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Old 09-20-2005, 11:39 AM
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3" PIPES(Too Big?)

Here's a question. {Hope it's in the right forum } Is it ever a bad idea to run big exhaust pipes? For example with my SB350 I'm aiming for 300-400 horse, would 3" pipe's be too large? I like a loud car... but would there be a back-flow issue? I've always heard bigger is better! But occasionaly have been told otherwise. P.S. why do I want to run an H-pipe? Any thoughts would be good. Thanx!!!

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Old 09-20-2005, 12:01 PM
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3 inch as true duals is a little big. I would think 2 1/2 would be more than large enough.

An "H" pipe equalizes the pulses, making more power.
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Old 09-20-2005, 12:43 PM
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Poncho is right on the money. Also depending on what year of Firebird you have (assuming a Firebird due to user name) dual 3'' may not fit.

Steve
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Old 09-20-2005, 01:23 PM
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If you have the room and the money, opt for an X pipe over the H. It will do the same thing as the H, but over a broader RPM range.
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Old 09-20-2005, 01:44 PM
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Anything over 2 and a half is too big in my opinion, you need a certain amount of backpressure.
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Old 09-20-2005, 02:17 PM
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Hmm I hate to go against conventional wisdom. My Jeep high bred Cherokee/J20 runs a duel 3 inch into asingle 3.5 on propane anybody who has seen it run will tell you it does not suffer because of to big an exhaust. Now to big of tire for the diff. gears 37X12.5X16.5s with 3.73/1. Here are some pics of the build.
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Old 09-20-2005, 08:47 PM
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Quote:
Originally Posted by Henry Highrise
Anything over 2 and a half is too big in my opinion, you need a certain amount of backpressure.
Henry, I hate to say this, but... You're wrong, However,
you're also partly right, but not how you think...

An engine is an air pump, can we all agree on that??
Okay then, now that we're all in agreement, can I point
out the error in your statement?? Back pressure is not
a good thing to have, right? Gas flow stalling in the
pipe because it's to big is a bad idea too, right?? So if
too big a pipe is bad, and too small a pipe is bad, how
are we to figure out what sized pipe we need?? Well,
we ned to go back to the sixties for the answer, when
the muscle cars ruled the roads. Look at the tube sizes
the factory used on the Hipo cars of the day. Those
engineers were trying to wring as much power out of the
engines, while retaining some level of reliability and NVH.

However, we don't give a rats ***** about the "N" (noise)
so it's entirely up to the individual how much they're willing
to put up with... But you've got to consider that if you're
spending your hard earned cash on a custom exhaust, you
better get it right, or a reasonable facsimility of right. Mainly
'cause you just spent a wad-o-cash on something you may
not be happy with...

Just as an asides, that 350 probably could get away with
a 2 1/4" system with either the crossover or X-pipe...
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Old 09-20-2005, 08:55 PM
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i have to agree with bigun. the bigger, the better. 2.5 will do the job but i don't think you'll gain, or lose anything with 3's. as for loud, i run a pair of magnaflo, 2.5 polished, straight throughs and it's just tollerable inside the car. as for outside......
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Old 09-20-2005, 08:57 PM
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Quote:
Originally Posted by Canzus
Henry, I hate to say this, but... You're wrong, However,
you're also partly right, but not how you think...

An engine is an air pump, can we all agree on that??
Okay then, now that we're all in agreement, can I point
out the error in your statement?? Back pressure is not
a good thing to have, right? Gas flow stalling in the
pipe because it's to big is a bad idea too, right?? So if
too big a pipe is bad, and too small a pipe is bad, how
are we to figure out what sized pipe we need?? Well,
we ned to go back to the sixties for the answer, when
the muscle cars ruled the roads. Look at the tube sizes
the factory used on the Hipo cars of the day. Those
engineers were trying to wring as much power out of the
engines, while retaining some level of reliability and NVH.

However, we don't give a rats ***** about the "N" (noise)
so it's entirely up to the individual how much they're willing
to put up with... But you've got to consider that if you're
spending your hard earned cash on a custom exhaust, you
better get it right, or a reasonable facsimility of right. Mainly
'cause you just spent a wad-o-cash on something you may
not be happy with...

