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Optimal Exhaust Diameter?

hello...wondering what the best pipe diameter for a N/A setup is?

also...i'm in the market for a new exhaust manifold, preferably one that will mate up nicely with the egr pipe, as i've heard that can be an issue. looking to replace the entire exhaust system, as mine is pretty well rusted out.

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Re: Optimal Exhaust Diameter?

I've heard 2.25 is best... with mandrel bent curves.... not crush bent. But I know that TwoPedalWarrior has 2.5 and his sounds and performs like a beast.

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Re: Optimal Exhaust Diameter?

2.5 is the best in my opinion on a 4 cyl you do run the chance on not having enough back pressure. On a kl.... well I don't own one but id say you don't have to worry about that. 3" would be for boost on a kl. IMO.

Im running a 2.5" cat delete, dual aero exhaust resonators mandrel bends and a dtm knock off muffler. With a header. If you click on my signature there should be a link to my exhaust video.That being said I believe there is a sticky with exhaust tone videos on here.

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Re: Optimal Exhaust Diameter?

I recall a thread about it on here and it said for optimal torque do 2.25 mandrel bent, and more top end do 2.5 mandrel. This is for n/a of course
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right on guys, thanks for the info.

what about a decent exhaust manifold? there's some gains to be made there too, right?

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Re: Optimal Exhaust Diameter?

Is this a kl or fs? I've seen major gains on exhaust for the kl
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its the FS

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Re: Optimal Exhaust Diameter?

If you go mandrell bend 2.5" with a decent header (you want a 5/8" exhaust flange or better) the 4 to 2 to 1 headers off fleabay youll lose a little low end power from lack of back pressure. If you plan on boost later than ignore that for now. I want to swap my 4 2 1 header to a 4 into one for higher flow and one of my collectors is dented.

If you delete your cat plan on it being really raspy. If you delete your cat and resonator plan on it being raspy. If you run a straight pipe with out a muffler... loud as hell and probably raspy. Although I have no experience with the latter.

I have tried to get as much rasp out of mine as possible but the only way to do that is a dual resonators, high flow cat, staight through muffler and piping all the same diameter all the way through. NO CRUSH BENT! Don't run glass packs right after the header as the glass will burn out over time.

My buddy did my exhaust and the only complaint I have is there are two pieces of my exhaust that are sleeved over a smaller piece of pipe and my muffler is ugly and probably not straight through.
Just my two cents. I have ran my fs with a hollowed cat and no resonators, hollowed cat and dual resonators and without the hollowed cat and dual resonators. What I have now sounds best. Imo but after I deleted my egr and swapped a cam its pretty loud. I want to get my other exhaust tip on and take it from there.

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Quote:
Originally Posted by friendsofdonr View Post
If you go mandrell bend 2.5" with a decent header (you want a 5/8" exhaust flange or better) the 4 to 2 to 1 headers off fleabay youll lose a little low end power from lack of back pressure. If you plan on boost later than ignore that for now. I want to swap my 4 2 1 header to a 4 into one for higher flow and one of my collectors is dented.
you're not losing anything from lack of back pressure...if an engine wants to exhale the exhaust, why would it want the exhaust to push back?

Lack of exhaust velocity will affect overall flow, and if you open it up, then at low rpms/loads, you don't fill the pipes, the gasses expand and slow down, then causing a restriction in flow.

2.25-2.5" on the exhaust will work just fine, and you likely won't notice any difference driving on the street, may be noticeable if raced, but I wouldn't expect too much of a difference on either end, I have 2.5 on my car currently, and usually run that size on my cars, but if I built an exhaust I would likely just do 2.25 since I would run a magnaflow and they only do 2.25 or 3...if you turbo, then bigger is better, but bigger is also louder, so it's then a balance game...
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Re: Optimal Exhaust Diameter?

Unless you are running a super short exhaust, N/A cars need some backpressure. Turbo cars benefit more from less back pressure.

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I'm not too familiar with the physics behind it, but doesn't back pressure allow for air to be sucked out of the exhaust so to speak?
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post #12 of 14 (permalink) Old 2-6-13, 23:24
 
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Quote:
Originally Posted by Scatman95 View Post
Unless you are running a super short exhaust, N/A cars have some backpressure. Turbo cars benefit more from less back pressure.
fixed, in reality, turbo cars have A LOT of backpressure, why turbo cars don't really gain VE, but SC cars do, but after the turbo, the less resistance is better...

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Originally Posted by Mika-MX6 View Post
I'm not too familiar with the physics behind it, but doesn't back pressure allow for air to be sucked out of the exhaust so to speak?
no, back pressure is how much pressure the exhaust is pushing against the engine exhausting it's gasses...

since the exhaust is compressed, it wants to expand. If the piping is sized right, the only way the gasses can expand is out, increasing the speed, which drops pressure, then creating a vacuum on the exhaust port, sucking the next pulse out.

The length of each part of the system affects it, though, length of manifolds/headers, length of any y pipe or crossover, and total length of exhaust, and any bends. Without going custom, all those lengths are set, so the easiest thing to change is the diameter. Too big will lose speed, too small is a restriction...

Last edited by matt89glturbo; 2-6-13 at 23:27.
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Originally Posted by matt89glturbo View Post
fixed, in reality, turbo cars have A LOT of backpressure, why turbo cars don't really gain VE, but SC cars do, but after the turbo, the less resistance is better...
I wouldnt say that, some backpressure but not "A LOT". A turbo'd cars exhaust has a different flow due to the turbine. the turbine itself is a restriction, which creates backpressure, but one that also creates flow and velocity.

And adding forced induction to almost any engine will give you an increase in VE, whether it be turbo or SC. It just may not be a large one if the rest of the engine hasnt been prepped properly.

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the turbine only creates flow and velocity through a shaft connected to a compressor, forcing more air into the system...since, the exhaust pressure goes so high, however, the VE does not increase, larger turbines/housings hold less pressure, so can start to raise the VE in the right conditions, but not as much as a SC of similar compressor size.

Of course this is all just the mathematical side, mathematically SC should be better with the raise in VE, and maybe (as I've never messed with them much) it takes less fuel to do the same job, but a properly sized turbo, I think, will always be faster, since they can boost non-linear to RPM.

from turbo magazine:
Up to this point, we had dedicated the Aux Volt Motec input to a 1-Bar MAP sensor providing barometric pressure compensation. We could free up the aux volt port to measure backpressure or compressor-discharge pressure by reconfiguring the load calc scheme to provide barometric pressure compensation using a manually entered parameter. By swapping out the 1-bar GM MAP sensor and installing a Delco 3-bar or Data Instruments 100-psi pressure sensor, we could datalog exhaust or compressor-discharge pressure to approximately 33 or 75 psi.

Project MR6 Toyota MR2 - Compound Forced Induction - Turbo Magazine
for the rest of the article...seems like a lot of back pressure to me, seems like a stock car was measured at 20-25psi, though, but can't remember where I read that, or what car it was...
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