Grand sport (1963) race car headers

[Duke W.]

Re: Grand sport (1963) race car headers

I still have the primary pipe length formula from GM somewhere around here. The primary pipe length is calculated using the desired RPM and the time (in seconds) that the exhaust valve is open. (seat to seat/duration) From this, you can calculate the correct length that will be required to allow the positive wave to travel to the end of the primary pipe and back as a negative to the exhaust valve. I'm sure Duke will understand this.

Use a wave velocity value of roughly 1670 fps for your calculations.

As RPM increases, the time, in seconds, that the exhaust valve is open becomes less and less. That leaves less time for the positive/negative wave to travel to the end of the primary pipe and return to the exhaust valve. That's why, as RPM increases, less and less primary pipe length is required.

If I remember correctly, engineering recommended 34" primary pipe length for the 302 Camaro Trans-Am engine with the "140" cam. I think the peak HP came in around 7200 RPM?? (I'll dig out the dyno sheets)

The only part that's complicated is the math. Not the theory.

Of interest is the fact that in 1967-68, for the Trans-Am Camaro "engineering exercise", GM engineering leaned heavily on a well known 2 stroke engine engineer. Very few people at that time knew more about exhaust wave dynamics than the 2 stroke folks.

The only thing that's arguable is the point the exhaust valve opens as it takes some time and crank angle for the blowdown wave to fully form and it is fairly long, but I think 34" is a good number for current vintage racing engines that are similar to the circa 1970 Trans-Am engines.

If one had the luxury of having sufficient resources to have multiple header sets one could be fabbed for short, tight tracks and one for big tracks with long straights. The former might be slightly longer with slightly smaller OD tubing than the latter to enhance the mid range since if you can get out of a corner quicker than a competitor onto a fairly short straight you will usually win the drag race to the next brake point.

Duke

[Jerry G.]

Re: Grand sport (1963) race car headers

So longer duration should equate to longer length??
I have all the data to input, it would be nice if there was an online program that let you plug in the values

[Duke W.]

Re: Grand sport (1963) race car headers

Depends... it's a matter of when the exhaust valve "opens" to somewhere in the TDC vicinity.

The link in Clem's post #25 has some very good exhaust pressure vs. crank angle plots, which illustrate the situation very well.

Duke