2" vs. 2 1/2" exhaust

[Ed Jennings]

Re: 2" vs. 2 1/2" exhaust

Much as I like the 327/350 as an all around engine for the street, you are probably correct. The 300 is certainly cheaper to overhaul to original specs.

[Duke W.]

Re: 2" vs. 2 1/2" exhaust

An engine equipped with headers will be more sensitive to tailpipe diameter dimensions because there are wave reflections from the end of the tailpipe that help broaden the rev range that will create negative pressure at the exhaust port, and wave action is affected by both pipe length and pipe diameter. That's why serious race cars have about two to three feet of tailpipe rather than just dumping the exhaust out of the header collector.

But in the case of common cast iron manifolds there is essentially no useful wave dynamics effects at the exhaust port, so the overall objective of this type of system is to minimize backpressure, and this can be done with large manifolds, exhaust pipes, and mufflers. Each increment of backpressure will reduce BMEP by an equal increment. The effects are most noticeable at high revs because of the square relationship of backpressure to exhaust flow, but there is also an effect at low revs, though it is less. Bottom line is that larger manifolds (I'm not talking headers, here) and pipes will make more torque/power across the rev range from off idle to the redline. When you install headers to take advantage of wave action, things get more complicated and "bigger is better" doesn't always apply, particularly to the header pipes, but also to the tailpipe. Too large a tailpipe will often loose some low to midrange torque because the primarly wave reflection from the end of the tailpipe is too weak by the time it reaches the exhaust port. Large pipes reduce overall flow friction losses, but they also reduce pressure wave peaks, and there is a tradeoff point.

Larger pipe diameters will tend to increase low frequency noise, which it tough to handle in most muffler designs. That's one reason why SHP engines have a "deep, thoaty" sound. And it's a reason why early Sting Rays have a much different sound than a '62, even though they have the same engines. The bigger pipes and lack of a cross over gives the Sting Rays a deeper tone due to less attenatuation of the low frequecies.

Install smaller pipes and some of the lower frequencies will be attenuated. Don't forget that the 300HP/manual trans early Sting Rays had the big manifolds and pipes, while the Powerglide versions had the small manifold and pipes. A side by side comparison of these two combinations with the same mufflers would yield a difference in both sound quantity and quality with the Powerglide version sounding more "tame".

Duke

[Dale Pearman]

Re: 2" vs. 2 1/2" exhaust

Whatever.

Dale.

[Robert C.]

Re: 2" vs. 2 1/2" exhaust

On the 70's L-T1 cars the exhaust manifold was 2" then the header pipe mushroomed to 21/2"s. Following to the rear, all the pipes were 21/2". The muffler

inlet was 21/2"s but the muffler outlet was pinched back down to 2"s.Do you know why?

[Duke W.]

Tailpipe size

As the exhaust gas travels down the pipe it cools and becomes denser, so velocity slows in proportion to the density reduction. This fact falls out of the continuity equation for gas flow.

Considerable cooling occurs in the mufflers, so the smaller tailpipes don't create significantly more back pressure. I think the internal tubes in the mufflers are also 2", not 2.5". Larger tubes would create less back pressure, but more noise. It's a tradeoff, and by 1970 I believe there were both federal and Califonia noise standards.

Duke

[James G.]

Re: 2" vs. 2 1/2" exhaust

I had a 62 Bal Air bubble top with a 327/250hp and 3 speed in college. That car ran two classes back of the 300hp cars at the drags, and I regularily had ET's in the high 14 second range right along side of the 300hp Chevrolet products. Now remember, that was with the WCFB carb. I really think performance wise, the 250hp engine was very under rated compared to 300hp motors.
Save the Wave,
Jim