marlinspike

Assume a car that makes peak hp at 5300rpm and has a 6000rpm redline.
With 2.47 gears, the car can hit 165mph which is at 5500rpm, which is right where the hp curve starts to come back down.
Wtih 2.65 gears, the top speed of the gearing would be 168. Would the car now be able to do 168 because of the multiplication impact of gearing on torque, or would the top speed be reduced because with 2.65 gears 5500rpm comes in at 154 (though, obviously top speed wouldn't be 154 since the air drag is lower - under the second theory I'm thinking it would be around 160 based on the power curves and drag specs)?

Fabrik8

Are you speculating or is this a fact based on an actual car...?

The power to the wheels would be increased by the lower gear ratio, like you've said. But, drag force increases with the square of velocity, and power is force times velocity, so the power required goes up with the cube of velocity. To put that in context, going twice as fast takes 8 times the power.

The top speed is almost completely governed by the intersection of the drag curve and the peak power output of the engine, and obviously the engine must stay within the RPM range to reach whatever top speed based on the gearing. So the answer is whether the change in gearing produces enough extra power to increase the top speed without reaching the engine redline. I think the answer is no, intuitively at least.

All of this can be calculated pretty easily with only slightly more info than you've already given.


I need to find a pen, a piece of scrap paper, and a calculator. And some motivation. The first three are easy.

marlinspike

I've got a spreadsheet that will approximate horsepower for a certain speed for the car. Can I assume the top speed stays the same if 2.47*(hp at 165mph for 2.47:1 gears) = 2.65*(hp at 165mph for 2.65:1 gears)

Fabrik8

You need a certain amount of power at the wheels for a given speed. So the top speed will stay the same if you scale the engine power up or down based on the gearing to keep the power at the wheels the same. It seems like that's what you're saying.

Again though, that only works if you keep the engine within the safe RPM range at that power output.

marlinspike

Ok, I just wasn't sure if I could do a straight multiplication of gearing by horsepower, but then since I know that works for torque and horsepower is a function of torque, it should have been blatantly obvious to me that that's how it works.

So 2.65 would be perfect for the car - after looking at the curves the hp at redline multiplied by 2.65 will be greater than the peak hp multiplied by 2.47.

Fabrik8

I'd have to scribble a little math to convince myself, but I think everything should come out in the wash based on the RPM difference.

For a quick refresh, HP= (torque*RPM)/5252

Fabrik8

So is the wheel RPM at redline with the 2.65 greater than the wheel RPM at peak HP with the 2.47? If it isn't, you're not going to match or increase the top speed.

Oh, don't forget the transmission gear ratio too. It should cancel out if you're just calculating a delta but it's not a bad thing to double check.

marlinspike

It is. 6000rpm with the 2.65 makes 167.7mph 5500rpm with the 2.47 (peak is 5300rpm, but 5500rpm is only 2hp lower and the engine will go to 5500) is 164.9mph.

Now, to find a 2.65 rear end with LSD and ABS.
I can find a 2.24 with both of those, and I could buy a 2.82 ring and pinion gear, which should be fun for acceleration but will drop top speed by gearing to 157.6. I wonder if the 2.65 ring and pinion could be put into the 2.24 since they use the same housing, hmm...to the parts catalog!

non-vtecpower

jss

somebody's playing the drag game on their iphone..