Suggestions for rear end ratio change

Post here for mechanical and engine topics such as fuel issues, transmission problems, rough idle, exhaust, electrical issues, etc

Moderators: GRNSHRK, ron, bfons

User avatar
Klaus
Posts: 221
Joined: Mon Dec 18, 2006 4:03 am
Location: Las Vegas, NV

Suggestions for rear end ratio change

Post by Klaus »

Getting closer to final debugging, next thing on my list is the differential.
Car started as 1981 633CSi automatic with open 3:45 differential. Now I am running a slightly older European spec M90 and a 5 speed out of an E30 M3, should be a Getrag 265? I did some slight modifications to match the M90 to the US spec M30 ECU in order to run an oxygen sensor and an oversized Magnaflow converter, also the transmission has been modified for the mechanical speedometer. Currently I am running 255/40R17's in the rear, but considering 275/40R17's.

While the M90 produces quiet a bit more torque over the whole power band compared to the original engine, I am running into an issue at cruising speed. At around 80-85 Mph I constantly find myself shifting from 5th to 4th gear, lugging in 5th gear getting up the smallest hill or reving 4200 rpm in 4th gear.
What would be a more appropriate rear ratio?Also a locking differential would be nice. Top speed is not too much of an issue, it'seems rare around here to hit 100 Mph on the freeway. I know it's a tall order, there must be somebody out there modifying big case sideloaders other then maybe Metric Mechanic.

I would like to hear some suggestions and experiences. Thanks in advance,

Klaus
GazM3
Posts: 1791
Joined: Wed Oct 26, 2011 12:55 pm
Location: Melbourne, Australia

Re: Suggestions for rear end ratio change

Post by GazM3 »

Hey klaus.

I assume the 5spd man is an overdrive box.

What do u want the car to do?
U have several available ratios. Most common are.
3.73
3.91
4.11

If going for an LSD and have grippy tyres the 4.11 gears will still give 120mph if needed and if u have the mechanical grip it will give really good results in off the mark acceleration but this ratio is not for everyone and most will complain that thier cruising rpms are too high.

If you have average tires and no lsd probably don't go more the 3.73 which is what I have in my e23 auto.

But changing from 3.45 to 3.73 won't be much of a change so maybe hedge your bets and go in the middle at 3.91.
BMW’s
84 E24 M635csi
90 E34 M5 3.6
94 E34 540i/6 SC E85
97 E36 M3 euro SC U/C
97 Z3 2.8 widebody

OTHERS
11 Audi S5 3.0 SC
19 VW Amarok V6
User avatar
hornhospital
Platinum Member
Platinum Member
Posts: 2929
Joined: Sun Jul 11, 2010 6:46 pm
Location: Silverhill, AL
Contact:

Re: Suggestions for rear end ratio change

Post by hornhospital »

If it's lugging at 85 mph in 5th gear with a 3.45 rear gear, something's wrong.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann"; '95 M3 "Ashlyn"
User avatar
slofut
Posts: 778
Joined: Thu May 30, 2013 1:29 am
Location: SW Ga

Re: Suggestions for rear end ratio change

Post by slofut »

hornhospital wrote:If it's lugging at 85 mph in 5th gear with a 3.45 rear gear, something's wrong.

+1 :shock:
'87 635csi, 5sp man, dk blu on pearl beige
'88 635csi, auto, black on grey
'63 BMW Isetta
'75 XJ6C, 2dr, warm 350
'86 XJ6, th700r4
'75TR6
'64 Olds 88 conv
"68 T120 Bonneville
baders
Platinum Member
Platinum Member
Posts: 1084
Joined: Sun Nov 23, 2014 7:38 am
Location: Perth Australia

Re: Suggestions for rear end ratio change

Post by baders »

slofut wrote:
hornhospital wrote:If it's lugging at 85 mph in 5th gear with a 3.45 rear gear, something's wrong.

+1 :shock:
+2

There's a problem there, but it's not gearing.
1986 635csi LOCUTUS / Arktis Blau Metallic / Pearl Beige / S38B36/ Close ratio dogleg 5sp
2002 E46 M3 / Topaz Blau Metallic / Black Nappa / 6 speed SMG software tuned / Bilstein PSS10 Coilovers / CMP RACP reinforcement
User avatar
Klaus
Posts: 221
Joined: Mon Dec 18, 2006 4:03 am
Location: Las Vegas, NV

Re: Suggestions for rear end ratio change

Post by Klaus »

"Lugging" was probably a poor choice of wording. Problem is more that I am reving pretty high in 4th and when I shift into 5th I could use a few more rpms, especially on hills. It's probably just a matter of 300-400 rpm, not really a major issue, would just be nice to use 5th gear other then as a cruising gear.

