Input needed - gears or stroke?

[ItBites]

Yes,

Torque at the rear axle is the product (multiplication) of the torque at the flywheel times the total gear reduction (transmission gear ratio X rear gear ratio). Losses are incurred in the driveline, but the method presented means a lower rear gear produces more torque at the rear axle, given the same torque at the flywheel and the same first gear ratio.

I don't know squat about Brand-X (Ford) trannies, but lets take an example case:

First gear ratio = 3.23 (example)
Old Rear ratio = 3.08 (typical chevy hiway gear)
Engine torque = 250 lb-ft (since you got a 289)

Old Rear axle torque (ignoring driveline losses) = 2487 lb-ft

First gear ratio = 3.23
New Rear ratio = 3.73
Engine torque = 250 lb-ft

New Rear axle torque (ignoring driveline losses) = 3012 lb-ft


Big increase (21%) huh?

Now, torque equates to tractive force (what makes ya go forward) by dividing the torque (lb-ft) by the tire radius (half the height of the tire). Remember to do this in feet, not inches of tire radius. So, if your tire is 26 inch tall, the radius is 1.083 ft. Using the example above you get:

Old tractive force = 2296 lb (force)

New tractive force = 2781 lb (force)

All this assumes the tires do not slip and is of course a snapshot at the point where the engine makes 250 lb-ft of torque.


You can take this one step farther and throw in about 15% drivetrain loss and then use F=mA to determine an approximate acceleration at this snapshot. I again don't know squat about Brand-X, but lets assume your mustang weighs 3220 lb (=100 slugs - trust me on the slugs part).

Old acceleration = .61 G

New acceleration = .73 G



Additionally, if you're looking to lower quarter mile times, the lower gear you go to will always lower times, until the point when you are at peak rpm in high gear before you reach the traps. So, in spite of other opinions in this post, a 4.11 gear might give you the lowest times as long as the engine is not yet at redline when you trap. Drag racers play this game all the time. They put in the gear that gives the max RPM in high gear in the traps. This way they use all of the engine torque (power) curve in every gear to get there.

I can explain this statement as well, but it involves integrating area under the curve and total energy (work) expended to reach the 1/4 mile in the least time (power), but I'll save that for another post.


Put in the lowest gears you can stand with a low torque 289. If you drive hiways this will be a compromise between cruise rpm and acceleration potential.