Gear ratio vs. rpm

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Gear ratio vs. rpm

Postby Kern Family Racing » Mon Aug 13, 2018 12:55 am

Anyone know or have a formula to figure out gear ratio to drop engine 200 rpm on top end? Right now I am at 11/70 on sprockets for a 6.36 ratio. I am hoping to pick up some mph as well.
Scott and Jack
Kern Family Racing
 
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Location: Greenfield, IN

Re: Gear ratio vs. rpm

Postby RNorton » Mon Aug 13, 2018 1:43 am

It really depends on whether you are overdriving the clutch or not....

If you are over driving the clutch and jackshaft speed is following engine RPM a good rule of thumb is as follows.

Current Engine RPM * (New Ratio/Old Ratio) = New Engine RPM

If you are not maxing out the overdrive decreasing jackshaft ratio can actually increase RPM depending on the setup. A lot of setups really wont see a large movement in this case however, only when you carry it to extremes. From a 4.58 gear to a 2.72 gear our finishline RPM jumped up 1000 RPM for instance. In a case were the clutch is not over-driving typically you'll add primary weight (or loosen the secondary spring if your peaking to high at the shift).

If we had some more info on your setup it might be possible to give you a better answer.
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Re: Gear ratio vs. rpm

Postby zrt-racing » Tue Aug 14, 2018 7:48 pm

Running at a 6.36 ratio I would assume the car is getting well into overdrive.
Formulas and such are great...but track testing is my suggestion for tuning.

An easy switch would be change the drive gear to 12 and see your results. My thought is this will get you close to the 200 maybe a little more. If its off figure out what change it made on your combo and go from there to divide to the exact ratio you need.

A little more info would be great, as well as what your trying to achieve.... many ways to reach a goal.
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Re: Gear ratio vs. rpm

Postby Kern Family Racing » Wed Aug 15, 2018 9:46 am

This car started as an outlaw car and has gear and chain size bigger than #415, but the jackshaft gear and chain are worn out. I have a new #415 11 tooth for the front, new chain, and I just need a new rear #415 rear gear so I was going to replace it anyway. I think I will drop the rear from 70 tooth to 67 tooth to get it just under 8000 which is where I would like to stay for a BZ1.
Scott and Jack
Kern Family Racing
 
Posts: 98
Joined: Tue May 31, 2016 7:12 am
Location: Greenfield, IN


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