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

GTgears

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.

Bill Verburg

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.

GTgears

I thought his ring and pinion ratio was 3.91. Did I get it wrong?

Bill Verburg

I think that part was about his BMW, but it wasn't perfectly clear either way.

GTgears

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.

Jonny A

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