Twin Cam gear ratios?

[Ducman]

Anybody have suggestion for starting point? Or is there a way to apply the following baselines (which I think are for FF) to my twin cam Formula B?

http://www.apexspeed.com/resources/baseline_gearing.htm


I can factor for tire size, R&P, RPM, etc, etc. Mostly wondering what you suggest that the FB should have as a redline speed per gear compared to the baseline FF. Same?, 10% less?, 10% more? Top gears factored heavier than low and middle or other way around?


I recognize there are many factors to consider, just trying to get myself in the ball park with an engine and box that was assembled for an unknown target track

Final ratios I have in it at the moment are:

11.250

7.425

5.581

4.813

4.125

and then 23.25" Hoosiers for now.

Thanks!

[billtebbutt]

Anybody have suggestion for starting point? Or is there a way to apply the following baselines (which I think are for FF) to my twin cam Formula B?

http://www.apexspeed.com/resources/baseline_gearing.htm


I can factor for tire size, R&P, RPM, etc, etc. Mostly wondering what you suggest that the FB should have as a redline speed per gear compared to the baseline FF. Same?, 10% less?, 10% more? Top gears factored heavier than low and middle or other way around?


I recognize there are many factors to consider, just trying to get myself in the ball park with an engine and box that was assembled for an unknown target track

Final ratios I have in it at the moment are:

11.250

7.425

5.581

4.813

4.125

and then 23.25" Hoosiers for now.

Thanks!

Ask the engine builder if possible, as he knows what's gone into the engine in terms of bits. I would think that a high end billet crank, rods, forged pistons, steel main caps, plus decent hardware to hold it all together etc *could* add a fair bit of room to the safe redline from an FF. However, you also really need to know where the thing makes the most power from the dyno sheet, and triangulate into that. Also, you can build a peaky engine with a narrow power band or one with a much flatter curve that allows far more flexibility in terms of gear choices

Early on in my racing career (if one can call it that) we built a motor for the MGB with a HUGE Kent cam that was designed to come in hard at about 6500 RPM and produce very little usable power under 6000. We found out (the hard way, but we were young!) pretty early that the design capacity of the cranks we were using was about 6700.......

cheers,
BT

[Ralph Z.]

I would expect the red line to be the same across all gears. Consult with the engine builder.

[Bobsy SR 6]

Don

A few thoughts from my experiences with my twin cam:
1) I switched from a 9:31 cwp to 8:31 which allowed me to utilizes larger (in physical size) gears. The smaller 1st and 2nd gears are prone to breakage due to the power in a twin cam. The ratios are the same but the 13-14 tooth layshaft gears have very little material at the hub area.
2) use ONLY “m” series gear sets. These are made of stronger material and are better suited to your application in a MK9 case
3)I utilized a 500 RPM lower shift point for 1st to 2nd gear shift as the engine will rev very quickly and likely over rev if the same shift point for all gears is used.

My twin cam is injected and makes around 190 hp and yours may be different. But these “rules” will still apply. As far as the actual ratios are concerned only testing will determine what is best.

Good luck and have fun

Tom