Final Drive Calculations
 

There are a few final drive upgrades available for our cars. My question is how do you calculate the increase in "power" for the various ratios? For example, how much more power will you gain at the wheels with a 4.4 final drive vs a 4.7 final drive?

I am interested in this too.

I think 4.1 FD is stock. In that case,

For 4.4:

4.4/4.1 = 1.07

For 4.7:

4.7/4.1 = 1.15

So the 4.4 FD and 4.7 FD would put out 7% and 15% more torque than stock respectively at a given RPM and in a given gear.

Be careful with this mod if you are an autocrosser, as it would significantly shorten second gear and possibly require a lot more third gear shifts.

Thanks for the info Josh. This is mostly an everyday/weekend car but I plan to get it out to the track a few times this season. Not all that interested in autocross. I think I can live with the change in gearing in terms of cruising around. I figure stock I am at 2100rpm at 70mph in 6th. With the 4.7 I would be at 2600rpm at 70mph in 6th (I think). That doesn't sound too bad to me in terms of everyday drive-ability. Opinions are welcome, however.

Assuming 150rwtq that means about 10ft/lbs with the 4.4 and about 22ft/lbs with the 4.7. Sounds fun to me!

Engine speed would go from 2100 rpm to (2100*1.15=) ~2400 rpm. I know someone has already installed a different final drive and their review was very positive if I remember correctly.

You don't gain power at the wheel you gain torque. It is just a multiplication ratio.

So whatever torque you get at the rear diff, multiply it by 4.7 instead of 4.4. I suck at math but this is how it works.

Engine torque * current gear ratio * differential ratio = Wheel torque

http://www.ek9.org/forum/transmissio...st-friend.html

here you go

I know this is a big argument, but you won't see any increase on a dyno. I went from the stock 4.10's to 4.77's on my s2000 and dyno'd both on a Dynojet and the numbers were the exact same.

I thinks to see the increase on a dyno you have to keep 4.10 on the dyno setting. But you did see an increase on your butt dyno no?

Yes and no. It was def quicker around town, especially with dealing with the low torque that car had. But after about 60-70 a stock one could pull on me because of the taller gear and torque advantage in the upper RPM band. I really enjoyed them for the most part. Only disadvantage is if you are a highway driver. That car with 4.77's was turning around 4k RPM's at 60 MPH, 5500 at 80.

That last part surprises me. Seems like you would pull on a stock car in any gear until you hit the Rev limiter.

It does seem that way. There is a huge mathimatical explenation to it on the S2k forums. But I am far from a mathimatical guy. So it's difficult for me to explain.

That surprises me as well. I thought it would give you the same increase in torque throughout the rpm band. From all reports I have read that is the case anyway. It would seem to me that if the more aggressive final drive was running at a higher RPM at any given MPH over the stock final drive the more aggressive final drive would be faster until it tops out.

Example: In an FRS with a 4.55 FD you should be around 3400 RPM at 70 MPH. The stock 4.10 should be at 2900 RPM at 70 MPH. Just looking at the RPM's, the 4.55 should be able to pull on the 4.10 at 70 MPH because the car makes more power at 3400 RPM then it does at 2900 RPM. The gearing advantage should, in my mind, amplify this effect further.

If I am wrong, what am I missing?

No one else?

Yeah me too, probably have to do the shifting in an another rpm range and he did'nt adjust to it.:iono:

Could be. I was hoping someone that has this upgrade in their FRS would chime in. From all accounts it gives power throughout the RPM band and through each gear. Which makes the most sense to me. Maybe racecomp or robi could give us a few words on their experience with this product?

I dont get it either? If your in a higher rpm band against the same or similarly powered car with similar or less modifications in a lower rpm band, your going to pull on it just for the sure fact that you would be able to accelerate faster due to being in that higher rpm range and having more usuable power. I know even with all the lightweight modifications i have if im cruising at 35 in 6th and a similar car whether it be stock or not is sitting at 35 in 2nd or third it will pull on me just like I said above because its in a higher rpm band thus more usuable power. So I dont understand how with a final drive modification which would have you cruising in a higher rpm range and thats supposed to increase torque throughout the gears, and a noticeable torque increase, a stock car would pull on it? Especially considering you wouldnt have to downshift since you would already be at a higher rpm than the similar car. Maby im missing something...?

Im planning on getting the mfactory 4.67 final drive down the road.

All I know is @robispec is using 4.7 final drive in his track car. It posted a very good time at Willow Springs (1:35), which is a high speed track. Lots of 3rd & 4th gear turns, with a large bank to a straight at the end of the track.

Based on that and that 4.7 final drive is going to give more torque, I'd say highway pulls would be faster with 4.7 final drive. Again this is all speculation.

We have the Mfactory 4.7 in our silver car now, and our red car is stock. We have some good comparisons and videos coming soon on this very subject. :)

Sweet looking forward to viewing the different comparisons between the 2

Every application is different, but remember that there's a reason why drag racers with muscle cars carefully spec out their final drive ratio and transmission.

Also, here's formulas to consider...

(Engine torque in newton-meters) * transmission ratio * final drive) / (tire radius in meters) = driving force in newtons . You can calculate that easy enough if you have the engine torque curve.

Driving force - running resistance = acceleration power . You can't easily calculate that in a useable way at home. If you change the final drive, your final drive multiplier increases. However, at a given vehicle speed you are now at a different engine rpm so torque might change. It's not that straightforward to predict which would be faster.

I have changed many a BMW diff to a higher ratio. I know its a bit apples to oranges but it was often a hard sell to customers but everyone came back smiling.

For me its a debate between 4.55 or 4.44, I drive highway more then I'd like to admit so I need to keep that mileage in check.

I've got my mfactory 4.67 sitting on shelf, waiting for my friend silverdub to install it, i'll surely make video and virtual dyno pull to show the differance. Simply can't wait to try that.

Samesies. Don't want to be turning 4-4.5K at 80mph, nahmean?

That would be awesome to see considering im planning on getting the same exact final drive looking forward to the videos after the install.

Luckly I dont do as much highway driving as I use to but I know that I will be again soon, ill just have to adjust to it. Its a modification I dont mind altering some driving mannerisms in order to figure out how to be efficient with it.

Is there a service manual for this car yet? I need to get ahold of the torque specs and what not for installing a new final drive.

There was a service manual many moons ago, grasshopper. It might have even posted on this website. Maybe.

^^^ its just one of those things, If I actually look at it I will want to buy the gear set even more. So by not looking I can maybe hold off a bit longer.

I love mine....bit noisy but wearing perfectly after 5k track miles

Did this ever happen?

Would mounting a taller tire such as 235/45R17 compliment or counteract with what a 4.55 FD sets to achieve?

I went to 4.88s in my automatic FRS and love it. 30 to 70 mph times dropped 1.5 seconds.

A larger wheel or taller tire would reduce gains.

Here is a good info
http://autofactory.jp/product/img/16/5.jpg

Edit: I realized how old is this threat after posting.