I don't know how the dyno sheet for your car looks, but the sheet shown in this thread shows the HP curve peaking around 6K rpm..... That's not quite at red line is it?

 

but depending on each gear the torque is different right? so you'll have to find the sweet spot by practicing at different rpms

 

For instance, in this dyno sheet you can see the power curve peaks at 5500 rpm and drops precipitously after that.... Obviously shifting at redline will result in weak runs.Aactually shifting at even the best part of the curve would not give very good runs because the fuel curve is so rich......

 

Blacksheep, keep in mind that just because your hp is stil climbing doesn't mean you should stay in that gear. If it climbs way after the torque drops off, you won't be able to shift back into the torque curve, which is really where your acceleration comes from.

 

Folks, like I said earlier, it doesn't matter what your torque or horsepower curves are doing, per se.

Determining the shift point has more to do with the gear ratio. Ideally you would be able to pick your ratios that would put your right in the band for optimal power delivery, but we can't swap out gears like we would like to.

So failing that, the next best way is to determine how much torque you're putting down to the ground. The gearbox operates on the principle of torque multiplication: torque is multiplied by the gear ratio to the rear wheel. For example, if your gear ratio is 3.5, whatever amount of engine torque coming into the gearbox will be multiplied by 3.5 coming out of the gearbox.

You can put down more torque to the wheels in lower gear, but you can't accelerate to high speed because the engine will run out of revs. So you need to shift to the next gear.

After I'm done, you will see precisely why. You will be putting down less torque, but you will be able to accelerate to higher speeds in higher gear.

So for now, max torque, max bhp, redline, whatever, it may not be true for your car. Like I said, sometimes you may have to shift above redline. Sometimes you'll have to shift way before redline. It depends on your dyno readout and your gear ratios.

 

Step 5: Raw Data

After reading the curve off the graph, here is all the data points, in increments of 100 RPM.

RPM TORQUE (ft-lb)
3000 162
3100 170
3200 190
3300 202
3400 218
3500 228
3600 236
3700 242
3800 245
3900 242
4000 239
4100 240
4200 239
4300 235
4400 231
4500 229
4600 231
4700 234
4800 235
4900 235
5000 236
5100 234
5200 228
5300 222
5400 215
5500 212
5600 210
5700 209
5800 210
5900 206
6000 206
6100 204
6200 196
6300 190
6400 189
6500 185
6600 182
6700 178
6800 173
6900 170
7000 165

 

Step 6: Gear ratios.

Usually the gear ratios are listed in the factory service manual. But I couldn't find one from the dealer. So I went looking for them on the Internet. I can't guarantee that these numbers are accurate. The best way is to open up your gearbox and count the number of teeth on each gear.

But failing that, this is the best I have to go on:

1st Gear Ratio: 2.928:1
2nd Gear Ratio: 1.950:1
3rd Gear Ratio: 1.407:1
4th Gear Ratio: 1.031:1
5th Gear Ratio: 0.720:1

I'm not really interested in the final drive ratio. I just want to find out how much torque is coming out of the gearbox because the only variable you can change when driving is selecting the gear. You can't change the final drive ratio, or your tyre size, and they all affect how much torque you actually put out.

So the way the gearbox works is this. Whatever amount of torque coming into the gearbox gets multiplied by the gear ratio for that gear, and that amount is put to the output shaft of the gearbox.

Say the engine is making 239 ft-lbs of torque at 4,000 RPM, this is how much torque it makes in each gear if you were revving 4,000 RPM:

1: 239 ft-lb x 2.928 = 699.792 ft-lb
2: 239 ft-lb x 1.950 = 466.05 ft-lb
3: 239 ft-lb x 1.407 = 336.273 ft-lb
4: 239 ft-lb x 1.031 = 246.409 ft-lb
5: 239 ft-lb x 0.720 = 172.08 ft-lb

The way to achieve maximum acceleration, assuming you have enough traction and don't spin tyres, is to put the maximum amount of torque you can get to come out of the gearbox. This is how gearing works. Your torque gets multiplied more in lower gear, but you can't go as quickly in lower gears.

So obviously, 4000 RPM will give you more acceleration in 1st gear than 2nd, more in second than 3rd, so on and so forth...

 

I can do it with just the gear ratios alone, but it will be very confusing to explain. So I'm going to go ahead and use the final drive and tyre size to calculate the vehicle speed. it will be easier this way.

So the final drive ratio is 4.529:1, and the tyre size is 235/45R17.

 

I beg to differ, it makes all the difference in the world what your torque and HP curves are as it relates to shifting gears. Regarless of the gear ratios you have, you must shift so that your run happens during the fattest part of your powerband. I other words, shift where your engine is making more torque, not before and not after. the best part of the power band is generally considered to be the area between peak torque and peak HP. Therefore you shift during those rpm areas, no matter what your gear ratios are.

 

I think what he's saying is that you can't determine shift points based on your power curve alone. You have to take into account the gear ratios, which from what I'm gathering would mean you might have a different ideal shift point for each gear.

 

Right. It all comes down to thrust force. You calculate thrust force by looking at how much torque is coming out of the engine, and which gear would give you the most thrust force.

There really isn't no set rule based upon where "the meat" of the powerband is. You have to take into consideration what the gear ratios are doing. Sometimes you'll have to hold a gear a little longer pass redline, like 1st gear, since 1st is really really short.

Ideally, you would select gear ratios that would put the shift point at redline each time for maximum acceleration, but sometimes you don't have a choice about what gear ratios you have.

Exactly! The gear ratios screw you up. If we have the same drop in RPM each shift, then the shift point is the same. But the gears don't have the same drop off so it will be different for each gear.

 

I'm throwing up a big Excel file later on. It will be painfully clear when you see it.

 

My optimal shift points are:

* slightly above redline
* 6900 RPM
* 6800 RPM
* redline

Can somebody tell me why?

 

1st gear is too short