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Exhaust system sizing for big HP

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Exhaust system sizing for big HP

Postby Orr89rocz » Wed Feb 12, 2014 2:35 pm

I have a question regarding exhaust pipe flow and sizing on applications over 1200 hp.

I have a 401" sbc with twin turbos making 1018 at tires thru 4500 stall th400. Probably 1200 or abit more at crank but theres more power left in it.

Its a street strip fun car so i have dual 3" all the way back to bumper. Xpipe and each bank has one dynomax bullet and one dynomax ultra flo round. Pretty quiet car but i have to think its restrictive.

I recall Vizard stating a typical engine needs 2.2 cfm per hp and an exhaust pipe flows 115 cfm per inch square of area. That said dual 3" should do 738 hp before being a restriction.
I cant fit dual 3.5" easily and thats even more heavy than what i have now. Turbos need to run 3" to firewall for clearance reasons and from there i could maybe force 3.5" duals back but i cant get them over axle easily.

Being inquisitive, i stumbled onto some 5" pipe. Calcs show flows of 2258 cfm, good to 1026 hp. I believe i can squeeze this in but need to do some measurements. If all is good, do you think this would produce substantial power gains?
I guess i could install some 3" cutouts and test that first to see if power improves. But i calculate 16% weight savings and the fact i got sewer pipe under my car that diesel trucks run intriques me. If it reduces post turbine backpressure i believe that will help my turbine performance as i already am making alot of power thru the smaller 64.5/74mm wheels. I already did 4" single to dual 3" on old motor combo and saw 2 psi less boost on same wastegate springs so it had to reduce backpressure in preturbine manifold. Thoughts?
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Re: Exhaust system sizing for big HP

Postby Dan Timberlake » Wed Feb 12, 2014 2:57 pm

I think I'd plumb a pressure gage into the pipe(s) up by the engine to see what I have at full throttle max revs.
Although performance after the change would be my final criteria.
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Re: Exhaust system sizing for big HP

Postby n2omike » Wed Feb 12, 2014 3:46 pm

I don't see the need for an X-pipe with a turbo application. With a turbo, you're just dumping gases... no scavenging or anything going on there. An X pipe just adds a lot of extra bends, and a reduction in cross section at the merge. Done 'right' on a n/a application, the gains are questionable. Done wrong, and they can be a major restriction. On a turbo car, I see zero gain, and a good chance at a loss.

I would want to run at least 4" mandrel bent pipe that was as straight as possible. What is the outlet size of the turbos? If you run larger pipe than the turbo outlet, I'd try and make a nice transition using a conic section if there's room, or at least a series of steps.

Just a Layman's thoughts. lol

Good Luck!
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Re: Exhaust system sizing for big HP

Postby Orr89rocz » Wed Feb 12, 2014 4:07 pm

3" vband connection on turbo. Pipes snake between two rear cyl header primaries which limits me to 3" there for install purposes. So i got about 5-7 ft of 3" before i can transition into a larger pipe. I could come off turbo in 3.5" for a foot or so but need to neck down to 3" again for the pipe that goes thru header and chassis

Dual 3" has more area than single 4". Already had single 4". Switched to dual 3" and saw 2 psi drop in boost pressure on same wastegate springs so that tells me i lost a good bit of backpressure in the system.
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Re: Exhaust system sizing for big HP

Postby MadBill » Wed Feb 12, 2014 6:23 pm

I'm normally a 'big pipes' guy (have a pair of 4" in, 3-1/2" out mufflers for my 495" BB Camaro if I ever finish it), but with your set up, the power difference from full length 3" to ditto 4" is likely less than another 2 psi of boost, so unless you are already on the ragged edge of disaster, it's likely better to leave it as-is.
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Re: Exhaust system sizing for big HP

Postby Orr89rocz » Wed Feb 12, 2014 7:31 pm

Well its not quite ragged edge but could be once air temps heat up outside. But i will add a meth inj kit for that or race gas mix. Reason i am thinking of doing this if it picks up 50-70 hp thats 1.5-2 psi boost worth of power and 1.5-2 psi less compression heat added to intake temps. Every little bit helps.

