Hunt for the perfect Shift

[blackwidow]

First off there are two things you need to find the optimal shift point.

 

1. a dyno plot of the torque curve your engine puts out

2. all the gear ratios you will be shifting through

3.(optional) is the differential gear, the reason this is optional is that the gear never changes which makes it a constant that does not effect where you need to shift. But i will show you why it is fun to have.

 

All the math I give you is pretty simple. It is just that the process is lengthy.

 

 

Finding true torque.

The equation for this is

Flywheel torque x gear ratio you are in.

for example if you are making 150 lbs of torque at the flywheel and have a first gear of 3.549, and second gear of 2.197.

 

150 x 3.549=532.35 You are making 532.35 lbs in first.

150 x 2.197=329.55 You are making 329.55 lbs in second.

 

As you see here as your gear ratio drops you don't make as much torque. Which is why your car doesn't pull as hard in 2nd as it does in 1st, and so on all the way through the gear box.

Now for some real fun. These numbers are to our theoretical drive shaft. Now lets take into account the optional differential gear. And put in 20% drivetrain losses. For ease i'll use the numbers from above with a diff. gear of 3.9:1.

 

(flywheel torque)x(gear ratio you are in)x(final drive ratio)x(.80)

150 x 3.549 x 3.90 x .80= 1660.93 lbs at the wheels

 

Now if you didn't know before this is what you are really seeing at the rear wheels. Now the next time your friends ask what your making at the rear wheels, tell them the real number.

 

For reference your dyno plot to the wheels already has the drive train loss in so use the same equation but without the drivetrain loss on the end.

 

 

Finding RPM drop when shifting

The equation for finding RPM drop is

 

gear your shifting into

---------------------- x RPM before shift = RPM after shift

gear your shifting from

 

If your shifting from 1st(3.54) to second(2.197) at 7000 rpm it would be

 

2.197

------- x 7000= 4333.33 rpm

3.549

 

So you would be at 4333.33 RPM after the shift. Pretty easy stuff.

 

 

Making the graph and finding the best shift

Now lets take our math we have learned so far and put it to use. Take the formulas i have given you and make a chart so that you can see shifting points and

torque numbers for the corresponding shift(ie. first to second). now i am going to use MONZTER's dyno plot from the N/A engine setups with dynos and the gear

ratio's from a factory 240z 4 speed as my examples. You can find the dyno plot here in this post

http://forums.hybridz.org/showthread.php?t=139485 , and the gear ratios here http://www.geocities.com/z_design_studio/transmission.html

 

 

Start at your red line and go down 1200 rpm in 200 rpm intervals, charting your rpm start and drop, and your torque start and drop side by side. Along with the drop in torque from the shift.

 

First column is 1st gear rpm

Second column is 1st gear torque at rpm

Third column is torque diffrence before the shift

Fourth column is 2nd gear torque at rpm

Fifth column is 2nd gear rpm after the shift

Sixth column is torque loss after the shift

Seventh column is torque diffrence after the shift

 

7200..... 514.61..... -107... 369.10..... 4456 ....-145.51 .....+157

7000..... 550.10..... -71..... 336.14..... 4333 ....-213.96 .....+89

6800..... 578.49..... -43 .....329.55..... 4209 ....-248.94 .....+54

6600..... 596.23..... -25 .....318.57..... 4085 ....-277.43 .....+25

6400..... 603.33..... -18 .....318.57..... 3962 ....-284.76 .....+18

6200..... 610.43..... -11 .....307.58..... 3838 ....-302.85 .......0

6000..... 621.08........ 0 .....318.57..... 3714 ....-302.43 .......0

 

 

So now that we have our first to second shift chart lets find that perfect shift point. The key to finding the best place to shift is to find the point at

which you lose the least amount of torque during the shift without losing to much forward momentum before the shift. I do this by comparing the torque diffrence before the shift( Which goes with your forward momentum) and the torque diffrence after the shift.On this particular setup I think he should shift right around 7000-7100 RPM, if he had the gearing I used here. While there are less torque losses after the shift above this rpm, you have to much torque drop off before the shift, which causes a loss of the forward momentum you were building. Once you find what you think is the optimal shift point vary it up and down about 400 RPM to find what suits you best. Oddly enough most drag racers have a rule of thumb that the best shift point is 10% beyond peak horsepower. Which if you look at his graph is is right around 7100 RPM. But you should take this as a coincidence and do the math yourself.On his setup he said his redline is 8000 RPM but he needs to shift well before that. What this shows is just because you can rev to 8000 doesn't mean you should shift there.

 

Now take this info and do every gear shift in your setup. Plus don't forget that since the ratios change from gear to gear the best place to shift in one gear might be different than where to shift in the next. But it should be pretty close.

 

After I did this I found this was went over on someons page but I don't think it elaborated as much, but just for more reading you can find it here http://www.datsuns.com/Tech/whentoshift.htm . I hope it helps some people out.