Hey Steve:

What the engine needs is a needle shaped like a dowell pin, and an SM is definitely out. I think the closest unmodified was an N54, I'd have to go back to my notes. Doubt Sunbelt would let out what we really used.

We were anal enough early on to actually mount a camera pointed into the carb during dyno runs to see what the piston was doing. In stock form the SU's are pretty much slammed to the top by 4-4500 rpm at WOT, so the needle isn't doing anything. Part throttle like most street driving is an entirely different kettle o' fish, and contrary to popular belief there is no part throttle (i.e. no squeezing on the throttle, that's for wimps) when racing a Z in ITS, unless you're driving to the grid (or you don't have the chassis nailed down). So, to make the needle effective, and thereby maintaining something like an optimum AFR, you have to slow down the piston. Very tricky balance between restricting airflow versus optimizing AFR. The AFR will vary somewhat just by the velocity over the bridge, so it's not like you want redline to equal piston at the top. You can let it get there sooner than that. Cool, huh?