Please School Me on Driveshaft Critical Speed

[ryeguy2006a]

Hey guys,

I'm working on getting my drivetrain set up for the future when I boost the car and am giving some thought to my driveshaft at this point. I bought a Quick Performance 9" rear, and I'm planning to run a Sonnax 1350 slip yoke in my 4L80e, and a 1350 yoke on the 9" side as well. I'm looking at my options now for the driveshaft and want to make sure that I'm going to be safe at whatever speed. On the Street or on the dyno. I'll likely order my driveshaft from PEM where I picked one up for my 68 Camaro after adding boost. They have a 3.5" "Drag Race" driveshaft that has a .125" wall tube with 1350 ujoints. But they also have the option for a 4", but I'm not sure I'll be able to fit that large of a driveshaft in my tunnel. My driveshaft was right around 50" long, but I'll have to crawl under and measure to confirm.

Please discuss all the things that I'm not thinking about.

Thanks,
Ryan