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

[Lonnies Performance]

You can use this as a guide.

[boss_hoss]

If you know the driveline shop you want to work with, I'd just call them and pick their brain. I'm on the Westcoast and got my driveshaft from Inland Empire driveline, I gave them all my specs and what I was planning on doing and their recommendation was spot on perfect for my car. I'm Sure PEM, Denny's or Driveshaft Shop will know what's best.

Here's a Critical Speed calculator that may help too.

http://www.wallaceracing.com/driveshaftspeed.php

[ryeguy2006a]

Thanks guys! That diagram is very useful Lonnie. It's also interesting that some of the calculators give different results. I was looking at the Spicer calculator and it's different than the Wallace Racing one.

[Lonnies Performance]

The chart I provided is for Strange Engineering Chromoly drive shafts.

What is your power level & how fast do you plan to spin it?

Personally, I go overboard on driveshafts (& the entire driveline) for peace of mind.

[ryeguy2006a]

Probably the fastest speed I'll go is on a Dyno, but I'm not ruling out a racetrack. As for power, I'm running a 5.3 LC9 and am planning to run a turbo in the near term. I ordered a 9" with 35 spline axles so the rear should be good. I'm leaning towards a 3.5" aluminum driveshaft from pem with a .125" wall. I measured tonight and my center to center length is 52" on my current driveshaft.

[Lonnies Performance]

What RPM?

[ryeguy2006a]

I can't see turning more than 7000 and I'll set my rev limiter to 6800. I have a .75 overdrive in my 4l80e as well.