Final Drive: 4.56 vs. 4.88

[Foulsmell]

Originally Posted by nikitopo

Your statement doesn't make sense. Auto tranny has bigger power losses and a much smaller top speed (130 mph vs. 140 mph of the manual tranny). You cannot get an improvement on top speed by changing the final drive to a shorter ratio. This is a well know fact valid for ages. Did you buy the car second hand? Your car should also have some other mods.

What he means is he can reach the that speed easier due to the increased acceleration. For example he doesn't run out of road or hit traffic.

[ZDan]

Gears are an overrated mod. If you want more torque to the drive wheels at a given speed, go to a lower transmission gear, i.e. delay upshifting. People have the idea that lower (numerically higher) gearing improves acceleration everywhere. It doesn't. You are forced to upshift sooner and put yourself in a taller transmission gear, and at that point you are accelerating slower vs. stock gearing in lower trans gear.

[Foulsmell]

Originally Posted by ZDan

Gears are an overrated mod. If you want more torque to the drive wheels at a given speed, go to a lower transmission gear, i.e. delay upshifting. People have the idea that lower (numerically higher) gearing improves acceleration everywhere. It doesn't. You are forced to upshift sooner and put yourself in a taller transmission gear, and at that point you are accelerating slower vs. stock gearing in lower trans gear.

Nonsense.

4.1 to 4.67 up shift is 300 revs sooner which equates to around 4 mph it's not a factor.

It took 2 whole seconds off my 30 - 90 mph time.

[nikitopo]

Originally Posted by Foulsmell

It took 2 whole seconds off my 30 - 90 mph time.

Yes that's true. But if you could measure 90 - 130 mph time (before and after change), you would see that now you are 2 whole seconds slower. So, you just have a shift of your power band to the lower speeds. That's the whole idea.

I don't blame this kind of modification. In fact it gives very good results if you're doing auto-x or tracking in small tracks. It also gives a better feeling and response in driving within town.


Personally, I have a 3.727 FD and on high speeds the car is very very strong. I was running lately side by side with a bmw 535d (313PS, 630Nm) and bmw couldn't get away. Reason is that my car was pulling using 4th and 5th gear instead of using 6th gear.


So it depends the application ...

[bfrank1972]

Originally Posted by Foulsmell

Nonsense.

4.1 to 4.67 up shift is 300 revs sooner which equates to around 4 mph it's not a factor.

It took 2 whole seconds off my 30 - 90 mph time.

In a nutshell, lower gears will benefit you from lower rpm starts, especially on cars like ours with the torque dip. What rpm did you start your 30-90 run? Lower fd gearing will get you into the power band (through the torque dip) faster, which is a tangible benefit for street driven cars over higher geared cars. To zdans point, what rpm did you end at when you hit 90mph? If near redline, then your gearing was optimized for a run to 90 mph. But you'd lose that advantage as you up shift and a higher geared car stays in a lower gear. It's why we rate cars with hp (rpm dependent) rather than just torque.


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[Toyota86.ir]

MT Has perfect gearbox ratio but AT are so long in 3 and 4 and 5.
the acceleration after FD swap depends on speed range.
for example acceleration in 0-176km/h in AT 4.88 is 5% better than AT 4.1 but acceleration from 0-200km/h in AT 4.88 is equal to AT 4.1

average result in all speed range give you maximum gain of 5% after FD swap even if you use 40% higher FD ratio.
you can imagine it with considering the FD 4.88 goes sooner to next gear and acceleration drop after shifting to next gear . stock FD 4.1 has better acceleration before shifting to next gear but after shifting to next gear the FD 4.88 will give better acceleration.

[Foulsmell]

Not sure of the rpm but it was started in 2nd gear

I still have to disagree on this. As I gather you don't have one fitted. Maybe you have experience of driving one but will not of done a specific test.

If we were to run the test till the OEM geared car had to change then 4.67 FD car will surge past.

The argument is flawed

At the end of the day no one bought this car to do drag runs because it's slow.
It's all about the cornering and general handling.

For real everyday fast road driving FD is a great option and will cover ground better than an OEM one.

[bfrank1972]

Originally Posted by Foulsmell

Not sure of the rpm but it was started in 2nd gear

I still have to disagree on this. As I gather you don't have one fitted. Maybe you have experience of driving one but will not of done a specific test.

If we were to run the test till the OEM geared car had to change then 4.67 FD car will surge past.

The argument is flawed

At the end of the day no one bought this car to do drag runs because it's slow.
It's all about the cornering and general handling.

For real everyday fast road driving FD is a great option and will cover ground better than an OEM one.

Actually there is no argument, it is physics. When I get some spare time I will illustrate here backed by mathematics.


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[Foulsmell]

Originally Posted by bfrank1972

Actually there is no argument, it is physics. When I get some spare time I will illustrate here backed by mathematics.


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I'll look forward to be educated 😂

[skye67]

My point was that by changing the rear end gear ratio It will get to 135 a whole lot faster than the stock car would.

[Foulsmell]

Originally Posted by bfrank1972

In a nutshell, lower gears will benefit you from lower rpm starts, especially on cars like ours with the torque dip. What rpm did you start your 30-90 run? Lower fd gearing will get you into the power band (through the torque dip) faster, which is a tangible benefit for street driven cars over higher geared cars. To zdans point, what rpm did you end at when you hit 90mph? If near redline, then your gearing was optimized for a run to 90 mph. But you'd lose that advantage as you up shift and a higher geared car stays in a lower gear. It's why we rate cars with hp (rpm dependent) rather than just torque.


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4000 rpm @ 30 mph 2nd gear to 6700 @ 90 4th.

[bfrank1972]

Put together a quick spreadsheet - the number are all hypothetical and simplified, assuming torque curve is completely flat from 0 to redline. Left has a final drive of "4", right has a final drive of "4.5", significantly greater than the left. Notice the distances and the delta at the far right. The lower geared car surges forward, but shifts up and the higher geared car passes it - they continue to swap positions the whole way. In real life, a lower FD would have some advantages, particularly moving the car through the torque dip into the upper power band in 1st gear, but results will diminish after that. Your scenario was ideal in that you ended at near redline. If you timed yourself to 110 or 120 you'd find a stock geared car would probably cut the time advantage you saw to very little. The 'oscillation' back and forth regarding which car is in the lead is magnified in higher gears. Also keep in mind, lower gears amplify effects such as shift times, rotational inertia, and frictional losses - albeit these differences aren't that huge considering the gear ratio changes we're really talking about.

https://drive.google.com/open?id=0Bw...UZhdWxHaWFleEE

[swarb]

@bfrank1972 thanks for putting that together. Can you open access or just post a pic of the data?

In the end, it is track dependent and most of us aren't drag racing or reaching the top of 5th(with the 4.10's). That extra shift where you lose time is probably at the end of a long straight. I would personally choose the extra torque response gained at every corner. It's track dependent of course.

Kind of reminds me of drag racing, et vs mph, at a certain point higher final drive means extra shift and/or less mph and eventually slower et.

[Foulsmell]

Originally Posted by swarb

@bfrank1972 thanks for putting that together. Can you open access or just post a pic of the data?

In the end, it is track dependent and most of us aren't drag racing or reaching the top of 5th(with the 4.10's). That extra shift where you lose time is probably at the end of a long straight. I would personally choose the extra torque response gained at every corner. It's track dependent of course.

Kind of reminds me of drag racing, et vs mph, at a certain point higher final drive means extra shift and/or less mph and eventually slower et.

This is exactly my point. Even on a shorter track the FD would be faster due to the better exit speed and unless there was a very long straight the OEM wouldn't catch up.