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Optimum Exhaust Diameter (header back w/ no cat)

  • 2.25" (stock)

    Votes: 3 4.0%
  • 2.5"

    Votes: 24 32.0%
  • 3"

    Votes: 48 64.0%
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Discussion Starter · #1 ·
Alright, let's end this. Hopefully people can use this in the future too.

Okay:
Specs:
1988 Bronco
5.0 (302) engine
automatic

I'm replacing the cracked manifolds with headers, as well as replacing the entire exhaust (flowmaster 40 or 50 series muffler). I live in a county the does NOT require emissions. I will therefore not be using a CAT.

The Question is: What diameter piping should I use? Stock is 2.25. I've heard that it's too restricting. I've also heard that 3 doesn't allow for enough back pressure. At the same time, I've heard that 3" is ideal. What about 2.5? Happy medium maybe?

Vote for which lends itself to the most power/aggresive sound/efficiency.

Let's do this.
 

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i have a 351m and i have 3" with true duals and it ran fantastic.
 

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dude, i got a 81 with 351 W. i use hooker headers with true dual exhaust. it goes from headers to flowmasters on each side turned down right before the rear wheels. i too have no cats. i forget what series flowmasters it is. the piping is 2 1/2". it sounds ****ing vicious. like :thumbup

i know alot of people with 350 engines and 305s andf 302s with dual 3" piping and it sounds loud, but it sounds like ****ing shit. if you do 3" with single exhaust it would sound good. but **** single go to dual.

as far as performance goes, come on, what do you expect from an exhaust. my truck ran fine. i drove others people cars with the 3" pipes and their cars somewhat lagged, but i dont know every inch on those cars as i do mine, so i cant start condemning exhausts.

one thing to remember, bigger isn't always better.
 

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Discussion Starter · #7 ·
MikE2 said:
Is this single exhaust or dual?
Long tubes or short headers?
Well, i wanted to do single initially, but that is gonna add at LEAST $150, so i was gonna go single. But it's still up for debate i suppose.

From what i've read, short headers are a little better for higher horsepower vehicles. Since this is a stock 302, i was going to long tubes.
 

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Systolic said:
From what i've read, short headers are a little better for higher horsepower vehicles. Since this is a stock 302, i was going to long tubes.
Its the other way around. Long tubes are more designed for high horsepower high RPM engines. If your going with short headers and single exhaust the bigger the pipe is beyond the Y-pipe the better power you will have. I started out at 2.5, then went to 3.0", and now its at 3.5" and power, responce and sound got better each time I made the pipe bigger.
 

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Sway's jealous of my
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I voted 3" single. Hands down its great for single exhaust on a 302.
 

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3" single is what I went with and it worked awsome on my stock 302, then on my 1994 302 HO mustang engine, and now my 351W fully built.... sounds good, looks good, and performs awsome!!!
 

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Discussion Starter · #11 ·
While I have your attention, what if i went with the header back true duals? Would that make any difference?
 

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Duals have a cool sound! a single IMO never does sound like duals....

When running dual properly on these trucks you should have a sample pipe made that connects between the two pipes to mount the 02... A GM is designed to sample from one bank, where as the ford is designed to monitor both banks together.... lots of guys run duals and keep the 02 just reading from the right bank... this is not the ideal way to do it!

A 3" exhaust will breath plenty for a stock 302... its cheaper, but in the end will not sound as good as dual 2 1/2s would...
 

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Discussion Starter · #13 ·
MrBob said:
Duals have a cool sound! a single IMO never does sound like duals....

When running dual properly on these trucks you should have a sample pipe made that connects between the two pipes to mount the 02... A GM is designed to sample from one bank, where as the ford is designed to monitor both banks together.... lots of guys run duals and keep the 02 just reading from the right bank... this is not the ideal way to do it!
Let me see if i'm understanding correctly. You're saying the GM's would have an O2 sensor on each side where a Ford would have one at the crossover pipe?
 

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Cadillac of Men
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run long tubes, single exhaust. its a 4wd truck. you dont need extra shit hanging off the belly waiting to get ripped off.

the theory is you want the exhaust pulses exiting as close together as possible, at lower rpm, the longer tubes help that happen. but there are others who say the opposite. basically, if you need the extra hp that badly, youre going to have enough funding (seeing as you prob spent 30k on the motor already) to dyno each set of headers and see what works the best for you.
 

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Geaux Tigers!!
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I vote(d) single 3". That's what I'll also be doing for my 351.

Tommy
 

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there used to be a chart with the optimum diameter for cetain size motors and certain Hp/Tq numbers. i remember seeing the chart on flowmastermufflers.com, but the site has changed since i went there last and i couldnt find it. i say 3inch just cuz it i beilieve more is better.
 

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broncos&bombs said:
keep in mind this is a 302, not a 351. a 351 will require a bigger diameter. a 302 can get away with a smaller one. engines need the back pressure.
Oh, I know. I still say a single 3" will suffice. It *may* be a tick on the large side given the low revving nature of you motor, but I doubt you'll ever notice a difference. If you're thinking it may be big, you could save a few bucks and run a single 2.5" setup. Of course only use mandrel bent pipe. I wouldn't waste my time on duals. Just added complexity without necessarily any performance benefit. However, they can sound different in which case it becomes a matter of personal preference. I'm just a fan of larger single exhausts (for a few reasons).

Tommy
 

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broncos&bombs said:
i suggest a 3" y-pipe, then 2.5" pipe the rest of the way. all mandrel bent.
Thats backwards. Flowing a 3" Y-pipe down in to a single 2.5" exhaust is going to be one big bottleneck restriction.
 
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