2.5" vs 3" pipe size for high powered FE; which is better?

[averagef250]

Last but least a study done in Britain in the late sixties and published in Hot Rod Magazine later showed that on muffler equiped cars the best power was when the mufflers front end was 3/4s of the way back from the manifold exit. I have done my cars and trucks that way ever since. If the car design does not let me move the mufflers I change where the pipes exit. The muffler entrance causes the scavenging pulses to reflect and they said that distance made more even power.

Don't put too much stock in any study done by a magazine or internet websites. Flow is not that important, scavenging is very complex and exhaust is actually comprised of flowing gas, sound waves and pressure pulses that all contribute equally to the problems inside the exhaust system. If you want a good read that actually explains how exhaust really works "The scientific design of exhaust and intake systems" is a very good book. That said, you can put some really crappy mufflers and crinkle bend pipes on your 400 HP FE and be fine. Exhaust tuning is for maximum effort engines, not street stuff. Go with what looks and sounds good to you!

[fomocoguy]

Don't put too much stock in any study done by a magazine or internet websites. Flow is not that important, scavenging is very complex and exhaust is actually comprised of flowing gas, sound waves and pressure pulses that all contribute equally to the problems inside the exhaust system. If you want a good read that actually explains how exhaust really works "The scientific design of exhaust and intake systems" is a very good book. That said, you can put some really crappy mufflers and crinkle bend pipes on your 400 HP FE and be fine. Exhaust tuning is for maximum effort engines, not street stuff. Go with what looks and sounds good to you!

Yeah, but you can't deny that if he runs pipe that is too big, the truck will fall flat on it's face at low rpms. It happens to people every day who don't realize what effect pipe size can have. I did it with an old Cutlass I had, and I'm sure there are at least a few who have done the same on this forum. I also know that I've talked to numerous people over on the ford460 forum that were running 400+ hp engines with flowmasters, then switched to a quality straight through design like magnaflow, and saw VERY large increases in top end pull. A poorly designed exhaust will without a doubt rob power from his FE, which I think is the last thing he wants to do after spending so much money to make that power.

[averagef250]

I agree totally with that, my example may be a little extreme to say you could run anything and be happy with it.

I think 3" pipes are fine for the street if your running something else rediculous like a 400 horse small inch small block with a 3500 converter and 4.10 gears. You have no low end torque anyway so might as well go oversize if you like the sound.

[bobbyinpd]

Dual 2.5" pipes are the way to go. That's what I run on my 390 and my brother just had a professional "X" style exhaust built for his radically built 390. The owner of the exhaust shop explained it to us and that's the best setup he said. He runs it on his 600 hp chevy as well. 3" dual would be over kill and non-profitable. If you are going single, then the 3" would probably be better, but I say duals all the way!

When I had my bump, all I did was a manifold, nice 750 holley 4 barrel with vacuum secondaries, long tube headers, ignition, and a full dual mandrel bent 3" with and x and Hooker aero chamber mufflers. Had it exiting the sides in front of the rear wheels. 3" duals is not overkill at all. My truck was an F250 with 4.11s and a C6 and that thing was an absolute rice-slayer. I know it had a cam installed by the PO, and I know it wasn't stock, but it wasn't big either Was very quiet at cruise and at idle, but when you got on it it would howl because of the x-pipe. Kinda sounded like a Late Model over at the local short track! We dynoed my truck at the shop I used to work at with both 2.5" and 3" mandrel bent exhaust systems(I loved writing things off for "Company Research") and with the 2.5" it made 417HP and 409 Torque, with the 3" it made 432HP and 407 Torque. I dunno about you but I would not mind making an extra 15 HP where you use it, like getting on the highway or pulling out into traffic for just giving up 2 FT/P of Torque. just my

[sideoilerfe]

...with the 2.5" it made 417HP and 409 Torque, with the 3" it made 432HP and 407 Torque. I dunno about you but I would not mind making an extra 15 HP where you use it, like getting on the highway or pulling out into traffic for just giving up 2 FT/P of Torque. just my

You're forgetting the RPM range of these readings. I'd rather have lower RPM torque than higher RPM HP in a truck any day. Torque gets you going anyway so HP without torque is pointless.

If You like 3" duals than that is cool, I just prefer more low end torque than high end HP in my Bump.

[bobbyinpd]

Here's the thing, my truck was more of a street strip toy than a tow vehicle or work truck, even though it towed, cruised, and worked better and harder than my BBC powered 93 suburban and my 96 PSD F350 Dually....when you have 300+ ft/pounds of torque anywhere above 2000 rpm and 5000 rpm, I'm pretty sure it doesn't really matter.....

[robroy]

Lately I've noticed that lots of folks recommend either a single 3" exhaust or dual 2.5" exhaust pipes.

