What does a final drive do?
The final drive is named as such because it is the last part in the powertrain that multiplies torque before it hits the wheels.
The equation for calculating wheel torque (ignoring losses) is:
Wheel Torque = Engine Torque * Gear * Final Drive
So, swapping in a final drive with a larger multiplier is an easy way to increase torque to the wheels across all gears. It's not all roses, though: this also causes your gears to become shorter, so on average you'll spend less time in them before upshifting, getting a gear with a lower multiplier, and thus having less torque. Shorter gears also means that at any given speed you'll be at a higher RPM, and thus limits your theoretical top speed.
Will it show on a dyno?
Well... no. But it's not what you might expect -- this isn't a crank pulley. Pretty much every dyno chart you see will list wheel torque, and they're totally lying
for reasons.
When you get dynoed, they input your gear and final drive multipliers to cancel them out. So the numbers you get aren't actually wheel torque, but more like engine torque minus drivetrain losses. They do this because it's a more meaningful number to compare tunes, but it also means gearing changes are nullified.
How does it all play out on the road?
With this graph, you can see how much go a 4.88 nets in each gear while wide-open:
As described above, you can see a ~19% increase in torque at first, but with the shorter gears causing a shift into 2nd a little bit sooner. If you average it out, the 4.88 gives about 8% more torque from 10-60mph, and a negligible 1% from 60-120mph. This should illustrate why the common advice is that a shorter final drive is better for street driving and maybe autocross, while advising caution for track cars.
This dyno is for stage 1 NA, and you can see the torque dip present in 1st. If you're stage 1, one pleasant side-effect of the shorter gearing should be that that you get through the dip a good bit faster.
Spencer Fabrication
There are two big problems with doing a final drive swap. The first is if you don't have a second diff, you'll be without your car for a few days at most places. The second is that diff work can be a bit more complicated than simple bolt-ons, so you'll find some horror stories online of people going to unprepared shops and having serious issues. One of the first shops I inquired at was asking if the FR-S had a 1.5 or 2.0 and didn't seem to understand the difference when I said it was a Torsen
. Doing some research, I found Spencer has done quite a lot of diff work for the 86, from installing completely new diffs to just swapping out the OE ring and pinion.
If you aren't local, Spencer will ship you a pre-built diff ready to just bolt on. He'll even buy your old one back from you. For me, a few hours of downtime was much more reasonable than a few days, and having my old pumpkin ready to install if there was an issue with the new one was great for peace of mind.
One thing I'd note is I had the unfortunate luck of ordering the new diff right in the middle of Mark moving his shop, so it took a long while longer than normal and time estimates proved pretty inaccurate. Communication was always quick, though, and I'm pretty confident it would have been a flawless experience if not for the shop move.
1500 mile impressions
The first thing I noticed was wheel spin. It almost counteracts my slightly wider and gripier tires, making it much easier to break the back loose.
Almost immediately after finding the wheel spin, I found that I had to completely re-learn shifting while in manual mode. Having the car for four years I essentially always knew my fastest gear available and never mis-timed a shift, but with the new gearing revs build quickly enough that it's like learning to drive a new car. Knowing how quickly it shifts so you get it started in time to not bounce off the rev limiter -- I must have hit it a good half dozen times before I got it right, and it took me a few weeks to get confident with it.
Gear whine is a thing. You generally only hear it then when coasting to a stop at lower speeds, and it's no louder than tire noise so it's not annoying. Any other noise, be it from engine, exhaust, or radio, will cover it up. I'm not sure what causes the whine -- I've heard theories from backlash tuning to the gears being cut differently (similar to straight-cut transmissions), but the general consensus is that it's normal and safe.
For street driving, I was pleasantly surprised to find that milage didn't seem to suffer. Being able to jump into a higher gear sooner seems to counteract the RPM increase; places I'd have been in 4th gear, I was now in 5th. places I'd be in 5th, I'm now in 6th. Street being fine, freeway cruising definitely takes a hit. My work commute has me about 70% on the freeway and my average mileage has dropped about 4mpg. Another thing I hadn't considered was additional heat while cruising at a higher RPM -- my oil temps on the freeway are notably higher than they were before, but still within safe ranges. I may opt for a cooler after the winter to help keep temps down on hot days.
As far as usability goes, the extra torque at lower speeds is absolutely noticable and great for passing people on the street. On the freeway I find passing ability to be a bust, with the car still feeling very underpowered at high speeds. Climbing revs faster definitely increases the fun factor, which I think is perfectly in line with what this car was designed for. While it is definitely a fun upgrade and I'm happy with the results, I'm not certain I'd call this "exactly what the AT needs" as others have, and price-wise I'm not completely convinced that it's worth it unless you can do the work yourself.