3.91 diff to 318i?

[jazma]

At the moment, my car has the original 4.1 diff. At about 60 mph, revs are ~3200. Imho, at those speeds the engine noise is quite annoying. When I press the clutch down at that speed and revs decline to about 2500, it's very much less noisy. This is the level where I'd like the revs to be. I don't do hard accelerations at all, so if differential swap is a tradeoff between acceleration and comfort, so be it.

For example, past week I've used my friends' Madza 323 as a everyday car and it's revs at 60 mph are about 2500. It's heck of a lot less noisy (=engine noise) than my bimmer.

If I'd change my diff to 3.91, how much would the revs decrease at 60 mph? Does anyone know? Would I get the revs to about 2500 at 60 mph and in fifth gear? I've used search function and seems to be that 3.64 is too much for m40. If I can't find 3.91, how about 3.73 then?

Any experiences regarding this problem would be very appreciated!

[Ant]

what do you have now ?

4.1:1 or 4.41:1 ( iirc close enough anyways... )

3.9:1 would give you the overdrive you crave, no taller than that though IMHO

[SPADGE]

Yep if you have a 4.1 now (which from what you said i don't think you have) then a 3.9 will drop it a little but not down to 2.5k. I suspect you have the 4.41 that Ant mentioned in which case it will make a decent amount of difference.

[Elecblondie]

By my reckoning you'll be doing around 3000 rather than 2500 I'm afraid, losing 5% torque. If it's a 4.41 diff, as mentioned, then it would be 2800 rpm, isn't that only the case for autos? I'm sure you could go a little taller if you weren't hugely fussed about the acceleration, I'd have thought the m40 had enough low down torque to deal with it, it just doesn't like reving too far, but then again I don't really know and as always would be happy to stand corrected.

[jazma]

Thanks. I've looked the letters under the diff few years ago and iirc the diff is 4.1. I looked for the letter S, but surprise surprise it didn't have it.
Maybe I should narrow my search to 3.91 diff. Bmwfans.info says that 316i French export models had 3.91 diff. Based on that, one might think that it would work also on 318i.

I could easily live with lower acceleration. Usually at 60 mph I use fifth gear when overtaking someone. With this new diff it wouldn't be any bother if I need to use fourth gear instead of fifth when overtaking. Even better if the 3.91 would improve fuel economy. The fuel prices in Finland are ridiculous.

[engine_ear]

I've been pondering the same, I do a lot of motorway miles and having the engine revving away at 3000+ rpm seemed a little high. Almost like you could do with having another gear
Done a few excel calcs on the ratios; with 14" wheels and 0.81 5th gear ratio (think thats right for the Getrag 240?) its ~2800rpm at 60 and ~3200rpm at 70 with the 4.1 diff. Seems to match indicated speeds in my 318 Touring
With the 4.41 its ~3000rpm at 60 and ~3500 at 70, seems closer to what you're getting?
(Its not going to match exactly, real life throws in all sorts of errors)

The 3.91 drops it to ~2600 at 60 and ~3000 at 70. For me that wasn't enough of difference to make it worthwhile.

Larger wheels would also drop engine speed. You'd have to increase the overall diameter though; increasing the rim size but with low profile tyres doesn't always mean a massive increase in overall size. I've no idea what you can fit under the arches tho!

I've been meaning to dig out a torque/power curve for the M40, see what sort of engine speed you could get away with and still maintain 70. No idea what effect it would have on mpg, you'd like to think a lower engine speed would be better but I'm a transmission guy not so much engines so don't really know

[jazma]

I tried to google a some sort of bmw revs calculator, but didn't find any. How did you calculate your numbers? I'll check the correct ratio next time I'm working on my car. Based on your numbers looks like it's the 4.41 diff.

At the moment I have 185/60/14 but soon I'll be fitting 8x15 RM's with 205/50/15 tires. Well, due to the costs of rufurbishment, my rm's have been at the point of "soon to be refurbished" for almost 3 years... Maybe I should have made some calculations before buying these rims. New 2" outer dishes, screws, plastic centres and badges cost almost 2x more than what I paid for the rims...

[engine_ear]

Road speed divided by tyre circumference gives tyre rotational speed at a given road speed. Then multiply up by diff ratio and gear ratio to give engine speed. The tricky bit is keeping the units consistant.

[jazma]

engine_ear: Thanks for the equation. Time to do some calculus when home from work.

Did some quick calculations with 1010 tires calculator. I don't know how to post an attachment, but there's the line "revs per mile". With the 185/60/14 tires the number is 914.6. With 205/50/15 the number is 901.6. And speedometer difference is 1.441% too slow with new tires.
Well, the speedometer diffence is right now about 7%. When driving 107 km/h, police radar shows 100 km/h.

I'll give it a shot with a new diff. There are few potential 3.91 diffs available and they shouldn't be that expensive.

I can post some results when I get some experiences. This seems to be a topic that isn't much widely discovered with stock cars?

edit: found and bought 3.64 diff from 1986 318i mainly because other candidates were not that convincing or not available at all. If it's from older 318i, it should work fine on facelift 318i?

[jazma]

Right, finally got the "new" diff installed. It really makes a difference. There is little difference in acceleration, but it doesn't bother that much. Most noticeable difference is that with the old diff it was possible to use 5th at ~40 mph/60 kmh and keep steady speed. If I try to do this know, there's instantly that sort of vibration that makes you shift back to 4th gear. But with 4th gear, revs are about 2000. At 65 mph and 5th, revs are about 2700.

Major flaw is that this diff whines little bit more than the old diff.