C2 Exhaust

[Justin N.]

Dear Members,

I have ordered the current production GM 2 1/2 inch manifolds for my 67 coupe. I used a carbide bit to smooth out some of the casting flaws and to port match to the exhaust gaskets. No, this won't be judged. I then had them both ceramic coated to avoid the obvious rust issues. My question: If the exhaust extensions are 2", is there any advantage to going to a 2 1/2 inch exhaust system including the manifolds? It seems irrelevant to switch to a 1964-1965 2 1/2 inch system to be restricted by the extensions. Thanks!

Justin #41362

[Mike M.]

Re: C2 Exhaust

go with the 2 1/2 headers and header pipes. it'll run better in spite of 2" exhaust tips. mike

[bill stephenson]

Re: C2 Exhaust

-------Mike is right, but maybe not for the reason he thinks.As exhaust gasses pass towards the rear of the car they cool down incrementally and because of that the gasses displace less space. Therefore, they dont necessarily need quite the same volume at the tip for full performance as at the manifold and header-pipe. To be effective the down sizing needs to be downstream a predetermined distance so that the exhaust is cool enough to occupy less area. The 67 327/350 downsizing right at the manifold would not count, as the gasses are still way too hot. How do I know this you might ask? Well, Im certainly not smart enough to figure it out for myself. No, I learned it from one of the best exhaust theoreticians in the business; none other than Doug Thorley of "Dougs Headers" fame. An extremely nice guy as well!............Bill S

[Robert C.]

Re: C2 Exhaust

OK Bill, I'll bite. Isn't cold air denser, thicker and takes up more volume than hot air? Seams your senario would be the opposite.
Bob

[Duke W.]

Re: C2 Exhaust

Bill's explanation is correct. Friction increases with velocity, but as the exhaust gases cool the velocity is reduced as density increases. Mass flow rate is the product of density, velocity, and flow area.

A straight through 2.5" core muffler with 2.5" tail pipes would provide some advantages - more as power output increases. The OE exhaust system is pretty efficient for SBs, but is a little restictive for SHP BBs.

Duke

[Justin N.]

Re: C2 Exhaust

Thanks much for all the help! I'll stick with the 2 1/2 inch system as I planned.

Justin #41362

[Alan Drake]

Re: C2 Exhaust

Yes, cooler uses less volume. The relationship to look at is PV =nRT
Pressure, Volume, nR are constants, Temperature
So as T (temperature) decreases the V (volume) side of the equation will decrease. This equation is from basic thermo dynamics. Just for information, after that level of information I'm lost.

[Duke W.]

Re: C2 Exhaust

It's called Boyle's law - named for the early English scientist Charles Boyle who discovered the relationship in the 18th Century. It applies to an "ideal gas". Air and most other gaseous state materials follows it quite closely until you get to very low or high temps or very low or high pressures.

The "V" is really "v", which is SPECIFIC VOLUME - the inverse of density, which is usually represented by the lower case Greek letter rho and eliminates the need for "n".

This relationship between pressure, temperature, and density is more clearly illustrated if you rewrite the relationship as follows:

P = rho(RT)

At constant pressure one can see that as temperature decreases, density must increase to preserve equality. "R" is called the "universal gas constant".

Duke

[Mike M.]

Re: C2 Exhaust

alan and duke: glad i didn't sleep thru the lecture on boyles law.the prof's always said we students would use all the trivial crap they threw at us. if the ole proff wasn't having his mail delivered by the ground hogs, i'd call and thank him for keeping me awake that day in the mid-60's. mike