BRZ 2018 AT to MT gear ratio
 

Hey fellas,
My name is Stav and I am new here. So, I've got a 2018 BRZ AT (and not MT due to medical issues), and I really like it, though I wonder if I can somehow get the same gear ratios as the MT? The roads and tracks I usually go to require a little more acceleration, at minimum such as the MT, otherwise it really kills the momentum, and of course, more fun.. Is there a decent way of achieving this without throwing thousands of dollars?

Thank you and have a great new year!

Not an expert by any means, but you might be able to swap out the final drive to get better acceleration at the expense of lower top end.

Take a look at the threads of people that change the final gear (FD) to a 4.3, 4.5 or 4.7, something like this. That would be the easiest solution.

Exactly what others have said, get a 4.7-4.88 final drive installed and it's performance will surpass that of a stock manual

Do you have plans of going FI ever? Do you have any other mods?

I am not sure but in some countries the manual 2018 cars actually jave more power than the auto model about 5kw if the manuals have the red manifold.


An alternate way for better acceleration may be to get a tune and possibly catless header. Probably arround same price as buying and getting someonecto fit the different differential and you will actually have nore power and less torque dip arroind 4000

Not sure what FI means, but my car is stock. Maybe get a supercharger in a few years, right now not really though, gotta enjoy that car to the fullest first. Besides, I'd like to avoid warranty void mods such as ECU/engine power etc.

Do you know perhaps if changing the final gear will interfere with the ECU somehow, perhaps raise a flag or warnings? I don't mind not getting to 200 kmh and upward, no need for that tbh in Israel, even in the track.

F-orced I-nduction. Short for both superchargers & turbo.

Changing manifold/ header to decated one + remap will eliminate torque dip and will add about 15-20 hp (depending on tune and what you have for starters). This is definitely worthwhile and in the UK often the 1st (and probably most recommended) power mod.

For AT as many mentioned above FD is the way to go.

Both combined will make an AT GT86/ BRZ quite a lot more fun to drive (either on road or track).

I ended up going with turbo kit at the end, but certainly would recommend these 2 mods if you planning to keep your AT NA.

Not sure about warranty rules in Israel, but mids typically affect warranty only for modified components. Thus, re-map may affect engine warranty, but NA manifold + remap seem to be quite safe mod with good reliability track record.

Sent from my SM-G930F using Tapatalk

FI = supercharger or turbo charger. If planning on FI at some point I wouldn’t go above a 4.56, 4th will top out at 115mph and autos stop pulling in 5th. 4.56fd dropped .4sec off 0-60 even with having to shift into 3rd to hit 60.

Idk the warranty laws in Israel. But in the states nothing voids a warranty. They have to prove the modification caused or atleast contributed to the failure to not fix that particular issue.

Also they have no way of knowing you have ever flashed your ecu, you can flash back to stock before going in for warranty work. Same with putting back on stock header or any mod really.

As for the FD swap unless the work was inside the FD specifically, that should have no effect on warranty for anything else. They would also have no way of knowing unless they drove your car and knew what speeds each gear should be.

Some other mods to consider aerodynamics and weight reduction.
Take out trunk junk -30lbs
Rear seat delete mine only dropped 25lbs but has a lot more noise reduction then before
Lightweight battery, I got the largest shorai dropped another 25lbs
And a full exhaust about 20lbs for 100lbs total
Light weight rims/wheels expensive but rotational mass + better grip/handling
Wilwood BBK -32lbs unsprung
Can get to 150lbs reduction somewhat easy without ruining DD comforts.

Aero to reduce drag
Jdm underpanels
Verus tunnel cover
Verus diff cover / diffuser
Verus front wheel deflector
Side skirts

All these add up to make a pretty big difference. Less drag/weight means you need less power to accelerate.

Those aero panels are all in my wishlist but I've been hesitant to pull the trigger on them being skeptical on noticeable improvements considering total weight added.

