rpm's,gear ratio's,hp and top speeds: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Jan. 19, 2009 9:19 p.m. Supercoupe New Reader

I'm trying to figure out a few things on my 09 Challenge car involving the gearing and I have no idea what I'm doing.

Question is: if I can't change my differential gears but triple my horsepower and have a higher redline, how much more top speed will I realize? Doesn't my gear ratio dictate the top speed? Won't my top speed just be the additional RPM's the new motor makes times the final drive ratio?

I understand I will hit the top speed faster ( more HP ) but won't it flatten out and become a point of diminishing return on my effort?

Sorry, I admit I don't truly understand the math behind all this.
Jan. 19, 2009 9:28 p.m. chaparral New Reader

Top end will go up in proportion to redline, assuming that the thing would redline in top gear before.

If you were originally doing 100 at 5000 RPM, and now you have tons of power and crank 7000 RPM in top gear, you'll do (7000/5000)*100 or a buck forty.

If you were only able to do 80 before at 4000 in top, and can now crank up to 7000 in the same gear, you're now capable of a buck forty once again.
Jan. 19, 2009 9:31 p.m. Keith SuperDork

Here, play with this: http://flyinmiata.com/tech/gearing.php

Your top speed is limited either by gearing or by power. Assuming you have unlimited power, it's determined by your redline and your gearing. But the need for power increases with the cube of your speed. Twice as fast needs eight times as much power, so you're likely to be limited by your power level.

We have a Miata that is theoretically capable of 305 mph :) Although I suspect it won't pull much past 200...
Jan. 19, 2009 9:49 p.m. Supercoupe New Reader

Okay, so if I increase the size of the rear tire diameter and can still achieve the higher rpm's my top speed should also be higher, correct?
Jan. 19, 2009 10:02 p.m. Keith SuperDork

Yup.
Jan. 20, 2009 10:37 a.m. neon4891 SuperDork

Like how my neon would have a theoretical top speed of 180 with a single cam spec tranny and Mopar PCM (7800rpm red line). Or 150ish if I rebuild the Perf. Spec. A tranny
Jan. 20, 2009 7:54 p.m. NYG95GA Dork

Plus there is an aerodynamic aspect. Most cars will "hit a wall" at about 130 MPH. (Even Neons. )

At that point it's all about pushing air out of the way, which ain't easy.
Jan. 20, 2009 8:55 p.m. Keith SuperDork

That's what makes the need for power rise with the cube of the speed. Put the car in a vacuum, it'll hit redline in top :)
Jan. 20, 2009 9:13 p.m. 11110000 New Reader

In reply to Keith:

No it won't - unles it's an electric car, no oxygen = no power!
Jan. 24, 2009 12:06 p.m. mrdontplay New Reader

11110000 wrote:

In reply to Keith:

No it won't - unles it's an electric car, no oxygen = no power!

OH SNAP!
Jan. 25, 2009 5:37 a.m. wheels777 HalfDork

Tire Height * RPM = MPH * Gear Ratio * 336

or

MPH = (THRPM)/(GR336)

If you have an AT you need to allow for slip. It will be 5 to 9 %. For max acceleration, you want to go with the lowest gear (higher number) you can and not hit the rev limiter through the trap or down the longest stretch. You can only reduce the tire size so much before the traction is limited and this is compounded by the increase in power. Too much gear with too little tire equals tire smoke. Dialing the gear ratio is one of the most important parts of setting up a car. That's why there are so many sets - ex. Ford 9" has 3.89, 4.11, 4.33, 4.44, 4.57, 4.86 close together. Helpful hint if ET is the goal - use lower gear in heavy car and let it hit the rev limiter in the trap, or use a higher gear (lower number) if you have a lighter car.
Jan. 25, 2009 10:13 a.m. Supercoupe New Reader

Thanks, Andy..would it be alright if I called you with a few questions about my project?

I'm trying to keep the body as original as possible and that limits my power selection.
Jan. 25, 2009 5:03 p.m. wheels777 HalfDork

Supercoupe wrote:

Thanks, Andy..would it be alright if I called you with a few questions about my project?

I'm trying to keep the body as original as possible and that limits my power selection.

Anytime