This is on a mild 350. Just trying to justify the cost. I already have the 2.5".

Assuming your exhaust had no bends, cat or muffler, at the 270 crankshaft HP level is where the 2.5" begins making measurable backpressure.

A pipe will flow 115cfm per square inch of cross-sectional area. You need roughly 2.25cfm of flow per crank horsepower.

A 2.5" Pipe is 4.9 square inches of theoretical surface area. That means that, with no restrictions, etc, you could be generating 250 horses before the 2.5" pipe becomes a restriction. A 3" Pipe is 7.05 square inches, giving you the ability to flow ~365 horses before it becomes a problem. Keep in mind, this is assuming no restrictions.

Well I measured my exhaust and it is 2.25. Yuk! It was my dad's 2.8 '87 Bird.

Since I need new exhaust I'm going with 3". I'll be installing a vortec 350 with a ZZ4 cam and a carb. I'll be making more than 270 hp.


Thanks for the info!

Stock 2.25" pipe is good for 175hp before becoming a restriction on a V8.

I picked up 35hp by going from stock LG4 exhaust to a banks y-pipe, 3" cat, and banks cat back way back in 1990 without any additional tuning.

I was looking at the same thing for mine but i already started buying up parts for a 2.5... I'll be posting them up for sale and starting over