Klaus
GazM3
Posts: 1791
Joined: Wed Oct 26, 2011 12:55 pm
Location: Melbourne, Australia

Re: Suggestions for rear end ratio change

Post by GazM3 »

The lower ratio really helps when u want to be lazy in top gear. Every car I've gone for a lower diff ratio other than traffic lights acceleration this is the other big plus. The lower power you have the more improvement you feel
BMW’s
84 E24 M635csi
90 E34 M5 3.6
94 E34 540i/6 SC E85
97 E36 M3 euro SC U/C
97 Z3 2.8 widebody

OTHERS
11 Audi S5 3.0 SC
19 VW Amarok V6
User avatar
86_6series Online
Platinum Member
Platinum Member
Posts: 1069
Joined: Mon Apr 02, 2007 10:08 pm
Location: West Nyack, NY

Re: Suggestions for rear end ratio change

Post by 86_6series »

Klaus wrote:"Lugging" was probably a poor choice of wording. Problem is more that I am reving pretty high in 4th and when I shift into 5th I could use a few more rpms, especially on hills. It's probably just a matter of 300-400 rpm, not really a major issue, would just be nice to use 5th gear other then as a cruising gear.

Klaus
Are you still doing 85 going up hills, needing higher revs to stay at that speed, without down shifting?
86-635CSi
96-MB C220 SOLD
98-740i
71-Continental MK3
User avatar
hornhospital
Platinum Member
Platinum Member
Posts: 2929
Joined: Sun Jul 11, 2010 6:46 pm
Location: Silverhill, AL
Contact:

Re: Suggestions for rear end ratio change

Post by hornhospital »

86_6series wrote:Are you still doing 85 going up hills, needing higher revs to stay at that speed, without down shifting?
Wondering the same thing myself. That M90 should be pulling like a freight train at 85 in 5th gear, uphill or not.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann"; '95 M3 "Ashlyn"
User avatar
Ralph in Socal
Platinum Member
Platinum Member
Posts: 2745
Joined: Tue Apr 14, 2009 3:32 am
Location: High Desert of SoCal

Re: Suggestions for rear end ratio change

Post by Ralph in Socal »

hornhospital wrote:
86_6series wrote:Are you still doing 85 going up hills, needing higher revs to stay at that speed, without down shifting?
Wondering the same thing myself. That M90 should be pulling like a freight train at 85 in 5th gear, uphill or not.
Baker Grade is about 19 miles long and has an inconsistent total climb over 3000 feet. Between Klaus and myself on the 15 Fwy. Going up the same grade in the opposite direction is worse since it's much shorter distance.

It is at that rpm range that the 265 OD trans is running while moving uphill that Klaus is not happy with. I'm sure the car jumps when in 4th gear but slowly accelerates while in 5th. Big difference!! Unfortunately it's the nature of the 265 OD box that there is a wider difference between 4th and 5th ratios. The only real solution is to install a CR box so that 4th and 5th ratios are much closer.

The sideloader ratios are limited to 3.07, 3.25, commonly 3.45, rare 3.64 and very rarely a 3.91. I suppose a 3.64 would improve your situation but the relative rarity of the 3.64 ratio will be your issue. I think a 3.91 would be overkill and reduce your fuel mileage tremendously and that it's too much of a trade off for the uphill acceleration you seek. You may feel differently as I know you have a heavy foot!!

Or slow down Klaus!!! You're already doing 85 so that's 15 over the limit. CHP and Lidar will get you if you're not careful!! (Look who's talking!!) Funny though, that if you're not doing 80 the semi trucks will run you over.

Ralph
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)
User avatar
Klaus
Posts: 221
Joined: Mon Dec 18, 2006 4:03 am
Location: Las Vegas, NV

Re: Suggestions for rear end ratio change

Post by Klaus »

Ralph, that is actually a pretty good description. I always forget that you know my stomping grounds! So far I have been very lucky with keeping the average at 85 mph, I run my Valentine 1 at the newest software and always make sure I have a rabbit near by.

Actually the M90 pulls like a freight train, once you are above 2800 rpm. I am just not a huge fan running uphill for 20 miles with fairly high rpms. It's comfortable at 80 mph in 4th, still pulling strong. But shifting into 5th at 85 mph you are running out of steam until you are crawling up to 90-92 mph, after that you are back to full power again.

I really have to look at the gear ratio's of the dogleg and overdrive transmissions and run some calculations. I guess the M90 usually runs a 3.07 rear with a straight 1:1 5th gear in the dogleg transmission?

