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Old 01-09-2006, 09:44 AM
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3" Exhaust

Guys

I just changed out my 327 - 350 HP for a 383 Stroker 500HP- was wondering if the new motor would benefit from changing the exhaust from 2".5 to 3" ?

fat40

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Old 01-09-2006, 10:03 AM
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The 2.5 exhaust system should be plenty for 500 HP. Do you have an "X" or "H" pipe? I can assume that it is a dual exhaust system? The 3 inch system would be OK also just your preference.

Steve
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Old 01-09-2006, 10:09 AM
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Hey whats going on? every bit helps, had 3" on my 72 nova nice fat pipes sound'ed wicked with 500 hp you want that baby to breathe (maybe 3.5")
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Old 01-09-2006, 10:17 AM
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Sound is not everything, 3" is too big except for an all out race engine. A street engine needs some amount of backpressure to operate properly and at peak usable power. I agree, 2.5" is as big as you want to go.

Vince
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Old 01-09-2006, 10:26 AM
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3" pipes

If I were you i would change it to a 3" with the 2.5" its choking, a litle to much back pressure. you gota let it breath I no its not alll about sound but it will sound sweet with that engine and that much power. I dont see the need to go wth 3.5" just 3" should be fine. I have 2.5" on my 350 ZZ4 with about 400 hp and im putting 3' pipes on mine as well. so yess i think your engine will slightly benifit form bigger pipes.
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Old 01-09-2006, 10:49 AM
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3" Exhaust

Thank's for the replies, I looks like we differ in what we think ? When we ran the motor on a test stand it had 3" on it and it performed great . I don't mined changing it to 3", but if it's just for a change and not going to make difference why do it , right . I have not try ed the motor in the car yet , it's in but not running.( I need to give more imformation the header tube size is 1 "5/8 . the collector is 3" the exhaust pipe size is 2"1/2



Fat40

Last edited by fat40; 01-13-2006 at 07:52 AM.
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Old 01-10-2006, 01:24 PM
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depending on the cars setup it doesnt manner u could run 10inch exhaust if u wanted to u dont wana choke the motor the bigger the exhaust the more she guna breath back pressure isnt really an issue it only cum into play when the exhaust is restricting flow and backs up into the carb and theres not a great air fuel mixture
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Old 01-10-2006, 01:43 PM
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Quote:
Originally Posted by Chevy boi SS
depending on the cars setup it doesnt manner u could run 10inch exhaust if u wanted to u dont wana choke the motor the bigger the exhaust the more she guna breath back pressure isnt really an issue it only cum into play when the exhaust is restricting flow and backs up into the carb and theres not a great air fuel mixture
We are talking about a street driven car.

That is the farthest from the truth I have heard. An engine needs some back pressure. Try not to make statements that are not true, Some people will take them as the truth.

Also try the spell checker it works very well.

Steve
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Old 01-10-2006, 02:33 PM
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What????

This topic has been talked about on several different occasions. The engine does not NEED back pressure! It need a pressure wave in the exhaust to carry the burnt gasses away from the exhaust port. 2.5" 3" 12" after the headers. . .who cares. At that point it is more about sound than power. You can get more power with a correctly tuned system. The only problem is that us normal people do not have the time or money to properly tune the system.

AGAIN the engine does not need back pressure! It needs a pressure wave to help carry the burnt gas away from the port. That is usually in the headers. For anyone who still disagrees I welcome any data to support otherwise.
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Old 01-10-2006, 02:40 PM
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The engine does need a certain amount of back pressure to preform properly and a 2 and a 1/2 inch system should be adaquate.
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Old 01-10-2006, 03:17 PM
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If the engine really makes 500HP 2" exhaust is not enough. 2.5" is borderline, 3" would be a better fit (IMO). Where does the engine make it's power? What size header tubes are you running? There is more to exhaust than most give credit for. The whole back pressure thing is mis-understood as you can see here. Someone please tell me what "good" back pressure does? You don;t want too large of tubes/pipes, but you darn sure don't want them too small.

What good is 3" exhaust if the primary header tubes are 1 1/2" with a 2" collector? As you can see it is a "system" just like the engine it needs to all work together. You want the headers/pipes to "pull" the bad gasses out, you don't want them backed up (back pressure).

You didn't give enough info to offer great advice, but assuming 1 5/8" - 1 3/4" headers with a collector of 3 - 3 1/2" I would think 3" exhaust would be best. Now if you want to spend the time and effort to actually see what your engine wants that's even better.
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Old 01-10-2006, 03:23 PM
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Why is backpressure needed? I hear this statement a lot but never really see any reasoning WHY along with facts or backup?

If backpressure is needed, why is it said that most vehicles at the track perform better while running open headers?

I personally have never understood the reason why having pressure for the exhaust leaving the cylinder to push against would in anyway help performance.
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Old 01-10-2006, 03:24 PM
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Here is the answer. Tested and True.

The most efficient exhasut system for a street motor, does not create much back pressure at high RPMs, but is not too large as to cause poor scavenging (pressure waves near the end of the exhaust pulling the innner pressure waves out.) at low RPMs

Pics always help best. Ignore the smaller exhasut size nnumbers, this is for a mild engine.






To make power, the engine will need to pump air at a faster speed. The higher the speed, the faster the exhaust will exit. The key is to make the exhaust both large enough to flow freely at the highest RPM, and also flow quickly at lower RPM (torque vs horsepower).

Because at 1000 RPM, there are less exhaust pulses per second than at 6000 RPM, if your exhaust is too large/designed for high RPM power only, exhaust will have trouble exiting efficently. You will lose power. Now if you reduced the exhaust to something smaller, you will increase exhaust velocity at lower RPM, but the engine will be choked at higher RPM (see the picture)

Everyone understand?
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Old 01-10-2006, 03:41 PM
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Thank you, I did not know the real answer to the question, but I do know that if you go to large of an exhaust you can harm the engine (burnt valves) and also lose power and torque. I have seen it many times when a car comes in with the muffler fallen off. If the size of the exhaust system does not matter why don't we see some huge exhaust systems on different types of cars, with space allowing them to fit? I mean the exhaust system not just the muffler (fart can) on the imports.
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Old 01-10-2006, 04:37 PM
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I think the whole point is being mis-understood. WHat part of the pipe is being shown in the "pictures" is that the header tubes or the exhaust pipe. There is a MAJOR difference. You don't want over sized headers, but you must have tube large enough for the engine and RPM. The rest of the exhaust system can effect that, it is a "system". There are so many variables you can't possibly cover every application. I would venture to say the picture above is supposed to reflect a header or primary tube, not an exhaust pipe (two different things). You don't wan't the gasses/pulses just "falling" out of the port, but you don't want them backed up waiting to get out either.

I still don't see back pressure in the picture I see, gasses/pulses. They are flowing one way and that's out. The last picture shows poor flow, un-organized pulses, that doesn't mean you need more "back pressure" it means the system is not right for the application.

If you look at it like an intake system you will get the picture. The same things basically apply, large runners are good for high rpm use or larger engines, small runner produce more torque, but tend to suffer at higher RPM. If this is a tow truck you are building and it won't see over 4500 RPM then a smaller exhaust may be perfect. As I said above we don't know all the details.

Edit: Also in the pictures, what RPM is that at? What is the use of the "test" engine. Once again you can scew data to support any angle you want to take. I have still yet to see dyno results proving backpressure makes more power. On certain engines X header sixe may be perfect on another it may be way off.
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