At which power level does 2,5\" dual exhaust start to restrict power output?

Yeah that pretty much is the topic....Using H - pipe and straight thru well flowing mufflers.
I am wondering whether I should replace my current (dual 2,5" with H pipe)system with 3" one and what would be the gain, if any? The 393 should put out something like 530-550 hp with open exhaust, max rpm 6700 or so.
I know this is an eternal question but has anyone got any new real life info?
[addsig]

In my other race group, turbocars are built that run 9's with 2.5" dual exhausts. The cars weigh 3600 lbs, and hp figures out to the 700+ range, so you should not have much problem with present setup. HTH.

did a test had 2.5 inch exhaust on car and ran it then put on 3 inch exhaust car ran 2 tenths and 3 mph over the 2.5 inch

2.5 mandrel bent exhaust is good upto 500hp
Also keep in mind the more cubic inches you have the larger size pipe you need to handle the extra flow. I would say with your 347ci you will be fine. You might even loose some streetable lowend torque with the 3 inch. However if you are running a drag car where the rpms exceed 6500 or more then 3 inch might be an option. An engine is nothing more than an air pump. Even if your engine runs at 100% volumetric efficiency you still only move 347 ci of air/fuel.

Assuming similar combustion efficiency, then shouldn't hp be more of a factor than displacement ? For example, a 500 hp engine could be a super-high revving 289, a blown 5.0L, a street/strip 393 (like ville's) or a relatively mild 460, but they all need to move pretty much the same amount of air to make 500 hp, right ?

Hey 654212spd, you said you dropped 2 tenths and gained 3 mph. Is this with the 421 you mention in the "your car" profile or an older combo ? If it is the 421, then I would think ville's 550 hp 393 would probably gain/lose something along those lines, maybe a little less.
Do you mind giving up 10-20 hp to stay with your 2.5" pipes?

<font size=-1>[ This Message was edited by: btc on 4/20/03 2:07pm ]</font>

When I had my heads put on the flowbench they flowed the best with stepped headers and a dual 2.75" free-flowing system,(with a balance pipe), as this pipe size was unavailable I went with 2.5" rather than 3" as this is only a 300hp 302. My head guy compared different configurations and this was what he came up with. Don't know if this helps, and flow benches are sometimes purely scientific and what happens on the car may be totally different.

[quote]
On 2003-04-19 22:50, 1badmav wrote:
2.5 mandrel bent exhaust is good upto 500hp
Also keep in mind the more cubic inches you have the larger size pipe you need to handle the extra flow. I would say with your 347ci you will be fine. You might even loose some streetable lowend torque with the 3 inch. However if you are running a drag car where the rpms exceed 6500 or more then 3 inch might be an option. An engine is nothing more than an air pump. Even if your engine runs at 100% volumetric efficiency you still only move 347 ci of air/fuel.
Actually, an engine just moves 347 ci of air during intake cycle. Remember what makes the engine work..it is the combustion where the 347 ci of air/fuel mixture expands to totally other volumes.That expansion is the power which pushes the piston downwards creating the power.
There is a formula to calculate the new volume ( pV=nRT), unfortunately only factor I can remember out of it is the "T"= temperature
[img]/phpBB/images/smiles/icon_frown.gif[/img]
Hmm... if I gained 2-3 tenths / mph that really would be worth it.[img]/phpBB/images/smiles/icon_bow.gif[/img]
[addsig]

p x V = n x R x T is the ideal gas law. It applies more or less to most gases.

p = pressure
V = volume
n = number of molecules (or m = mass of air)
R = gas constant
T = Temperature

since mass in = mass out and R is a constant, then p*V/T is constant. So intake pressure x Volume flow in / inlet air temperature = exhaust pressure x Volume flow out / exhaust temperature

This only works if you calculate the temperature in Kelvin (Kelvin = Celcius + 273). After a bit of number crunching, you'll see that in the exhaust volume flow is about 3 times higher than the exhaust volume flow.




<font size=-1>[ This Message was edited by: Helmantel on 4/20/03 11:13pm ]</font>

I agree with what you are saying I was just trying to be generic in definition. Sorry [img]/phpBB/images/smiles/icon_frown.gif[/img] Things such as cam overlap and X pipes help scavenge the exhaust as well. Making up the difference. Sometimes we think to big or not big enough [img]/phpBB/images/smiles/icon_smile.gif[/img]

I would be nice to measure the actual backpressure with a pressure gauge, say just before the mufflers. Then you can be sure wether or not your exhaust system is a restriction or not.
I guess sticking in a tube perpendicular to the exhaust pipe will give nothing but vacuum readings though, due to the gasses flowing by.