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Discussion Starter #1
ok guy, i just bought some stuff and a 351w. with the goodies i'd like to put on it, i'm wondering if 3 inch exhaust will be too big. with mightymach and i playing with his comp. dyno, it says about 415hp.

so a 415hp 351w gonna have TOO much exhast with the 3 inch pipes because of not enough backpressure?

right now i got a 282hp 351w and got 2.25 inch pipes all the way through. could i run a 415hp 351w motor with this exhaust without hurting anything. or do i have to get the 3 inch to see the most gain.

would a 2.5 pipe make much difference? it would suck is a 3 inch is too big cuz i have to completely re-do the exhaust AGAIN just for the .25.
 

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Go with 3". Backpressure is your enemy with a high HP motor. I've got a 3" setup on my 347 stroker and it seems to like it just fine even on the street.
 

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3" is all good... just don't expect to install 3" tailpipes. I use 3" on my 302 without tailpipes, and all is good. I don't even drop the exhaust at the track. However, it does thump on your head a little at idle.


If you want to run tailpipes, 2.5" is your only choice. With a hot engine like that, anything less than MANDREL bent 2.5" tailpipes is going to cost power. Tailpipes get seriously crushed and kinked when made on a traditional bender... the larger the pipe, the worse it gets. For a nice mandrel bent set, you've got to go to the aftermarket. Flowmaster makes a nice 2.5" set for only $99. It's the best money you will spend on the car.

If you run tailpipes, don't run a long muffler. Short mufflers give more room for that first bend up over the axle. I use Edelbrock #5525. They are very small for a 3" unit, and work really well.

A buddy of mine runs 3", and welded a set of header collector rings to the ends of his mufflers. If he want 'dumps', he just bolts on his turndowns. If he wants tailpipes, he bolts on his 2.5" mandrel bent flowmasters.


Good Luck!
 

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Discussion Starter #4
thing is our have quick 302. will a 415hp motor be able to handle that much exhaust?
 

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I wouldn't worry about having too much exhauset. My wife's 2.5L 6cyl BMW has true full-length 2" (50mm?) dual exhaust and it doesn't seem to hurt the low-end. Those Germans think hard and long about EVERYthing.
That is a TON of exhaust for 150 cu in and would arithmetically compare with 2.8" exhaust for a 302, (based on the area of the cross-section relative to displacement). Come to think of it, if a 2" circle has 3.141 sq inches and a 3" has 7 sq inches, how does actual flow compare between the popular exhaust sizes?
 

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I dont think 3-inch will be to much , Probably could go larger and not hurt anything, my dirt bike has a 2-1/4 pipe on it and it is only a 450 cc 4 stroke single cylinder and 55hp. With exhuast displace and rpm have alot to do with it not neccecarly hp but you wont get that 415 with 2.25all the way, oh another example a chevy van with a 305 has near 3.5 and it isnt 250 hp. 3 inch will be good>


lowside
 

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On 2002-02-04 01:59, XolieX wrote:
ok guy, i just bought some stuff and a 351w. with the goodies i'd like to put on it, i'm wondering if 3 inch exhaust will be too big. with mightymach and i playing with his comp. dyno, it says about 415hp.

so a 415hp 351w gonna have TOO much exhast with the 3 inch pipes because of not enough backpressure?

right now i got a 282hp 351w and got 2.25 inch pipes all the way through. could i run a 415hp 351w motor with this exhaust without hurting anything. or do i have to get the 3 inch to see the most gain.

would a 2.5 pipe make much difference? it would suck is a 3 inch is too big cuz i have to completely re-do the exhaust AGAIN just for the .25.
Engines need some backpressure to run good is an old wives tale that keeps floating around that is totally inaccurate. Exhaust backpressure does not increase torque at any RPM level. At high RPM it restricts flow and kills horsepower. At low RPM it causes reversion in the intake tract which hurts low end torque. If at some time a dyno test has supposedly proved that adding backpressure can build more torque, then there was something else wrong with the tune-up or combination that the added exhaust pressure was simply masking. The primary header tube diameters and lengths effect torque a great deal, but once the exhaust is past the collector flow volume is what's important. Always go with the biggest exhaust you can afford or stand to listen to.
 

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Discussion Starter #8
thanks alot guys. one more question, this is regarding my 66 mustang now with a 200 6cyl. the original pipe size is 1 3/4 all the way through. the engine only has a slightly bigger cam and that's all. i was wondering if i went orignal size head pipe into a delta 2chamber flowmasters and going dual out tail pipes with 1 3/4 also, would that be too much. you are all saying that most cars could use as much exhaust as they cold take. but it seem like a old inlin 6 is a different story.

anybody hear a 6cyl wih a flowmaster.\? this is the fiance's car.
 

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I'd say you could probably run 2.0 2.25 single to the back and split it 2.0 or 2.25 dual past the muffler, remember with exshuast it will flow at its greatest restriction so single pipe to the back and split it after the muffler it will be like a single . I Also have a sonoma pick up with a 4.3 (262cid)and run from the cat outlet 3.0 to a 3.0 inlet flowmaster and dual 2.5 outlets and 2.5 on out the back and it runns much stronger and sounds good , I think it would be cool if you ran a single 2.5 from exhaust manifold to the bumper it would sound good and look cool and be different. The bigger the pipe the deeper and better the sound. At idle it will probably sound like a mild v-8!

lowside
 

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with some chasis, it may be very difficult or impossible to run 3" pipe without doing chasis mods.
There is also a lot more resonation with that big pipe.
watch for the cops.
 

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On 2002-02-05 15:58, R Code wrote:
I wouldn't worry about having too much exhauset. My wife's 2.5L 6cyl BMW has true full-length 2" (50mm?) dual exhaust and it doesn't seem to hurt the low-end. Those Germans think hard and long about EVERYthing.
That is a TON of exhaust for 150 cu in and would arithmetically compare with 2.8" exhaust for a 302, (based on the area of the cross-section relative to displacement). Come to think of it, if a 2" circle has 3.141 sq inches and a 3" has 7 sq inches, how does actual flow compare between the popular exhaust sizes?
Flow rates increase to the 4th power (exponentially) as the exhaust diameter increases.

For example: 2 inch diameter exhaust will flow 16 units of air (2^4). 3 inch exhaust will flow 81 units of air (3^4). One inch difference in the diameter provides over 5 X the flow (81/16 = 5.1).
 
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