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Diff Ratio - going the other way?

Old 10-18-10, 12:46 PM
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Diff Ratio - going the other way?

Today I was contemplating what to do about the diff in my FC. This may have something to do with the fact that I keep tripping over the several spares I have while making my way through the garage in the morning. :-)

Currently I have a 4.3 ratio open diff in the car. I have a clutch-type LSD core that I am planning on swapping in eventually. The problem in that I do not have another 4.3 ring and pinion and the 4.88 or 5.12 units are just way out of my budget. I would prefer to keep the existing diff unmolested in case my LSD rebuild talents are found lacking and the new one grenades (ie. keep a known good spare in case Plan-B is needed).

I also want to keep my setup more or less in-line with SCCA ITS class rules, which mandate stock tranny ratios and using the stock diff casings. This is in case someday in a sudden fit of insanity I decide to burn even more money on this hobby of mine by going door-to-door racing.

So I started thinking about the effective gear ratios at the rear end, esp. for the 4.88 and 5.12 units. Now on my home track (VIR) I'm mainly a 3rd-4th kind of driver. There are two turns where second potentially be useful for me (T4 and Oak Tree), but I find that the extra acceleration I get on the launch is countered by the necessity of doing the extra shift mid-turn and it's effectively a wash. Second is just a little too low for me.

The other thing I find is that I find I run out of revs at the top end in several places, mainly in 4th at the end of the straights. I could grab fifth, but b/c of the big step up in gearing it doesn't give me a lot of additional torque - the car is NA with stock internals, so I don't have a whole lot more top end. A GTUs tranny ($$) with the shorter 5th may get me a little more, but I would really like to be able to stretch 4th a bit more.

So here is the question - instead of looking for another 4.3 rear, should I just flip it around and go lower instead? If instead I go find a 3.909 ring and pinion from an auto then 2nd would seem to become a lot more useful and I could stretch 4th maybe another 10mph down the straights. 5th would be pretty useless, but I don't use it now anyway.

I put together a quick spreadsheet with the final-drive rations for the different tranny/diff combos and the numbers look pretty reasonable. So has anyone else thought about this or even tried it?


Thanks,

-b
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Old 10-18-10, 04:34 PM
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The drop in RPM's from 2nd to 3rd will be a problem if you try the 3.9 gear. I have a 3.9 all set up waiting for the cash to buy a Roltek dogring 4 speed. The price of building a 5.12 the right way is 2/3rds of the indestructable 4sp based on a T10 and the driveshaft speed won't be an issue.

Here is a good free gear program that I use.

http://locost7.info/gearcalc.php
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Old 10-18-10, 11:19 PM
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I just installed a 3.90:1 diff in my track FD. I had two issues: 1) dying 4.10:1 stock diff and 2) running out of gear on the main straight at a local track. I was happy with the results. FYI, I purchased a JDM pull off recently and found, much to my horror after installing the diff, that the bolt holes from the drive shaft of my 93 FD did not line up. I turned the drive shaft 45 degrees and drilled new holes. It's probably easier to do when your not under the car.

Guy
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Old 10-19-10, 12:50 PM
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Originally Posted by jgrewe View Post
The price of building a 5.12 the right way is 2/3rds of the indestructable 4sp based...
Roltex shows $3080 for the 4spd, then you need a bellhousing adapter ($200 est), bellhousing ($150 est), custom driveshaft ($400 est), correct spline clutch disk ($150 est), custom tranny mount (free if you have fab skills), shifter ($500 est), etc. You are easily at $4500 and that's assuming you do the labor yourself and have no shipping (I know you know that).

I think my 5.12 which I bought from you, so it is built right, on sale right now for only $1300 obo (shameless plug) is much less than 2/3rds the cost of that Roltek... plus it's plug and play, 3 hours in the shop and you are back on track with what feels like a whole new car.

I do agree a dog box is the way to go if you don't mind spending the money, my Jerico is unbelievable compared to a stock box.

OP: There is a good reason Speedsource ran the 5.12 in their ITS cars. If it's not in the budget, pay someone who knows what they are doing to put the LSD in the 4.30, shouldn't be that much assuming you removed from the car, and worth the expense of not blowing up your 4.30. Next time you get your tranny rebuilt order the .756 5th gear from Mazdacomp and have it changed out.

How'd you get an open 4.30 anyways? I didn't think a 4.30 open diff was ever avaiable?
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Old 10-19-10, 03:47 PM
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Originally Posted by wrankin View Post
A GTUs tranny ($$) with the shorter 5th may get me a little more, but I would really like to be able to stretch 4th a bit more.

I put together a quick spreadsheet with the final-drive rations for the different tranny/diff combos and the numbers look pretty reasonable. So has anyone else thought about this or even tried it?

-b
the GTUS trans isn't that bad either, you just need the two gears for 5th, the rest is the same.

we did try the stock final drive back in the day. we put a haltech in the stock case and moved the rev limiter up to 9400. the ecu also gave us a much broader powerband than the stock ecu (peak is about the same), so it got out of the corners quicker, AND we didn't shift as much.

for us the difference between the s5 limiter and 9400 is a shift to 4th at the end of the main straight at thunderhill, and then back to 3rd for turn 1, so the fact that we ran out of power band wasn't a problem.
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Old 10-19-10, 05:01 PM
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Originally Posted by j9fd3s View Post
for us the difference between the s5 limiter and 9400 is a shift to 4th at the end of the main straight at thunderhill, and then back to 3rd for turn 1, so the fact that we ran out of power band wasn't a problem.
What kind of speed did that equate to with your tires and gearing? I'm concerned about this with my current build, too, especially if I go with short tires I'm just not sure how high into the revs the turbo will make power... so I'm thinking 8krpm will be about the max for me, which may make me shift like made... (FWIW, my street '06 C6 Z51 on R888s hits a GPS verified 135mph at Thunderhill before braking for T1... I think there's a 140mph there if I can get a better launch, and feel better about the brakes...)
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Old 10-20-10, 12:13 PM
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not sure, no speedo and no datalogging!

the gear calculator says about [email protected] 8250 rpms.... it would actually hit the limiter in 3rd right at start finish, so taking it higher just avoided the upshift downshift.

out car was a non turbo though, with double the power you should be way into 4th....
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Old 10-21-10, 08:43 PM
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Originally Posted by SCCAITS View Post
How'd you get an open 4.30 anyways? I didn't think a 4.30 open diff was ever avaiable?
Car came that way when I bought it. I understand it was a 4.3 gear set retrofit into (maybe?) a 91 diff. Heck, for all I know it could be an expired VLSD - I've never opened it up. But it's definitely acts like an open unit.
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Old 10-21-10, 08:49 PM
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