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daywilmOffline
Post subject: Transmission Gearing  PostPosted: Jun 05, 2013 - 11:30 PM
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Looking for opinions from some of you more experienced guys. I'm building on a replica frame and using the BMW v-12 in the OEM position. I'm planning on mating a Borg-Warner T5 or Tremec 3650 5-speed to it and using a Casale v-drive to direct the power back to the BMW differential. I'm trying to get the gearing as close to OEM as possible. To handle to power of the v-12 I'm looking at a tranny from either a V-8 Camaro or Mustang. Second through fifth gears are close in either choice but first gear is different. Camaro is 2.95 and Mustang is 3.35. I've calculated the first gear final drive of a real Diablo as 8.85 (first gear 2.31 X transfer case 1.59 X diff 2.41). My V-drive is 1.09 and the BMW diff is 3.15. So using the Camaro tranny I get a first gear final drive of 10.13 (first gear 2.95 X v-drive 1.09 X diff 3.15). Not that close to the OEM 8.85. The Mustang is even worse with a 11.50 (first gear 3.35 X v-drive 1.09 X diff 3.15).

I guess my question is, would the Mustang gearing give me too short of a first gear range? I don't want to get into a situation where first gear is only good for getting the wheels rolling before quickly having to shift into second (or only good for pulling stumps).
 
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dleachOffline
Post subject: RE: Transmission Gearing  PostPosted: Jun 07, 2013 - 06:37 AM
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Shouldn't you be trying to approximate the OEM gear ratios from the BMW V-12 (not the Diablo).

Don
Building a 6.0
 
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daywilmOffline
Post subject:   PostPosted: Jun 07, 2013 - 06:59 PM
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Well, my thought was, I'd like the car to run more like a Diablo than a BMW luxury tank. I know what you're saying, though, match the gearing to the engine but the original auto tranny only has 4 gears and I'm dealing with 5, so there is some extrapolating that would need to be done. I'm not really trying to find the "magic" ideal, I'm just trying not to get too far from ideal. I've driven cars where 1st was so short it was worthless and others where you had to slip the clutch just to get it going. I'm trying to avoid those two scenarios.
Thanks for the input!
Mike
 
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dleachOffline
Post subject:   PostPosted: Jun 08, 2013 - 01:32 PM
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I hear you. I had a car where first gear would get you about two car lengths...lol.
I've had the other type as well. Its a balancing act. Good luck.

Don
Building a 6.0
 
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wire2Offline
Post subject:   PostPosted: Jun 12, 2013 - 07:25 AM
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Location: Milton, ON, Canada
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What about doing the calcs in reverse order. Like, how fast do you want to be going with it in 1st gear at say, 5,000 rpm? Then work back to a ratio to accomplish it. Typical tire diameter will be ~ 26", 1 turn is 6.8', so 776 revs/mile.
Your effective diff/V drive ratio is 3.43 so speed at 5k rpm will be 38 mph.

Here's a handy calc, just plug in the numbers;
http://www.apexgarage.com/tech/gear_ratios.shtml
 
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daywilmOffline
Post subject:   PostPosted: Jun 14, 2013 - 03:25 AM
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Thanks Wire. That sounds like a good idea. It will either verify I'm on the right track or the train has derailed.
 
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