2017 diff swap

[gtengr]

Originally Posted by mrg666

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.

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.

[olsonpg]

Originally Posted by mrg666

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.

Sounds like a viable trade off to me.

[CSG Mike]

I swapped my stock 4.3 diff for a 4.1 diff.

[mrg666]

Originally Posted by gtengr

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

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.

[gtengr]

Originally Posted by mrg666

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.

I think the journalist you're quoting is incorrect. Topspeed is 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 top speed 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.

[Teseo]

Originally Posted by CSG Mike

I swapped my stock 4.3 diff for a 4.1 diff.

The car with the supercharger?

[CSG Mike]

Originally Posted by Teseo

The car with the supercharger?

The car with the turbocharger, but it was swapped over a year before the turbo went in.

[nikitopo]

Originally Posted by gtengr

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

This number is very low for the US car.

[mrg666]

Originally Posted by gtengr

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.

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.

[gtengr]

Originally Posted by mrg666

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.

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.

[Somerandom18]

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

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[ermax]

Originally Posted by mrg666

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.

Calculating RPM at a given speed isn't a complex formula like you seem to think. Google it, geez.

[Tcoat]

Originally Posted by Somerandom18

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

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MPH vs KPH?


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

[gtengr]

Originally Posted by nikitopo

This number is very low for the US car.

Originally Posted by Somerandom18

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

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.