Originally Posted by JOE APO (Post 3004449)

Has anyone attempted to swap out their stock differential with the new 2017 rear diff? I've been thinking about doing this to mines due to the more aggressive gear ratio,but i'm not sure if the new diff has different dimensions that would cause some sort of issues. If you've tried this or have some insight, comment below. Thanks guys!:party0030:

Originally Posted by stevesnj (Post 3004504)

Final drive is a great way to go. It's a great analogue upgrade that is simple and effective as is the trans gear upgrade they did in the 2017's. The combination should net noticeable results. The 2017's have 4.33 gears now I believe. Not everyone needs to go FI. What is your goal with the car?

Originally Posted by mrg666 (Post 3004519)

Originally Posted by JOE APO (Post 3004449)

Has anyone attempted to swap out their stock differential with the new 2017 rear diff? I've been thinking about doing this to mines due to the more aggressive gear ratio,but i'm not sure if the new diff has different dimensions that would cause some sort of issues. If you've tried this or have some insight, comment below. Thanks guys!:party0030:

Originally Posted by stevesnj (Post 3004523)

What don't you understand?

Originally Posted by Teseo (Post 3004552)

Watch "BRZ 4.1 to 4.88 Final Drive Thoughts" on YouTube
https://youtu.be/UZwglkOkom0

I'd really want to do the swap but on my opinion price keep me away

Originally Posted by nextcar (Post 3004538)

If you can get one out of a totaled car, great. Otherwise it can be much cheaper to change gears in your existing differential...

Note: the 2017 only went from 4.10 to 4.30, and only on the manual cars.

This small of a change is very, very subtle. I went all the way to 4.56 and I sometimes wish I had gone to 4.88 on my manual daily driven car. I personally would not think the change to 4.30 is worth the expense.

Gas milage takes a very small hit. With AC off, level freeway driving I still get 32 US MPG. Overall I average 26 mixed city/highway.

Engine turns faster, but benefits from mechanical advantage - e.g. it may actually be under less stress.

Originally Posted by gtengr (Post 3004540)

An extra ~165 rpm at 80mph in 6th is negligible. Top speed is drag limited, so the gears will actually help in that department.

Originally Posted by mrg666 (Post 3004564)

Top speed is determined by the combination of power, drag, and gear ratio. Since drag is proportional to square of speed and power is a function of rpm, obviously, power will be balanced with drag force at a certain speed. If that limiting speed balancing power and drag is below the theoretical speed determined by the gear ratio and engine specs, the top speed will be limited by drag. Do you have the numbers and equations to back up your claim for our cars? It will not be as easy as saying 3000x4.1/4.33 -3000 ~ 165 rpm.

Originally Posted by mrg666 (Post 3004501)

Final drive replacement is a tradeoff, not really a power upgrade. Your engine will be working harder to do the same work. Acceleration will be quicker but sustained speeds will be at higher rpm. You might like it in the city since the car will feel quicker. But freeway driving will be worse; engine will be running at higher rpm on freeway, it will be noisier, and gas mileage will be lower. Even on track, you will be compromising your top speed for acceleration.

You want more power. So the right solution is getting more power. I recommend one of the followings.
- header + tune
- forced induction

If I had the 2017, I would go back to 2016 final drive and upgrade with FI again.

Originally Posted by gtengr (Post 3004577)

Yes, I have numbers and equations to back up my claim.

Originally Posted by mrg666 (Post 3004589)

Where are they? Show that 4.1 to 4.3 final gear ratio change will increase the top speed.

Edit:
Here is what Topspeed writes:
"Subaru has yet to release any details on actual performance, but for automatic-equipped models, expect the same 6.7-second sprint to 60 mph and a top speed of 143 mph. On models with the six-speed manual, the 0-to-60 time probably drops down to 6.6 seconds, but with the smaller final drive, the top speed will probably decrease by a couple ticks to 141 mph. Of course, the BRZ is still rear-wheel drive, so at least it has that going for it."

I won't go into say what this means, tired of nonsense here.

Originally Posted by CSG Mike (Post 3004585)

I swapped my stock 4.3 diff for a 4.1 diff.

Originally Posted by Teseo (Post 3004630)

The car with the supercharger?

Originally Posted by gtengr (Post 3004629)

Subaru claims the top speed for the US car is 216 kph (134 mph).

Originally Posted by gtengr (Post 3004629)

I'll take Subaru's published numbers over something you were able to scrounge up in a biased internet search. And that journalist you're quoting is incorrect. Topspeed is in fact contradicting themselves because in their test of a 2016 BRZ they claim a lower top speed than that of the 2017. How can they say the 2017's gearing would knock the 2017 down from 143 to 141, when their claim for a 2016 is 139?

And all of those numbers disagree with what Subaru has published. Subaru claims the top speed for the US car is 216 kph (134 mph). It's drag limited by inspection because there are still rpm's left in the motor's operating range at that speed. Higher numerical gearing = higher rpm in the powerband during a drag limited top speed run, and that means the engine is making more horsepower and can overcome incrementally more resistance.

