diff ratio info.

[mr bump]

I up graded my diff in my 72 cs last year to a pre 79 E24 side loading LSD diff along with the dogleg box out the same car.
The diff ratio is 3.07 and is a 10 bolt crown wheel and lsd unit. Its a great motorway cruiser with the 3.07, but I want to pep it up a bit.
So im after a 3.64 or round that type of ratio. The original ratio in my cs when it was auto was a 3.64 but its only an 8 bolt crown wheel and wont fit to the 10 lsd unit.
Does anyone know if a later E28, E34 ect crownwheel will fit into the e24 sideload casing or if therewas a 3.64 available in the E24 range of engines?

 

[Honolulu]

Mmmm. Search function? I'm sure this has come up a dozen times.

 

[DerSchwede]

E12 525 and 528 until '78 had 3.64 rear end gearing

 

[mr bump]

E12 525 and 528 until '78 had 3.64 rear end gearing

They did but the crownwheel only had 8 bolts not 10. Looks like the best I can get is an e24 628 which had a 3.45 which should improve the sprint a little.
Or E28 complete rear sub frame and fabricate mountings which is a lot of work and looks to be the last resort.

 

[mr bump]

Mmmm. Search function? I'm sure this has come up a dozen times.

Thanks.

 

[DerSchwede]

They did but the crownwheel only had 8 bolts not 10. Looks like the best I can get is an e24 628 which had a 3.45 which should improve the sprint a little.
Or E28 complete rear sub frame and fabricate mountings which is a lot of work and looks to be the last resort.

The auto versions of above models had the same gearing until '81 so that should ease the search. I have a 3.07:1 LSD with 10 bolts as a spare but would be happy to swap it for a unit with 8 bolts.

E28 subframe is a downgrade since you have to give up the vented rear discs then - and you don't want to do that, do you? ;-)

With a bit of luck you should be able to find crown and pinion gear with 3.64 and 10 bolts at either the local dealer or similar.

 

[kasbatts]

Just get a machine shop to re drill machine your LSD housing for the 8 bolt crown wheel

pretty sure that's what my Diff guy did if memory serves me correct?

 

[sfdon]

You need to get the diff out of a e30 325i lsd
Add one clutch disc and machine the cover 50 thousands.
You get 40% lockup and a 3.73 ratio.
Or go with stock and get 25%.
Pete at Pete's Gear Shop in Hayward CA
Is making them now for guys here in NorCal
Call Pete for a quote.
Nice guy and straight shooter.

 

[bdigel]

Hey Guys
diffs out of 82 and newer cars will NOT bolt in to an earlier cars subframe
you need to modify your subframe or put a newer subframe in the car, as a matter of fact I am doing that to my project right now with an e28 subframe

the early car have whats called a sideloader, while the later cars are called a rear, the rear loader are a much better diff, and have more ratio combo than you can think of, as well as a better selection on LSD units . The side loaders are prone to over heating and boiling the fluid out while under spirited use, and that's one reason both my e12 and e9 have later rear suspension's

I also think its a poor idea to machine the top of the LSD unit to fit extra clutch's, the extra clutch put more force and could end up exploding the machined piece, Diffy Dan and performance gearing have new pieces made out of chrome moly to take the extra force and make the room for the additional clutch's

 

[sfdon]

I really need to clarify my point-
Take the pumpkin out of the 325i diff- that is the LSD unit and it will go into a side loader.

I appreciate your point about the 325i diff not fitting.

 

[rb1971]

I second calling Pete if you want to move to newer tech in the older package. My new diff is quiet and seems strong, and he was a pleasure to deal with. (Don't call him until he's done with the next diffs for Don and I, however!)

 

[pmansson]

What gearbox do you have?

The 3.64 with the 4-speed (or CR 5-speed) box will drive you crazy on the motorways. OK with the OD box, but if you come from 3.07 it will make a major difference going to 3.64.
I have 3.45 with OD on a couple of 3L engined FI cars and that is OK. I prefer a higher rear end but this is personal. On a beefed up 3.5L car even have a 3.15 LSD with the OD box, and that is very comfortable for long distances.
All to do with what you want. Red light performance or a quiet car on the freeways.....

 

[rb1971]

In my car - which I freely admit is heavily modified - I am running an S38, 265CR and 3.73. In my view it is fine on the freeways, although I suppose that depends on how "free" you want to be (i.e., that RPM you want at what speed). Based on a run last Friday up to Point Reyes and back, I am at about 3250-3500RPM when I am at about 140KPH indicated (or around 85MPH).