The question now is if there are any parts available to change the ratio in a big sideloader and if there is a possibility of adding Limited Slip. The only ones I have seen coming up for sale once in a while are 3.07 LSD's, which wouldn't make much sense with the overdrive transmission. I could consider switching to an E28 style rear subframe, but all the used differentials will still have high mileage and might need rebuilding or modifications, on top of that I still would have to deal with brake and swaybar changes.

Klaus
GazM3
Posts: 1791
Joined: Wed Oct 26, 2011 12:55 pm
Location: Melbourne, Australia

Re: Suggestions for rear end ratio change

Post by GazM3 »

3.73 and LSD unit sounds like the ratio u want klaus. Are the ratios and diff centres transferable to the sideloader? Never had to investigate this.

I've got a dogleg with a 3.45 LSD behind my m635. I've got a few extra rpm to play with and top gear cruising is a busy 2900rpm @62mph. 3.25 would probably be best ratio with the dogleg.

I'm actually next year sometime installing the getrag 420g 6spd in the 6 series so I can have the short spaced gearing with some cruising ability. I'll have to make up an adaptor for the bellhousing which will be fun.
BMW’s
84 E24 M635csi
90 E34 M5 3.6
94 E34 540i/6 SC E85
97 E36 M3 euro SC U/C
97 Z3 2.8 widebody

OTHERS
11 Audi S5 3.0 SC
19 VW Amarok V6
User avatar
Ralph in Socal
Platinum Member
Platinum Member
Posts: 2745
Joined: Tue Apr 14, 2009 3:32 am
Location: High Desert of SoCal

Re: Suggestions for rear end ratio change

Post by Ralph in Socal »

GazM3 wrote:I'm actually next year sometime installing the getrag 420g 6spd in the 6 series so I can have the short spaced gearing with some cruising ability. I'll have to make up an adaptor for the bellhousing which will be fun.
Gawd Almighty if you can achieve this Gaz. I've been holding on to a 420G for a couple of years waiting for someone to do this or to find a 540 for it's motor.

Anyway Klaus, there is no known LSD unit that can be installed into a sideloader to make an open unit LSD. I have done some research and have yet to find anyone offering such an item. I'm open to finding out I'm wrong as I have several open sideloaders that I would be very happy to convert. I also have a couple of factory sideloader LSD units available if you are interested.

Later e28 LSD internals are not compatible with sideloaders so that's not an option. At this point I know only factory sideloaders are available in LSD form.

Ralph
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)
GRNSHRK
Platinum Member
Platinum Member
Posts: 3710
Joined: Thu Apr 08, 2004 3:24 pm
Location: Gilroy (SF Bay Area) CA

Re: Suggestions for rear end ratio change

Post by GRNSHRK »

I guess the M90 usually runs a 3.07 rear with a straight 1:1 5th gear in the dogleg transmission?
True :wink:

3.64 would be optimal for the 265 OD gearbox, wish I had one #-o

BTW, please check your home email [-o<

Oh, and here's a pic for ya, dug this one up, Paul did actually \:D/
Attachments
Klaus &amp; Bobbo
Klaus & Bobbo
Bob Klaus MB.JPG (749.27 KiB) Viewed 10012 times
:mrgreen:
Bobbo
1980 633 CSi Cypress Green/Pearl Beige
2017 X5 M Sport Xdrive 35i Carbon Black/Ivory White
2005 330 Ci ZHP Cabrio Imola Red/Bone/Black
Image
User avatar
Klaus
Posts: 221
Joined: Mon Dec 18, 2006 4:03 am
Location: Las Vegas, NV

Re: Suggestions for rear end ratio change

Post by Klaus »

GazM3 wrote: I'm actually next year sometime installing the getrag 420g 6spd in the 6 series so I can have the short spaced gearing with some cruising ability. I'll have to make up an adaptor for the bellhousing which will be fun.
I thought I read somewhere that the 420g is quiet huge compared to the 260 or 265, something that might severely interfere with the tunnel? Can not say I have ever seen these boxes in one picture for comparison, also making up an adapter might take some effort. Would be interesting to see if it could be done without too many drastic modifications, keep us posted please.

Ralph, I send you a PM, going to be in the valley this weekend.

Bobbo, email must have slipped by, I see if I can find some pictures for you later this week.
Nice picture, I remember the event, Wynn was sure in his element...