Perhaps I've misunderstood, but are these two choices supposed to be equitable? I'd guess that a single 5" pipe would equal two 2.5" pipes. Or am I missing the idea here?

Thanks!
-Robroy

[My427stang]

If you calculate the area of a circle:

A 2.5 inch circle (1.25 radius) = 4.90 square inches X 2 pipes = 9.8 sq inches

A single 3.0 inch pipe area calculates to = 7.07 sq in

A 5.0 inch pipe = 19.65 sq inches

So technically for the same cross sectional area (CSA) you'd need a 3.5 inch pipe for a CSA = 9.62 sq inches to get close to matching the dual 2.5s

Area of a circle grows pretty quickly, so its not that same as adding the two together. You can play with this link to see

http://www.calculateme.com/cArea/AreaOfCircle.htm

[robroy]

Oooooh. I guess I should have paid more attention in my geometry class. Thank you! -Robroy

[SteveC]

there is a kid out at my college who put a 5 inch exuahst on his 90's ford truck and it sounds like a rice grinder and pretty much it falls flat on its face every time he gets on it.

[robroy]

So, I've re-read this thread through several times and it looks like the majority of folks are recommending 2.5" pipes.

Since posting this thread I found out from Proformanceunlimited that the calculated engine power is 436 horsepower and 463 ft/lbs of torque at 4,000RPM.

With these increased specs, is 2.5" still the recommended size by the majority of us?

When it comes to loud/quiet, my preference leans towards quiet. Maybe this helps make the choice for the 2.5" pipes? (They should be more quiet than the 3" pipes, right?)

Thank you very much folks!
-Robroy

[fomocoguy]

So, I've re-read this thread through several times and it looks like the majority of folks are recommending 2.5" pipes.

Since posting this thread I found out from Proformanceunlimited that the calculated engine power is 436 horsepower and 463 ft/lbs of torque at 4,000RPM.

With these increased specs, is 2.5" still the recommended size by the majority of us?

When it comes to loud/quiet, my preference leans towards quiet. Maybe this helps make the choice for the 2.5" pipes? (They should be more quiet than the 3" pipes, right?)

Thank you very much folks!
-Robroy

The 2.5 inch pipe will be quieter, without a doubt. I still stand by my opinion earlier that you'd be well off with 2.5 inch mandrel bent pipe, a quality h pipe, and some high flow mufflers. Here, check out this FAQ page from Magnaflow: http://www.magnaflow.com/07techtips/faq/question10.asp

[bobbyinpd]

So, I've re-read this thread through several times and it looks like the majority of folks are recommending 2.5" pipes.

Since posting this thread I found out from Proformanceunlimited that the calculated engine power is 436 horsepower and 463 ft/lbs of torque at 4,000RPM.

With these increased specs, is 2.5" still the recommended size by the majority of us?

When it comes to loud/quiet, my preference leans towards quiet. Maybe this helps make the choice for the 2.5" pipes? (They should be more quiet than the 3" pipes, right?)

Thank you very much folks!
-Robroy

The 2.5 inch pipe will be quieter, without a doubt. I still stand by my opinion earlier that you'd be well off with 2.5 inch mandrel bent pipe, a quality h pipe, and some high flow mufflers. Here, check out this FAQ page from Magnaflow: http://www.magnaflow.com/07techtips/faq/question10.asp

2.5" will be quieter by no doubt I agree, I would still say go with the 3" mandrel bent and an x, or at the very least 2.5 mandrel bent with an x. remember X > H the X-pipe will scavenge better at the times you need it, like hard acceleration, and it sounds a lot cooler too, and it will help quiet the exhaust a bit better than an H or nothing.

[Jake11]

Bigger is better. Like she said.
As far as all this torque stuff, I put headers on my 390. Drove it for a week with NO PIPES at all. Open.
Other than the racket it runs the same with 2 1/2" duals.
Jake

[fomocoguy]

Bigger is better. Like she said.
As far as all this torque stuff, I put headers on my 390. Drove it for a week with NO PIPES at all. Open.
Other than the racket it runs the same with 2 1/2" duals.
Jake

I'm sorry, but bigger is not better, not by any means. Call any reputable high performance exhaust shop and ask them. Call any muffler manufacturer and ask them. When it come to exhaust, bigger is NOT always better. I've learned it the hard way, and so have lots of guys here, plus thousands of hot rodders all across the country. I had a 72 Cutlass with a 350 "Rocket" and thought it would be great to install headers and 2.5 inch duals with single chamber flowmaster race mufflers. It ran pretty good before, but was a dog off the line after the change. I know his engine is making a lot more hp than that ol beater was, but it is a track and dyno proven solid fact that bigger is not always better when it comes to exhaust. Give me some proof that a 400hp engine makes better power with 3 inch duals than 2.5 inch duals and I'll eat my words.