4.88 Final Drive review / Spencer Fabrications
 

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:

http://i.imgur.com/gJMW98N.png

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 :bonk:. 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.

This is one of those "stealth" mods I've been considering. I feel like it would give the AT the extra "oomph" it needs, but I'm not totally sold simply because it would kill some gas mileage while cruising on the highway. A lot of the driving I do is highway so I'm not sure that's a con I'm willing to live with.

Thank you for the in depth write up, and the chart you've posted.

For MT i'd always prefer close gear set to FD .. if it didn't cost much more then FD :/

Good write up!
 

I just reinstalled my 4.88 FD a month ago, and I love it. I bought a second rear carrier for it this time, and installed the rear carrier myself. That way I can switch back anytime I want to. But now I don't want to. It's way too much fun. (Anyone interested in a slightly used 4.30 FD?)

I have taller tires than stock, so my gearing isn't quite the same as yours, I have 225/45/17's, instead of the usual 215/45/17. I know what you mean by wheelspin though, I now have to use the Sport/TC button when it rains to allow a little wheelspin, otherwise the ECU is cutting the engine every time I take a corner.

As far as high-speed MPG goes, I have to disagree. My highway MPG is just as good, now that I've found the right tune for it, but it really suffered up till then. I also have to disagree about power at highway speed. I think it is also a tune issue, but I also have a Phantom SC, so there's that. In 4th gear at 55 MPH it pulls up to 100 MPH so smooth and so fast it's almost scary. My current tune has much less valve overlap in the AVCS tables than any other tune I have seen for these vehicles, yet it has more power, and economy than before. Because of what I've found out, I've completely revised how I tune cars. Each car/driver has to find the best Exhaust tune, and then the best Intake tune, in order to have the tune best suited for each vehicle. Since each car has different Intake and Exhaust restrictions, and engine load profiles, each tune must be different.

I've also found another difference. Shifting manually under light acceleration is different now because I used to have to rev up each gear, in order to have enough power to pull the next gear. Now I can short-shift gears AS LONG I SHIFT BEFORE THE TC LOCKS. I'm still not sure what RPM the TC locks at, but, if I wait too long it does a whiplash shift, and if I shift too soon, it bogs. Since I'm always trying to catch these shift points, my city MPG is just horrible.

If only more people realized that a 4.88 FD would make your car more enjoyable to drive on the street versus some pricey catback or huge wing with stickers....:D

This^ The 3.73 rear diff mod was probably one of the best mods I did to my old Mustang right next to going FI. FD kits are expensive and uncommon for this chassis... Although my Mustang did have a simple live axle in the rear, that probably helped lower cost lol

Mechanical LSD will get rid of all of that lack of grip ya'll are speaking of.... :)

It's not helpful that many part makers are dishonest in how they present gains, and we all know car people are among the worst when it comes to cognitive bias around their latest upgrades.

I referenced lightweight pulleys in my original post because review-wise they have a similar level of wishy-washiness to final drives -- a lot of anecdotes with very little or no supporting facts on either the for or against side. I believe people just don't understand what a FD does, which is why I made this post.

I've never experienced a clutch LSD, can you explain how one would help here?

Would swapping the 4.3 out of a junkyard auto FR-S in to a MT BRZ make a significant difference over the 4.1? Would I need to go to 4.5X to feel anything significant.

If I plan on going with a JRSC in the future should I just leave it alone?

Auto frs only gets the 4.1 I believe, with any FI stock is actually pretty good. One thing I noticed with my friends sc brz is that his car can pull totally fine in 6 gear......>_>


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4.3 is only in the 2017 MT so far. Good luck finding one of those!

Regardless, you won't feel a significant difference going 4.1 to 4.3; I wouldn't bother. Much better things to spend money on.

Thanks for this post...exactly what my next upgrade may be !!!

I won't do a full explanation because you can search for that, but in a nutshell, as much as the stock torsen + abs tries to maintain grip to both wheels, it's still reliant on the torque bias ratio, so when one wheel has no traction, that wheel gets the power. abs creates a little bit of resistance from what I noticed when going up steep driveways, but it can't work with a lot of power, it would roast your brakes.

Mechanical LSD's begin to lock both wheels as one begins to loose traction. Pair that with good tires and it would help you out a lot.

Do some research though, mechanical LSD's aren't for everyone. They require more maintenance and make lots of noises.