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Author Topic:   Rpm and Gear Ratio.
bncars10sen
Dirt Roller

Total posts: 13
posted January 22, 2003 12:10 PM
We've got a 1979 Olds Cutlass, Running a 355 2 barrel, We ran a 350 turbo trans last year and we are thinking of using a 3 speed sag. We have put in a set of Richmond 4.56s in the rear. We run a semi-banked oval 3/8 mile track on gumbo clay. We usually run a set of cooper 225 70 R15s. Can anyone tell me what the Rpms would be in third gear with the manual 3 speed trans? Id like to run about 5800 RPMs if possible. What tire size should i use and do i have to change the ring and pinion again to correctly achieve the desired Rs? Any help would be GREATLY appreciated!!! Thanks Fellas!

KPLugnut
Dirt Freak

Total posts: 408
posted January 22, 2003 12:39 PM
Here is a helpful link to gaer ratios of transmissions:
http://www.racersguide.com/trans_ratios.html

Just multiply your rear end ratio by your 2nd gear ratio in whatever 3spd trans you have, and that is your final ratio. Compare that to your previous final ratio to see how much you've changed it.

Hope that helps,

KPLugnut

racer17j
Dirt Forum Champ
Total posts: 1565
posted January 22, 2003 06:52 PM UIN: 54865418
what is the roll of the tire is 85 about right if so your gonna have to change the rear end if you want to run in 3rd gear you'll be lucky to hit 5000 on a 3/8 try like kp said and figure what your 2nd gear is to find your final i'm guessing if your tires are around 85's i would say running around a 6.0 ight get you in the 5800 range

Flatlander
Dirt Freak

Total posts: 310
posted January 22, 2003 09:56 PM
Just a ? why do you want to run such a high gear on gumbo thats gonna kill you I' bringit up to abot a 656 or so and crankit the heavier track will reduce your RPMS with the type vehicle you describeI'd say a 656maybe a 640 and you are gomna love the ride IMO

rico 08
Dirt Freak

Total posts: 372
posted January 25, 2003 12:21 PM
the 3/8 i run at 6.00 is a really good gear anything over 6.20 usually puts you turning more than 7,000 rpm he may not want to turn that much with a 2brl.

Flatlander
Dirt Freak

Total posts: 310
posted January 25, 2003 08:56 PM
I'm assuming as a purestock or bomber it weighs 3400 # or thereabouts it's gonna be hard to turn 7000 with a heavy track like was described IMO I ran a 656 with a 2barrel and so did a lot of others turning in the 7000range and still had power on a slick track or a run and park track I agree But I think that lugging a motor is just as bad or worse than overturning it

racer17j
Dirt Forum Champ
Total posts: 1565
posted January 26, 2003 12:13 AM UIN: 54865418
one thing you want to keep in mind with a bomber motor most guys are runnning stock stuff and small cams that start to fall off at higher rpm's.ost guys around here will run 620-633 and turn around 65-6700 with an 83 in tire they run 650's on 1/4's and turn about the same i'm gonna agree with rico here anything over 620 might be over kill and the only thing that will be any faster will be the pistons coming thru the pan

77k
Dirt Newbie

Total posts: 1
posted February 17, 2003 04:15 AM
try 241 gear in 1st=607 final.these gears are not to hard to find , if you are runing more than stock cam it should give you about 5800 some stock cams will not let you hit that much.

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