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.

 

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

Hmm... if I gained 2-3 tenths / mph that really would be worth it.

 

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

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

 

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.

 

I read a article about a system that was better then open headers, the ide was that you should keep the speed up with the gases with a 2.5" to the x-pipe or H-pipe, and then open it upp to 3". Btw,they tested it with a Dyno also.

 

On 2003-04-20 07:15, Helmantel wrote:
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>

Thanks for the info Helmantel! ...is there a typo in your post - should it say exhaust volume flow is about 3x bigger than in take volume flow?

 

Oh sorry, that should of course be exhaust volume flow is 3 times larger than intake volume flow.

 

The cross sectional area of the different size pipes is...

2.5" pipe = 4.91 square inches
3.0" pipe = 7.07 square inches

x2 for dual exhaust...

2.5" duals = 9.82 square inches
3.0" duals = 14.14 square inches

It's a very significant difference. I really like the 3.0" exhaust on my mustang... It provides a very deep tone. However, if tailpipes are part of the plan, you'll find it very difficult to do with 3". However, a place called Asphault Jungle makes them for Fox mustangs. (they make the whole system, front to back)

You could run 3" back to a set of 3" mufflers, then weld some collector rings to their outlets. This would allow you have REMOVABLE tailpipes.

All you would have to do is weld the tailpipes to a set of ordinary reducers. This way they could be quickly unbolted when they weren't needed. You could even re-use your existing 2.5" units.

Good Luck!

_________________
Mike Burch, 66 mustang real street
302 4-speed 289 heads, 10.63 @ 129.3
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220
http://www.fortunecity.com/silverstone/healey/367

<font size=-1>[ This Message was edited by: n2omike on 4/23/03 6:53am ]</font>

 

Space with 3" system is not a big issue.I am rfunning the pipes/tailpipes beneath the rearend housing.It is extremely straigtforward, easy and simply way.And...the pipes are NOT visible from under the car.It is a perfect way without any negatives. No sharp bends, easy to remove, simple & easy to build.
Part of the picture is that the car must sit quite low in order to hide the pipes- there is only few uglier sights than a full lenght visible exhaust under the car.
Something I am also thinking is the exhaust gas temperature/volume in the exhaust pipe itself.Since the temp/volume is much greater in the beginning of the system/header and is only "warm" when it exists the system, it means that the volume at the end of the system is perhaps only half at the end and thus would require only half of the size exhaust there? If this logic works then 2,5" mufflers at the end of the system would be more than enough?
My mufflers are located just before the rear bumber.I was thinking running 3" system to the mufflers and use the existing 2,5" tailpipes/mufflers like N20Mike suggested.
On the other hand somebody wrote here that using 2,5" system,which provides more exhaust speed / scavenging produced more power in the dyno...go figure.

 

Thats what im thinking of doin. 3" xpipe to the mufflers then the flowmaster 2.5" tailpipes. Sound oK?