a/x trans gear set options, what works? Options

[Trekkor]

What is your favorite combo.

I need to stay in the 4500-6500rpm powerband.

I can tach out at 7-7300rpm on the limiter.

I have a spare trans I can build up now and then install... whenever.

KT

[Demick]

We had a discussion about shift points a couple of years back.

Shift point discussion

Here is one of my posts from that topic which discusses optimal shift points based on the torque curve. Although the example is based on a /4 torque curve, I think it applies to Andy and Richs' discussion:


If you want to be scientific about your shift point, here it goes:

First, you need a torque curve for the car. I will use Geoff's curve that he posted (I reposted it below so you don't have to look back and forth).

The 914 torque curve is almost flat up to about 4300rpm (I will assume it is flat at 108 ft-lbs).

Second, you need to know how much your gear ratio changes for the shift. For autocross, people are most interested in the 2nd to 3rd shift point, so that is what I will use here. Stock 914-4 transaxles have a 1.889 ratio 2nd gear, and a 1.261 ratio 3rd gear. This means that your gear ratio changes by about 33% when changing from 2nd to 3rd gear. Two important things happen in this change: 1. Your RPM will drop by 33%, and 2. For a given torque generated by the engine, your torque at the rear wheels will drop by 33%. Since it is the torque at the rear wheels that actually accelerates the car, this is a very important number. This also means that we need to make 33% more engine torque in order to maintain the same acceleration when we shift from 2nd to 3rd.

Now, back to the torque curve. Pretty much wherever we make the shift, we will drop back into our 108 ft-lbs of torque region. In order to have gained 33% torque in this transition, we need to have had about 81 ft-lbs of torque before the shift. On the graph, this corresponds to about 5400 rpm. This is the ideal shift point from 2nd to 3rd gear to maintain maximum acceleration.

Working through the same math, ideal shift points for 3rd to 4th is 5300rpm. 4th to 5th is 5250rpm.

Note, this applies to Geoff's '76 2.0, and will change dependent on the torque curve for your car, and any difference in gear ratios you have.

Makes sense?

Demick