Just wondering if anyone has seen gains switching from smaller pipe to more appropriately sized systems on fairly stout boosted motors
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Re: Exhaust system sizing for big HP

Postby exhausted » Wed Feb 12, 2014 8:02 pm

Orr89rocz wrote:Well its not quite ragged edge but could be once air temps heat up outside. But i will add a meth inj kit for that or race gas mix. Reason i am thinking of doing this if it picks up 50-70 hp thats 1.5-2 psi boost worth of power and 1.5-2 psi less compression heat added to intake temps. Every little bit helps.

Just wondering if anyone has seen gains switching from smaller pipe to more appropriately sized systems on fairly stout boosted motors


Well this is fun, you can always help the turbos with larger exhaust pipes. I built a lot of the Underground Viper and Lambo kits and they realy like the x pipe. The issue as always is dragging the stuff off on the ground. That is the limiting factor... :)
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Re: Exhaust system sizing for big HP

Postby johnny5 » Wed Feb 12, 2014 8:16 pm

Are your wastegates atmospheric? How much exhaust gas is flowing through the wastegates maybe some food for thought.
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Re: Exhaust system sizing for big HP

Postby cuisinartvette » Wed Feb 12, 2014 8:43 pm

spintech and a number of other places offer oval tubing to save space
Just to get your head spinning :mrgreen:

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Re: Exhaust system sizing for big HP

Postby Orr89rocz » Wed Feb 12, 2014 10:19 pm

exhausted wrote:
Orr89rocz wrote:Well its not quite ragged edge but could be once air temps heat up outside. But i will add a meth inj kit for that or race gas mix. Reason i am thinking of doing this if it picks up 50-70 hp thats 1.5-2 psi boost worth of power and 1.5-2 psi less compression heat added to intake temps. Every little bit helps.

Just wondering if anyone has seen gains switching from smaller pipe to more appropriately sized systems on fairly stout boosted motors


Well this is fun, you can always help the turbos with larger exhaust pipes. I built a lot of the Underground Viper and Lambo kits and they realy like the x pipe. The issue as always is dragging the stuff off on the ground. That is the limiting factor... :)

Thats what i am thinking. Bigger cant be wrong atleast cant hurt. We shall see, i may try to get some data on current system as far as backpressure and such. Maybe add some quick cutouts at the down pipe ends to see if it picks up

Oval is out its gonna be to pricey.

Wastegates are 40mm and dumped back into downpipe. I'm at 24 psi now with control down to 6 psi, so i think the gates are good.
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Re: Exhaust system sizing for big HP

Postby justahoby » Wed Feb 12, 2014 10:28 pm

cuisinartvette wrote:spintech and a number of other places offer oval tubing to save space
Just to get your head spinning :mrgreen:

I have welded up 6 exhaust systems so far up nicely with a Lincoln 125, and flux core 0.035 wire.
I am not a weldor , but used to weld on a construction site back in 1998 on metal stud framing and been welding since. I have often thought of using alternative pipe shapes in tight areas, and this may tempt me to give it a go. :mrgreen:
I always thought it was not as good velocity , but that's just an opinion I can't prove..
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Re: Exhaust system sizing for big HP

Postby MadBill » Wed Feb 12, 2014 10:35 pm

Orr89rocz wrote:...Oval is out its gonna be to pricey. ...


Depending on the areas of interference, a pipe with smoothly 'crushed' sections can flow a very high percentage of that of am unmolested one...
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Re: Exhaust system sizing for big HP

Postby justahoby » Wed Feb 12, 2014 10:38 pm

MadBill wrote:
Orr89rocz wrote:...Oval is out its gonna be to pricey. ...


Depending on the areas of interference, a pipe with smoothly 'crushed' sections can flow a very high percentage of that of am unmolested one...

That was my question , thank you MADBill.
High percentage as 65%, 75%,90%?
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Re: Exhaust system sizing for big HP

Postby MadBill » Wed Feb 12, 2014 10:58 pm

Unless it's really flattened for a substantial portion of the length, 85-90%, maybe more.
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Re: Exhaust system sizing for big HP

Postby Orr89rocz » Wed Feb 12, 2014 11:05 pm

Soon as you crush a circle you lose area. Area is flow capacity. Most oval tubes are advertised as equals to a certain diameter round size, but dimensionally they are not simply one normal diameter just bent flat. Does it just act like a choke in a merge collector and just increase velocity?
I do have a small section of flattened pipe to give clearance around valve cover but i certainly dont imagine flattening 5" round for clearance everywhere underneath car as it would take away from its capacity
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