Also suggest getting an aluminum driveshaft to save 12 pounds that is also not insignificant rotational driveline inertia reduction. I've done that, 4.56 FD, Wilwood BBK, trunk junk removal, and aftermarket wheels/tires and run E85 with the OFT tune and it is night and day compared to stock. More weight savings could be had by getting a lightweight water pump pulley, emptying the wiper fluid, and running with a low gas tank.

Drag reduction actually adds up and makes a big improvement. I wouldn’t do it as a first mod but once the low hanging fruit is picked it’s definitely worth it. Just look at modern supercars. Most have just a flat under panel from bumper to bumper. Underbody drag reduction is awesome because it lowers drag but also increases down force. Usually you have to sacrifice one for the other.

The JDM panels only add about 2.5lbs. Trans tunnel covers about 4lbs but in a major drag area. Also spread evenly across the lowest point of car.
I dont have the numbers off the top of my head but just from those I think the added weight breaks even from drag reduction at 25mph and then is essentially equavalent to 100lbs weight reduction at 100mph.

If that's true, that's good enough for me. Any links to the assessments to back it up?

The weight is a guesstimate from holding In hand. Could easily hold the far edge with just thumb/index and hold level/straight with no effort. Surprisingly light.

A lot of articles and formulas for calculating force required based on drag coefficient and frontal area. And you can find the cd reduction with some googling. JDM claims panels reduce cd by .01 , trans tunnel should be by another .0075 and wheel deflectors by .0025 or .02 total. Which is actually a lot of drag forces in lbs.

don’t have every link / article I read. But spent everyday researching and calculating things for a few weeks when I was on that “stage” of my build before purchasing, noticed a solid difference 60+mph especially 100+mph.

As speed doubles drag forces quadruple. 5lbs of drag at 20mph, is 20lbs at 40mph, 80lbs at 80mph, 320lbs at 160mph. Benefits add up a lot at higher speeds.

Thank you everyone for the help!
Is there a particular/"tried and tested" method to do the FD changing? and how do I select to which ratio I change?

Gearing change isn't going to do a whole lot for you overall. Should improve 0-xx acceleration by a tenth or two, but from a roll at speed, it's totally dependent on starting speed. From some speeds you'll have to be in the next taller transmission gear, so you'll accelerate slower. At some tracks, "better" (numerically higher) gearing will help, but at others it will hurt. Very much course-dependent. Long/short: gearing doesn't magic you better power/weight... Also, changing diff gearing won't close the gaps in the AT's ratios, which are wider vs. MT. RPM drops will remain the same whatever you do with final drive gearing. You're still going to lose more rpm between shifts vs. MT, and hence fall a little further down the power curve.

Weight reduction: Lose 100 lb. and you'll be about a tenth quicker and 1mph faster in the 1/4-mile. That's good, but for sure if you delete rear seats and trunk junk, you'll notice the noise difference way more than any actual acceleration improvement.

Aero drag, 10% drag reduction will gain you about 0.5mph in the 1/4, something, yeah, but not a lot...

Unfortunately there's no easy way to meaningfully improve acceleration performance with most NA cars, this one in particular...

My diy rear seat delete is significantly less noise then stock seat/trunk junk. Rear seat -32lbs then I put 8lbs of sound insulation/cardboard/chloroplast/carpet back in.

And yes drag reduction won’t make a huge difference in 0-60 and the 1/4th when trap speeds are around 90mph as aero doesn’t make a noticeable difference until about 60mph and a substantial difference until about 100mph. Aero is Great for better mpg, Hwy pulls, and faster tracks. Also most of
The drag reduction for this car also adds downforce.

When I was NA I went from 7.8sec 0-60 and 16.5sec 1/4 to 6.6sec 0-60 and 14.8sec 1/4 with a full tank of 92. Still relatively slow. But substantially faster then stock.

But 0-60 and 1/4 is a horrible way to measure auto 86s acceleration. When almost no one cares about stop light/off the line speed. Think 60-120 is a better comparison for real world applications.

