Final Gear Ratios
 

I know this may not be the right subforum, but I had a question after Jay Leno mentioned it.

I have no need (anymore) to go 130+ in my car. What everyone seems to be seeking is more acceleration. Instead of bolt ons, Cai, etc, couldn't you change the final Gear ratio to achieve a little more acceleration while sacrificing top end?

Just a layman's (and Jay Leno's lol thoughts)

Oh, and first post, love it here!

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Increasing the ring and pinion ratio will sacrifice top speed in favor of acceleration. Like you said though, you don't really need to run at top speed. However, it will raise the RPM for a given speed. So if previously you were cruising at 70 mph at 3K RPM, now you might be cruising at 70 mph at 3.4K RPM. That kind of thing.


It's a pretty popular mod for the A/T guys; there are plenty of threads about it.

Oh! So a little more work for the engine and worse gas mileage?

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You'll gain in feeling, but not on actual results. You'll accelerate stronger in each gear, but overall you'll use a higher gear (3rd instead of 2nd, 4th instead of 3rd, etc). How often it depends the final drive ratio. We discussed it many times and you can search the forum history. If you need more acceleration, the best approach is to have more power or less rotating mass.

Yep. As with most vehicle mods, it's a trade-off. It's supposed to make a decent improvement in acceleration though, so for some people it's worth it.

So for the power I want, it's about 5-6k plus odd bits I don't know about.

So what's this rotating mass you're talking about?

I promise to do more searching, but I seem to find posts that only partly answer my questions. I'll work harder!

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Rotating mass is everything that rotates due to the engine turning. Engine internals, pulleys, the flywheel, transmission internals, driveshaft, diff internals, axles, brake discs, wheels, and tires. These all represent a significant amount of weight, and power spent turning all these things around is power that could be going to the ground to move you forwards. Obviously, some things you're not going to be able to change. However, you can lighten the pulleys, flywheel, driveshaft, brake discs, and wheels pretty easily. Less weight requires less power to move the weight and allows more power to be put to the ground.


But there are tradeoffs. A heavy (OEM) flywheel has a lot of inertia, so when you push the clutch in to shift, the RPMs drop slowly instead of plummeting. This makes for smoother shifts with less fucking up. A lighter flywheel has less mass, so less inertia. Now when you push the clutch in the RPMs drop like a rock. Your shifts need to be on point, every time, or they're going to suck.

Excellent explanation of the tradeoffs of rotational mass reduction. This will theoretically net you faster acceleration, however there are many other factors involved. The same goes with changing the final drive, you will accelerate faster at lower speeds with a higher final reduction ratio since you will be in the power band sooner, but you also sacrifice acceleration at higher speeds when you're stuck in 5th or 6th.

Edit: For Automatic owners this is usually a very good tradeoff, since it makes the longer gearbox ratios more similar to the manual in terms of acceleration. Since 6th is so overdriven in our tranny, it's really only useful for cruising and offers very little in terms of acceleration since you're near the top speed of the car well before redline in 5th with the stock ratios.

Agreed. There are tradeoffs to everything. I love my drivetrain bushings; they remove a lot of slop out of the drivetrain. But at the cost of higher NVH. I'm ok with that, but it's good to know what the cost will be before committing to something.

SO what are you really wanting? Be specific so you can get sound advise.....
I have run just about every ratio you can think of.
Just remember you cant have both ways. If you want a gear to race light to light on the street or to give you the ability to drift the car with ease no problem but you wont be going 140mph anymore more like 120mph to get those results but a lot goes into picking the right ratio. Tire size, weight, RPM's, Manual or auto trans, just to name a few..

Thanks Justin...

I don't have it 100% decided overall, but here's what I've got so far. All stock 6mt, with possibly a cat back and some lesser performance mods at most.

Maybe 115, 120MPH top speed (depends on how much it helps acceleration), but I've gone way over 150 plenty of times, and having a son now, that no longer interests me. Also, I'm not going to be "stop light to stop light," but I guess you could put it that way. I can't see on this phone if I've answered all your questions... If not, follow up coming

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I would like to replace my wheels and tires at some point, but sticking with 17's. And just to be clear, we are talking rear end gear right?

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I would run a 4.56 ratio then it will still let first gear be usable and have a lot more bottom end. 120mph will be easy to reach in 6th 5th gear will max out around 104mph. The most I would do is a 4.88 but that is a extreme street gear and first is pretty useless.
When your ready to get a set call Mark at spencer fab in N.H. he will hook you up.

Yes rear diff gears,
I ran a 13.58 1/4 mile with a 4.56gear


[ame="https://www.youtube.com/watch?v=o6uvBexBYX8"]Copy of 13.58 FRS Pass - YouTube[/ame]

I'm in love! What mods do you have? So I'd call him, get the gear, and have someone locally (not me!) install it? Before I bug him, ballpark what might the gear cost?

Oh, and any changes to the speedo? Seems like there would be.

Thanks so much! *Exactly* what I'm looking for!

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So something like a light weight crank pulley?

Paging @justatroll

I've been leery of that, I don't know why exactly. Partially I can't imagine it doing much good. Like I know anything though

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"Crank Pulley"???!!??

Slooowly I Turned.... STEP BY STEP :bellyroll:

Hey Justin, long time to talk :)

Why 4.56 ratio gives a so small top end in 5th gear? I was thinking 5th gear would give something closer to 120mph @7400 rpm. I am using a gear ratio calculator: http://www.apexgarage.com/tech/gear_ratios.shtml

Is the actual speed much lower than the theoretical? I have a 3.7 FD and in 5th I max out around 145mph. In my car, the actual speed is the same with the theoretical.

Here is also a picture with the calculations:


http://i67.tinypic.com/10xzllx.png

We said less rotating mass in general. If you read the complete thread, you 'll find specific examples. First step is to use a much lighter set of wheels and tires. If you also question this, then you don't know what you are talking about.

what other mods do you have to run that time!?

Ya I screwed that one up what I meant was 4th topped out not 5th..

:thumbsup:

This is the list I have to run 13.58


1: PPE merge collector header and overpipe
invidia N1 full exhaust


2: Airaid CAI


3: Delicious tuning Flexfuel kit and STG 2 tune....


4: 4:56 gears


5: Hoosier drag radials.... That's it


6: You must have no fear and worries about breaking your car!!!!!
I try to break it every pass.... Launching at 8000 RPMS and dropping the clutch.


good luck




and no I will not tell you how I went 13.47 because I plan to go back to N/A and smash the current record.




NOTE All of these times the car had full interior and A/C

e85 tune, 91 or 93 tuned?

E85

DR. BRZ My tuner flew in for the weekend to go to the track with us you should come down to San Antonio raceway park and hang Friday and Saturday...

And a stock crank pulley?
Headlights on or off? (cuz that makes a bigger difference than a CF driveshaft in a 1/4 mile) :)