DIFF RATIO

Sun Mar 01, 2009 10:12 pm

Hi, Hello to you all from a new boy here! I have a dilemma I hope someone can help me with. I have 2 immaculate E30 Tourers. One is a 318i, the other is a 320i. They also run on LPG, which is great. However, now my wife has had enough of hers and has bought an MGZS (boo!). My question is this, I love the 320i the most but at 80mph it's revving it's nuts off even compared to the 318i, which surely can't be right. Does anyone know what the likely diff ratios are in both cars, and are diffs cheap for the 320 to perhaps put something like a 3.20:1 in, 'cos I'm guessing it must be pulling something like a 3.9:1 now? Thanks very much.

Sun Mar 01, 2009 11:16 pm

your 320i should have a 4.1 ratio diff (65mph @3k)

the 3.64 should give 75mph @3k but it will be very slugish round town

not sure what diff is in the 318 but it will be quite close to the 320i ie 4.1 or 4.27

any lower than a 3.64 in a 320i would be like pulling off in second everywhere and god knows what it would be like on a steep hill start

3.91 is probably your best bet

Sun Mar 01, 2009 11:55 pm

Time for a 2.5 engine swap, complete with appropriate diff!
The 2.5 runs beautifully on LPG. I've done close on 100,000 miles on gas with mine.

Mon Mar 02, 2009 1:01 am

Just out of interest. My 1.8 M42 slushbox is about to become a 1.8 M42 manual. Any idea what diff is likely to be in my car, and wether its ok for what engine is going in. Not concerned about 0-60 times or top speed. Sorry to thread hijack!

Tue Mar 03, 2009 1:02 am

Bump!

Tue Mar 03, 2009 1:09 am

Thanks guys for the detailed answers. You've answered my questions perfectly. I did consider another 325i, as I used to have one. And I do agree with you that the 325 is superior in most ways. However, although the economy of the 320 and the 325 is similar if driven sensibly, I used to get astoundingly rubbish economy if the 325 was used in anger, whereas the 320 just drops to surprisingly bad!! I've also learned (unless I was just unlucky) that both the 325 and the 318 engines are a lot weaker than the 320 which appears bullit proof and doesn't suffer at all from the head problems that seem to plague the other two. I have done 150 000 miles on the 320 and in that time it's only ever needed a clutch an exhaust and servicing. The 318 has been a pain in the butt, averaging around 20 000 miles between some mechanical drama or other. Incidentally the 318 is a 'H' plate and the 320 is an 'F' plate. Now, if you want performance, it's true, the 320 is no match for the 325, but if you just want a great car that sounds fantastic, and is as smooth as silk, then the 320, although only 200cc bigger than the 318 is the best car. My 318 averages 35mpg on petrol. The 320 manages 32mpg. I could never get that out of my 325! Maybe it was just the way I drove it... Anway, to get back to point. Are the diffs you suggest available cheaply, and how can you tell the ratio in a scrapyard? Old Jaguars have a tag on the diff casing. Do BMW use something similar? Cheers.

Tue Mar 03, 2009 1:17 am

the BMW diff also has a tag on the casing of the diff for the ratio. it should be on one of the back plate bolts . as for price it really depends on where you go for one

Tue Mar 03, 2009 1:28 am

Thanks jbh. The search begins then, 'cos with a more relaxed cruise, the 320 would suit me perfectly. All the best.

Tue Mar 03, 2009 1:29 am

jagman wrote: Are the diffs you suggest available cheaply, and how can you tell the ratio in a scrapyard? Old Jaguars have a tag on the diff casing. Do BMW use something similar? Cheers.

Yes about £30 tops for a non LSD diff

you got it, the ratio should be on a tag, e.g. 3.64. If its "S"3.64 etc then its an LSD, in which case they are worth ££££

Andy

Tue Mar 03, 2009 1:30 am

I've got an open 325i diff if you want i can check the ratio on it for you. I'm in central Scotland though but could courier if need be if you cant get anything local

Tue Mar 03, 2009 1:44 am

Thanks Andy, very useful to know...

Tue Mar 03, 2009 1:46 am

Thanks jbh, will that be 3.64 then or a 3.91? How much do you want for it. In the meantime I'll crawl under my bus and see what it's got in it now, now I know where to look! I'd also be able to arrange a courier I use all the time if you'd be happy to prep it for carriage?

Tue Mar 03, 2009 1:50 am

im pretty sure it would be a 3.64 as its from a preface lift but ill double check tomorrow. call it £25 and ill box it up and drain it for being couriered . that sound fair to you ?

Tue Mar 03, 2009 1:53 am

Sounds perfect. I'm new here so don't know how we get it all sorted out re payment and exchanging details, etc. Let me know tomorrow if that's ok with you and we'll get it sorted. Cheers.

Tue Mar 03, 2009 1:55 am

will keep you up to date even if you want give me a call tomorrow on 07856001234

James

Tue Mar 03, 2009 2:03 am

Will do. Cheers James.

Thu Mar 05, 2009 4:06 pm

sory to hijack thread.

i dr a 316 manual and not sure what shd the diff be. wat diff options can fit without need for mod to improve acceleration from standstill?

thks

Thu Mar 05, 2009 8:50 pm

Your 316 will already have one of the highest ratio E30 diffs available (probably a 4.27), and the problem is that this gives you a relatively low maximum speed in first.
As soon as you change into second, your 4.27 effectively becomes a 2.31, with the attendant reduction in acceleration.
If you fit an even higher diff ratio, you may accelerate quicker up to the now even lower maximum speed in first, but will be forced to change into second gear at a speed where before, you were still accelerating in first.