Vigilante torque converter efficiency question


Dec 25, 2003
I recently made a surprising discovery. Direct Scan data indicated my 87 GN was going 96 mph (in 3rd gear) and the engine was turning 5225 rpm. If I did the calculations correctly (3.42 rear gears, 1:1 3rd gear ratio, 26 in. tires), at 100% torque converter efficiecy my speed should have been 118 mph. I expected to see a loss of approximately 7-8% instead of approximately 18%. Am doing something wrong or is my converter very ineffecient? BTW, my transmission is newly rebuilt.
Thanks for your comments.
How tall are your tires? That can affect your speed reading. Also what was your mph in 1/4 and how did that match to what DS said? What stall do you have? Do you have a multi-disc vigilante? If so why not lock it up at about ~60mph and have no convrter slip and pic up some ET and mph? Its possible your trans could be slipping or maybe the oncverter going bad. What kind of TV pressure does your trans have?
That's pretty average from what I've seen on lower RPM engines. The higher you spin it, they seem to get better. I couldn't live with this because I run a stock cam.
My Vigilante torque converter has a brake stall of approx. 3200-3300 rpm. I bought it from a Turbo Buick vendor's inventory. In other words, the converter was recommended for my application. The 18% speed loss at 5225 rpm seems excessive. Does anyone have similiar data for a 9/11 or other brands of converters?
FWIW, my converter is a single disc lock-up and I have not tried to lock it at wide open throttle.
The larger diameter the converter, the more torque the engine has, the closer you are to peak torque, and the taller the gearing in the car the longer it will take (more rpm's) to get to the most efficient range. If its meant to be locked it probably wont come close to the efficiency of a smaller lighter non lock up converter until its locked. My combo doesnt couple well till 5900 rpm with the non lockng AC converter. At 5000 rpm im probably slipping 40%! Basically the engine just hovers at 5200 rpm and the car accelerates like a bullet.
What bison says makes sense to what I've experienced. I had a 9/11 and it worked well for many years and never failed. It did tend to hover around 5200 also until I locked it up. Currently I'm trying a Yank to see if I like it or not. The tach has more movement on the shifts than before and no longer hovers. It may simply be that it's a tighter convertor too though....