Drive Ratio with TC

[turbojimmy]

I know this topic has been beaten to death, but I'm confused by the speed calculator on this site.

I have an 11HP engine with a 40-series torque converter. I have a jackshaft with two possible sprockets: 10T and 14T. I thought I'd get better acceleration with the 14T, at the expense of top speed, so that's what I have installed currently. It seems to work okay, plenty of torque and enough top speed to get hurt.

For fun, I was going to see how the 10T sprocket did. But, when I use the online calculator, it shows a better top speed with the 14T (53 MPH) rather than the 10T (38 MPH). This is with a 54T axle sprocket and 19" tall tires. This doesn't make any sense to me. Shouldn't the smaller 10T gear result in a higher top speed?

I'd pop the 10T on the axle to see what happens, but I'd need to cut the chain and then I can't go back to the 14T.

 

[redsox985]

Well, think about it. If gear ratio is 1:5.4, your 10 tooth sprocket, then it takes 5.4 turns of the jackshaft to spin the tires once but if the ratio is 1:3.857, your 14 tooth sprocket, then it takes 3.857 turns of the jackshaft to spin the tires once. Assuming your motor is at the same RPM then we can throw that factor out the window and as long as the ratio between the TC and jack shaft remains constant it's all about ratios between the output of the jackshaft and the rear axle. The closer you are to a 1:1 ratio the faster you go.

 

[turbojimmy]

Well, think about it. If gear ratio is 1:5.4, your 10 tooth sprocket, then it takes 5.4 turns of the jackshaft to spin the tires once but if the ratio is 1:3.857, your 14 tooth sprocket, then it takes 3.857 turns of the jackshaft to spin the tires once. Assuming your motor is at the same RPM then we can throw that factor out the window and as long as the ratio between the TC and jack shaft remains constant it's all about ratios between the output of the jackshaft and the rear axle. The closer you are to a 1:1 ratio the faster you go.

That makes sense. I think.

So the 10-tooth sprocket is actually the one that will improve torque at the expense of top speed.