peeeot (6/17/2009)
...I didn't think about calculating torque produced vs. torque needed. How do I do that? I knew there was a tradeoff between rpms and load, but specifics are vague in my mind... Torque produced must always overcome torque needed, otherwise your engine will lose rpm's at full throttle, and eventually die. High ratio gear sets shine at low speeds. If you are happy with your engine's pulling power at a light, or if you pull a trailer, fine. If you need more torque at low speed, gearing is the answer.
My Service Manual says a '55 292 produces 280 ft/lbs of torque. Let's just use this figure. Std transmissions output 1:1 in high gear. Now comes the fun part:
There is a direct ratio between gears and torque output. At 3.0:1 gear reduction (your gear), rear wheel torque is trippled to 840 ft/lbs, which is great! Want more torque? A 4:30:1 gear produces 1,204 ft/lbs of stump-pulling torque (280 x 4.30 = 1,204). That's nearly one and a half times more pulling power (in high gear) using the same engine and transmission. The trade-off is engine speed. Basically, if your car goes 70 mph at 3,000 rpm, with a 4.30:1 gear, your new engine speed would be ~4,300 rpm.
Now for First Gear: A V8 3-speed tranny's First Gear has a ratio of 2.57:1. Using the above figures:
* Your 3:1 TIMES 2.57:1 TIMES 280 ft/lbs of torque= 7.71x280 = 2,159 ft/lbs. (This is your maximum torque at the light to your tires).
* 4.3 X 2.57 X 280 = 3,940 ft/lbs of torque! This gear nearly DOUBLED the engine torque for a great launch at the light.
Tire diameters can change ratios just like gears. Your tire diameter calculations start where the gearing leaves off. Racers use very large tires with rear end gears of 4:30 (commonly).
Another side advantage to using these high ratio gears, the rear ends tend to stay together. If you had 600 ft/lbs of torque (or more) you can see where a 3:1 gear has a harder job transferring torque than a 4.3:1 gear. The 4.3:1 gear spreads out the same torque over an additional 1-1/3 driveshaft turns than the 3:1 gear set.
Sorry to be so long winded, but I hope this helps you understand the difference in gear sets.
- Dave
Royal Oak, Michigan (Four miles north of Detroit, and 12 miles NORTH of Windsor, Canada). That's right, we're north of Canada.Ford 292 Y-Block major overhaul by simplyconnected