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Transmission gearing choices

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Old 01-15-2010, 04:49 PM
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Jonny A
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Default Transmission gearing choices

I am trying to work out changing the transmission gearing to come up with a better power band from 1st through 6th.

I currently own an 87 BMW 635 csi with a 5 speed and 3:91 LSD and can change it to a 6 speed with a lot more gear choices.

The BMW forums are not giving me any real world feedback or ideas.

I have owned two early 911s and hope to soon own an SC targa similar to Ed Hughes "ruby".

My current gearing is 3.82/2.20/1.40/1.00/.81.

First is to short with to much drop in rpm into second. 2nd-5th are much better, but the rpm drop is 1750/1500/and 1000 .

Bruce Anderson and Rossi Martini recommend an even gearing drop of around 1500 per gear change for a better gear set.

I have found a set of gears that will give me that rpm drop through all 6 gears. 2.97/2.10/1.46/1.00/.74/.50.

Would 1st be to tall ?

My car has a lot of lower end torque.

Any input would be greatly appreciated.

I am hoping for some one who has a gear set up similar in rpm drops and how they fell it works for the street.

I do not track this car.

Thanks everybody.

Jon
Old 01-15-2010, 05:09 PM
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GTgears
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Hello Jon,

I'll offer you my two cents. Given your final drive ratio, 1st gear isn't really a problem. In fact, it's probably just about right, as would be second gear and maybe 3rd. It's 4-5-6 where I see an issue. That 6th gear is like a 200mph 6th. Really what you've got there for 5th is about as tall as we ever go on either a street car or a track special.

What I look for in the higher gears is a descending rpm drop. As wind resistance goes up (with higher speeds) it becomes more important to keep your engine functioning in the sweet spot between peak torque and peak horsepower. I don't know a lot about the 635 engine but I suspect it's got decent midrange torque with a peak horspower and redline that's not much more than 6000rpm?

What I would suggest at this point is look for a stack that gives you 1300-1500 rpm drop on 3-4, 1100-1300 drop on 4-5 and around 1000 rpms on 5-6. I don't know if that option exists for your car, but that's going to give you the fastest acceleration and if you stay around .75-.70 for 6th gear you'll have good highway cruising rpms and still have a top speed higher than you'll ever hit on any street or most any track short of Daytona.
Old 01-15-2010, 06:02 PM
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Bill Verburg
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Originally Posted by Jonny A
I am trying to work out changing the transmission gearing to come up with a better power band from 1st through 6th.

I currently own an 87 BMW 635 csi with a 5 speed and 3:91 LSD and can change it to a 6 speed with a lot more gear choices.

The BMW forums are not giving me any real world feedback or ideas.

I have owned two early 911s and hope to soon own an SC targa similar to Ed Hughes "ruby".

My current gearing is 3.82/2.20/1.40/1.00/.81.

First is to short with to much drop in rpm into second. 2nd-5th are much better, but the rpm drop is 1750/1500/and 1000 .

Bruce Anderson and Rossi Martini recommend an even gearing drop of around 1500 per gear change for a better gear set.

I have found a set of gears that will give me that rpm drop through all 6 gears. 2.97/2.10/1.46/1.00/.74/.50.

Would 1st be to tall ?

My car has a lot of lower end torque.

Any input would be greatly appreciated.

I am hoping for some one who has a gear set up similar in rpm drops and how they fell it works for the street.

I do not track this car.

Thanks everybody.

Jon
It would be extremely useful for you to provide cwp(maybe 8:31), drive tires(maybe 245/45?), max rpm, intended use, for any meaningful suggestions to flow.
Old 01-15-2010, 06:42 PM
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GTgears
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I thought his ring and pinion ratio was 3.91. Did I get it wrong?
Old 01-15-2010, 06:44 PM
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Bill Verburg
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Originally Posted by GTgears
I thought his ring and pinion ratio was 3.91. Did I get it wrong?
I think that part was about his BMW, but it wasn't perfectly clear either way.
Old 01-15-2010, 09:26 PM
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GTgears
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Originally Posted by Bill Verburg
I think that part was about his BMW, but it wasn't perfectly clear either way.
I thought he was talking about gearing for his BMW and came to us Porsche people because he had found the BMW forums to be useless when talking about gearing.
Old 01-15-2010, 10:46 PM
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Jonny A
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Bill: Yes, I am talking about my BMW. I did not list my tire size since I had already worked that part of the equation out. Sorry for the confusion. They are 265 wide by 18" diameter. My tires are Yokohama ES100s.

I have asked for help with you guys, because this Porsche forum is great in my opinion. Everyone has a lot of great knowledge and ideas.

Thank you GT Gearing. I have quite a few choices for gearing, but none of them ideal. My other main choice is 2.66/1.78/1.30/1.00/.80/.63. This gives me roughly the same rpm drop of my current 5 speed with a better 1st gear ( not a stump puller gear) and the added 6th gear of .63 for better freeway driving. This setup gives me a rpm drop of 1750/1500/1250/1000/1000 from 2nd on.

I am figuring this rpm drop at 5000 rpm gear changes which for the street is a realistic shifting point for this car. The redline is 6250.

As I said, I use this car for street driving only and am trying to enjoy this car with as much fun as you can have without getting into trouble with police.

I changed out to a 3.46 lsd and it helped with 1st gear, but left too much rpm drop to 2nd. With the 3.91, at least I feel like I stay in the power band better.

I am trying to get higher gearing for 1st and lower gearing for 6th to equal out all the gears.

Thanks

Jon



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