2.5" exhaust ?s

[kacy]

My truck has some unknown dual setup that is 2.25 out the manifolds then steps up to 2.5. It has rusty pipes that go around the tcase to a pair of pypes muffler I picked up cheap they sound good but the pipes rattle on everything. I have been looking for a new 2.5" system with no luck. So before I go try to piece one together I was wondering what is everyone using on gas trucks for towing?

[wilkin250r]

What size is the engine? I'm thinking 2.5 is a little big unless you're pushing max horsepower at higher revs.

[cadillac_al]

I always liked the Dynomax kits from summit. They are 2 1/4 though.

[kacy]

Its a stock 350 that I am getting ready to swap vortec heads and LT1 cam. I'd be ok with a single 3in system if it will promote more torque.

[mongocanfly]

I'm running 2 1/2" off my headers to a set of mag-flow mufflers dumping in front of rear axle...not to loud but has a throaty sound..

[wilkin250r]

I'll be 100% honest, I don't have much experience with chevy motors, but I DO know engines in general (lots of motorcycle performance), so I can tell you the basic theory.

Smaller exhaust means the gasses have to move faster. This is both good and bad. Air is not weightless, it has mass. And thus, when moving, it has momentum. The greater the velocity, the greater the momentum. That momentum can actually be used to do work, to help create power.

I won't get TOO in depth with cam profiles, overlap, and scavenging. The important thing to know is that exhaust velocity is good, and smaller pipes have better velocity at lower RPMs, but they are restrictive at higher RPMs. So smaller pipes are good for low-RPM performance and torque, larger pipes are better for higher RPMs and maximum horsepower.

However, terms like "smaller" and "larger" are all relative to engine size. 2" would be huge on a VW bug, but would be tiny on a 454.

[kacy]

Originally Posted by wilkin250r

I'll be 100% honest, I don't have much experience with chevy motors, but I DO know engines in general (lots of motorcycle performance), so I can tell you the basic theory.

Smaller exhaust means the gasses have to move faster. This is both good and bad. Air is not weightless, it has mass. And thus, when moving, it has momentum. The greater the velocity, the greater the momentum. That momentum can actually be used to do work, to help create power.

I won't get TOO in depth with cam profiles, overlap, and scavenging. The important thing to know is that exhaust velocity is good, and smaller pipes have better velocity at lower RPMs, but they are restrictive at higher RPMs. So smaller pipes are good for low-RPM performance and torque, larger pipes are better for higher RPMs and maximum horsepower.

However, terms like "smaller" and "larger" are all relative to engine size. 2" would be huge on a VW bug, but would be tiny on a 454.

I get that and i designed the exhaust on my vette with this in mind, now I am hoping to build a system that will work for a 350 with some mods and still be useable with mods should I switch to a 454. I was thinking 2.5 from manifolds/headers to single 3" for high flow cat to 2.5" straight through mufflers. This should allow for scavenging to help with torque and flow enough if I need to ring it out to 5k. Right?

[Desert_Eagle02]

I have a 454 with dual 2.5 from headers back with long glasspacks. Sounds mean but not obnoxious and I get a lot of compliments on it. I didn't notice any performance differences. Had flowmasters on it before the glasspacks.

[HotRodfuelhose]

Originally Posted by kacy

My truck has some unknown dual setup that is 2.25 out the manifolds then steps up to 2.5. It has rusty pipes that go around the tcase to a pair of pypes muffler I picked up cheap they sound good but the pipes rattle on everything. I have been looking for a new 2.5" system with no luck. So before I go try to piece one together I was wondering what is everyone using on gas trucks for towing?

This video explains all you need to know. https://www.youtube.com/watch?v=bXQ-cXeri1Y&t=604s

[lowbrow454]

This video explains all you need to know. https://www.youtube.com/watch?v=bXQ-cXeri1Y&t=604s

Really good video and explanation.

To add to that this was taken from the Flowmaster website:
What size exhaust tubing should I use for my street application?