Just as an asides, that 350 probably could get away with
a 2 1/4" system with either the crossover or X-pipe...
Actully, backpreassure is nessasary unless you want to run all the time at WOT, otherwise the motor don't work efficently. you only need a little back preassure but you need it there. What the back preassure does is maintain cylinder preassure.
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Old 09-20-2005, 10:34 PM
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oddly enough my rig with the single muffler is quite at idle it does have a real nice deep rumble when you step on it tho. It would be fairly quite if I could just get the header bolts to stay tight LOl. One question I have for those advocats of duel 2.5s combined your pipes are 5 inches yet most people say with a single 3.5 I am to big please explain! Now one problem with my set up is heat in the cab, and I mean HEAT!!! My buddy black Bart ace fabricator and I are brain storming the best way to take care of this problem at this time.
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Old 09-21-2005, 09:11 AM
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bigun -- you're effectively running a single 3.5", as that's the "bottle neck" -- pressure will "back up" the two 3 inchers after a few seconds. Back pressure helps cylinder scavenging, so some is needed. The higher rpm an engine runs the less back pressure is needed.

A pair of 2.5" pipes isn't the same as a single 5". Do some simple math! Area of a circle is pi x radius (squared). So compute the surface area of a 2.5" pipe and multiply by two ( 2.5/2 [squared] x pi = 4.91 x 2 = 9.82 sq. in.). Then compare to the surface area of a 5" pipe (5/2 [squared] x pi = 19.64 sq. in.). A single 5" pipe is MORE THAN TWICE the size of two 2.5" pipes. In contrast a single 3" is 7 sq. in whereas two 2.5" is 9.82 sq. in, and a pair of 2.25" 7.95 sq. in. A pair of 2.25" pipes is no better than a single 3".

You're running LPG, which throws another wrench into the equation. LPG burns much cleaner than gasoline, but requires more volume to produce the same amount of power. Because it's cleaner and more complete burning, scavenging shouldn't be as big an issue as with gasoline. Another "problem" is how your engine is built. Is it a high compression engine with a cam leaning toward LPG or is it a dual fuel setup? If the engine is optimized for LPG that would be a critical factor in how much back pressure it should have. I'm not sure how much it should have, to be honest, or what operating range you typically run in.
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Old 09-21-2005, 04:14 PM
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Quote:
Originally Posted by farna
Back pressure helps cylinder scavenging, so some is needed. The higher rpm an engine runs the less back pressure is needed.
Sorry, you're going to need to explain this theory...
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Old 09-21-2005, 05:40 PM
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3 in for a sb is way too much. you will have a dog on the bottom end. on all the street cars i build i use 2 in. most are just crate motors with a cam. bigger is not better on a street car. same goes with big carbs. they hurt more than they help.
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Old 09-21-2005, 06:07 PM
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My 400M in a F-150 4x4 ,while built for low end and torque,runs much,much better with 2 1/4 duals than with 2 1/2 duals.When I first put the truck together,I put 2 1/4 pipes on it. When I replaced that exhaust,I had it made out of 2 1/2,thinking it would run stronger.Even playing with the jets in the carb,timing,etc couldn't get the truck to run as good as it did with the smaller exhaust.I used the same brand and style of mufflers and the same routing of the pipes.I went back to the 2 1/4 set up and the truck again ran strong.You could feel the difference in the two systems.

George
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Old 09-21-2005, 06:22 PM
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Quote:
Originally Posted by Canzus
Sorry, you're going to need to explain this theory...
Basicly, what he means is just what I said about cylinder preassure. What the cylender preassure does is allow for a better and more compleate burn.you need a little back preassure. without backpreassure, the burn suffers. This is also the reason they make headders with diffrent size primaries and collectors, if it was the bigger is better theory, then they would use the biggest collectors/ primaries as a standard size, and have only that size. The smaller primaries will give you more torque and power in the low end which is best for street use but, larger primaries are better used in conditions that WOT is normal.
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