Gear Ratio Change
 

I have the automatic FRS and plan on a rear end gear ratio change.
I want as much performance as possible but keep the engine below 3000 rpm's at 70 mph. I also will be going to a better tire, not decided yet but thinking 235/40/17.
Here's what I came up with, please check my figures and correct me if i'm wrong, and please offer any advice you might have.

Stock
tires-215/45/17, 24.6 dia.
6th gear tranny, .582
rear end, 4.10
RPM @ 70 mph, 2281

Proposed changes, option 1
tires-235/40/17, 24.4 dia
6th gear tranny, .582
new rear end gears, 4.80
RPM @ 70 mph, 2693

option 2,
same tires, but 5.10 rear end gears
RPM @ 70 mph, 2861

I'm thinking the 5.10 gears would be best.

Comments, suggestions please.

Are you going to track your car? The final gear also depends on what kind of track you will use it on. Shorter gears will cause you to shift more, and the AT has tall 5th and 6th gears, so it would be bad to be using 5th and 6th a lot on the track. The 5.1 is interesting, but for my local track I top out 4th gear after the longest straight. So if I use the 5.1, I would be using 5th gear then. Shifting gears all the time also slows you down, so I look at being in 2-3-4 gears at the local track I go to. I'm using a 4.55 FD. I'm interested in looking at the 4.8 though.

Yes, my car will be a track car/daily driver. My current track car has a 6 speed sequential gear box with paddle shift. 100 shifts to make a lap at Oregon Raceway Park, so I don't mind shifting. Unless there would be some adverse effect to using 5th and 6th on the track. My two favorite tracks, ORP and Chuckwalla Valley, have 1/4 mile straights.

The big question is, is road RPM more important, or gearing properly for the track more important?

All of these numbers will be off if/when you make further changes to the car.

I don't think that is right.... stock config @ 70 mph should be turning just a bit above 3000 rpm...

Anyone mind providing a quick down 'n' dirty explanation on gear ratios? Maybe not here to keep the thread on point with OP's concern, but maybe its own thread.


My BRZ is the first car I'm actually modifying so this forum has become a wealth of knowledge. But proper gearing ratios is something I only ever see a few informed people conversing about on here. The way I see it either I'm the only willing to ask or everyone else already knows, and my money is on the former.

Shorter gears = more shifting, but more mechanical torque multiplication.

Longer gears = less shifting, but less mechanical multiplication.

It's like comparing 3rd gear power to 4th gear power, except ALL your gears are getting "lower (shorter)" with a higher final drive.

The price is your RPMs are always higher for street driving.

I just came back from town and 70 mph in cruize control was tacking just under 2300 rpm.

Proper gearing for the track is more important.

How big of a job is this/how reliable is it? I stumbled across this topic and had the thought that I could do the same thing to my manual trans for its final drive. I drive 90% on the highway at about 120km/h and hover around 32-3400 rpm which seems high over the long term. Ideally it would be under 3 imo. Any thoughts or opinions on that? Thanks

Oh whoops, automatic, duh

Will you ever go forced induction in the future? Are you going to be investing in a transmission cooler?

More shifting means more heat in your transmission (AT).

it just seems to me that its one of those high dollar/low gains kind of things that should be pretty low on your list of priorities. if you are concerned with highway driving at all in this car, it probably shouldnt be considered. dump that money in to suspension or tires or power or brakes etc and shed more time while still having a mild cruising rpm.

Yes I will be adding a tranny cooler, oil cooler, and with time wider tires, brakes, coil overs, and yes, some kind of forced induction.

Keep your stock final drive; you'll want taller gears when you're FI.

preach that ! especially on high boost levels the stock gearing gets too short; I'm fed up with it so I'm slapping a 3.73 in this weekend :popcorn:

Even with the automatic?

Yes, especially with the automatic.

Now I'm confused. I took it to the track today and spent all day in 2nd and 3rd. There is only one place at the end of the straight where I would momently need 4th. Wouldn't I want it geared to be able to use all 6 gears?

just go faster. thats usually the best way to fix that.

Why would you want to use all six gears if you didn't need to? That's just more shifting that needs to be done, which means more time that needs to be taken to shift, which means less time putting power to the ground.

And your gear usage was just for that track you were on. Say you get a final drive ratio that lets you use all six gears on that track. What happens when you go to a faster track and you're topped out in sixth gear on a straight while people go around you?