Klaus
GazM3
Posts: 1791
Joined: Wed Oct 26, 2011 12:55 pm
Location: Melbourne, Australia

Re: Suggestions for rear end ratio change

Post by GazM3 »

420 fits just. but the top bellhousing bolts will be tricky.
BMW’s
84 E24 M635csi
90 E34 M5 3.6
94 E34 540i/6 SC E85
97 E36 M3 euro SC U/C
97 Z3 2.8 widebody

OTHERS
11 Audi S5 3.0 SC
19 VW Amarok V6
User avatar
Klaus
Posts: 221
Joined: Mon Dec 18, 2006 4:03 am
Location: Las Vegas, NV

Re: Suggestions for rear end ratio change

Post by Klaus »

Just a quick update, ended up buying a 3.64 big case differential, just need to find some time to go through it and install it.

Klaus
GRNSHRK
Platinum Member
Platinum Member
Posts: 3710
Joined: Thu Apr 08, 2004 3:24 pm
Location: Gilroy (SF Bay Area) CA

Re: Suggestions for rear end ratio change

Post by GRNSHRK »

Klaus, is this a sideloader unit :-k

I know you are considering switching out to an E28 rear subframe, and it's my understanding that "big case" diffs are rear loaders :-?

let us know what you're up to [-o<
:mrgreen:
Bobbo
1980 633 CSi Cypress Green/Pearl Beige
2017 X5 M Sport Xdrive 35i Carbon Black/Ivory White
2005 330 Ci ZHP Cabrio Imola Red/Bone/Black
Image
User avatar
Brucey
6 Series Guru
6 Series Guru
Posts: 10077
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: Suggestions for rear end ratio change

Post by Brucey »

if it were me I'd fit the 'correct' (for that engine) 3,07 diff. This will let you do your uphill stint in 4th at sensible rpm, and still give you 5th as a cruising gear for when you are not climbing.

If you run the shorter gearing (eg with a 3.64) you will be doing ~3000rpm all the time on the freeway when you should be cruising.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
Ralph in Socal
Platinum Member
Platinum Member
Posts: 2745
Joined: Tue Apr 14, 2009 3:32 am
Location: High Desert of SoCal

Re: Suggestions for rear end ratio change

Post by Ralph in Socal »

GRNSHRK wrote:Klaus, is this a sideloader unit :-k

I know you are considering switching out to an E28 rear subframe, and it's my understanding that "big case" diffs are rear loaders :-?

let us know what you're up to [-o<
There's only 1 size of side loader differential for the early cars.

The later rear load differentials come in 168 mm, 188 mm and 210 mm. Termed small case, medium case and large/big case diffs. Small case diffs are found in 4 cylinder cars from the e10 (probably before that as well) all the way up to the e46 from my knowledge. Medium case diffs are found in small and large 6 cylinder cars while large case diffs are in the e23 745 turbo, M cars and the later and larger 5 and 7 series cars and now the SUVs.

Hope I clarified this misconception about diffs. There are many more!!

Ralph
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)
GRNSHRK
Platinum Member
Platinum Member
Posts: 3710
Joined: Thu Apr 08, 2004 3:24 pm
Location: Gilroy (SF Bay Area) CA

Re: Suggestions for rear end ratio change

Post by GRNSHRK »

Duly noted Brucey, but I "think" Klaus has a 265 OD gearbox, not the dogleg CR box :-k

Ralph, that was more of a question for Klaus, he threw me off by saying "large" case 3.64 diff :-"

I realize that all sideloaders have the same case =D>

I'd still like to pick up a 3.64 sideloader LSD myself :shock:

With the 3.45 gearset and OD gearbox, the gearing is just "slightly" long :-({|=
:mrgreen:
Bobbo
1980 633 CSi Cypress Green/Pearl Beige
2017 X5 M Sport Xdrive 35i Carbon Black/Ivory White
2005 330 Ci ZHP Cabrio Imola Red/Bone/Black
Image
User avatar
Ralph in Socal
Platinum Member
Platinum Member
Posts: 2745
Joined: Tue Apr 14, 2009 3:32 am
Location: High Desert of SoCal

Re: Suggestions for rear end ratio change

Post by Ralph in Socal »

Brucey has a very good point about what the engine will be screaming about while you're on the Fwy.

I have a 3.07 LSD in my 83 635 auto and I love it. I was going to switch to a 3.45 before I was really using the car. Now I just let the auto shift down (I've adjusted so it shifts down sooner rather than later) and the car flies up Cajon Pass. I can also cruise on the Fwy at 80 mph and the motor is barely doing 2200 rpm. Cruising is very nice and comfortable in that range. MPG is tremendous also.

