BRZ 2018 AT to MT gear ratio

[Septor]

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?

[ZDan]

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...

[86TOYO2k17]

Originally Posted by ZDan

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.

[86TOYO2k17]

Originally Posted by Septor

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?

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.

[ZDan]

Originally Posted by 86TOYO2k17

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.

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.

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.

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.

Also most of the drag reduction for this car also adds downforce.

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

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.

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

[86TOYO2k17]

Originally Posted by ZDan

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 10mph, 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.

[ZDan]

Originally Posted by 86TOYO2k17

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.

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.

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.

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.

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.

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...

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.

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!

[86TOYO2k17]

Originally Posted by ZDan

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.

[Septor]

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!