Recommended differential for 5 sp swap. 635CSI

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ramp
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Recommended differential for 5 sp swap. 635CSI

Post by ramp »

Newbie to this variant. I'm purchasing ShapeShifter's 635CSI. It seems the only remaining task to complete the 5 spd transfer is a more user friendly differential, 3.19 to 3.46 or 3.91 LSD as noted in Jon Ackerman's build sheet. Is there any history on this subject? Performance/mileage etc. Thanks
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Brucey
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Post by Brucey »

all standard european-spec cars up to 6/87 (when they increased weight and (I thnk) reduced low down torque, so were forced to lower the gearing) were fitted with very high gearing. The only exceptions to this were the M cars and those fitted with the dogleg gearbox.

A typical arrangement would be a true overdrive top ratio in the gearbox (4spd auto or 5speed) and a 3.07 diff.

This gives a top gear that does about 2000 rpm @ ~75mph and would be good for about 200mph if the engine could pull it. The idea is that you can cruise down zer autobahn at 100mph plus, pulling just 3000rpm or so. They work really well like this, but you can't drive this way everywhere.

If you fit very much lower gearing it can be fun, but it may render first gear almost completely redundant; the only time you might want to use it is when starting off, and then only if you are pursuing the last couple of tenths off the line, and you will likely be fighting traction all the way. Once underway, there would be no need to select a ratio that low; -hence, in fact, the dogleg shift pattern.

One way of looking at it is that with the same diff, gears 2-5 on a dogleg are not a million miles away from 1-4 on a typical overdrive box. They are also the gears you would use when (say) giving it some on a twisty road, and they are also in the same shift pattern.

In detail the dogleg gears are (IIRC) slightly closer together, but unless you have put in a very peaky cam, this won't trouble you greatly. I think it alters the shift points by 200 or 300 rpm or something. The big choice is really if you want an extra low gear for traction-troubling starts or a taller gear for high speed cruising.

If you put a short diff ratio in with an overdrive gearbox, you can end up with a completely useless bottom gear (remember the ratios are a bit further apart, and 4th is still 1:1) and still not have a really good high speed cruising gear. This hurts fuel economy slightly on long runs, and not everyone likes the less relaxed engine note, but on the plus side the engine will obviously pick up pretty quickly as soon as you prod the throttle.

A cynic might say you could acheive the latter by simply driving along in 4th if you wanted....

If all you want to do with the car is drive round a racetrack as quickly as possible, you would pick the dogleg box and a short diff ratio, and gear the car for a top speed of maybe 125mph or so. There are not many racetracks where a 6er is going to go much faster than that down the straight. The problem would be that you would be doing nearly 4000rpm at 80mph in top which would not be good for use on a daily basis.

Personally I prefer to keep a high top gear for motorway work, but I know others quite like the lower gearing. There are no rights and wrongs here, you just have to ask yourself what you like, and how you will be using the car.

cheers
Last edited by Brucey on Mon Mar 11, 2013 10:01 pm, edited 1 time in total.
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ramp
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Post by ramp »

Thanks for the quick comprehensive reply. I do not plan on racing the car. My intent would be to have her as drivable on normal road use. It has been suggested to me that a 3.46 ratio would achieve this.
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2I6es
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Post by 2I6es »

Ramp, I am not an expert, merely a seat of your pants driver. My 1986 CSi was changed from 4 speed auto with a 3.45 diff to a 5 speed manual dogleg with a 3.07 diff by the previous owner.

I picked the vehicle up and did a 10,000km drive back to my home. On the trip the car returned ~9km/l for the trip. I cannot tell you how fast it goes as I only quickly looked at the tacho on two short high speed runs. One showed 5,200rpm the other 5,350rpm. I think one was around 210kph(130mph) and still pulling strongly.

Only engine mods are block is bored to 3.6litres with forged Mahle pistons and the ECU has a chip from an Alpina B10 running things. It's torque (pulling power) is strong even in 5th gear.

So, why am I telling you all this, simple. You must first decide how the vehicle is to be used. If you are going to use the car for cruising and trips away, stay around the 3.00 diff ratio (better fuel mileage). If you are going to stick around town with maybe a little track work go around 3.45 diff ratio(better pickup off the mark but slightly more fuel used = less mileage). If it is mainly track with a little town work head for the 3.9x diff ratio(your not worried about fuel mileage but acceleration will be great, possibly at the cost of losing 1st gear like Brucey said).

Forget my rant, I somehow missed you stating that you wanted a town car for cruising.

Cheers Vinny
Last edited by 2I6es on Mon Mar 11, 2013 4:17 pm, edited 1 time in total.
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Brucey
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Post by Brucey »

you perhaps shouldn't choose your diff without mentioning what gearbox you have; they have different top gear ratios. Which gearbox do you have?

cheers
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Post by GazM3 »

The non dogleg is much more common so ill assume this.

I'm a big fan of low gearing.
In the m635 they have 3.73:1 and first gear is entirely usable. Sure if u launch he clutch hard at 3500 rpm she will spin up the tyres but if u just ease out the clutch u can use full throttle in first no problem. U are sitting around 70mph at 3000 in top and you don't even need to change down

With the m30 and a functioning LSD 3.73-3.91 would be nice.
If u do lots of country driving maybe knock done to the 3.46:1.

The loss of economy on high speed cruise is as a rough rule of thumb about 1/2 the % gearing loss. Ie change gearing by 18% u lose about 9% of economy on cruise. Around the city the economy loss is about 1/4 of the gearing if that.