For an extreme example, consider an autocross course. The runs are around a minute long, very tight and windy, and you're usually in second and occasionally third gear. If you geared your car to use all six gears on that course, you would constantly be shifting up or down, which would make you a lot slower than the person staying in second the entire time, and potentially upset the car in a corner. Now compare that to the back straight in Le Mans, where you can genuinely max out sixth gear before you hit the end of the straight. If your car is geared for the autocross course, you're only going to be going about 60-80 mph while other cars are passing you doing 150+.

more usable gears is always good, especially in cars with narrow powerbands


ideally you'd want your top gear to max out at the point on the track where your car can reach it's theoretical maximum speed (HP vs Aero) and no more. Having more gears means you get to stay in the peak area of your power band longer.

look at formula 1


your autocross example is also not totally correct, because with more gear choice you get to choose the best powerband for corner exit. Time spend shifting can easily be made up by having the car in it's powerband 100%. Instead, we often sacrifice power-out by staying in 2nd gear because dropping to 1st will yield too little time spent under the curve. By having more gears you can accommodate more corners.


i mean ideally you'd want an 8-9 gear auto DSG box.. then you'd never even know the car was shifting gears.. it would just always be in the best place.

the gears in our cars are made entirely for economy. Our 6th gear tops out at like 180mph or something, the car would never even reach those speeds on any race track without maybe some serious drafting.


check out the Cusco Final Drive thread to understand why.

Except in an automatic it would be shifting constantly. You'd accelerate through a corner and it would drop a gear, then shift up a few seconds later, then drop again for the next corner, then back up, then up one more as you enter a small straight, then down two for the sharp corner at the end, etc... Plus autos shift in response to your actions, not in anticipation of your actions.

I would love a tightly geared sequential manual for autocross. You'd be able to set up for the next turn, not have the transmission react once you're in the turn. And you'd only have to worry about up or down, not fighting with shift gates. Or an aggressive CVT that would keep the car at 5.5K+ the whole time.

our powerband is roughly 1k... I'd even say 800rpm


here is random dynoplot I pulled off google


http://image.modified.com/f/projectc...no-results.jpg


1st to 2nd gear the car drops 3500 rpm, that puts you roughly 20-25 horsepower below had the shift only dropped you 1500


ohh whats that, a turn coming up and you're not even back to your powerband while I'm already in it? and we need to brake a little? Guess who is going to power out of it faster?


if c-stock allowed a final drive change I'd be over that cusco unit like white on rice.


especially in autocross where you have no idea what kind of corners are going to be thrown at you, that extra shift in some long straight will be outweighted by being able to power out of the previous 4-5 turns with more power than the other guy.

here is a "long" powerband car... a VW TDI


this is where your gearing barely matters, you'll pull at any place.


http://www.malonetuning.com/misc/dyn...e-tdi-run3.png

I managed to kill my previous post; not sure how I did that.

I see at least 2K of usable power there; from ~5K to 7.4K rpm.


That's building a car to be competitive in a certain class under certain requirements though, which is not what OP was talking about.

there is always power...... :bonk:


and it's all usable... even 1hp is usable power. :bonk:




but the area under the curve between 6200 and 7200 is far greater than 5200 and 6200

right now for slow speed autocrosses we have a choice, either stay in 2nd for whole track or drop to 1st for some corners. Both options SUCK because we either drop out of our powerband and lose 20-25% of our peak engine power or we drop into 1st to hang in it for split second (not to mention upsetting the car)


for fast speed autocross we have the same thing but only 3rd and 2nd


but if we had 4 gears for the same speed spans then we'd have better ability to mesh with the course.


so instead of hanging at 5000-6000 rpm coming out of a turn, bogging down, you'd hang in 6400-6500 and power right out with 20 more horsepower under your foot.

If it's good enough to get a horse up to speed, it's good enough for me. :D

It turns out that I didn't look at the X axis of your graph; I thought the major increments were 1000 RPM, not 400 RPM. I would like to amend my answer to 6400, not 5200.

Agreed, but putting all six gears in to cover the same range of speed effectively ruins the car for anything beyond that speed, which was my original point in response to the OP's question that I had quoted. We've kind of gotten off on a tangent.

Now, if I were building a trailered car purely to win in a certain class at a certain level, then yes, I would do that.

agreed, 6 speeds for a 80kph span would be overkill (unless, of course, it was somehow automated).

I see your point.

I would be driving in manual mode with the paddles. I am used to this having a track car with a 6 speed sequential gear box. To keep it in my torque range between 6 and 11,000 rpm one lap at ORP would be 100 shifts.

Good info, keep the suggestions coming. I am not trying to dispute anyone, just trying how to figure this out and learn the differences between my old track can and my new FRS.

i still dont see how an auto frs would be something to upgrade from something with a sequential gearbox

You'll need to identify how fast your courses are and where on the rev range you're in at each corner. This is something we can help you guys out on if you can send us data. ;)

It's not an upgrade, but something the wife can drive.

If you want to see my real track car look at


https://www.facebook.com/pages/Lunat...75620119162747