Choosing the proper diameter of the tubing for your exhaust system is as important as selecting the right muffler. Your tubing diameter choice will affect the sound level and performance characteristics of the muffler, but keep in mind that bigger is not always better. Using tubing that is too large can actually hinder exhaust scavenging by slowing the velocity of exhaust pulses in the tubing resulting in a loss of torque. And torque is what moves the car. As a general rule, most stock to mildly modified V8 powered street applications will use 2.25” – 2.5” diameter tubing. There are many variables that need to be taken into consideration when choosing the correct tubing size for your application, such as: engine technology (flat head engines vs 60’s engines vs present day engines) cubic inch, catalytic convertors, head flow, vehicle weight, how the vehicle is driven and numerous others. Light to moderately modified street applications will generally use 2.5” and rarely some can use dual 3.00” tubing, again depending on the above named factors, sound desired etc. *We generally don’t encourage the use of dual 3” exhaust systems for just street use. Here are some guidelines for exhaust tubing size vs power that can be used for street applications: Street V8 applications – dual exhaust 2” tubing200 hp – 250 hp 2.25” tubing 250 hp – 375 hp 2.5” tubing 350 hp – 500 hp *3” tubing 500 hp – 600 hp

I've had both 2.25" and 2.5" exhaust on both stock and mild 350's including one with vortec heads like you're getting to swap in and honestly I didn't really notice much performance difference either way. The 2.5" sounded "healthier" but as far as making more power, if it did it wasn't noticeable.

[kacy]

Originally Posted by lowbrow454

Really good video and explanation.

To add to that this was taken from the Flowmaster website:
What size exhaust tubing should I use for my street application?

Choosing the proper diameter of the tubing for your exhaust system is as important as selecting the right muffler. Your tubing diameter choice will affect the sound level and performance characteristics of the muffler, but keep in mind that bigger is not always better. Using tubing that is too large can actually hinder exhaust scavenging by slowing the velocity of exhaust pulses in the tubing resulting in a loss of torque. And torque is what moves the car. As a general rule, most stock to mildly modified V8 powered street applications will use 2.25” – 2.5” diameter tubing. There are many variables that need to be taken into consideration when choosing the correct tubing size for your application, such as: engine technology (flat head engines vs 60’s engines vs present day engines) cubic inch, catalytic convertors, head flow, vehicle weight, how the vehicle is driven and numerous others. Light to moderately modified street applications will generally use 2.5” and rarely some can use dual 3.00” tubing, again depending on the above named factors, sound desired etc. *We generally don’t encourage the use of dual 3” exhaust systems for just street use. Here are some guidelines for exhaust tubing size vs power that can be used for street applications: Street V8 applications – dual exhaust 2” tubing200 hp – 250 hp 2.25” tubing 250 hp – 375 hp 2.5” tubing 350 hp – 500 hp *3” tubing 500 hp – 600 hp

I've had both 2.25" and 2.5" exhaust on both stock and mild 350's including one with vortec heads like you're getting to swap in and honestly I didn't really notice much performance difference either way. The 2.5" sounded "healthier" but as far as making more power, if it did it wasn't noticeable.

If there was no difference between the 2.25 and the 2.5 systems you had it only make sense for me to use the 2.5 so it can be reused on the 454 when it goes in.

[DieselSJ]

87 454. Mine was 3" from the factory - 2 1/4" off the manifolds into a 3" cat and went to dual 2 1/4" pipes after the muffler. I swapped in a new 3" magnaflow high-flow cat and a huge 3" in/out magnaflow muffler and 3" over the axle and out the back. You can hear it just enough at idle to know it isn't stock, no drone or resonance on the highway (in fact it is dead quiet on the highway). Picked up a consistent 1.5mpg over the stock setup.

In your case, either a dual 2.5" with a x-pipe or a single 3" would work great, IMHO.

[lowbrow454]

In your case, either a dual 2.5" with a x-pipe or a single 3" would work great, IMHO.

If there was no difference between the 2.25 and the 2.5 systems you had it only make sense for me to use the 2.5 so it can be reused on the 454 when it goes in.

I agree. Personally if it was mine that's what I would do.