2.5 Inch exhaust to small?

[FETorino]

Hot gas needs more room than cool gas. Go 3" to the mufflers for sure. The restriction from the muffler back created by the 2.5" pipe won't be that bad. Of course if your 3" muffler's cfm rating (if it exists) is less that 750 each then that is your cork anyway.

From one exhaust article I read and saved a while back.

Once the available flow exceeds about 2.2 cfm per hp, the gains possible by increasing muffler capacity drop to less than 1 percent.

Knowing that 2.2 cfm per open-pipe hp means zero loss from backpressure allows us to determine how much muffler flow your engine needs. Just make a reasonable estimate of its open exhaust power potential and multiply by 2.2. For instance, a V-8 making 700 horsepower on open exhaust will require 700 x 2.2 = 1540 cfm.

A section of straight pipe the length of a typical muffler, rated at the same test pressure as a carb (10.5 inches of mercury), flows about 115 cfm per square inch. Given this flow rating, we will see about 560 cfm from a 2.5-inch pipe. Houston we have a problem.



Read more: http://www.popularhotrodding.com/eng...#ixzz2eyrgMmkk

[Ron Sutton]

Originally Posted by FETorino

Hot gas needs more room than cool gas. Go 3" to the mufflers for sure. The restriction from the muffler back created by the 2.5" pipe won't be that bad. Of course if your 3" muffler's cfm rating (if it exists) is less that 750 each then that is your cork anyway.

From one exhaust article I read and saved a while back.

Once the available flow exceeds about 2.2 cfm per hp, the gains possible by increasing muffler capacity drop to less than 1 percent.

Knowing that 2.2 cfm per open-pipe hp means zero loss from backpressure allows us to determine how much muffler flow your engine needs. Just make a reasonable estimate of its open exhaust power potential and multiply by 2.2. For instance, a V-8 making 700 horsepower on open exhaust will require 700 x 2.2 = 1540 cfm.

A section of straight pipe the length of a typical muffler, rated at the same test pressure as a carb (10.5 inches of mercury), flows about 115 cfm per square inch. Given this flow rating, we will see about 560 cfm from a 2.5-inch pipe. Houston we have a problem.



Read more: http://www.popularhotrodding.com/eng...#ixzz2eyrgMmkk

This is correct is you're looking for optimum power. If you're not looking for optimum power, what is the purpose of the question?

[Vegas69]

Drivability, dumps get old from a noise and dust perspective on a street car.

I take my previous statement back, 3" over the axle with modifications to your trunk or under with black powder coating is my pick with this hp level.

[DaleTx]

X2 ^^^

My car has a 670HP 427” and makes peak power at 6,900 rpm. I ran 3" exhaust with 2.5" tail pipes at first, and then switched to 3" tail pipes. After having tried both set ups… I would recommend 3” all the way for sure. I didn’t think the 2.5” would restrict it that much but when I made the switch… there was a noticeable difference up in the higher rpm range. The engine responded well with the larger diameter tail pipes. I don’t have numbers to back this up… just seat of the pants feel. Also, the 3” tail pipes sound better and have a deeper tone than the 2.5".

[Flash68]

No way I'd run a 2.5 inch exhaust on that engine with that power. I don't need science to tell me that.

[FETorino]

Originally Posted by Flash68

No way I'd run a 2.5 inch exhaust on that engine with that power. I don't need science to tell me that.

But you're just more in tune with your emotional side than the rest of us. You just feel things.

The rest of us have to rely on hard facts.

[GregWeld]

Originally Posted by Flash68

No way I'd run a 2.5 inch exhaust on that engine with that power. I don't need science to tell me that.

Agree....

[GregWeld]

Gee ---- In the old days if we wanted max hp we uncorked the headers!


Seriously - how often do you need "max" hp on the street....

[Flash68]

Originally Posted by GregWeld

Gee ---- In the old days if we wanted max hp we uncorked the headers!

Sure, but then Rob will link some article that shows you that uncorked isn't always best for max power. You know, cold hard facts and stuff.

[FETorino]

Originally Posted by Flash68

Sure, but then Rob will link some article that shows you that uncorked isn't always best for max power. You know, cold hard facts and stuff.

Next time I'll just post "I feel a 3" pipe will really wake that motor up" since that seems to be what you want.