And since I do quite a bit of 80+mph all over SoCal and mpg is more critical for me, I am happy with the current 3.07. I have other vehicles set up for the quick around town play so I'm quite content with status quo.

I'm somewhat to blame for Klaus picking up that 3.64 LSD as I told him they are very difficult to find and they will not go down in price. I feel that he made a good investment with the purchase but the eventual install is up to his driving preference.

Ralph

PS - Not directed at you Bobbo as I did see you reference "rear loader". Just adding facts to prevent propagation of any misconception.
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)
User avatar
Brucey
6 Series Guru
6 Series Guru
Posts: 10077
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: Suggestions for rear end ratio change

Post by Brucey »

GRNSHRK wrote:Duly noted Brucey, but I "think" Klaus has a 265 OD gearbox, not the dogleg CR box :-k
yes, I know that. Euro 635CSi with M90 engine standard spec is that gearbox and a 3,07 diff. [Option is a dogleg.] . IMHO the OD gearbox is (unsurprisingly) best used with 5th gear as a true overdrive if you are doing lots of miles on the motorway/autobahn/freeway.

In general terms I think it makes sense (from an overall real-world performance perspective) to have a 'max/top' gear ratio that lets the engine reach max power at the top speed of the vehicle. [If nothing else, provided the engine has a decent torque curve, this gear will offer reasonable acceleration when you need it over a wide range of road speeds.].

With the OD/3,07 combination, that 'max/top' gear is 4th.

However I think that with a OD box and a 3,45 rear end you don't have that gear at all (you have a gear each side I think).

If you make that gear 5th, the engine rpms are then much higher than they should be when cruising at a decent lick. I've owned cars that had 20mph/1000rpm gearing in top gear and they flew along nicely but it was just very very tiring on long journeys.

If you go for really low gearing, this can be just the job on a racetrack, because most racetracks don't have straights that are long enough for cars to reach their maximum speed; arguably the perfect 'racetrack top' gearing is one that just lets you hit the rev limiter in top (having got a tow) at the end of the longest straight. In a modified road car, this might only be about 125mph at a lot of tracks. A 3,7 to 3,9 diff and a dogleg box will give you that. Some folk use this kind of gearing on the street but it would be my idea of absolute purgatory if I had to drive the car on a long journey.

I have often wondered if the idea of a 3,45 diff with an OD box is to give 4th as 'a racetrack top' ratio. It might work well like that, but IMHO it does give some pretty crummy gearing for road use with a standard engine.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
GRNSHRK
Platinum Member
Platinum Member
Posts: 3710
Joined: Thu Apr 08, 2004 3:24 pm
Location: Gilroy (SF Bay Area) CA

Re: Suggestions for rear end ratio change

Post by GRNSHRK »

Hey Brucey,
Euro 635CSi with M90 engine standard spec is that gearbox and a 3,07 diff. [Option is a dogleg.]
Really :shock:

No way that I would doubt your knowledge, but I have never heard of a series I Euro 635 that DIDN'T have a dogleg CR tranny and a 3.07 rear end [-X

Live and learn I guess :P

Interesting bits about when and where you can top out, good points but you also mentioned being able to cruise on the motorway (freeway) at lower RPM's is often the actual goal :-k

Great discussion, now where is my big German friend Klaus \:D/
:mrgreen:
Bobbo
1980 633 CSi Cypress Green/Pearl Beige
2017 X5 M Sport Xdrive 35i Carbon Black/Ivory White
2005 330 Ci ZHP Cabrio Imola Red/Bone/Black
Image
GazM3
Posts: 1791
Joined: Wed Oct 26, 2011 12:55 pm
Location: Melbourne, Australia

Re: Suggestions for rear end ratio change

Post by GazM3 »

Good description Brucey. Short gearing is a personal preference. I'm one of those donkeys that prefers the 20mph/1000rpm gearing. I have the dogleg /3.45 combo on the m635 and will set my 2002 up same way. The e23 has an auto OD and has 3.73 ratio and it's about perfect compromise on low down acceleration vs cruising rpm.

There is nothing like cruising on the road at 130kmh with the engine spinning at 4000rpm. Feels like you are in a speedboat and no downchanging required for overtaking or hills. Fuel economy is still surprisingly good. Can still get 10ltr/100km which gives me a safe 600km range on the open road between fills. The e23 on same conditions gets 8 ltr/100km
BMW’s
84 E24 M635csi
90 E34 M5 3.6
94 E34 540i/6 SC E85
97 E36 M3 euro SC U/C
97 Z3 2.8 widebody

OTHERS
11 Audi S5 3.0 SC
19 VW Amarok V6
Post Reply