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Rear end gear ratio

36
27
Exp. Type
Autocross
Exp. Level
3-5 Years
Utah
Long story short. The 9" full float rear end I want has a 11 week lead time (Thanks Biden). While I know transmission ratio and engine torque curves are all taken into consideration. I'm trying to narrow down the rear end ratio that comes the closest since I need to buy today to have it 3 months from now.

Engine is going to be a high revving 363 or 427. I know both have different curves but we'll be bouncing 3500 RPM and up on both.

Tranny is going to be a 5 speed with most likely the following ratio's (or pretty darn close)

1st 2.90
2nd 1.90
3rd 1.37
4th 1.00
5th .58

I will have the power and suspension to handle 180+ mph, light weight to handle a higher ratio and of course the brakes to stop it.

For the 9" I'm thinking the 3.50 or 3.40 are best, one given slightly higher top speed other faster accel. 3.25 is too low and 3.70, too high. Basically looking for opinions, what you run or better yet validation. If I have to change it down the road if I really need to then fine

Thanks
 

Dave_W

Cones - not just for ice cream
450
449
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
This is from the 2021 SCCA Solo Rules book:
1639103482257.png
I think there are 2 major consideration in gearing - not running out of rpm at the end of a straight, and being in a good rpm range for accelerating out of a corner, especially if it leads to a long straight. If you can find the radius of the "important" corners on the tracks you'll be running, you can estimate what your apex speed would be at your target lateral g. Then calculate the engine rpm (from tire circumference, rear gear, and transmission gear) that you'll be in at that speed, and see if it's in a good part of your torque band. Tire Rack lists revs/mile and circumference for many tires, or you can calculate from "marketing" size (e.g., 285/40-18).
 
3,946
4,006
I'm not sure I've ever seen a "high revving" 427, 6 to 7K was about it, even for those side oilers with the crossbolt mains. They were torque monsters though.

Here's something to consider from the dragster guys, back in the day it was thought that you wanted the engine to run out of steam at the end of the strip, that proved false, you wanted the engine to pull THROUGH the lights and a bit beyond, also, since they launch from a standing start, a lower 1 and 2 gear was more important than the top end ratios because you had to get the car moving, this was discovered by accident when a guy broke his M22 and replaced with a wide ratio M21 only to discover it was faster. (the M22 Muncie was the preferred road race trans because it had a center bearing that prevented galling, and the gear ratio spacing was closer)
Just some more points to ponder.
 
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