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Engine, Exhaust, Transmission Discuss the FR-S | 86 | BRZ engine, exhaust and drivetrain.


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Old 06-13-2019, 07:11 AM   #1
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Higher final drive ratio and increased engine braking

So logic suggests that a higher FD ratio will increase engine braking. Has anyone noticed this, specifically in terms of a trail braking effect (i.e when you lift off going into a corner, does the car rotate better/ easier?)

This could be a massive added bonus for me of changing the FD.
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Old 06-13-2019, 08:33 AM   #2
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It's relative to gearing-RPM-road speed and "stuff".

You would be better off working out you final drive to suit the track, more so than worrying about engine braking.

Playing with final drives is tricky, do you have your FD so you are near redline in 6th at the end of the main straight, or have it so you can punch through turn 3 into 4 in 3rd without downshifting, or, shortshifting up and coming off the boil (the torque happy place).

In the end it's a balance of where you want the 6 in your hand to be at certain a RPM and road speed.

I would take some notes IRT where and why you want to change your FD, then work from there.

Smarter people than me, well some anyway...... https://www.ft86club.com/forums/showthread.php?t=45152
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http://www.ft86club.com/forums/showthread.php?t=133311
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Old 06-13-2019, 09:35 AM   #3
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It will be track dependent. If you need more rotation just tap the brakes.

Last edited by TommyW; 06-13-2019 at 01:29 PM.
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Old 06-13-2019, 10:04 AM   #4
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I'd say that change of braking won't matter much. Changed FD will just move gears down a bit, so that more of them can be used to stay in powerband. You still won't overrev past redliine, and still probably will keep engine in effective powerband above low rpms, downshifting when needed, so roughly estimated braking from engine's pumping losses and friction at those similar rpm-s will be somewhat same, just at other gear. Yes, at some preset speeds you might be at different gear (that matched up with changed FD will provide somewhat same overall gearing) so slight difference in gearbox frictional losses & rear diff/fd frictional losses, and rpms at specific track locations may differ due different shifted gear switch locations but in general - difference meager enough to not care/bother.
Main bit one should care, if specific FD gearing for specific track layout will allow to accelerate faster due more usable gears and/or can reduce gear switches due more track coverable at some gears in optimum powerband, or if laptime will be lost due extra gear shifts needed, or they being at unfavorable places and such.
Track aside, for DD most favor FD change for NA power twins, enhancing acceleration/subjective feel of it, at some drop in fuel economy & increase in noise due higher rpm-s.
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Old 06-13-2019, 10:14 AM   #5
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I noticed no difference though I made a fairly small change going from 4.1 to 4.3.
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Old 06-13-2019, 11:42 AM   #6
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I noticed higher engine braking going from the 4.1 to a 4.88. It's definitely obvious! I also put in an MFactory 1.5 way LSD at the same time..
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Old 06-13-2019, 11:50 AM   #7
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A *big* change will indeed be obvious *in the same gear* and *from a given speed*. However *overall*, it's a wash, as there will be some braking zones where you will be in a higher transmission gear, which will *reduce* engine braking significantly.

Overall it's gonna be a wash and IMO any off-throttle effects should NOT drive one's choice of diff gearing, at all.

Also note that the "benefits" of gearing are often tremendously overstated anyway. Gearing doesn't magic you any better power-to-weight, and overall acceleration won't be improved. zero-to-XX mph times should improve, but from any rolling speed it 100% depends on the starting and ending speeds. If shorter (numerically higher) gearing requires you to be in a taller transmission gear from a given speed, you will obviously accelerate more slowly from that speed.

Optimal gearing for track use will be determined by what speed you're going out of the most critical corners. "More" isn't always going to be better...

Last edited by ZDan; 06-13-2019 at 12:50 PM.
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Old 06-13-2019, 12:22 PM   #8
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fika84: but is engine braking in eg. 4th @4.1 vs 5th @4.88 that different? With different FD great chance for you to be at different gear at specific speed ..
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Old 06-13-2019, 12:27 PM   #9
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Quote:
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fika84: but is engine braking in eg. 4th @4.1 vs 5th @4.88 that different? With different FD great chance for you to be at different gear at specific speed ..
From what I've noticed, every gear has increased engine braking. Obviously the higher the gear, the lower the engine braking because the lower the torque. But increasing torque 15+% has the same effect on or off gas.

I find myself not needing brakes to slow down behind traffic nearly as much as I used to.
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Old 06-14-2019, 07:26 AM   #10
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Thanks all. I wouldn't base my choice of FD on its effect on engine braking. I do sprints and hillclimbs (1st and 2nd gear only with the occasional very short shifts into 3rd), so lots of tight stuff. I often lift off into corners not to slow down, but just to shift weight to the front to avoid understeer, so like I said, higher FD could be a nice added bonus if it helps rotation as well as increasing torque for the hills.
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Old 06-14-2019, 08:41 AM   #11
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Why not use slight brake touch for even more mass transfer for more rotation then? Imho simpler to learn or adjust driver inputs then mod car in this case, as former will be applicable in other cars too, but later costs money and not without cons.
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Old 06-16-2019, 12:25 AM   #12
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Thanks all. To be clear: I wasn't going to change the FD to help rotation, I was just wondering if it might be a desirable side effect.
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