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?