Are there any computer programs I could buy that could help me figure out what would be the best trany and rear end ratios to have for my application? Give me torque and RPM and horsepower in any given gear at any given speed? And vary the trany gears and rear end gears?

 

Something like this? - http://www.fatboyraceworks.com/gears/

 

http://www.smokemup.com/auto_math/#gear

 

I've done several programs and spreadsheets for that. In fact, some of us built a nice spreadsheet for selecting gears by specifying your RPM range and how many gears you have and what you want as a top speed. Unfortunately, corvetteforum doesn't have any place for us to put such files. I can't even locate the file myself.

It's relatively easy to do the calculations and create a spreadsheet. Select an RPM range based on your horsepower curve, for example 3500 to 5200 RPM. Multiply both of these numbers by 60*85 then divide by 63360 to convert them to miles per hour at the rear wheels. 85 is the circumference of the stock tires...adjust this for your application. For example you will get 281.72 mph at 3500 RPM and 418.56 mph at 5200 RPM.

Now select your top speed, for example 150 mph and divide it into 418.56 and you will get 2.79...this is your 1:1 gear top speed. If you are going to get an overdrive tranny then you will need to divide by your overdrive gear for example a .85 overdrive will require a 2.79 / .85 = 3.28 rear end. I'm giving an example of a 4 speed non-overdrive in the following...

Now we need to know what speed you will be going at 3500RPM. Think about this...the final ratio of 2.79 indicates that the engine will turn 2.79 times for each rev of the wheels. Thus the effective RPM at the wheels is 3500 / 2.79 which we can use the multiply by 60*85 then divide by 63360 formula to get 101 mph. So, your next to last gear needs to get you up to 101mph. Just use the formula again (adjust for final ratio) 5200 * 60 * 85 / 63360 / 101 / 2.79 = 1.49 or the most common gear ratio is 1.50! For a 4 speed this will be your 3rd gear.

Now we need to know what speed you will be going at 3500RPM in that gear. The effective RPM at the wheels is 3500 / (1.5 * 2.79) which we can use the multiply by 60*85 then divide by 63360 formula to get about 67 mph. So, your 3rd from the last gear needs to get you up to 67 mph. Just use the formula again (adjust for final ratio) 5200 * 60 * 85 / 63360 / 67 / 2.79 = 2.24 or somewhere around there...this is 2nd gear in a 4 speed tranny.

Now we need to know what speed you will be going at 3500RPM in that gear. The effective RPM at the wheels is 3500 / (2.24 * 2.79) which we can use the multiply by 60*85 then divide by 63360 formula to get about 45 mph. So, your 4th from the last gear needs to get you up to 45 mph. Just use the formula again (adjust for final ratio) 5200 * 60 * 85 / 63360 / 45 / 2.79 = 3.33 or somewhere around there...this is 1st gear in a 4 speed tranny.

If you have more gears then your RPM range will be smaller, for example a 5 speed will be like 3800 to 5200 and a 6 speed will be like 4000 to 5200.


Set up the spread sheet with these cells:

B3=Start RPM
B4=End RPM
C2=60 //this is the minutes in an hour.
D2=85 //this is the tire circumfence in inches.
E2=63360 //this is the inches in a mile.
F4=150 //this is the desired top speed.

Then all cells in the C column from row 4 down = =B4*C$2 //this converts the RPM to RPH.
Then all cells in the D column from row 4 down = =C4*D$2 //this converts the RPH to IPH.
Then all cells in the E column from row 4 down = =D4/E$2 //this converts the IPH to MPH.
Then all cells in the F column from row 5 down = =$B$3/($G$4*H4)*$C$2*$D$2/$E$2 //this calculates the final RPM in that gear.
Then all cells in the G column from row 4 down = =E8/F8 //this calculates the final ratio in that gear.

Now for the tricky part...if you are not using an overdrive tranny then put a 1 in H4 and all cells in the H column from row 5 down = =G5/$G$4 //to calculate the actual gear ratio.
If you want an overdrive tranny then calculate the final drive then replace the cell at G4 with the rear end ratio and put the overdrive ratio in cell H4 and replace cell H5 with 1.

This is what it looks like for a 4 speed:

MPH CIRC. IPM
RPM 60 85 63360
Start 3500
End 5200 312000 26520000 418.5606 150 2.790404 1
5200 312000 26520000 418.5606 100.9615 4.145743 1.485714
5200 312000 26520000 418.5606 67.95488 6.15939 2.207347
5200 312000 26520000 418.5606 45.73886 9.151093 3.279487

This is what it looks like for a 5 speed:

MPH CIRC. IPM
RPM 60 85 63360
Start 3800
End 5200 312000 26520000 418.5606 150 2.790404 1
5200 312000 26520000 418.5606 109.6154 3.818448 1.368421
5200 312000 26520000 418.5606 80.10355 5.225244 1.872576
5200 312000 26520000 418.5606 58.53721 7.150334 2.562473
5200 312000 26520000 418.5606 42.77719 9.784668 3.506542

This is what it looks like for a 6 speed with .85 overdrive:
MPH CIRC. IPM
RPM 60 85 63360
Start 4000
End 5200 312000 26520000 418.5606 150 3.282828 0.85
5200 312000 26520000 418.5606 115.3846 3.627525 1
5200 312000 26520000 418.5606 98.07692 4.267677 1.3
5200 312000 26520000 418.5606 75.44379 5.54798 1.69
5200 312000 26520000 418.5606 58.03368 7.212374 2.197
5200 312000 26520000 418.5606 44.64129 9.376086 2.8561
RPH IPH MPH CALC FIN RATIO GEAR

The whole point of having gears is to maximize total horsepower, so no matter what gear you are in the power will be there for you...all you gotta do after that is choose which gear you want so that your RPM is within your power range.

I wish there was a way to upload files. After setting up the spreadsheet you just need to change the RPM values and Wheel Circumference...and overdrive and final drive if you are going with an overdrive tranny. For example I played with it and found that a .76 overdrive tranny is ideal for a 6 speed with a 3.67 rear end to have a top speed of 150mph with all gears engineered to get you there as fast as possible.

 

send me the fileinfo@force-efi.com I will host it and post a link until and if my hosting service complains. If the mods are OK with this...

 

Email has been sent. I highlighted the cells that can/should be changed. I'm sure the mods are good with it...they simply didn't have any hosted member pages left when I registered with the site and got tired of paying for a hosting service that totally lacked features.

I hope the spreadsheet helps.

 

Spreadsheets can be quite helpful (I've got some farily complex ones myself), but if you wish to more accurately model performance to identify optimal gearing for a specific application you'll need a good sim program which accounts for available traction and aero drag among other variables, particularly when estimating a combination's terminal velocity.


TSW