Tastes vary, so people may want to go a lot faster or be a lot quieter, but I have now run in this car the following ratios: 3.07, 3.45, 3.64, 3.73. I find the higher ratios to be more suitable without sacrificing freeway performance. If I did a lot of highway driving or cared about MPG I might go back to a 3.45, but never lower.

I would think without knowing for certain that a different motor would not change the RPM numbers for a given speed, and those running a more civilized exhaust set-up and tires would have even less noise.

 

[HB Chris]

John,

Is your speedo calibrated? I am amazed that with a CR 1:1 box and 3.73 that your revs could be that low. My coupe has a 265OD and 3.45 and at 78mph I am at 3000rpm. My diff is 10% taller and the OD is also good for about a 500rpm advantage so I figure about 700-800 rpm overall more efficient. I would be closer to 4000rpm with your setup.

Chris

 

[rb1971]

Chris - I think it is pretty close. I don't know how accurate these speedos were originally, but it was rebuilt by Palo Alto Speedometer 2 years ago and we have done some side-by-side tests up to 65 with newer cars. Also, more qualitatively, at 140KPH I was the fastest car coming towards the Golden Gate Bridge out of Marin which tends to be a bit of a speedway (and I slowed down since my car is a somewhat cop-attracting color) - so I don't think I was going under 85.

-John

ETA: I'm happy to test this out with a co-pilot if one presents him- or herself. As a general matter the highway noise doesn't bother me but as anyone who has seen it or been in it knows, mine is really no longer an executive GT car.

 

[DerSchwede]

With std 1920mm tyre circumferance you will have following car speed at 3000 rpm:

4-sp and 265 dogleg box (1:1 in highest gear)
3.64: 59 mph
3.45: 63 mph
3.25: 67 mph
3.07: 70 mph

With an OD 265 5-speed with 0.812:1 in 5th, the engine speed decreases by 19%, hence a 3.45 OD setup will go about 77-78 mph at 3000 rpm as stated previously by HBChris

It is evident that many old speedos read far too much, ofter it will show 90 when you go 80... Some on the other hand are almost spot on.

 

[JFENG]

FWIW: These figures match what I got when I checked.

I used a GPS application on my smart phone to get an accurate read on speed and am in agreement with these figures shown below in 4th gear (OD 5spd).
BMW I6's are pretty smooth engines and if you have a stock exhaust (I do), I could tolerate 3500rpm for long periods of highway driving. But, I'm older and my hearing isn't particularly sensitive. I sometimes use -15dB ear plugs, and it depends on your reference. The E9 is far and away the quietest old car I have and I don't compare it to my DD.


BTW: Mario Langsten is now providing rebuilt diffs in a wide variety of ratio's, and are said to have some improvements on the originals.

John

With std 1920mm tyre circumferance you will have following car speed at 3000 rpm:

4-sp and 265 dogleg box (1:1 in highest gear)
3.64: 59 mph
3.45: 63 mph
3.25: 67 mph
3.07: 70 mph

With an OD 265 5-speed with 0.812:1 in 5th, the engine speed decreases by 19%, hence a 3.45 OD setup will go about 77-78 mph at 3000 rpm as stated previously by HBChris

It is evident that many old speedos read far too much, ofter it will show 90 when you go 80... Some on the other hand are almost spot on.

 

[rb1971]

A thought occurred to me this morning - I am running 17" wheels so the total diameter with tire is about 19", giving a circumference of about 60". If you are running 14" inch wheels with higher sidewall tires (2.5"), your circumference would be about 52". All numbers approximate.

60/52 = 1.15. I suspect that means my ~3500RPM observed at 140KPH would be ~4000RPM in a car with stock tire sizes.

Giant caveat: I stopped being an engineer in 1995 so all of this could easily be wrong.

 

[rsporsche]

i have a differential gearing spreadsheet that i got from diffsonline.com you input the diff gear ratio, then the tire size and the gear ratios of your tranny .... and it gives you the speed per rpm / per gear. its a pretty cool thing. i can't post it on the site, but if you want a copy, send me your email.

 

[DerSchwede]

You can find similar calculators on almost every car related site.

This one for instance:

http://www.cargister.com/calculator-gear-ratio