Several Gear questions about 2004 Cobra Gears (for my RX-7)

[Kevin Doe]

I know this is probably a really stupid question, and I could probably find the answer by searching, but I don't really have the time right now otherwise I would.

First questions:
Right now I have the 3.55 gears in my car (I have a 04 cobra diff in it), and have been thinking about swapping out for a different ratio. I don't want to go all radical or anything like that. With the 3.55s my 4th gear takes me to 150 mph, which is pretty unnecessary. I was thinking of a 3.73 or a 3.90 or something like that. I kinda recall people saying the 3.90s whine really bad for some odd reason, but I don't recall why. Would a switch from a 3.55 to a 3.73 be a noticeable change, and worth the time to swap?

Second question:
I've done a diff swap in a ranger before, but reused the same gears. All I did was re-shim the carrier bearings, checked backlash, and gear mesh and all was well. For a gear swap I'll probably need to re-shim both the carrier bearings, and the pinion gear to get it all right. If I need to change the shim on the pinion, do I need to use a new crush sleeve every time? So I should have multiple crush sleeves on hand when setting it up?

Third question:
Can anyone tell me the specs for setting up an aluminum housing 8.8? I'm looking for bearing preload (amount to add to the shims), and torque specs on the bearing caps, and pinion nut?

Fourth question:
Where is the best place to get all this stuff from one place? I'll need the entire intall kit, shims, crush sleeves, gear paint. The only thing I won't need is the dial indicator, I've got one of those.


I know I'm asking a lot of stuff, but I really appreciate any help you could offer.

[Kevin Doe]

I forgot to add one more thing. Are there any gear brands that I should stay away from, or any that you'd recommend?

[bangingears]

just my input, stay away from richmond, they are hard to set-up and require some not common shims.

second, when assembling the rear end to check everything, don't use the crush collar, just CAREFULLEY tighten pinion nut till ya get 20-25 in. lbs and start with re-using the factory pinion shim, almost all gears that i have installed usally end up in the .026''-.030'' shim range.

third, take a junk inner pinion bearing and take a die grinder to it so it easily slides on and off the pinion so you and quickly change shims with out having to use a press every time.

[Kevin Doe]

I made up a quick spreadsheet to look at some of this gear stuff. Here is what i came up with.

Inputs to the spreadsheet. In input the tire diameter, trans ratios, and final drive ratio. I also put in my torque curve from my dyno. The other thing is traction limit cold and hot. You'll see how its all used in the following plots.
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/Plot3.jpg

This is a plot of vehicle speed vs engine speed. Shows the current and proposed gearing, and how the speed/speed relationships change. Looks like it will bump my rpms up a bit, @ 200 rpms depending on gear just cruising. I have a big cam that likes to buck at low rpms, so it might be a nice thing to bring the rpms up a touch more than they are now. It may help drive ability.
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/Plot1.jpg

This last one is the most interesting, I think. It shows wheel torque, in each gear, for the proposed and current final drive ratios. I plotted a traction limit on there for different temperatures. I found this out somewhat empirically. I know that when its ~50 outside, I spin 2nd gear as soon as I get into the power band. So I looked at the data, and figured out what the wheel torque was at that rpm in 2nd. Then that was my 50 degree traction limit. I also know that when its 85 outside I don't spin 2nd, so I know the traction limit is higher, so I just kinda picked something that was higher than the current 2nd gear wheel torque. This is based using my suspension setup, and with my street tires (Azenis).

What I think this tells me is that I'll spin 1st regardless of the gear change. 2nd will spin when cold quite a bit easier. 2nd will want to spin a bit more when 85, but probably managable. 3rd should still hook great, and give me a lot more torque to the wheel. It also shows that with the tight 4-5 gear split, that 5th gear basically becomes what 4th was, so I should have the same acceleration in 5th that I used to get in 4th. In the 1/4 mile I'll be shifting into 4th regardless, so it won't make for any extra shifting.
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/Plot2.jpg


I'm not sure if I'm skewing this in my head, just trying to talk myself into something that isnt' necessary or what.

[Will]

Fourth question:
Where is the best place to get all this stuff from one place? I'll need the entire intall kit, shims, crush sleeves, gear paint. The only thing I won't need is the dial indicator, I've got one of those.

http://www.americanmuscle.com/99-04-mustang-gears.html

I have ordered from this site several times. Great prices and free shipping. I think the longest I waited for something to arrive at my house was 2 days tops.

[bangingears]

on your plots, just curious as to the coefficent of friction, to different tire compunds. It seems to me like your "traction limit" would be greatly affected depending on compound and surface area. Care to shed some light?

[Kevin Doe]

That traction limit is assuming the Falken Azenis RT-615s that I was running, with the suspension setup I have. Change the tires, tire pressure, shock adjustments, spring rate, etc in any way and that will change. I did not assume any coefficient of friction, or anything like that. I know what when the car comes into powerband at 4500 in 2nd, the tires spin when its cold ~50 degrees. So I looked in my spreadsheet (a part thats not shown here) and found the wheel torque that corresponded with 2nd gear, and d4500 rpms. I used that wheel torque as the low temperature limit. I also know that when its ~85 degrees outside that I hook pretty well in 2nd. So I looked to see what the highest wheel torque I saw in second was, and I made it a little bit higher. Its not exact, but it just gave me a general idea.

I was using it to make sure that with the gear swap, I wouldn't start spinning the shit out of 3rd all the time and make my car un-drivable.

[Kevin Doe]

I ended up going with 4.10s. I have about 100 miles on them now. So far I love them. No track times yet, but I'm happy with them from a seat of the pants.