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Information on 5:12 Differential Ratio

Old 11-27-03, 12:43 PM
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Information on 5:12 Differential Ratio

Looking for any help anyone can provide on acquiring, installing, maintaining and benefits of a 5:12 diff ratio for a second gen. On another website I read that the improved acceleration capability may be worth up to 2 seconds a lap. I am concerned about cost, durability, temp rise (in diff) and availability.

How is this achieved? I have read that it is a first gen diff with a welded on extension to the pinion shaft. This sounds a little scary to me. I am running the car as a cross between EP and a World Challange spec and my longest races are 3 hours. Thanks in advance for the help.
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Old 11-27-03, 11:36 PM
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it is a modified 1st gen piece as you described and most EP cars I have seen use a cooler to help it last. as for performance it is likely to make your car faster.
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Old 11-28-03, 12:32 PM
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most IT cars can't suck enough air to make use of the 5.12, they simply can't rev fast enough.

At least that's what I've heard from other IT racers.

But one thing I didn't hear is, how the heck do you run a cooler for a differential?
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Old 11-28-03, 03:02 PM
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Originally posted by Barwick
most IT cars can't suck enough air to make use of the 5.12, they simply can't rev fast enough.

At least that's what I've heard from other IT racers.
Don't tell that to all the fast guys here in the Southeast. If fact, part of the reason the fast guys use the 5.12 is because it is faster. The 4.88 is nice, but won't get you past a 5.12 car on the tracks we run (Sebring, Daytona, Road Atlanta, VIR). It loses off the corners and lacks the necessary RPM at the top end. With the .756 5th gear, 8400-8500 RPM down the front stretch will yield about 145-147 MPH with the right weather.

But one thing I didn't hear is, how the heck do you run a cooler for a differential?
Tilton and other manufacturers make a fluid pump for just this type of application. You mount a smaller oil cooler somewhere in the rear of the car that might see some airflow, and plumb the pump and lines to and from the cooler. Oil gets picked up from the drain hole and returns through the fill hole.

Disclaimer: The above modification has been deemed illegal for IT-rules competition. Your situation may vary.
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Old 11-28-03, 03:08 PM
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2nd Gen Differential Oil Cooler

Simply plumbed it is as follows:

1) out from drain plug at bottom of diff.
2) through filter element
3) through pump, has to be able to handle gear lube and temperatures up to 300 F.
4) through cooler element
5) back in through filler plug hole

Will need to add fittings and hoses. Doubt that diff cooler liquid to air rad can be plumbed on to a street car as no place will be out of the way and have air flow. I will mount mine drawing air through rear plexi window and exiting through screen in place of license plate mount.

Concern over diff temp with new gear, is that even with stock diff I have seen over 200 F temps.
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Old 11-28-03, 03:20 PM
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Re: Information on 5:12 Differential Ratio

Originally posted by 23Racer
I am concerned about cost, durability, temp rise (in diff) and availability.
Cost - gear set is only $300 more than the 4.88 through Mazdaspeed, machine work to have the pinion lengthened is $300, most shops will completely deassemble/assemble your diff for about $250-$300 plus hard parts.

Durability - have yet to see a 5.12 gearset fail. Ever.

Temp rise - not a concern if you change your fluid regularly (1-2 weekends).

Availability - The gearset is readily available, but the machine work is hard to come by. The only sources I know of are SpeedSource (941-578-7071, www.speedsourceinc.com ) and ISC Racing (863-324-4539, www.iscracing.net ). Both are in Florida.
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Old 11-28-03, 03:26 PM
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Thanks

Thanks Chris for the information. I will contact Speedsource and ISC on Monday. I will tell them you gave me the lead and maybe they will give you a break someday.
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Old 11-29-03, 11:19 AM
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I should note though that I have the 5.12 rear in my race car (although I haven't driven it yet). I think the guy's reasoning for giving me that tip is because I race at Waterford Hills www.waterfordhills.com and it's a very tight track, so that the engine needs to rev like crap at the low speeds (average speed for the track record in IT7 (12A first gen with ITA rules) is 68 mph or so). So at those low speeds the 5.12 might not be the best. Check out the track, there's only one real straight. It's so tight that really that back straight is the only place you can check your guages.
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