Exhaust system sizing for big HP

[Orr89rocz]

I could go 3.5" to firewall from turbo but still needs to neck down to 3" to go between header primaries on last 2 cyl due to chassis limitations. From under headers, i could go back up 3.5" perhaps but need tight radius turn to make it fit. Then into a flowmaster Y merge 3.5" to single 5". Squeeeeeze that down driveshaft tunnel

But i may just leave it 3" to firewall and try 3.5" under car to Y pipe

[BrazilianZ28Camaro]

I could go 3.5" to firewall from turbo but still needs to neck down to 3" to go between header primaries on last 2 cyl due to chassis limitations. From under headers, i could go back up 3.5" perhaps but need tight radius turn to make it fit. Then into a flowmaster Y merge 3.5" to single 5". Squeeeeeze that down driveshaft tunnel

But i may just leave it 3" to firewall and try 3.5" under car to Y pipe

Its a bit weird but, could you run 3.5 or 4" turbo to firewall then install a "Y" to two 2-2 1/8" pipes and after the headers join then again to 4" ???

Should work I'd say.

You might want to delete a pair of mufflers also, just one correct sized pair should do the job.

Mufflers are a restriction no matter what they say.

[Orr89rocz]

I may be able to run them in a y and spit back together for packaging purposes. Mufflers i have limited space as it is, so i got 2 small ones per bank to quiet this thing down as much as possible. I like how it sounds but i could delete the front pair. We shall see. Got to do some thinking once my other project is done

[CamKing]

How does the change in exhaust temp change the CSA requirment of the exhaust pipe ?

[Orr89rocz]

Well exhaust temp changes velocity pressure and sonic velocity, all used in sizing and tuning exhausts.

More heat is greater expansion so makes sense to me that it would want larger csa. As gas cools towards rear it slows and gets more dense. May not require as large a pipe but more surface area in long distance runs has more flow losses, so perhaps can make up for it in stepping up in size

[CamKing]

Has anyone ever done a test on the dyno, where the only thing you changed is the length of exhaust pipe between the collector and the muffler ?
If so, what did you find ?

[CamKing]

Has anyone ever done a test on the dyno, where the only thing you changed is the length of exhaust pipe between the collector and the muffler ?
If so, what did you find ?

When we did this, we found that lengthening the pipe between the collector and muffler. gave the exhaust more time to cool off before it got to the restrictive muffler, and the power picked up. Any lose from the added surfice area of the extra exhaust was out-weighed by the advantage of cooling the air going into the muffler.
For this reason, I think your best bet is to increase the exhaust size out of the turbo, even it you have to step it back down to 3". At least by the time you have to step it back down, the exhaust will have had some time to cool.

[Orr89rocz]

Makes sense to me. I think i have a plan now. Just got to decide if i want to execute it. Just got car running i'd hate to change everything again lol. It still stands to reason that larger pipe downstream still can help here

Thanks for the input

[CamKing]

It still stands to reason that larger pipe downstream still can help here

I'm not sure.
If you've got "X" feet of 3" tube, and the exhaust is hottest right out of the turbo, the 3" tubing becomes less of a restriction, as the air cools while moving down the tubing. If that's correct, increasing the tubing size "X" feet beyond where the 3" tubing is most restrictive, won't help the restriction.

BTW, I've got a ton of turbo experience, but we just let the exhaust exit out the turbo, so this is new to me.

[MadBill]

Flow resistance is proportional to pipe length, so for example a single 'half walnut' clearance dent in a primary pipe may have virtually no effect on power. Conversely, even though the flow resistance per foot of tailpipe may be less near the rear as the gas cools, a larger pipe for several feet there will still contribute to a reduction in the overall system B.P.

[Orr89rocz]

Whats the best way to determine if it is a restriction?

Pressure taps along the system? EGT readings along system? At peak hp rpm i assume.

Find a way to optimize pressure drop across turbine, reduce the backpressure in the post turbine system. Obviously backpressure is no good in any engine as that increases pumping losses correct? So if i get pressure data i can tell how bad of a restriction it is, assuming pressure gauge doesnt pick up pulse spikes and can average the values, and assuming lowest pressure is best

And madbill that is what i was thinking, length of pipe losses may start to become more of a factor regardless of exhaust cooling. If exhaust gas temp is 1000-1300 F out the turbo, high pressure, and it cools to 300-400 degrees by end, i know theres alot of pressure drop in that constant volume of pipe. If i step up a larger size, the cooling effect may be more pronounced and reduced length of small pipe cuts losses so should alleviate pressure

[MadBill]

Pressure taps are the best way. You could could check right at the turbo outlet and at the potential enlargement point(s). E.g., if the B.P. was 8 psi at the former and still 5 at the latter, a bigger pipe rearward would be a benefit, but if was 2, the potential gains would be very small. BTW, if you use a mechanical gauge to measure B.P., make provision for damping out the pulses, say a pair of vice grips squeezing the hose enough to damp out the signal. (although a turbo should have a little smoother flow)

[ptuomov]

Surprisingly its not very loud. Didnt need ear plugs in dyno room unlike my 320 whp ls1 car lol. 4 mufflers help but the exhaust thrust coming out was significant. Blowing banners on wall 15 ft behind car with force. I hear mostly turbo noise. I didnt want to dump wastegates but wouldnt be hard to do

I blew a baby stroller out thru the garage door once with the exhaust pipe flow, that was a single 4" pipe. Luckily no baby in it. It really flew out, but it's one of those more expensive light-weight ones that ladies like to show off. Now that I think of it, it probably wouldn't have flown that far with a baby in it, the way the BMIs are trending.

[user-9613590]

There must remember that the flow from turbine is not just down the tube;it's strong swirl and middle of the pipe is poorly used.That's the main reason why ridiculous big downpipes work.After the swirl is dampened and flow is along the pipe the area can reduce.

[swatson454]

I blew a baby stroller out thru the garage door once with the exhaust pipe flow, that was a single 4" pipe. Luckily no baby in it. It really flew out, but it's one of those more expensive light-weight ones that ladies like to show off. Now that I think of it, it probably wouldn't have flown that far with a baby in it, the way the BMIs are trending.

Funny stuff, man