I had a dramatic gear change in an old m30 e28
Auto from 3.25:1 to 4.10:1 and the city economy hardly changes at all as with the tall gears and the Auto the engine was always low in the rev range. It was a real dull car to drive with the tall gears and with the short gears it was much better.
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Brucey
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Post by Brucey »

IIRC the M635 has slightly more closely spaced gears than the standard ratios in the overdrive 'box.

I'm sure there was a good post somewhere with all the various ratios in it.

[edit; ratio ranges as follows;

265/5 dogleg CR 3.72 (1) to 1.0 (5) Range = x 3.72
265/6, 260/5, 260/6 OD 3.83 (1) to 0.81 (5) Range = x4.73
280/5 (OD CR as per M635) 3.51 (1) to 0.81 (5) Range = x4.33
(4HP22 (EH and non-EH versions) 0.73 (4))

So 3.2,3.1, 3.4 diffs respectively on the three manual gearboxes would each give the same bottom gear, but at ~75mph in top, 3100, 2400, 2700 rpms respectively.]



cheers
Last edited by Brucey on Tue Mar 12, 2013 10:36 am, edited 1 time in total.
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Sprocket

Post by Sprocket »

My 265 has a 3.71 ratio rear gear, as I was told when I bought it. It may be 3.91, I need to look more closely, as I have not seen 3.71 mentioned as an option.

As stated above, first gear with this set up really is optional, and quite annoying in sedate driving conditions. Second is a little too tall for smooth take-off, and first can be ridiculous in a spirited (not racing) start. You literally shift from 1st to 2nd after about 2 seconds, and it is tiresome for me and passengers.

Yes, I would love to find a 3.09 diff near me and switch. I also have the original diff that came with the car before the auto to manual swap, but that is even higher (I believe) at 3.91.
Area 52

Post by Area 52 »

Old dude here: I swapped from a 3.46 to a 3.25 in my e23 w/265-6 as most of my driving is in town. First gear was pretty much useless as shifting from 1st to 2nd had to be completed before you even get through an intersection. The 3.25 is much better for what I need. At freeway speeds the rpm dropped a couple hundred with no loss of torque climbing hills in 5th.
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Post by bpoliakoff »

IMHO the ideal rear end ratio for an OD gear box is a 3.63. They are semi rare, but out there
81 Euro 635 M90 with motronic. Currently under a complete nut and bolt restoration. Pictures at
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GazM3
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Post by GazM3 »

I like diff ratio discussions.
Ask 10 people get 10 different responses LOL.
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90 E34 M5 3.6
94 E34 540i/6 SC E85
97 E36 M3 euro SC U/C
97 Z3 2.8 widebody

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ramp
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Post by ramp »

I pick the car up this weekend. Its a few hundred miles home. We'll just have to see how she feels. Thanks all for the input.
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Post by winfred »

i have 3.73s in almost everything i own, e30 with built 2.8 stroker m20 and a supercharger, m635 with 260/6 (to replace the 280 that probably failed despite only having 100k miles when i got it) and even my 3/4 ton 4x4 suburban has 3.73s, if i had my way my ridiculously fast highly modified cummins turbo diesel powered dodge truck would have 3.73s instead of 4.10s too
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ClassyPower

Post by ClassyPower »

Im assuming these gear ratios for the rears are similar to that in american cars? The 3.73s, 4.11s, 3.42s are all for better take off, but worse fuel mileage on the freeway and those such as 2.93 & 2.13 are for good highway mileage but horrible on takeoff?
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Post by jps635 »

I developed this when researching the same subject, used the on-line gearing calculator. It served my purposes but I don't take any responsibility or apologise for any errors.

I wanted more driveablity in 4th and 5th at around speed limit, but to retain the ability to still crack 100 mph in third (when swap my ECU). I eventually settled on 1 3.45 ratio, which are hard to come by here anyway, in the large case.

The other option which I sometimes regret was to keep the 3.07 ratio and stick in a dog-leg.

Chris
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Post by PorscheDude »

I am shapeshifter aka porschedude here. The car Mark has bought is my 89 635.
http://forums.bimmerforums.com/forum/sh ... ther-Shark!

It is a B35 with a 265/6 and still has the stock 3.91LSD.
IMO, the 3.46LSD is the perfect compromise between acceleration and good highway manners.

The 3.91 is great for getting around and was fun for about a week. First is almost useless. I feel the 89 is about as fast as a US spec M6 till about 70. The M6 walks away after this due the s38.

I think a 3.07 or 3.25 would really slow the car down although it would be great if the car was only a highway cruiser.
'89 635CSi Black/Silver Gray #3269085
Bilstein Sports/H&R's, ACS Type II's, 5 speed, 3.91 LSD, UUC Evo 3, Lt FW

'95 E34 Touring Blk/Blk MTech Bumpers, Euro Trim, Euro Lights, BBS RX.

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Post by PorscheDude »

Mark, Did you ever replace the Dif?
'89 635CSi Black/Silver Gray #3269085
Bilstein Sports/H&R's, ACS Type II's, 5 speed, 3.91 LSD, UUC Evo 3, Lt FW

'95 E34 Touring Blk/Blk MTech Bumpers, Euro Trim, Euro Lights, BBS RX.

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'86 951 Blk/Blk '94 968 Polar Silver/Black
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Post by bpoliakoff »

From what I have heard on various forums over the years, a 3.63 diff ratio is the one to use with an O.D. gear box. A bit hard to find but they are out there.
81 Euro 635 M90 with motronic. Currently under a complete nut and bolt restoration. Pictures at
flickr.com/photos/bertsphotos.
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