Starting line ratio?

[J Kuchel]

What is the best starting line ratio for a super stock car? Is a 12 a good ratio? Just looking to improve my performance. Thanx

[Jeff Lee]

Assuming adequate tire & suspension, I believe it should be in the ballpark of 14 minimum.
Pending dyno results, I'm guessing my new or revised SS/H entry will have a 17.6 ratio (3.02 x 5.86). Automatic equipped cars may be less and it all depends on what engine you have. For example, I certainly wouldn't take something like a Pontiac torque monster to such extremes.
The new 2011 Mustang GT with manual 6-speed and optional 3.73 rear gears is 13.46 ratio! (3.61 low gear x 3.73 rear gear). I'm having a hard time understanding that, especially with a factory low aspect ratio tire on 19" rims...

[Adger Smith]

Jeff,
I've been told that if I go to a stick and I have a hard running automatic combo I will want 20 % more low gear in the stick to make up for what the torque converter does. My comment about the 2011 Stang is: Is it For street driving? Look at how little torque the engine has down low in the RPM band. Some engineer thought it might have to be that way just to get it to pull smoothly away from a stop sign.

[Alan Roehrich]

For an automatic car, you can generally figure that the tallest low gear normally found in a PowerGlide is a 1.76, and the 3 speed transmissions usually are between 2.5 and 3.0, although there are some 2.2x low gear sets.

I try to look at the ET and MPH I want to run, then use a "power-speed" slide rule or a "dream wheel" to see what rear gear puts me at the maximum finish line RPM I want to see, then find the tire I want to run on the calculator or dream wheel and see what rear gear it takes to make that happen.

For example, using the ATI "power-speed calculator", say I decide I need to run 145 MPH at around 8500 RPM with a 32" radial slick. So the calculator shows that a 5.57 gear with a 32" at 8500 RPM gives you 145 MPH, approximately, at zero slippage. You can figure around 4-8% slippage with a good converter, some are better than that, but some are not, and either add that to your RPM if you can stand it, or subtract it from your gear if you can't stand any more RPM.

Figuring 4% slippage and taking that off of the gear, if you don't want to exceed 8500 gives you a 5.38 gear, since it actually comes up to 5.34, and there is not a 5.34 gear for every rear end. If you add 8% to the 8500 RPM I wanted to turn, you get 9200 RPM, that's too high, at least for what I'm looking at. If you add 4% for slippage, you get almost 8900, that's still pretty high, but you might live with it.

That helps you choose your final. For an automatic car, figure the converter makes around 2:1, but you cannot count that as part of your starting line ratio. So you take the 5.38 gear, and figure a 2.75 low gear (common for a 3 speed automatic) and you get about a 14.8 low gear. That's pretty stout, for a big block car (I saw you were looking for a big block intake).

With a 2.5 low gear and that 5.38, that's 13.45:1. To get above 13.5:1 with a 2.5 low gear you have to go back to the 5.57, and you still get less than 14:1 but your are close at 13.92:1. If you wanted to try to hit it real hard, and you could stand the 4% more RPM, if you got a good tight converter, you could try the 5.57 with the 2.75 for a starting line ratio of 15.32:1. That's a real stout launch ratio, especially if you've got a good converter that multiplies torque at 2:1 or better on the flash.

Someone probably has a better way to do it than I just wrote up, hopefully they'll follow up my post with an improvement.