Posted this on LS1Tech, didn't get the intelligentdicussion I was hoping for!

[BRD-PREY]

I always get an honest look at issues on here.

My thought is a 2" or even 1.5 inch exhaust cutout would be as effective as a full size 3" cutout. I figure that since its not an exhaust divertor that it only needs to aloow enough additional flow as to remove excess backpressure.
The original 3"-4" and muffler are still flowing so the cutout just needs to provide that extra flow above what they are capable.
This would make it much easier to fit under the car.

If this is correct it would be a significant improvement in ground clearance!

Please chime in if you see a flaw or agree with this conclusion.

Thanks
ed

I talked to a design engineer with Borla and he seems to think it would be fine with a smaller cutout.

[joecar]

I don't see why it wouldn't sufficiently relieve back pressure... and if it was short, it might act like a venturi (increase flow velocity).

If it was thought that 2" would be restrictive (maybe in the upper rpm range), then maybe have two cutouts.

There isn't a whole lot of back pressure to relieve, maybe up to 2psi in the worst case.

Do an experiment:
attach an exhaust back pressure gauge at the from O2 bung;
measure pressure with cutout closed (2 psi), and then open (0 psi).

I think the Borla engineer is right (he, if anyone, would understand flow dynamics).

$0.02

[turbo_bu]

Probably also has to do with where the cutout is at. Typically people put it right up near the headers (or manifolds) which also helps to eliminate the piping losses. Thus, if further away from the muffler, then it might be more size sensative. Though sensative is a relative term. As joecar said, it's not a lot of pressure, so just the fact of letting some of it out with a slightly smaller cutout will help anyway.

[BRD-PREY]

The cutouts are about 18 to 24" back from the collector.