Redlinerevver

360 ci bored 350
th350 with shift kit
running 29" tall slicks
rpm range 3200-7200

HI. Im not sure what gear to go with with a ford 9" rear. I have been thinking 4.56 , 4.71, or 4,86... Im going to use a drag spool and 31 spline axels. my car has ran a 11.92 at 113mph with the 3.73 geared stock 10 bolt then on the next run the posi unit exploded. im running a 9.7:1 compression 350 with 210 procomp heads and a circle track cam. the cam power bands 3200-7200 Im pretty sure i was going across the line around 5300 rpm with the th350 in 3rd.

AlkyIROC

4.56 would do well, maybe something just a little deeper. 4.86 would be too much for that rpm range and tire size.

I use 4.86 gears with a 32" tall tire. 6000 stall converter and I shift at 7400. I tried some 4.57 gears and although they worked, I crossed the finish line somewhere around 7000-7200 rpm and calculated converter slip was higher than I wanted because of the 4.57 gears. It was over 10% and with the 4.86 gears, it's closer to 4% because the engine rpms get into the powerband quicker.

I use a powerglide. I launch at 4500 rpm, the rpms quickly jump to 6000 and I shift at 7400 somewhere around the 330' mark. The rpms drop back to 6400 after the shift and I then do a high gear pull to the finish crossing it at 7400. My engine runs best at those high rpms because that's where my cam makes the most power. I've thought about some 5.00 gears but feel they may be a little too much although my engine could spin a little higher with no problems.

The goal is to get the proper gearing so that you cross the finish line at or just after the shift light comes on while in high gear (not OD). Converter stall can affect what gears should be used just as much as tire size and rpm range.

IHI

iTrader: (1)

I'd be thinking ont he 4.88 gear range if your currently only turning 5300 and the convertor slip is optimal and overall tune is good...right now it's a small motor/tall tire/wrong gear combo obviously...need to overcome the huge roll out of the 29" shoes with gear reduction

Redlinerevver

Thanks for the help guys. I may be running a bit more rpm at the line than 5300 maby around 5700.. my last time at the track I was talking to a guy he had an older chevy 2 and he was running a 10.7 in the 1/4 he had 29 in tires and the same stall i had and tranny but he had a 383. Mines a .060 350 so Im thinking of going with a 4.71 gear ratio because the torq.. Would that be the way to go or stay with the 4.56's? My car is the slowest just leaving the line.

Redlinerevver

Im not realy understanding the converter lock. I have a boss hoss 3200-3600 stall it is stalls around 3400 in my car

AlkyIROC

Converter slip?

1/4 mile MPH can be easily calculated. Knowing the circumference of the tire, and how many revolutions it will make in a mile, finish line rpm, you can calculate the MPH the tire should be turning. Because of wheel spin and converter slip, a vehicle will never run that MPH. The difference in MPH is the percentage of slip.

My 32" tall tires have a circumference of 100.5". They will rotate 630.3 times in a mile. Crossing the finish line at 7500 rpm means the tires should be turning 147.0 mph but my timeslip says 143.1. That calculates to 2.635% converter slip. With the 4.57 gears, it took longer to get above the stall speed so converter slip was higher.

The more an engine operates above the converter's stall speed, the less slip the converter will have. Below the stall speed and the converter has to slip or the engine will stall.