What Size Exhaust

[AU Doc]

I know this sounds like the same question we've all heard hundreds of times, but I had to put my gearhead card away while I was in school, getting married, and having kids. So please humor me



Do I understand correctly that 3" exhaust is recommended when doing an LS swap? The exhaust on my car now is laughably bad, and I'd like to go ahead and upgrade while I still have the old gen I sbc in the car. With that in mind, 2-1/2" used to be the magic number for "most" street engines to balance flow and velocity for exhaust scavenging. I don't mind giving up a little hp/tq now since the sbc will be coming out in a year or so, but I'd really like to only do the exhaust once if I can.

I guess back of the envelope is to calculate the CFM through the motor, and make sure the exhaust is sized appropriately. But there's lots of nuances I don't think that takes into account.

[astroracer]

Unless you are pushing mega HP numbers 2 1/2 will work just fine if you are running a dual type system with an X pipe. 2 1/2 is a lot easier on the pocket and easier to route.
I had an '02 CE Trans Am that I ran dual 2 1/2 pipes and Magnaflows on and it dyno'd, in mostly stock, form at 375 at the motor. No exhaust restriction issues showed up on the dyno.
Mark

[Mr Nick]

Rule of thumb is mandrel bent 2.5" exhaust can support up to 500 hp.

[Shawn01754]

I read an article with some testing done and I remember it said 3" off the headers into a x pipe, then into the mufflers. If you want to drop it to 2-1/2" tail pipes it would support up to 600 hp. That what i did but Im not sure.

[AU Doc]

If I remember correctly, oversizing the exhaust tends to lower exhaust velocity at lower RPM, which usually shows as hit to low-end torque. I've not ridden in a car with an LS-based engine. Are they a higher winding engine compared to your standard street genI sbc?

[rustomatic]

Is the engine stock? What power numbers are you working with? This will determine the appropriate sizing. Most people go way too far with big pipes, frequently twice what they need . . .

[Shawn01754]

Old article but interesting
http://www.pontiacstreetperformance....p/exhaust.html

[nokones]

I am running a 3 1/2".

[AU Doc]

Thanks for all the great feedback! As well as the great article! Lots of good info.

I suppose the horsepower is the difficult question here. The engine I have in the car will be getting heads and a more appropriate cam in the near-term. I'd like to think somewhere in the ballpark of 400 hp at the motor, but that is going to be on the generous end since I don't know anything about the bottom end of the engine.

Long term, I'm planning for an LS swap, and the end-game is a turbo (or two). I'm leaning towards 3" to the mufflers. We'll see from there. I know that's going to be WAY oversized for now, but I think it will be pretty close - if not a little small - once the LS and turbos go in.

Any additional thoughts?

[Mr Nick]

5 years ago when I swapped my auto for a 5-speed, I put a 600hp clutch in it since I thought "someday" I'll swap my 400hp SBO for a hot BBO. That day hasn't come and I swapped the non-street friendly 600hp clutch for a mild Centerforce Dual Friction last year...

My point is, build your combo for what you have now, unless you are d@mn sure you're going to put a twin turbo LS in it in the near future. You'll end up hacking up part of your exhaust anyway doing the turbo install. If I were in your shoes, I'd do a nice aluminized (lower cost than stainless) 2.5" exhaust now, and then when you do the TT-LS, upgrade your exhaust as needed.

[AU Doc]

That, sir, is a good point!

This may be a good opportunity for me to learn to weld as well. I can try to put together a 2.5" exhaust. If it turns out nice, I can do the 3" later.

[Mr Nick]

I may buy myself a beginner MIG welder for Christmas this year. I'd love to learn how to weld.

[jknight16]

I'm of the opinion, no science here, that running 3" from the collector through the mufflers with 2.5" tailpipes would be a good compromise of function and packaging, particularly with the more challenging rear link setups and panhard bars/watts links. Let's not forget, 2.5" is still pretty darn big pipe.

[TT302Z28]

My Camaro is running a 496 BBC through 2.5" dual pipes w/ a cross after the trans. I doubt I would pick up much more power at 3" and will have more issues with routing with the larger pipes.

[kimosabi]

I'm glad I went 2.5". Like others have mentioned, with an x and depending on what mufflers you choose it flows plenty. The 500hp mark is not an absolute, just a general guideline. 3" tend to be too loud for us guys with sensitive ears. lol

[downl50]

Great question -- there is a hotrod tv episode where they dyno a 496 (I believe) with 2.5 and 3" and show some HP gain with the larger. I'm installing a ~575hp sbc, and doing 3" to the muffler and 2.5 to the tips with some large magnaflow mufflers. I get the sense this will be a decent compromise.

I found this on Chevelles.com -- who knows what to make about the claim but interesting setup:

"(1966 Chevelle)
Starting at headers:
Car Chemistry 3-disc collector inserts

(Ultraflow #24215 2.5 12" Bullet) <---No room on Camaro

Dr Gas X-Pipe 2.5 in/out

Ultraflow #17236 2.5 Mufflers 24" case

2.25 Tailpipes

Quiet is an understatement. Pure cam & header "ping" inside.

Full-throttle is more like an RPM-based ever-deepening
"tuned" sound, somewhere between a NASCAR & an F1 exhaust.

This particular combination is the final culmination of 30-plus
years of experimentation & a lifelong interest in IC gas flow &
exhaust systems. This system could support a 1500 hp/2000
cfm engine w/o any appreciable difference vs. an open exhaust."

[65 drop top]

Another thing to keep in mind when people use hp numbers for deciding on which exhaust, those hp numbers are at peak. How often do you drive around a 6k rpm's? 99% of driving is at part throttle for a street car.

That being said, I ended up going dual 3" with my 600+ hp Chevelle. I tend to go to the extremes. It sounds bad ass, my neighbors several blocks away might not agree.

[gator68428]

I added an H-pipe to my dual three inch exhaust on my 450-500 flywheel HP Pontiac 428 and picked up solid gains at peak torque (see graph below). Mustang Dyno.

I run 4 mufflers. First 2 are single chamber 3" Flowmasters (before rear axle) and the second two (below trunk) are C6 ZR1 dual mode mufflers (with 2.5" inlets) that are essentially straight pipes when open (hence the addition of the two Flowmasters before--i like the sound better with them), and nice and quiet when closed. The middle two of the four Vette tail pipe tips stay closed for street driving. The butterfly valve is very robust and reliable.

Though i will caution about 3" on a first gen F-body as i had ground clearance issues for a while. Not much room there. And to make matters worse the Hooker super comp headers for Pontiac are lower than necessary. The TCI engineering x-member was designed for 2.5" exhaust I'm pretty sure. So I recently hacked up the headers and x-member and re-welded to get that extra inch of ground clearance back.

[gator68428]

Oh, and FWIW, it's about a 30HP loss with the butterfly valves closed in the C6ZR1 mufflers.

[AU Doc]

That's good info! Now I have to go Google the corvette mufflers. I'm not familiar with them.