Final Drive and Power Band

[OBEEWON]

I have a turbo the doesn't suck...enough air...fast enough...

Im wondering what people have done (primarily people with SR's), gearing wise, to keep thier car in the power pand created by a larger turbo, cams and other modifications that reduce low end response blah blah whatev.

I'm looking at a 4.33 gear as apposed to the 4.08 stock. I have a .86 2871 and it sucks for drifting. As soon as I shift out of 2nd into 3rd I drop out of the powerband and catch grip, even trying to carry a 2nd gear slide is difficult because if any correction is needed that doesn't involve wot...game over. Its very very frustrating and limiting my learning curve.

So post up your findings, setups what have you.

[g6civcx]

You need to do a baseline dyno run. Then we can decide what RPM we need to keep the engine at for maximum efficiency.

If you want less RPM drop between gears, you need to adjust the effective gear ratio.

The effective gear ratio = gearbox ratio X diff ratio.

Since most people don't ever play with gearbox ratios, the only variable you have left is the diff.

In order to help you, I need the following info:

1) dyno readout, or at least what RPM you want to keep the engine at, and what your redline is

2) gear ratios for each gear in the gearbox

3) tyre size


From then on we can calculate the optimal diff ratio for you. Then you can try and find the closest one available.

[jspaeth]

This is rather coincidental.

I just got done making an excel spreadsheet where I tabulated my torque versus RPM from my dyno sheet, and then used my gear ratios and tire size to calculate:

Torque at rear axle vs. RPMS (in each gear)
Torque at rear wheels vs. speed (in each gear)
Acceleration versus RPMS (in each gear)
Acceleration versus speed (in each gear)


It helped me to better understand how much of an effect gearing has.

Perviously, I though I should be downshifting when doing so would result in more torque........but I was just going off of the dyno

Now,doing it properly and taking gearing into account, you can look at which point an upshift would produce more acceleration....

It turns out that for my car, for example, you should ALWAYS wind out 1st, 2nd, and 3rd all the way to redline.

Even though the torque drops off up there, the difference in gear ratios makes it worthwhile to stay in the lower gear.


If you are interested, I can send you my spreadsheet.


Keep in mind this is a highly "theoretical" calculation that doesn't take wheelspin, drag, etc. into consideration, but is useful for looking at changes in gear ratios and the like.


Justin

[g6civcx]

Torque at rear axle vs. RPMS (in each gear)

This is the only thing that matters. To keep the calculations simple, you can ignore the effect of rear tyre size because the rear tyre size is a constant that will affect all gears equally.

If you change out tyre sizes a lot then we'll need to talk, but for usually I don't consider tyre sizes because it makes the calculations more complicated.

Torque at rear wheels vs. speed (in each gear)

This is excellent. Just make sure your calculations are correct.

This is all that matters. Whatever puts the maximum amount of torque at the output shafts of the diff will make the car go. Redline/fuel cutoff/torque peak/short shift, there's all misconceptions made up by people who don't understand physics and mechanical engineers.


Acceleration versus RPMS (in each gear)
Acceleration versus speed (in each gear)


This is a good idea but I'm not sure how you can estimate acceleration without factoring in grip and aero drag. The most you should do is just calculate thrust at the rear wheels.

Thrust forces affect the car in the same way (ignoring other factors like grip/wind/cornering loads/etc) so you don't have to try to estimate acceleration. Calculating thrust will be just fine.

It helped me to better understand how much of an effect gearing has.

Excellent.

Perviously, I though I should be downshifting when doing so would result in more torque........but I was just going off of the dyno

Now,doing it properly and taking gearing into account, you can look at which point an upshift would produce more acceleration....

You compare staying in gear versus shifting up. Which one gives you more torque at the wheels? If you get more torque by staying in gear, you stay. If you get more by shifting up, you shift up.

Torque at driving wheel(s) = engine torque X effective gear ratio

Engine torque depends in engine RPM, which is controlled by wheel speed. For the same wheel speed, the engine will be at different RPMs for different gears. The gear giving you the highest torque at the rear wheel is the one you go with.

It turns out that for my car, for example, you should ALWAYS wind out 1st, 2nd, and 3rd all the way to redline.

Even though the torque drops off up there, the difference in gear ratios makes it worthwhile to stay in the lower gear.

You are exactly correct. People may have figured this out by the seat-of-the-pants, but they don't know why.

So they take the shift-at-redline approach to every car they drive.

I'm telling you right now, if you tried to do this with my V8 the engine falls flat on its face. Shift point varies around 4,500-4900 RPM for each gear.


If we have reps I would give you +1000000 points right now. Good job.

[jspaeth]

Well, basically, I just did acceleration for fun.

You are just taking the axel torque, converting it to force (by dividing the torque by the lever arm, which is the radius of the tire), and then dividing by the car's mass to get acceleration.


I was hoping to see how well this works for trying to "estimate" a 1/4 mile time or 0-100 time, but it obviously is very theoretical, and dramatically overestimates (Improves) these times.....

1) The data for tq (acceleration) vs. speed are tabulated, not continuous (differentiable or integrable)....

2) So what I did was, to divide the difference in speed between 2 cells (2 different RPM levels in a certain gear) by the acceleration at that speed in that gear to ESTIMATE time elapsed to go between those 2 RPM levels.
....I tabulated my tq. vs. RPM every 100 RPM

3) Then you can add these up and get an estimate of times to do blah blah blah......but it always comes out way faster than the car actually is.


But yes, for some reason, when trying to think about this stuff, I wasn't even thinking about gearing.

Another thing I realized:

1) The gear ratio is so different between 1st and 2nd that for any reasonably concievable torque curve on a 240, you always will make more torque in 1st gear, so you should take it to redline.

2) Seems like this will probably be true in 2nd gear as well, in most cases.

3) 3rd gear and up, the gear ratios from one to the next get closer, and there are cases where it begins to potentially become beneficial to shift before redline.


Again, the calculations are idealised, but are useful for visually seeing the difference due to changing gearsets, tire size, etc.



*** Also g6civcx, the acceleration I was calculating is just the acceleration due to thrust.....does not include frictional losses, rolling losses, drag losses etc.


---I also factored in drag, by calculating the drag force as a function of speed (estimated 240sx Cd*A is 6 ft^2......at least that's what wikipedia gives for an s13).

The numbers show, for example, that the drag force is only ~10% as large as the thrust generated in 4th gear at that speed (for my tq curve anyway).

[!Zar!]

OP: What turbo are you using? Because no matter what your final drive is, you won't be spinning the turbo any faster because your engine is still feeding it the same amount of air regardless.

Naturally, a higher final drive will make it easier to spin. So you won't have to be at full boost to spin the wheels as easily.

[OBEEWON]

I just saw the responses. Good info J. I would be interested to see your spreadsheet.

Zar Im on a 2871r. The big laggy one.

[jspaeth]

I just saw the responses. Good info J. I would be interested to see your spreadsheet.

Zar Im on a 2871r. The big laggy one.


:hahano:

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