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post #50 of (permalink) Old 07-05-2018, 03:58 PM
Big Dave
Join Date: Jan 2006
Posts: 12,456
I hate horsepower ratings as they are meaningless unless you know at what RPM the HP was measured. If in the case of a 540 BBC that makes over 400 foot pounds of torque just off idle and it climbs slowly to a table top 650 pounds of torque that it holds from 4,200 to 6,400 RPM how is that going to help you? Or I could tell you that it makes peak horsepower of 775 HP at 6,800 RPM. That any better? How often are you running around town at WOT?

It is torque (the twisting force that breaks axles, input shafts, ring gear teeth, etc.) that breaks parts, not peak horsepower. The transmission builders know this as well as I do; but they post horse power numbers that have no bearing on the torque rating of the tranny strictly as a sales gimmick, not as a warranty. Most good tranny rebuilders put their tranny on a reverse engine dyno (big electric motor takes the place of your engine called a Spin-Tron) and they twist it up to see if it stays together at a predetermined torque value. That is how trannies are rated.

GM takes an Alison diesel truck transmission that weights in at 330 pounds and is as big as a SBC and uses that as a 100% value (it is rated at 740 foot pounds of torque) then they assign a number like 40, 60, 65, 70, 80, 85, or 90 to a GM factory built transmission to give you an idea that it is that percentage as strong as the Alison. So a 6L90 has 90% of the torque capacity of the Alison, the 4L80 has 80% of the strength, the 700R4 is actually a 4L60 transmission after 1993 when GM changed the names of all of their transmissions to reflect torque rating.

So theoretically it can live behind a motor that has 444 foot pounds (which I can tell you it won't last long with big tires a posi and hard launches). It can live behind a 444 foot pound of torque motor if you go through it and replace the input shaft, the Sun Shell, the Sparg and the clutches), maybe. Once again depends upon weight of the car and the car's final gear ratio (the immovable object part) compared to the torque curve of the motor (the irresistible force part of the equation).

Big Dave
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