Some diy / cheap methods in forums on here if you search. But weir performance or rhdjapan have Fd kits. Should have everything but shims. Pull the pumpkin take FD kit to trans shop have them do install, and put pumpkin back in. Maybe 400-600$ labor and 650$ or less for kit.

As for what FD to choose. This is somewhat complicated. Kind of like hitting a moving target. But I’ll try to explain a few things. First in a perfect “vacuum” (not factoring traction limitations, rolling and wind resistance, and other variables) the percentage increase of acceleration should match the percentage increase of FD to the new top speed of each gear but each gear will top out at the same percentage less.

For example theoretically say 4.1 FD makes 2nd top at 60 and reach 54mph in 6seconds. 4.51 Fd (10%) will make 2nd top out at 54mph and in 5.4seconds but now you have to shift into 3rd to hit 60.
Another example when I switched FD to 4.56 before 2nd would top at 63 and do 60 in 7.3sec. After 2nd topped at 57 and hit 60 in 7.0sec flat bcs I needed an extra shift and had to accelerate at the bottom of third. But I hit 57mph about .6sec faster then before. The main difference I noticed was before the car would stop pulling once I shifted to 4th at 93mph, after it shifts to 4th at 82 but pulls all the way to the top to 114mph.

With all the being said. For tracking
The easiest method of choosing is picking a FD that tops you out in a gear just before optimal shift point on the longest and or most frequent straights for fastest acceleratetions and least shifting.
The most optimal is choosing a FD that puts you at optimal shift points just after turn out on the most frequent turns.
Now why it’s hitting a moving target is those speeds while always change not only on different tracks but as you beecome more skilled and or modify car to be faster those speeds should slowly be increasing. And then a less aggressive or maybe much more aggressive FD will be optimal.

For regular DD, street driving, canyons just decide how fast you want to go and choose a FD that makes the top of 4th max at that speed.

For tracking/auto cross maybe hold off until you have more
Mods and skills and a better idea of what speeds and FD ratio would be best accordingly.

That is cool! But still, -24 lb. or -32 lb. just isn't going to make any difference as far as seat-of-pants feel, and barely any difference in measured acceleration or lap times.

It will affect mpg a lot more than acceleration or lap time performance. Even there the difference will be quite small.
I modded the performance analysis spreadsheet I had made for my FD's Texas Mile run to simulate my stock-drivetrain '17 BRZ. I looked at what the elapsed time and max speed would be after a straight at full throttle after exiting 60mph and 80mph turns:

cD = 0.31: 60mph + 1500ft = 11.97 seconds @ 104.1mph
cD = 0.28: 60mph + 1500ft = 11.95 seconds @ 104.8 mph
10% lower cD => 0.02seconds quicker


cD = 0.31: 80mph + 2300ft = 15.26 seconds @ 120.1 mph
cD = 0.28: 80mph + 2300ft = 15.17 seconds @ 121.2 mph
10% lower cD => 0.09seconds quicker

Optimistically you *might* see as much as a 0.25 second lap time reduction at a faster track like Watkins Glen from a 10% reduction in drag coefficient.

Decreasing lift/increasing downforce will influence laptimes much more so than drag coefficient.

For 60mph - 120 mph times for 17+ BRZ, I get:
cD 0.31 60-120 = 19.50 seconds
cD 0.28 60-120 = 18.74 seconds

10% lower drag = 0.76 seconds or 0.4% quicker 60-120 mph.

Background: I did performance analysis at an electric vehicle startup in the 2000s/early 2010s. Developed spreadsheet for 500hp FD that predicted its standing mile performance within <1mph, predicted = 184.2mph, actual = 184.8mph

-25lbs for rear seat delete will make no noticeable difference, but its a small piece to the puzzle. cost roughly 40$ to drop 25lbs and substantially reduce noise. trunk junk -30lbs for free. Shorai battery was kind of pricy about 300$ for -24lbs for the largest one + custom diy mount. Take out rear floor mats and user manual. round it to 85lbs for 340$. header back removed another 16lbs also added a lot of HP. Also probably a waste of money admittedly but cf driveshaft another -12lbs. 113lbs total. Assuming at least 230crank hp from headerback+tune. stock 2815lbs / 230hp = 12.24lbs per hp. 2815lbs - 113lbs / 230hp = 11.75lbs per hp which is equal to 239.6hp from new power to weight ratio, or about 10 hp for 120lbs weight reduction (almost a passenger) and that will be noticeable.

