TMac50 Ratio Guide
First, let me say that there are many ways to go about getting good ratios for both bracket and h2h cars, I am going to try to explain how I do it. It may not be exactly like what others are doing but it works well for me, just takes a little time to get it just right. And each time you modify the car and add/remove horsepower you will need to revisit your ratios. So, a word of advice, write down hp along with ratio’s for the car as you get them worked out. None of the numbers I use here for examples will work in any of the cars, I am just using them to illustrate my points.
Step 1)This is important, so please always keep it in mind when working with ratios. After you have the gearbox installed go to the garage and click on "View Dyno", then click run. The resulting chart shows both the horsepower and torque curves for the car you have selected. As you can see the torque and horsepower cross at some point with the horsepower going above the torque. Make a note where this point is on the rpm scale. You will need to know this later.
Step 2) Make a few practice runs to determine if the car is spinning or not when you launch. Start by launching at wide open throttle and engaging first when the light comes down. If it is spinning then adjust first gear to a lower number. Like from 10 to 9. If it is not spinning then slowly adjust the first gear up till it does. Like from 10 to 11. This part only takes a few minutes and is going to give you a good starting point. When the car spins at launch adjust to just where it just barely doesn’t spin anymore. I only go out 3 decimal places but in here you can get crazy as you want with ratio’s so knock yourself out. Now take a look at the 60’ times and see where you are launching at wide open throttle. If your intent is a bracket car you will leave 1st gear right where it is and go to step 3. If you intend to do some h2h racing follow along. Take a look at the point where the rpm and hp cross on the dyno run. From that point to where the hp peaks note a point about in the middle. Practice launching from this point a few times by staging and bringing the rpms to this point. As soon as the light comes engage 1st and move the throttle to wide open. Do this enough times to get a feel for it and get a little consistency on the 60’ times. It will be slower right now than it was launching at wide open. Now adjust 1st gear to a bigger number going a bit and testing and going a bit and testing. You will do this till you reach a point where the car is spinning. Then back it off till you are just barely not spinning anymore. Your 60’ times should be a good bit better now than they were when launching at wide open throttle. Now practice launching from different rpms in about 250 rpm increments and adjusting the same way for each point. Once you find the launch rpm that best suits your throttle application speed and gets you the best 60’ times you are done with 1st gear.
Step 3) High gear. I adjust high so that it hits the rev limiter right as I cross the finish line. Not rocket science here but important to get it right. You will have to come back to this after you get the middle gears adjusted so you aren’t done with it yet.
Step 4) Middle gears. Don’t matter how many middle gears there are they all adjust the same. Next gear should never drop far below, if at all, the point where hp and torque cross when you shift. Adjust each gear this way through the box till all gears are done. This will probably require multiple attempts through the box to get an even spread between the gears. I like the gears to progressively drop less on rpm so this takes me a while. You need to adjust high again probably so do that now.
Step 5) Fine tune it. Go through the middle gears again adjusting in small increments looking for better et’s. I do each gear and just do a little change at a time till I get the best et I can. I go through the box several times as a change in second can affect where third should be, etc.
Step 6) You are done. Go tear it up on the race track and have a great time!
