The best transmission isn’t always the adjustable gearbox, then again occasionally it is.
First we need to remind about shifting. you want to shift “at even power” that means where ever you are on the rev range, you want to shift with as high possible amount of hp left, but not to a lower HP figure. to put it simply I put it in detail
Lets think for a second that our gearing lets us sift to next gear, and you will end up 1000rpm lower rpm. In other words if you shift at 6000 rpm, you will end up on 5000 rpm
RPM…Power
4000rpm - 150hp
4500rpm - 160hp
5000rpm - 170hp
5500rpm - 180hp
6000rpm - 185hp
6500rpm - 180hp - shift here
7000rpm - 170hp
7500rpm - 158hp REDLINE
7750rpm - 145hp Rev limiter
In power band line this you want to shift from 180hp to 180 hp or should we say, from 6500 to 5500 rpm. (This isn’t any particular car, its just a scale I made up for example)
If we start talking from D class, it might be very beneficial to use race gearbox. As you don’t have too much power to work with, you need to make the best possible use out of that power.
Therefore you might want to have fairly short gears, so that you could keep as close to the power peak as possible.
When we start talking about B and higher classes, My personal opinion would quite often be race gearbox is waste of PI, as I have so much more power. Only exception is if the car has just simply terrible stock gearing. (looking at the car list and nothing really pops out for good example) Or if you have the 1.6 turbo engine swap which requires “slightly different” gearing, as you have to short shift so much with that engine. (Or so I have heard, I haven’t experienced that myself as Xbone hasn’t arrived to my country yet. so I’m still in FM4).
Then you have to (again) remember that powerband, is it really wide (old musclecar) or is it quite peaky like Honda engines tend to be.
In case of old muscle car, you might have only 3 gears to work with, but then again you have power from 3000 rpm to almost as high as it goes, 3 gears MIGHT be just enough In case of peaky engine, you really need to be close to that peak as you have only 500 -1000 rpm wide range where you have any power which needs very close ratios, and therefore more gears.
Now that roughly defined “Do I need adjustable gearbox?” question. Now if you have decided that you do need it, we shall start looking how to set it up.
First. Do some full stop launches. you want set the first gear so, that you have small amounts of wheelspin, just so that you can get moving without bogging down. If you get a lot of wheelspin, that isn’t too bad, if you can control it with throttle control. Then drive couple laps, look for situations where you are in corner, note that you should NEVER shift right after or in corner. cause every time you shoft there is a small “pulse” of power sent trough the driveline, which might brake the traction. adjust the gears slightly longer than this. Also, you should never have to shift up right before entering a braking zone. that’s couple tenths off your laptime every time you have to do this. Adjust the gear so that you either have tho shift bit earlier, or you can whine it up bit higher so that you don’t have to shift for the next gear at all.
Anyhow, My personal preference is to stay with the stock / sport gearbox as often as possible. Unless I’m driving a car where isn’t too much power to work with. and PI spent on race gearbox is notably cheaper that PI spent on power.