I did all the equations but don't have them written down or feel like doing them again... but what was the drag forces in lbs going from .29 (stock 86) to .27 (jdms advertised new cd with spoiler + underpanels) at 100mph? i remember it being well over 100lbs which again 120lbs = 10hp on this platform with a few NA power mods.

Also conservatively trans tunnel covers + rear diff covers + rear diffuser + front wheel deflectors should be at least another .01, if the jdm spoiler+underpanels can reduce by .02 those combined should do at least as much.

not saying all this is necessarily a good bang for buck ratio, or that aero is a starting spot. but if trying to stay NA, worrying about warranty, and or wanting to push every ounce of performance out of this platform. Aero and weight reduction do add up and make a noticeable difference.

You have to include driver weight! Totally stock power/weight with half a tank of fuel is ~14.2:1 with 165 lb. driver. Lose 120 lb. and it's 13.6 lb/hp. 4% - 5% difference.
I have trouble detecting 10% at seat-of-pants, but for sure a stopwatch would notice 5% given a consistent driver and conditions.

Drag = 1/2 rho v^2 cD A. In awkward 'murican units, drag at 100mph in lb = 1/2 * (0.00237 sl/ft^3) * (146.7 ft/s)^2 * 0.29 * 20ft^2
0.29 cD, 20 ft^2 frontal area, aero drag at 100mph is 147.9 lb.
For 0.27 cD, 137.7 lb.
10.2 lb. difference in aero drag.
The horsepower freed up by reducing drag by 10.2 lb. at 100mph (146.7 ft/s) is:
hp = (10.2 lb)*(146.7 ft/s) / [(550 ft-lb/s) / hp] = 2.72 hp
Around +1.6% vs. rwhp available.

Maybe, maybe not... IMO there really isn't a lot of "low-hanging fruit" to reduce aero drag on a car that's already very slippery from the factory. Pretty much all that's easy to get has been got...

Weight reduction maybe, aero drag reduction, I still don't think you're going to find anything meaningful there.

I do appreciate the idea of doing a little here, little there, *everywhere* to get a cumulative benefit! But for me, due to laziness and being right at the weight limit for my class anyway, I just drive the car to the track and run it, pretty much... My only mods are lowering springs and camber plates and tires, but those mods are worth *seconds* per lap! Hard to get enthusiastic about spending more time and $$$ for *maybe* a tenth or two...
If I had a lot more time and tools and garage and storage space (and skill), I'd be a lot more interested in doing mods for small benefits, or even just for the f of it!

Curb weight is with full tank gas included. But I guess driver weight should be added. Also I did with 230hp crank as most people have header back + tune befor looking for more “speed” performance.

I guess I over remembered the benefit of lowering cd.

I have a garage/tools, and do all my own mods for fun as a hobby and have draggy to see before and after improvements at various acceleration points. I knew each weight + drag mod alone wouldn’t make a huge seat of pants difference going in so I did battery, trunk junk, cf driveshaft, jdm underpanels, trans tunnel covers, underdrive accessory pulleys, and fluidampr crank pulley all at same time. The accumulated effect was pretty noticeable. Probably half as noticeable seat of pants wise as my ace header + custom e-tune was.

Sorry for the late response, been a little busy.
You all wrote some interesting ideas and quite detailed information which highly enlightened me to be sure!
It seems to me though that since my car is Daily + track + mountain trips there isn't really any 1-hit wonder solution, but rather more of a combination of things, and changing the FD will not matter that much as I thought.. Guess I'll just wait out the warranty for forced induction or simply when I can't give a damn about it anymore.

Thank you all for your time and tips!