Back Pressure?
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New User
Joined: Apr 2003
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From: Gulf Coast
Back Pressure?
I just got 2-1/4" True dual cats and flowmasters put on my 302 (with 3" tips of course 
) I've heard from a few people that it won't provide enough back pressure, but I haven't noticed any real loss of power. Are these people smoking crack, or is there something to it?
) I've heard from a few people that it won't provide enough back pressure, but I haven't noticed any real loss of power. Are these people smoking crack, or is there something to it?
Elder User
Joined: Sep 2003
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From: Quinton NJ
If anything, the setup will help your truck "breathe better". anything is better than your stock system. Now if your running like 4inch tubing all the way out, then I would say not enough back-pressure. hope this helps
Post Fiend
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From: Iowa
Backpressure is something you want to get rid of as much as possible. The bigger piping the better. What you want to do is match the piping to the size of the collector of the headers, or the outlet of the manifold. Anytime you change the size of the pipe, it disturbs the airflow and causes turbulence, which increases backpressure. This is why those little ricemobiles use the huge exhaust tips, the turbulence makes it louder.
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Sorry don't mean to go off topic...but how do the true duals with cats sound? Ive heard of a lot of people with true duals with out cats and they say it sounds good, but to stay emissions legal im going to use cats when i get true duals so i just wanted to know if it still sounds good.
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From: Copley, OH
Actually, rusty is right about backpressure always being a bad thing but sometimes it's a necessary evil. If you go too large on diameter you lose exhaust gas velocity and end up with pumping losses that exceed the gains from the lower back pressure. A pair of 2.25" duals should be perfect (it's the size of the stock exhaust system on a 5.0 HO Mustang)
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From: Copley, OH
Velocity matters all the way to the tip. You lose velocity, you lose kinetc energy which causes back pressure as the upstream gasses have to push harder to get out the pipe. The tuning effects are generally only important in the headers but flow velocity is important everywhere.
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Post Fiend
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From: Iowa
The exhaust is already moving in the larger pipe, so why would it have to re-accelerate it? As long as it's moving at the same speed, what's the difference? With long tube headers, the pulses peter out in the collector, when they mix with the other pulses, after that it's pretty much constant flow, at higher rpms anyway.
You're right, you wouldn't want to attach 4" piping to a 2.5" collector. That would cause turbulence and increased backpressure. I'm just saying the sweet spot is the size of the collector, because you're not restricting the flow at all by going compressing it into a smaller pipe, and you're not causing turbulence by dumping it into stagnant air in a humongus pipe.
So if you've got a 3" header collector, use 3" pipe. If you've got 2 1/4" manifold outlet, use 2 1/4" pipe. Simple!
You're right, you wouldn't want to attach 4" piping to a 2.5" collector. That would cause turbulence and increased backpressure. I'm just saying the sweet spot is the size of the collector, because you're not restricting the flow at all by going compressing it into a smaller pipe, and you're not causing turbulence by dumping it into stagnant air in a humongus pipe.
So if you've got a 3" header collector, use 3" pipe. If you've got 2 1/4" manifold outlet, use 2 1/4" pipe. Simple!
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From: Gulf Coast
Originally posted by MrDeath8666
Sorry don't mean to go off topic...but how do the true duals with cats sound? Ive heard of a lot of people with true duals with out cats and they say it sounds good, but to stay emissions legal im going to use cats when i get true duals so i just wanted to know if it still sounds good.
Sorry don't mean to go off topic...but how do the true duals with cats sound? Ive heard of a lot of people with true duals with out cats and they say it sounds good, but to stay emissions legal im going to use cats when i get true duals so i just wanted to know if it still sounds good.
It's not as throaty or loud as it could be, but it sounds good for a city driver.
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From: Copley, OH
Velocity decreases as diameter increases. Velocity in a pipe is a function of dia^2 but pressure drop is a function of dia^4. Small changes in pipe diameter make significant changes in flow velocity but they make very large changes in back pressure. Increasing pipe diameter from 2.25" to 2.5' cuts back pressure by about 1/3 but only reduces flow velocity by about 1/5
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From: irwin,pa
when running true duals all the way out- one way to kepp the velocity up due to the long lenght of pipes is to use a x pipe the next size up from the tubing you are using- alla- THE STATIC REGAIN METHOD BY IMPLODING THE SIZE of the orifice the going back down to the size you started out with!
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From: alaska
Re: Back Pressure?
Originally posted by rdnjr
I just got 2-1/4" True dual cats and flowmasters put on my 302 (with 3" tips of course ) I've heard from a few people that it won't provide enough back pressure, but I haven't noticed any real loss of power. Are these people smoking crack, or is there something to it?
I just got 2-1/4" True dual cats and flowmasters put on my 302 (with 3" tips of course
) I've heard from a few people that it won't provide enough back pressure, but I haven't noticed any real loss of power. Are these people smoking crack, or is there something to it?
Junior User
Joined: Jan 2004
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You have more than enough exhaust system for a stock 302! Don't listen to the idiots who don't know what they're talking about. Hell, the exhaust system on 400 horspower 400 cubic incher in my '65 GTO is only 2.5 inches all the way to the rear bumper and this thing puts almost 320 horse to the rear wheels.
Postmaster
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From: Kearney, Nebr
I put true 2.25" with an H pipe, no cats and 28" glasspacks on my 90' 302. Didn't noticed a change in power until I put the headers on a couple years later. You can only get some much out of the exhaust. 2.25" is the way to go.
____________________
90' F150 Longbox xlt Lariat 4x4 302 4-speed w/ 4.10 rear
Headers/true duals 2.25" w/ 28" glasspacks coming out at 45's
____________________
90' F150 Longbox xlt Lariat 4x4 302 4-speed w/ 4.10 rear
Headers/true duals 2.25" w/ 28" glasspacks coming out at 45's
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From: Iowa
Originally posted by MrDeath8666
Sorry don't mean to go off topic...but how do the true duals with cats sound? Ive heard of a lot of people with true duals with out cats and they say it sounds good, but to stay emissions legal im going to use cats when i get true duals so i just wanted to know if it still sounds good.
Sorry don't mean to go off topic...but how do the true duals with cats sound? Ive heard of a lot of people with true duals with out cats and they say it sounds good, but to stay emissions legal im going to use cats when i get true duals so i just wanted to know if it still sounds good.
Not to talk about chevys on a ford site but iv got a friend with a 96 vortec 350 with true duals and one cat on each side and it sounds sweet!!!!