I'm looking for help on whether to go with 2.5" or 3.0" exhaust on my 69. It is an L78 bored .030 over. Other than that it is stock except for forged aluminum rods and pistons. I'm about finished with the build. It's a 4-speed car, if that matters. I am leaning towards Hedman headers. The car is a driver. Mainly I have read that 500hp is the cutover to 3.0", and I should be well below that. Any problems with staying at 2.5" or should I move up?

I'd recommend reading this for some great info. http://www.pontiacstreetperformance.com/psp/exhaust2.html

Quick view is that based on all the things I have read, the real "power retention" key is keeping the "up-to-the-muffler" size at 3" and that w/ 2.5" tailpipes there's not much of a power drop for a CI application like yours. The above article goes into details.

Hopefully other exhaust seasoned members will add knowledge.

I would say for a mild build to go 2.5.

You do not need 3" for a motor that is street driven only and you will have more low end tq with the 2.5.

Quote:
The temperature measurements showed that exhaust gasses cool significantly before they reach the tail pipes.

JMO, I think ^this^ statement is a stretch at best, think about the velocity of exhausted gas fumes, are these people serious. :rolleyes:

The temperature of the exhaust gasses at various locations was also measured to provide additional data.

How else would they determine the change in temperature along the exhaust system? Direct measurement of transient characteristics is a relatively time-honored method of gathering data.
Did you believe any of the other measurements of the article?

Did you believe any of the other measurements of the article?
:yes: Yes. Hey if I'm wrong I'm wrong http://www.powwows.com/gathering/images/smilies/57.gif, I still find it hard to believe at 6-7K R.P.M. on a 90* day it's getting a real chance to cool down significantly.
It was also demonstrated, that the use of 2.25" tailpipes, caused only a small reduction in performance.

It's all relative to power level IMO, bigger power = more loss with the use of a smaller diameter tail pipe, however this is just my opinion. :thumbsup:

I'd recommend reading this for some great info. http://www.pontiacstreetperformance.com/psp/exhaust2.html

Quick view is that based on all the things I have read, the real "power retention" key is keeping the "up-to-the-muffler" size at 3" and that w/ 2.5" tailpipes there's not much of a power drop for a CI application like yours. The above article goes into details.

Hopefully other exhaust seasoned members will add knowledge.

I wish they made kits with 3" in and 2.5" out.

I wish they made kits with 3" in and 2.5" out.

you can buy 3" headpipes from Pypes for about $60 and seperate 2.5" tailpipes.

Let's look at the numbers..
If you look at the relative area of the tubes in the system, going from a 3" tube (6.611"sq) to a 2-1/2" tube (4.529"sq) entails a 46% reduction in area.
Exhaust gas comes out of the engine at about 1200°F, and let's assume it enters the collector at 1000°F. In order to satisfy Boyles law we only need to cool the gas by 46% before the tailpipe, to about 685°F, and that's still mighty hot.
When you are dealing with temperatures in the range of 1000°, the difference between a 70°F and a 90°F outside temperature is virtually nil.

In that specific test article they used 2.25" tail pipes.

3.0" exhaust pipes and mufflers but using only 2.25" tail pipes.

In your opinion would ceramic coatings affect the heat dissipation rate in this particular type of test, would it be marginal, as others I am also here to learn. :)

FWIW, I absorb everything performance oriented and sort out fact vs. fiction for myself, thanks for any and all information you are willing to share. :thumbsup:

Thanks for all of the help, as always it is much appreciated.