How do you determine optimum shift points in a 6 speed?

Do you need a dyno graph to determine at which RPMs it makes peak hp and tq?

Do you need a million plus trial and error runs at the track?

And, do you guys use the same RPM in each gear as a shift point?

I, myself (a 1/4 mile noob) shift at 5800 RPM in 1-2 shift, and 5500 RPM each additional shift. I never remember to look at the tach to see what RPM I'm trapping at, I keep my eyes on the road ahead of me.

I have a super rammed 383 stroker in a 89 ZF6 Z51 car with 3.33 rear gear with a 222/226 @ 0.050" cam.

I have brought it to 6300 RPM but it made no power up there, so I set my rev limiter to 6000 RPM and try not to bounce off it.

 

You can get a copy of Drag2000 drag strip simulation software from Motion Software, Inc. and plug in your torque curve and car's setup to determine optimum shift points.

Or, if you make consistent passes, you can bump each shift point in turn till you get the best trap speed. Start with a 1-2 bump of 400rpm till trap speed drops off, then back off 200rpm. Repeat with the 2-3 shift point.

 

A dyno graph would be good along with a spreadsheet that has your tranny gear ratios, tire size, etc. This way you can figure out at what shift point you would end up in the optimal spot in your power band.

My car makes it easy. It makes more power at higher RPM's than it does at lower RPM's, period and it pulls hard all the way to the red line. That means I shift right under the redline every time. If your car falls off before your rev limiter we'll have to do some math, or trial and error works too.

 

Yes, using a dyno curve is one way to determine theoretical optimum shift points. BTW the optimum shift point is neither the max torque nor the max horsepower rpm!!!!

All you need to determine the optimum shift point is the dyno horsepower curve and the transmission gear ratios. The optimum shift point is that point which maximizes rear wheel torque or horsepower at all speeds.

So the procedure is basically to look at your hp curve for the rpm you are at, then look at the horsepower for the rpm you would be at if you shifted. If the horsepower would drop when you shift, then you need to shift at a higher rpm....so you keep doing this until either you find a shift point or you get to redline. This procedure maximizes the area under the hp curve.

Stated another way, the shift point is when the horsepower is the same after the shift as it was before the shift!!! This means that your optimum shift point is going to be somewhere above your hp peak..

The gear ratios in a 6 speed are 2.68:1, 1.80:1, 1.29:1. & 1>00:1. So the drop in rpm when you shift will be determine by the ratio of the two gears, that is 1.80/2.68=.671, 1.29/1.80=.716, & 1.00/1.29=.775. So the shift points(unless they are all at redline) will be DIFFERENT for each shift.

To illustrate, lets say you shifted at 6300 on all shifts, you would drop from 6300 to 4231 on your 1-2 shift, from 6300 to 4515 on your 2-3 shift, and 6300 to 4883 on your 3-4 shift. Since it is unlikely your hp would be the same at 4231,4515, & 4883 rpm, the shift points will be different for each gear change, unless we are at red line.

This procedure can be speeded up using spread sheet or graphical techniques, but for the few curves I have looked at you can find the shift points pretty quickly by a couple trial and error calculations. On a stock LT4 it is redline.....

Clear as mud???? But I gave it a shot.....hope it helps..

 

I cant recall which magazine did it- I have subscriptions to CarCraft, Hot Rod, and Chevy High Performance- but one of them just did an article on this in the last two months. They showed how to find optimum shift points. I will see if I can find it.

 

On an SR with that cam, my guess is that somewhere between 5,800 and 6,000 max. 99% of the SR's max horsepower is about 5500- 5600 RPM. You may want to put a 6200 limiter pill in your MSD, as it probably wants to pull pretty hard in 1st and 2nd, even though it's done by then.