When "better" gearing is slower

[ZDan]

Originally Posted by RedReplicant

Yeah, something weird is going on there with both. The GPS (AIM) accel probably makes more sense to look at in this case, unless it isn't tightly mounted to the car.

Yeah, this makes more sense...

[ZDan]

Originally Posted by Goingnowherefast

Wait hold up, we have to mention that this change isn't JUST gearing and must be taken with a massive grain of salt. More things change when moving tire sizes. Mainly, the shape of the tire changes as well as weight. Thus, pressures will have to be optimized for both.

Claiming that this data is due to gearing only is not true.

I never stated that "this data is due to gearing ONLY".

FWIW A052s work well over a pretty broad range of pressures. 245/40-17 and 235/45-17 aren't such radically different sizes that "optimal" pressures should be all that different. FWIW I was running them at 35psi hot.

You are right that it's not *only* a test between overall gearing. The biggest difference is that while the 235/45-17 wheels+tires weigh the same as the 245/40-17 wheels+tires (both sizes = 16 lb. wheels + 22 lb. tires), the shorter 245s have to rotationally accelerate slightly quicker, as at the same linear road speed they rotate/spin faster. But this can be calculated to be a *tiny* effect on total acceleration.

Should also note that while I ran both 235/45-17 and 245/40-17s at the back of the car, I had 245/40-17s up front the whole time. So any *exceedingly minor* differences due to "different optimal pressures" betweed 235 and 245 (I'd put this at round about 0.0seconds), and the shorter 245s having to spin up more for same road speeds (maybe 0.04seconds?) would be cut in half vs. running 235/45-17 square vs. 245/40-17 square...

[EAGLE5]

I think it's a pretty big "it depends". If you can shift perfectly, the delay of a shift is less. If the gearing is just right, you can keep the engine at peak power more of the time, but simply changing the final drive isn't the same as gearing perfectly for your situation, be it the race track, drag strip, or road.

On the street, I'd prefer a far shorter final drive ratio. If I hit 5th gear at 80mph, I'd have a lot of fun getting there.

[nikitopo]

Originally Posted by Petah78

Fact is, shorter gearing will put down more wheel torque, gear for gear. More torque = faster acceleration = more fun. On the street, I will only hoon up to the end of 2nd and shut it down. I dun care if end of 2nd is 55mph or 65mph. The fact that I can feel more torque and accelerate faster to the end 2nd = more fun. But that is me and I can't say it applies to everyone.

Originally Posted by ZDan

It doesn't apply to me, I still don't get the idea that it's somehow more fun to accelerate slightly harder to a slightly lower max speed in a given gear. Fun for me has been more related to having the right ratios for a given track at 10/10ths, or for a given set of backroads curves at 6/10ths but in that case I'm not generally revving it to redline every time anyway so gearing not really critical...

It doesn't apply to me either to constantly adjust settings on a race track, because after a point this is what you are looking for. Have the perfect gearing for the certain track, have the perfect suspension adjustments, have the perfect tire pressures and so on. For me it makes more sense to go into a nice road and just drive, even though it is pretty clear that I'll never drive at 9/10ths or 10/10ths.

One additional point I would like to make is that some of us don't want to go catless and make the car illegal (i.e. toxic) just for fixing the torque dip. So the other solution for 2nd gear is to increase the revs, using a shorter final drive ratio, and be more time on engine's rev sweet spot.

[ZDan]

Originally Posted by nikitopo

For me it makes more sense to go into a nice road and just drive, even though it is pretty clear that I'll never drive at 9/10ths or 10/10ths.

At 7/10ths or less, diff gearing isn't going to matter all that much. Whatever the diff gearing, at a given speed you'll have options for a more "revvy" transmission gear to be in vs. "normal" cruising gear.

One additional point I would like to make is that some of us don't want to go catless and make the car illegal (i.e. toxic) just for fixing the torque dip. So the other solution for 2nd gear is to increase the revs, using a shorter final drive ratio, and be more time on engine's rev sweet spot.

This is no solution. Changing diff gearing doesn't magically give you "more revs" everywhere. At a given speed, you may have slightly more engine revs, or you may have significantly lower engine revs vs. base diff gearing. In either case the rev drop between transmission gears is the same, so you don't get "more time on engine's rev sweet spot".

[nikitopo]

Originally Posted by ZDan

This is no solution. Changing diff gearing doesn't magically give you "more revs" everywhere. At a given speed, you may have slightly more engine revs, or you may have significantly lower engine revs vs. base diff gearing. In either case the rev drop between transmission gears is the same, so you don't get "more time on engine's rev sweet spot".

I get more time on engine's rev sweet spot. Imagine that I need to accelerate after a slow corner. Let's say from 36mph. Before I had to accelerate from 4000 rpm in 2nd gear (FD 3.7). Now I accelerate from 4700 rpm in same gear (FD 4.3), which is over the torque dip. If I rev to redline and switch to 3rd gear, then I am above 4900 rpm which is again after the torque dip. So the improvement is really in slow corner exiting. The slower the corners, the shorter the FD ratio is needed. Not a coincidence that rally cars go for a FD 5.0 or 5.3 ratio

[ZDan]

Originally Posted by nikitopo

I get more time on engine's rev sweet spot. Imagine that I need to accelerate after a slow corner. Let's say from 36mph. Before I had to accelerate from 4000 rpm in 2nd gear (FD 3.7). Now I accelerate from 4700 rpm in same gear (FD 4.3), which is over the torque dip. If I rev to redline and switch to 3rd gear, then I am above 4900 rpm which is again after the torque dip. So the improvement is really in slow corner exiting.

