Originally Posted by Mulvaney:
i was just needing a little guidence i was running a 13 54 gear last night at thunder valley and exploded my engine six laps later at 13 200 rpms i have bought another engine but the trans ratios are different so i am trying to figure out if i was running 12.3 overall gearing last night is up or down lower rpms
example 10.6 lower or higher rpm than 12.3? also in my book on this other engine has primary ratio and final gear reduction ratio witch one do i use when figuring out my gear primary is 1.565 and final is 3.00
thanks in advance for the help
First thing we need to know is year and make and motor. Unless it was a pretty new motor you were probably over reving it.
Gear ratio is number of times engine turns over for every one revolution of the rear wheel. 10 to 1 means engine turns over 10 times fo each wheel revolution. 15 to 1 means the engine turns over 15 times for each wheel revolution,
This is where it gets kinda of confusing. We say the 10 to 1 ratio is higher than the 15 to 1 ratio. The 10 to 1 will go faster at the same rpm than the 15 to 1 will. Its just like in your street car low gear has more power and high gear has more speed.
We like to get or gear ratio so that just at the end of the straight our motor is a couple of hundred rpm's short of the advertised rev limit. They put a rev limmiter in most ign boxes to help keep from blowing up the motor. When the motor hits the rev limmiter it will start to stutter if it does you are just a little low in your gear go down 1 tooth on your rear sprocket. If it starts stuttering at the flag stand and continues all the way to the turn you are way low, make the front sprocket 1 tooth larger. The gear ratio change is much greater when you change the front sprocket. At litttle Salem we gear for an axil speed of about 1000 to 1050 rpms. If your motor rev limmiter is 11000 rpm, we would take 11000 and devide it by 1025 to get our desired gear ratio. 8.8 to 1 If the rev limit is 13000 rpm's 13000 divided by 1025 is 12.6 to 1.
NOW to figure your ratio you need 3 things your primary ratio that is the gear ratio between the motor and clutch, the next thing is the ratio of the transhmishion gear you race in (usually second gear) . multiply these two ratios together that will be the total engine ratio. To find your sprocket ratio devide your axil sprocket teeth by your engine engine sprocket teeth. 54 devided by 13 equals 4.15 to 1. If you wrote your engine gear on the trailer wall multiply it by your sprocket ratio and you get your final or overall ratio.
Read down a few links about cross multiply and divide to help find gears for other tracks.
Honest Dad himself
