Final Drive choice - Buttonwillow
 

So I'm going to get an LSD and would like to change the final drive as well while I'm at it.

Anyone have any experience with final drives at the track specifically Buttonwillow? That is my go to track since it is near by and I want to optimize the final drive for that specific track. It's more of a medium speed track, mostly 3rd - 4th gear corner, may hit 5th gear on one small straightaway. Don't care about street usability since I rarely take the freeway.

I'm going to keep the engine stock so I'm trying to optimize everything else for more money than simply getting a supercharger lol.

Thanks!

Not really an answer (more of a side question), but isn't the theoretical quickest final drive then one where you're top gear winds up hitting the highest maximum speed attainable on the track?


Shorter gearing leads to more torque at the wheels, so you can get up to a quicker speed with the same engine power before drag limitation becomes an issue. However, if you go too short you run out of gearing first and wind up sitting at the rev limited on track. Of course if you go this route on a slowish track, you'll wind up shifting like a maniac everywhere. Not sure if that's the best route. You'd also be optimized for one track, and your gearing might be too short somewhere else.

A piece of advice. Consider 5th your final gear. 6th is such an overdrive that the car just falls flat on its face. So in your case, choose a final drive that will almost top out at your longuest/fastest straight on that track. Optimize for the fastest track of all the tracks you plan to go to, its worse to run out of gear than it is having to shift often.

Might go with 4.88 based on this. Since I can sometimes hit fuel cut at the top of 4th but there isn’t enough road or ideal to change to 5th I should be able to almost top out at 5th with 4.88 but not hit fuel cut.

Unless you're going to have more than one diff to swap out between tracks, I'd take WSIR, Cal. Speedway, and Laguna Seca into account.

If you optimize just for BW, you'll either run out of gear at the others, need to go to the overdrive 6th, or likely be sitting at the top of 5th for extended periods (which would add to heat-building and be awkward going through, say, T8 at WSIR where it can get bumpy and it's often better to be one gear up to increase confidence/control if your going flat through there.

What is your power level stock? That is going to make a big difference in what you choose. I went 4.55 and while it was great on the street it was less than ideal for most tracks and especially autoX. Shifting takes time, the more shifts the more time. My car was over 300 whp on E85 so that makes a big difference. Gearing felt too short for the track. I’d recommend 4.3 if you have an older car and just leaving it if you have a new car. OS Giken LSD is a great upgrade!

For my local tracks I went with the 5.29. I still don't have enough room to top out 5th on our tracks. I haven't had the car at Mosport GP so I can't comment on a larger track. I would figure the car just doesn't have enough power to top out 6th there anyway. The tracks that I go to the car typically can almost hit the top of 5th. Car has stock power as of now maybe with the addition of the header this year that will change things and 4.88 might be better. It's hard to say at 7000rpm the car just about tops out at 200kph with stock sized tires with the 5.29 but I have never had it up that fast.

i also have a 1.5/2way MFactory diff installed as well set to 1.5way. You will definitely like it.


i just did some quick math to lookup some of the gearing to help you decide.

if you top out 4th without enough room to justify the shift the that would mean your about 173kph assuming a 7300rpm redline. Moving to a 4.88 your top speed in 5th would be 176kph. That would only be a 3kph (~2mph) gain. You may be better off going to the 5.29 where your 5th gear will top out at 163kph so you would make the shift into 6th which then tops out at 212kph. That would seem worth it to me and follows the same route I took. Now the one thing people don't really consider is the slowest corners. For example if you are shifting into 2nd for the slowest corners now you will be leaving it in 3rd. In the end I noticed after the change I was making about the same shifts per lap as I was before the FD change, so the time lost on more shifting is not there. But I was in a better spot of the powerband around the slower corners which made up for a lot of time. Do you know what your slowest corner exit speed is?

I'd be more concerned with the FD producing gear spacing that allows you to pull out of most corners in the powerband then worrying about being able to top out a specific gear on the longest straight. There is always going to be some compromise but gearing for topping out on the straight seems pointless to me on a road course :iono:

This makes more sense. Got to see my corner speed to see which final gear puts me in the best powerband range.

Final drive selection is extremely track-dependent.

I go to two tracks where I had to shift into 6th on the main straights. When NA, I would lose speed at that point! Even with the JRSC, switching to a 3.73 FD produced the best lap times. Going anything greater than stock to maximize acceleration was a loss because the shift time and lack of acceleration once shifting into 6th cancelled out the acceleration gained.

Curious, what is your pace at Buttonwillow that makes it seem the FD is costing you time? With what tires?

What modifications are done to your car, and what kind of times are you putting down?

Stock motor, shocks and springs, aero. Average around 2:07 so not fast. Just wanted to do the final drive with the LSD all at once.

Keep your FD stock.