ETA: Here is my formula for calculating rpm at 80mph in 6th gear: =($G$4*$D$1*$A21)/($B$1*0.00094697*3.1416)

I charted this a while ago in 5mph increments to make quick inspections for changes to tire size and gearing. I've also calculated rough hp losses due to drag, rolling resistance, and drivetrain at 135 mph, and it is in the ballpark of the claimed top speed.

Originally Posted by mrg666 (Post 3004639)

You are still not showing any equations and/or data that shows top speed increases with 4.3 final gear ratio. A ton of blah blah though that I won't even read.

Here two more references
2013 BRZ Top speed = 140 mph
2017 BRZ Top speed = 134 mph

We are done.

Originally Posted by mrg666 (Post 3004639)

You are still not showing any equations and/or data that shows top speed increases with 4.3 final gear ratio. A ton of blah blah though that I won't even read.

Here two more references
2013 BRZ Top speed = 140 mph
2017 BRZ Top speed = 134 mph

We are done.

Originally Posted by Somerandom18 (Post 3004643)

Why did a police officer clock my 2017 at 148 when apparently the top speed is 134. Lmao

Sent from my Pixel XL using Tapatalk

Originally Posted by nikitopo (Post 3004638)

This number is very low for the US car.

Originally Posted by Somerandom18 (Post 3004643)

Why did a police officer clock my 2017 at 148 when apparently the top speed is 134. Lmao

Sent from my Pixel XL using Tapatalk

Originally Posted by gtengr (Post 3004642)

I posted my equation? Since you're saying I'm wrong, you should be able to compare it to the equation you use and easily explain why it's wrong. I'm not taking magazine top speeds over what Subaru published, but even if we use those numbers, it doesn't change my point: higher numerical gearing = higher 6th gear RPM = more horsepower put to the ground = incrementally higher top speed if it's aero drag limited.

Originally Posted by mrg666 (Post 3004501)

Final drive replacement is a tradeoff, not really a power upgrade. Your engine will be working harder to do the same work. Acceleration will be quicker but sustained speeds will be at higher rpm. You might like it in the city since the car will feel quicker. But freeway driving will be worse; engine will be running at higher rpm on freeway, it will be noisier, and gas mileage will be lower. Even on track, you will be compromising your top speed for acceleration.

You want more power. So the right solution is getting more power. I recommend one of the followings.
- header + tune
- forced induction

If I had the 2017, I would go back to 2016 final drive and upgrade with FI again.

Originally Posted by mrg666 (Post 3004660)

Are you able to read? Or are you playing the dumb? I am not questioning your simple rpm calculation. I showed the same number in my post. It is correct.

I am asking you to backup your increasing top speed claim against two references I posted (and I can post more). Show the calculation.

Originally Posted by Tokay444 (Post 3004664)

Having never been able to hit top speed on any track I've driven, I'll take the extra acceleration.
Also, if you compare engine load, I bet the engine is working less hard to do the same work. Rpm=/=effort.

Originally Posted by ermax (Post 3004666)

The car makes power basically all the way to redline. So if you are running higher RPM then you have more power to pull a higher speed. If the car was limited because it was hitting redline in 6th gear then sure it would have a lower top speed.

Originally Posted by mrg666 (Post 3004671)

You are not redlining at 6th, you cannot. Because drag limited (not enough power), remember? That is why I said increasing power is more important.

Originally Posted by ermax (Post 3004672)

Okay now I'm starting to think you are the one playing dumb. Reread all the posts in this thread and then return.

Edit: As explained several times. The higher final drive would put you higher in the rev band. The car makes power all the way to redline so the higher final puts you higher in the band producing more power. More power, more top speed. You agree that the car will accelerate faster. The gearing keeps the revs in the power band. Do you think this simply stops once you put it in 6th gear?

Originally Posted by gtengr (Post 3004577)

Yes, I have numbers and equations to back up my claim. I considered engine rpm, trans gearing, diff gearing, and tire size to get the speed of the car. 4.10 vs. 4.30 results in 167 rpm difference at 80mph in 6th.

The official claim is that top speed is drag limited to 134 mph. 4.30 gearing would put the engine a couple of hundred rpm's higher in the power band than the 4.10, which would allow the car to overcome incrementally more aero drag and rolling resistance.

Originally Posted by mrg666 (Post 3004667)

Of course, that is totally your decision and your car. How can I argue with that?

Originally Posted by Tokay444 (Post 3004694)

Except I've hit over 240km/h, completely stock.

Originally Posted by gtengr (Post 3004653)

The actual number isn't all that relevant to my point. And on top of that, claiming a top speed higher than 134 makes my point more obvious by inspection of the speed vs. rpm vs. engine power plot.

Originally Posted by Tcoat (Post 3004652)

MPH vs KPH?


If they clocked you doing 148 MPH how do you still have the car?