It's not that simple. The acceleration is faster in any gear but not at any speed. At speeds in which the shorter final drive forces you into a higher gear, the acceleration is actually slower than with the stock final drive. So what you are doing is making the car faster at some speeds, and slower at others. Thus it might help your lap times at some tracks, where you spend a lot of time at speeds where it helps, and it might hurt your lap times at others, where you spend a lot of time at speeds where it hurts.

Read through the links for more details, particularly this post which tells the actual speed ranges where it helps and hurts.

 

Ken have you installed a shorter final drive in your car or is your stance on this issue purely academic?

Why not put a taller (i.e. LS/RS) final drive in then?

 

I am always interested in finding out about different parts and setups and techniques for my car, that will benefit on the track as well as on the street. But I want to consider downsides as well as benefits. It's rarely as simple as some people make it out to be.

I have not installed a shorter final drive in my ITR. Aside from the cost - certainly a significant downside - the fact that the stock FD accelerates substantially (~18 percent) faster than the shorter one at speeds of 79-88 mph, where a lot of driving takes place at the tracks that I frequent. It's faster because I can stay in third gear rather than being forced to upshift to fourth and be slower with the shorter FD. This makes me believe that the shorter FD will overall be no faster, and may in fact be slower, than the stock one.

Some people perceive that the shorter FD is faster, because it reaches redline faster. But most of reaching redline faster is because that occurs at a lower road speed, rather than actual acceleration benefits. So it seems like it improves acceleration in any given gear much more than it actually does - and that is before taking into account the poorer acceleration it provides when it is forced to upshift to a higher gear at a lower road speed than the stock FD.

If I were more interested in drag strip performance than in road courses, I would probably get the shorter FD, because its acceleration benefits are more consistent at lower speeds. But acceleration below 50 mph is of little importance to me.


What that will do is shift those "speed bands", just like a shorter one does. I like the stock FD because of its ability to keep me at the top of third gear at the speeds noted above. The stock FD will be quicker at 79-88 mph, where it is at the top of third gear, than the taller FD or the shorter one.

 

Glad i found this since i just bought a Tranny. I mean always had a good idea of what i thought the difference was but was off by quite a bit. Good info guys :thumbup:

 

This is very interesting.

Since an upshift will keep you on the cam, though, do you think that there's a possibility that being in fourth with room to accelerate might be a better choice than being at the top of third w/no room to move w/out shifting?

I guess you can tell that's prompting the witness. The feeling I get is that I'm better off at the bottom of fourth because there's room to get faster w/out unsettling the car.

In my case, I really think that having the JDM final drive helps me out at VIR North and Summit Point Main, the two tracks I frequent.

Interesting idea, though. Maybe you should drive my car sometime.

 

im piecing together my swap for my LS... so far, i have a b18c5 bottom end and a b16a top end, obvoiusly just need a tranny... ive been teetering back and forth which to get usdm or jdm ITR tranny... i was just gonna get the usdm cuz its a bit cheaper and for a daily driver, should be enough.... anybody wanna say that the JDM one might be worth my while?

 

There are two different issues: (a) which gearing setup provides better acceleration, and (b) the TIMING of the shifts.

Regarding (a), there is no doubt that a stock R&P in third gear provides better acceleration than a shorter R&P in fourth gear.

However, regarding (b), the shift points occur at different speeds with the two setups. At any particular track, it's possible that the shift points in one setup or the other may come at the "wrong" time in certain turns, so that it's not really where you want to be shifting (because, as you imply, unsettling the car may be a problem in certain turns). However, this will vary from one track to another; even at a given track, changing the shift points is likely to help at some turns, and hurt at others.

 

:Moves Ken from the Expressive column to the Analytical column:

 

Well according to AssPenny from Honda-Tech its the best mod he has ever done to a R

 

Good insight, Ken.