[750hp]

Bernie, I wanted the longest possible mufflers within my sidepipes. I ended up with perforated sheet rolled to 3.5" diameter, fibreglass packed tight around this, with a 5" stainless pipe holding it all together. After one afternoon at Qld Raceway, a lot of the fibreglass packing melted away, making the car too loud for the street.
Version 2 now has stainless wool packing around the perf tube, then a really thick fibreglass matting outside this. Just to be sure, there a spiral welded into the exhaust so that all exhaust is forced out and through the perf tubing, rather than running straight out the pipe. It might cost a couple of horsepower, but the chance of making the pipes a little quieter is well worth a couple of horses.

Jason, no - it's not a GT, but it does have a reasonable size motor.