Monday, August 15, 2005

Road racing an automatic

Road racing an automatic is historically speaking a big no-no. No engine braking, no blips, uncontrolled shifts, not being able to hold at certain rpm for arbitrary periods of time, they're all good reasons to declare an automatic an ill choice.

But this is 21st century, and even GM's crappy transmissions are a subject of being electronically controlled. There are tables controlling when to up- or down-shift, depending on Throttle Position Sensor (TPS) and speed. There are also separate tables for cruise and overheating conditions, which are are a nice touch, albait useless for the purspose of road racing. Now though, with the help from HPTuners or EFILive packages, we can adjust it any way we want. Let's take a look at what we can do with them.

Normally they look something like this:
1->2 10 10 11 12 13 15 17 19 21 24 27 29 32 33 35 37 37
2->3 20 21 23 25 27 29 31 34 37 41 45 51 59 66 74 75 75
3->4 37 37 37 37 44 52 69 83 99 108 112 117 117 117 117 117 117
2->1 9 9 9 11 12 14 16 18 20 22 24 27 29 31 32 34 34
3->2 17 18 19 21 23 25 27 30 32 35 39 44 51 59 67 72 72
4->3 32 32 34 36 37 45 61 75 89 99 106 110 111 111 112 112 112

This is what I figured out by simple trial and error at the track:
1->2 15 15 15 15....
2->3 67 67 67 67....
3->4 115 115 115 115....
2->1 15 15 15 15....
3->2 65 65 65 65....
4->3 115 115 115 115....

What we basically did, we eliminated partial throttle! If you think about it for a second, at the track, we want to very rarely shift early, you want to shift when you 'run out of revs.' This simulates it, as we set WOT shifts to partial shifts. The important part of it is the first column, which is for 0% TPS. That means, that even if I take my foot of the gas completely, the transmission will not suprise me with an upshift.

Another interesting things you might notice, that while the stock table ends with a 75mph shift, the racing table ends with a 65mph. Why is that? Because at the track, the trans fluid gets so overheated (yes, even with B&M 24000GVM trans cooler!) that it turns into 'water.' It seems to lose so much viscosity, that the shifts that were normally intended to initialize at 6000rpm, now would start at 6200-6300 and would not complete sometimes until my revlimiter at 6600rpm. Thus, I lowered the speed/rpms at which I wanted it to start shifting, and everything was fine again. It was suprising however how much this can change at the track, and never on the street.

So what was the result of all these experiments and strange changes? It worked great! I went to the Autobahn Country Club in Joliet, IL for a track day on the 'North Loop', and the car was more fun than ever. Granted, the long automatic gears aren't the greatest for roadracing, but it would feel good. There is a sequence of T3-T4-T5 which by the end of the second session I figured out how to take without any braking whatsoever. All these corners are about 50-60mph, and with the transmission in 'track only' mode, I'd plow through all of them holding ~5000rpm, letting off throttle completely in some moments, and feathering it in others. Laying into it completely coming out of T5 and setting up for T6 was extra fun, as I'd rocket out of there at the top of second gear which feels very powerful in my car (right about peak hp and before torque starts to drop off).

The gear spaceing was actually not bad considering how the track was laid out. Most turns were 50-60mph turns, with T1 being about 75mph (I tried it out at 80mph few times, but brakes just weren't up for it going into T2 and I'd overshoot the turn in point) and T2 being ~40mph. If fit the car well coming out of these corners hard, charging up the top of second gear. The 3 straights were fun two, two of them topping out about 85-90mph, and the main one was about 105-110mph (depending how chickenshit I'd go through 'the kink' T1. So I'd come out on power, and utilize the powerband fully. I wish I had another gear between 2 and 3, something like Supra gears were (60-90-125mph) as that would put me at the peak power at all times. But, I still claim it's the most fun thing you can do with your pants on.


At 12:06 PM, August 29, 2005, Anonymous Anonymous said...

you know that some little known race car designers have advocated automatic transmission for road racing. guys most people never hearf of like Jim Hall, Dan Gurney, and Andy Granitelli. all of these designers and buiolders have campained at least one automatic car in various roadrace catagories and the ruling body eventually outlawd the use of an automatic transmission.

At 3:17 AM, December 26, 2007, Anonymous BobDoLe said...

also note that in hptuners, you program a separate "performance" table which you CAN switch to if you wire in a momentary button.

instructions from the guys at hptuners for the switch:

switch from regular driving to autox mode at the push of a button...


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