I'm in the process of ordering a Liberty five-speed transmission to be coupled to a Winters QC incorporating a 3.08 ring and pinion ratio. The rear tires have a 30" diameter The engine a normally aspirated, 365 cu. in. SBC producing 711 HP and 533 ft. lbs. of torque at 7,000 rpm. My plan is to run a turbo motor at some future date, and I would like to select the transmission ratios to accommodate both configurations and make the required changes in final driveline ratios via the quick change....to the extent possible. My problem is exactly what gear ratios to specify for the Liberty to achieve an optimum RPM split.
I have produced a spreadsheet that gives me the speeds in each gear and the RPM drop at each gear change, but am not sure how to interpret the data, based on my (admitted) inexperience. My general approach has been that at the release speed from the push vehicle (assumed at 25 MPH...too low? too high?) the engine needs to have enough RPM to accelerate the car. For the normally aspirated motor, I have assumed this to be about 2500 RPM, but this would likely be higher with a turbo motor. Thereafter, it seems logical to me that the shift from first to second and second to third, etc. should have successively smaller RPM drops since the engine needs to remain in the higher end of the power band as the mechanical advantage decreases in each higher gear. Engine peak power and torque are made at 7000 RPM, and the negative slope of the graph after 7000 is much steeper then the positive slope before 7000, so I'm assuming it's better to execute the shift at 7000 rather than running the engine faster but at a lower HP and torque value.
I've tried some ratios in the spreadsheet with the following results:
Quick change ratio 3.23
30" tire
1st gear 2.42, speed at 7000 RPM=80, RPM drop in shift to 2nd=2144
2nd gear 1.68, speed at 7000 RPM=115, RPM drop in shift to 3rd=1707
3rd gear 1.27, speed at 7000 RPM=153, RPM drop in shift to 4th=930
4th gear 1.10, speed at 7000 RPM=176, RPM drop in shift to 5th=640
5th gear 1.00, target speed at 7000 RPM=192
Is there a guideline that defines what the respective RPM drop should be in each successive gear or does anybody out there have any experience that would be helpful? Liberty offers a wide variety of ratios in small steps, and while I can always make a change if I make the wrong choice, I would prefer to base my gear selection on more than just luck.
Thanks for your help.
Roger McMullin
rogermcmullin@comcast.net