Gear Change = What RPM ?

chevyman · Post by **chevyman** » Thu Mar 08, 2007 5:13 pm

I Have a 3100 # car with 456 gears and a powerglide tranny w/180 gear set behind a 540. The car currently goes thruogh the traps @ 6450 1/8 mile with 32x14 tires. I need to increase my trap rpm to 6800 to 7000. I shift at 6800. I am looking to go to 33x15 tires and a 488 or 513 gear , but need to know how to calculate what my rpm will be with gear changes. Any info would be very helpful and greatly appreciated. Thanks in advance.

WRIGHTWAY · Post by **WRIGHTWAY** » Thu Mar 08, 2007 8:35 pm

might try this

http://www.accuautoparts.com/index.jsp#

joe

John Wallace · Post by **John Wallace** » Fri Mar 09, 2007 12:59 am

I have some calculators that might help.

Calculators

Might try this one:

Gear Ratio Change

Or this one:

Gear Calc

chevyman · Post by **chevyman** » Fri Mar 09, 2007 6:27 am

Thanks for the help. Mr. Wallace your calculators gave me what I was looking for. Thanks again

BillyShope · Post by **BillyShope** » Fri Mar 09, 2007 6:34 am

The calculators John has provided are fine, but you've added a tire change into the mix, which complicates things. It's just a matter of including the tire ratio in the calculations. Here's your options:

6450(32)(4.88)/33(4.56) = 6693 rpm

6450(32)(5.13)/33(4.56) = 7036 rpm

or, if you don't change tires:

6450(4.88)/4.56 = 6903 rpm

6450(5.13)/4.56 = 7256 rpm

Your trap speed will change only slightly, but I'm sure you already realize that. You're after an improved ET. You can play around with the eighth mile performance calculator at my blog and get an idea of how much the speed might change. Just diddle with the rear wheel power that you input until you get the trap speed with your present setup (the ET will be off a bit, but don't worry about it) and then change the axle ratio.

http://home.earthlink.net/~whshope

Maximum Race Engines · Sun Mar 25, 2007 6:38 am

You are making two changes at once, however I am a believer in going as tall as possible, maybe not as wide, but tall and run more gear. Each gear ratio we have found to be good for about 250 rpm. BUT, there are other factors. Anytime you change tire heights, you change more than effective gear ratio. The tire heights affect pinion angle, control arms or 4-link bars or ladder bars all change in angle, as does ride height. These factors alone can effect trap rpm as well as how the car leaves and accelerates. If you have the car leaving hard, I would recommend going to the gear instead of the tire, as nothing changes in the geometry of the suspension, just rpm. Now keep in mind, when you add gear, you also take slip out of the converter, which these calculators don't factor in. When you pull gear out, you add slip to the converter because the engine is under more of a load to pull the gear, so it will slip more. On average each gear ratio is roughly 250rpm, so I personally would leave the tire height, and go to the gear. Now my next question would be, what is the current converter slippage? I can figure this out knowing trap rpm which u gave, and mph at the quarter. The gear doesnt make a huge impact on slippage with 1 gear size difference, but drop it 3 sizes and watch what happens. If the converter is too tight, as far as slippage goes, you can add a little more gear and then loosen the converter a bit. If it is slipping a bit more than optimal, you can get awy with adding even a bit more to help it tighten up but yet gain the rpm. Also, if it is too tight, you can just looosen the converter and pick up rpm that way. What does the converter flash to on a foot brake? Transbrake? Knowing this can determine , with slippage, how loose or tight it is.