The whole point is you can just as easily pick corner speeds that favor the taller final drive ratio.

But anyway, from 36mph you could be in 1st gear with the 3.7 diff and be at 6700rpm and have a *huge* power advantage over having to be in 2nd at 4700rpm with the 4.3. Yeah, you have to upshift at 40mph but to 90% of the fans of shorter diff gearing isn't that supposedly a benefit?!

So with the 3.7, pulling from 36mph, you get to have higher revs, making way more power, and you get to shift sooner, sounds like all the benefits that most ascribe to "shorter gearing" to me!


My point has never been that shorter gearing is "better" or "worse", it's just that it is entirely dependent on what speeds you're accelerating from and to. Particularly for street cars being driven at (I hope!) no more than 7/10ths, if you want to rev higher, be in a lower gear. Sometimes you'll be "better off" with shorter diff gearing, and sometimes you'd be better of with taller diff gearing. For overall performance over a broad range of speeds, it is a *wash*.

[nikitopo]

Originally Posted by ZDan

But anyway, from 36mph you could be in 1st gear with the 3.7 diff and be at 6700rpm and have a *huge* power advantage over having to be in 2nd at 4700rpm with the 4.3. Yeah, you have to upshift at 40mph but to 90% of the fans of shorter diff gearing isn't that supposedly a benefit?!

So with the 3.7, pulling from 36mph, you get to have higher revs, making way more power, and you get to shift sooner, sounds like all the benefits that most ascribe to "shorter gearing" to me!

Issue with 1st gear is that it is not close to the rest of the gears, so when you change gear at 7100 with 39 mph you drop again in 2nd gear at 4300 on a 3.7 diff. So, not that much of an advantage given additionally the extra gear change. There is a Cusco transmission gear kit that moves 1st and 2nd gears closer to 3rd gear, but I didn't want to take the risk to open my transmission. It was much easier to swap a 4.3 rear diff from the new cars

[steverife]

I had a Mustang and swapped from a 3.27 FD to a 4.10 and it was pretty much like doing an engine swap or getting a new car.

But yeah, a nominal tire diameter change for an application where you can keep the car in the power band isn't a huge deal. I've never really heard otherwise.

[ZDan]

Originally Posted by nikitopo

Issue with 1st gear is that it is not close to the rest of the gears, so when you change gear at 7100 with 39 mph you drop again in 2nd gear at 4300 on a 3.7 diff.

Why not change at 7400?

So, not that much of an advantage given additionally the extra gear change.

But still, so many who want to do "gears" want it for a) "higher revs!" and b) "more shifting!". Well here you go, higher revs and more shifting with the 3.7 vs. 4.3!

But again the larger point is you can *always* find a speed range where you're "better" with one gear ratio vs another, and you can always find a counter-example at a different speed where the advantage is swapped around!

For me, wider 1-2 shift is fine as it allows for easier/smooover engagement from a dead stop with the 1 ratio and the 1-2 shift is zero priority for me as I'm pretty much never "on it" at 10/10ths in that speed range.

Curious:
Why did you go to 3.7 in the first place?

[timurrrr]

Originally Posted by ZDan

the 1-2 shift is zero priority ... never "on it" at 10/10ths in that speed range.

Hello from autocrossers

Originally Posted by ZDan

Curious:
Why did you go to 3.7 in the first place?

+1, I think the 3.7 FD negatively affects the gearing, not positively.
It might make one gear better than stock, but you can achieve the same result with a shorter FD in a different gear but also have other gears utilizing the torque curve better for street driving.

[nikitopo]

Originally Posted by ZDan

Curious:
Why did you go to 3.7 in the first place?

It was the default final drive ratio of the BRZ "RA" grade in Germany. It had better fuel economy and a 159g/km CO2 which was quite low for a sports car at the time. The more expensive grade with the 4.1 ratio and the Torsen diff (the 3.7 option was open diff) had 180g/km CO2. In general the RA grade was a bit cheaper and had the mindset that you'll take it and modify it more.

[ZDan]

Originally Posted by steverife

I had a Mustang and swapped from a 3.27 FD to a 4.10 and it was pretty much like doing an engine swap or getting a new car.

Gimme your trans gears, tire diameter, and power vs rpm and I promise I can show you speeds from which you'd accelerate quicker with the 3.27 gears! But I wouldn't dispute that you were better off overall with 4.10 gearing vs. 3.27!

I think that the FT86 with stock 4.1/4.3 diff gearing is *much* closer to the sweet spot than yer Mustang was with a 3.27. I had a '95 Z28 M6 with 3.42 gears and that was way too tall for that heavy a car, even with low-revving torquey 90s LT1...