f20kills

At most tracks here in SoCal it is not a bad combo..ie. BW, CVR and WSIR.
One less shift...most have to hit 5th gear while I can keep it in 4th gear


BTW, nice work John!

speedengineer

The optimal final drive ratio (including secondary shaft gear) is extremely dependent on the track - it depends on what your corner exits speeds are more than anything. If a track allows you to put the power down hard right at the start of a straight, that is optimal. The AP1 gearing will be faster than the AP2 gearing at some tracks, vise versa at others. However, the AP1 5th and 6th gears will likely always be better than the AP2 5th and 6th, however the difference is minute, since you rarely are in 5th and 6th until near the end of the straights.

speedengineer

I wasn't going to share this, but what the hell, why not. Did some simulations last night for different gearing combinations. The plot below shows the time it takes the car to travel down a 2350 ft straight away as a function of the starting speed (aka corner exit speed). This is for my car with 24.9" diameter tires, 2645 lb actual race weight.



There is some difference between the two secondary gear ratios (1.208 and 1.16, however the 5th and 6th gears have only a very small effect (blue and green are nearly on top of each other, as are red and purple).

In real life (as opposed to the simulation), there wouldn't be the giant 1/3rd of a second differences in time like you see at 63mph and 87 mph. This is because the code always tries to be in the optimal gear for max wheel torque, however a real person would just leave the car in one higher gear when exiting that corner, so the traces would smooth out slightly.

JUSTINTHECOASTIE

^^interesting with good info

robrob

I have to agree with this and comment about my experience with lower gearing. For 2011 I installed 4.44 gearing and was surprised what it did to my lap times. I know Summit Point Main, my local track, very well and had my optimal shift points figured out. I knew the lower diff gearing would have the effect of reducing the spread of all 6 gears which would make the transmission more efficient since 6th gear was way too high for racing with stock gearing. But of course what happened was some of the corners that I had perfect gearing for were changed so that I had to either shift in the corner or run a higher than optimal gear through it which slowed my acceleration out of the corner.

Summit Main's Turn 10 is by far the most important corner on the track because it's a high speed corner that leads onto the longest straight and start/finish. My new gearing has me in 5th in that corner at too low an RPM for max acceleration. I tried the corner in 4th but end up tapping the rev limiter before corner exit. For me and my car the stock 4.10 gearing fit the track better than the lower 4.44. This is why big money race teams play around with gearing to find what's best for each track (and driver).

In general and theoretically 4.44 gearing is better than 4.10 in the S2000 but that may not be the case when you compare them at a particular race tack.

mLeach

I know what you're saying, but I thought I'd point this out because it's a common misunderstanding. A final drive will not affect the RPM spread between gears. It will change the speed where the gear change occurs, but the spacing is entirely transmission gear ratio dependent. If a gear change has a 2000 rpm drop with one FD, it will always have that issue with any FD just at different wheel speeds.

that is why this conversation is even happening. The AP1 has much better gear ratio spreads. putting the secondary shaft of the ap2 lowers it's primary ratio giving it a much better thrust curve while keeping the closely spaced gears.

speedengineer

Actually, that is inaccurate. The final drive ratio does indeed affect the RPM drop between gears. The numerically higher final drive ratio will make you RPM's drop less when performing that shift. This is beneficial because in means you are able to put higher average horsepower to the ground. However, this benefit has to be balanced out with the penalty of additional up-shifts, where no power is put to the ground during the shift.

EDIT - oops, I was wrong, ZDan is correct that there is the same RPM drop regardless of final drive ratio.

robrob

You're right. After thinking about it I realized there's no problem with pairing an AP2 motor with an AP1 transmission. I edited my earlier post to correct it.


I agree with this. The reason it's true is because the final drive affects 6th gear much more than 1st gear--it's not a linear relationship.

ZDan

Final drive does not affect rpm drop between transmission gears. rpm drop is strictly due to the difference in the two transmission ratios. Upshifting from 2nd gear at 9000rpm in an AP1 will put you at just over 6500 in 3rd whatever diff ratio you have. 9000*1.481/2.045 = 6518

ZDan

It *IS* a linear relationship. 4.77 gears will increase rpm in 6th gear at a given speed by a factor of 4.77/4.10 over stock 4.10 gears. They will also increase rpm at a given speed in 1st, 2nd, 3rd, 4th, and 5th by 4.77/4.10.