaust. I would have though the same thing too if the larger area is acting like an atmosphere.
There are so many ways to go about it. I should go though my physics books from school lol!

Must of missed it, link me please

blade7

Basically I said that when I ran a practically straight through system on my road car it definitely was more laggy, though it did seem to pull harder at the top end,sticking baffles in the tailpipes brought response back. Not exactly a conclusive test but it seemed to suggest that away from the dragstrip there's more to exhaust design than just going bigger.

refresh951

This is a very good read on the subject:

http://www.veryuseful.com/mustang/te...Scavenging.pdf

Dave W.

I think some of you are overthinking the issue. It's been said many times on this forum and other turbo forums, and it's still true-
a turbo car makes the best power and spoolup when post-turbine backpressure is kept lowest. Bigger is better, although there is a point of diminishing returns.

I can't imagine why an EVO shop would recommend a 2.5 exhaust on a 700hp evo. Maybe there was some mis-communication? Perhaps they were talking about intercooler pipe size? Yes?
I visit EVO forums often and no one is running a 2.5" exhaust. Seriously, the stock EVO exhaust is 2.3" so how would a 2.5" exhaust be considered an upgrade? It's not, everyone just goes up to 3", except for a few JDM fans who get 70mm size exhausts without realizing it's not quite 3". However there has been much debate among some top shops regarding the best IC pipe size, 2.5" vs. 3", and there's been no consensus. There's many high HP EVO's running 2.5" IC pipes with no problem. If you want to go a little bigger just use 2.75" aluminum (aka aluminium) pipes.

IMHO the debate over velocity vs pressure drop is simple. Anywhere air is flowing in steady-state conditions (such as turbo inlet pipes, IC pipes, and post turbine exhaust pipes) keep velocity low enough to prevent pressure drop. OTOH when air is flowing in pulses, such as the intake mani runners and exhaust ports/headers, higher velocity has the potential to be tuned for better output.

333pg333

See I don't quite get what you're saying with 'a turbo car makes the best power and spoolup when post-turbine backpressure is kept lowest. Bigger is better, although there is a point of diminishing returns.' ?

If it's best with no backpressure then in theory no exhaust is best...yet why are there diminishing returns?
Is there any reversion in the exhaust system? I thought there is and therefore size is important. Are there any cars (drag?) that run turbos with no exhaust at all due to not having to worry about noise?

Dave W.

'Diminishing returns' means that you'll get a nice bump in power when you upgrade from stock size exhaust to a 3" exhaust, but you won't gain as much power going from a 3" to a 3.5" exhaust. The difference between 3.5" exh to 4" exh is very small, not worth it unless you're running a very inefficient turbo at high boost.

There's a lot of reversion in our exhausts, but it occurs due to backpressure in the exhaust ports/headers/crossover pipe. All pre-turbo. The turbo is what makes backpressure high. Most decent modded 951's with upgraded turbo's have backpressure/boost pressure ratio around 1.5:1 which means if you run 15psi of boost you have 22.5psi backpressure in the crossover pipe. Stock turbo's at high boost can see upwards of 3:1 ratio or 45psi backpressure at 15psi boost. That's where the reversion comes from.

and Yes, you'd get the best power if there was no exhaust after the turbo, except it might light the car on fire. That would be bad.
Many drag race turbo cars run with open downpipes, basically just a large open pipe to route exhaust away from the car. Some of the better setups use a side exit exhaust, or even a hood exit.

333pg333

Ok, I had it wrong. The new exhaust on the car is in fact 3” and so is it on that EVO I mentioned. Brain fart or whatever!
I’m happy enough with that to start. We can measure the backpressure after that and make changes if needed.

mikey_audiogeek

Well said!

Cheers,
Mike

TurboTommy

That's a huge mother exhaust wheel.
Just to be clear; that's the smaller section diameter of the wheel where the exhaust comes out.
That can't be right.
You probably mean 72 mm is the overall diameter of the wheel which is something about a stage 5?
With that you can get away with 2.5" down-pipe and gradually increasing to 3.5 or 4.0
Air should always flow to an ever increasing flow area. So, same on the intake; never have bigger intercooler pipes than the throttle body. It creates pressure drops like somebody already mentioned.