Gearing for low power/low torque cars. What should you be looking for?

What does the newbie need to know about this? Figure a typical NA car with under 200 BHP and 200 torques. From being here, I know you want a big final drive, but how do you figure that out (and suitable ratios) and why? Math is welcome, I usually can't wrap my head around things until I do some calculations.

Thanks,

J

In an ideal world, you want gearing such that the torque is equivalent when you switch from one gear to the next. So the important things are the torque curve and redline/shift point.
The real world takeaway is that when you shift you want the torque at the wheels to be approximately the same. If you shift and there is a big increase in torque you'll see potential wheelspin and unnecessary wear on the components. If you shift and there is a big loss in torque you'll sound like a VW Golf shifting from 2nd to 3rd and lose a bunch of momentum.

People like to post and look at stuff like top speeds in each gear and things like RPM drop between gears but those are all just fluff and approximating or making assumptions about the engine's operating circumstances/torque curve.

Final drive is torque multiplication. As long as you have four good useable gears you will likely be fine for US rallying if you find something reasonable close with a 1st gear that can get you into the 40MPH range and a top speed of 110-120 MPH in the highest gear.

But the mathematics are semantic and wanking off really without a good and accurate dyno and torque graph.

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Grant Hughes

Quote
NoCoast
In an ideal world, you want gearing such that the torque is equivalent when you switch from one gear to the next. So the important things are the torque curve and redline/shift point.
The real world takeaway is that when you shift you want the torque at the wheels to be approximately the same. If you shift and there is a big increase in torque you'll see potential wheelspin and unnecessary wear on the components. If you shift and there is a big loss in torque you'll sound like a VW Golf shifting from 2nd to 3rd and lose a bunch of momentum.

People like to post and look at stuff like top speeds in each gear and things like RPM drop between gears but those are all just fluff and approximating or making assumptions about the engine's operating circumstances/torque curve.

Final drive is torque multiplication. As long as you have four good useable gears you will likely be fine for US rallying if you find something reasonable close with a 1st gear that can get you into the 40MPH range and a top speed of 110-120 MPH in the highest gear.

But the mathematics are semantic and wanking off really without a good and accurate dyno and torque graph.

Grant. said easier we want the most acceleration we can finger out in the 25-90mph range where 98% of the time we're in..
Realistically the time spent above 100 isn't that important, but the bulk of everything is that 25 to 90..

I say the gears are there "to sustain" acceleration including up hills..exactly as you siad to avoid that typical MPG box where the motor is going whaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa _(grunch) duh uh uhhhhhh uuuuuuuuuuhhhhhhhhhhhaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaawha a wha whaaaaaaaaaaaaaaaaaaaaaaaaaaaa...

How close the ratios gotta be depends on the motor's torque and if its a peaky motor, the gears better be close of it will fall down....

Final drive depends again on motor torque output, ability to rev and tire size AND in many FWD gearboxes since VW and Opel rearranged things about 1989 the gearsets...

Used to be, and usually still is in RWD cars the spread of gears in a rally box for a low powered thing might start at 2.25 to 2.4 first and 1:1 top and that combined with a 4.88 to 5.1 final drive..
VW and Opel both decided to make the pinions bigger, meaning more bigger teeth cause its part of a shaft in the box, and then just shortened the upper gears so OVERALL ratio in TOP was around 5.0 to 5.1 overall
(around 4.27 final frive and UNDER-driven 5th at 1.16)

But as usual the question is too broad..
More details, whatcher have?

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John Vanlandingham
Sleezattle, WA, USA

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There's also the practical reality that without shelling out piles of bucks, you don't really get much choice in gear ratios anyway unless you're flexible about choice of car (that is to say, some have better ratios than others)

I don't have it yet. I'm buying a prepped car (John, you posted something a few weeks ago about time, and the lack thereof; that's why I'm buying a built car), I've looked at a 2wd Lancer, looking at a VW Scirocco, and a 2wd Mazda 323 as of right now. It'll be something like that.

I'll probalby live with whatever trans/drive it has for awhile. I know there's VW stuff available. Eric Burmister has posted info at SS about the best Mazda boxes. Not sure what's available for a Lancer.

This is meant mostly as a build my knowledge type post.

Thanks,

Jeff

323 w/ drivetrain swap from the Protege LX (90-95) or Ford Escort GT (91-95) or better yet, Kia Sephia (95-97) would work great. There are good final drives available, decent LSD and the engine sounds fantastic when it's strung out. This is just my bias, the reality is, VW is probably the way to go, tons of rally parts available, plenty of other competitors to borrow parts from when you have a break down, etc... Gear is certainly key when you don't have a ton of excess power, but so is proper driving technique, knowing how to keep the car in the power band, reguardless of where it happens to be.

-Jon