Mailbag: Solving Recurring Driveline Vibration

You’ve got questions. We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re troubleshooting drivetrain vibration.

Q: I have two questions about my truck. It’s a 1994 Chevy C1500 pickup with a 4.3L Vortec V6 engine. I purchased it from  my uncle, who converted it from a manual transmission to a TH-700-R4 automatic with  a shift kit and “Vette” style servo. The flexplate and torque converter were sourced from a wrecked 1987 Chevy Astro with a 4.3L (non-Vortec) engine, but I’m unsure whether they’re right for my application.

Since the conversion, the truck has suffered from drivetrain vibrations. I took it to a mechanic who diagnosed it as having a bad torque converter, so I purchased a new converter and had it installed. Once installed, the vibrations went away for a few months and everything was great. But recently, the vibrations returned. They begin just off idle and continue until about 2,000 rpm. They stop briefly, and then begin again at around 3,200 rpm and continue until the transmission shifts. I don’t know what to do–it shifts great, the fluid is clean, and no debris has been found in the pan. What do you recommend?

My second question is about rear-axle gear ratios, My truck has the original 8.5-inch rear-end with a 3.08 ring and pinion. It’s sluggish from takeoff and just doesn’t have that “torquey” feel I’m looking for. But from 4,000 rpm and up it really moves! I was told 4.11 gears would really wake it up down low. What do you think?

A: According to our manuals, Chevrolet used the same flexplate in both the Astro and C1500. And since the vibration was eliminated with the installation of the new torque converter, we’d suggest that you recheck the condition of the flexplate and converter. Also, retorque the fasteners that hold everything together. Using a good thread locker will prevent the mounting hardware from loosening.

For your axle ratio, we recommend 3.73 gears. Combined with the 700-R4’s low first gear ratio, 3.73s will help move your truck from a standing start with some authority while providing you with that “torquey” feel you want.