318 Track day car ...what diff to use?

Post Sat May 09, 2009 1:20 am

Just a quickie, what the best E30 diff to use to help with the gearing for my 318 E30?
It's currently screaming it's knackers off down the back straight at Snetterton, and needs to be about 5-700rpm lower if possible.
Thanks in anticipation
Rob

Post Sat May 09, 2009 6:30 am

4.10 i think but i could be wrong and it is a bit moor.

Post Sat May 09, 2009 8:23 am

Thanks Leon,
It's not too much of a problem if the change drops the RPM a little bit more than I suggested.
Our engine quite torquey (well, for a 1.8 M40) What model had the 4.10 diff?

Post Sat May 09, 2009 1:01 pm

The production bmw guys use the standard gearbox and diff (4.1) in the championship, and I've not heard of them hitting the rev limits in top gear at Snetterton.

Post Sat May 09, 2009 2:11 pm

It's not hitting the rev limiter, the engines more suited to torque than revs, hence me enquiring about the options.

Post Sat May 09, 2009 2:44 pm

Rob, if you can tell me what gearbox, what diff ratio, and what size rear tyres you currently have on the car, then I can tell you exactly what speed you are doing at what RPM in which gear. After that I can tell you exactly what diff you should look for.

I'm not going to assume "standard" as I don't know what work has been done on your car. Could be the diff has already been changed for a high ratio job, or you have non-standard size tyres.

If you don't know your current diff ratio then have a look at the tag that's usually attached to one of the bolts on the aluminium diff cover plate. 'Might have a sticker on the cover plate if its a later model part.

Geoff

Post Sat May 09, 2009 4:28 pm

Munich,
Does this mean your keeping the car now?

Ste.

Post Sat May 09, 2009 8:52 pm

It does appear I might be buying my mate out of it!
We've acquired him an E36 saloon IS model for buttons, and I am hoping to keep "our" one.
Should know by Monday.......

And thanks Geoff, I'll update once I've been under the car.
It's currently got a standard box, Piper cams, intake, scorpion exhaust, but not sure of the diff....to be continued!

Post Sat May 09, 2009 9:13 pm

.... And rear tyre size please. It's often forgotten that the last gear in the drive-train is your wheels.

Post Sun May 10, 2009 11:21 pm

195/50/15's GeoffBob.
It's quite possible I am expecting too much from our 1.8, but it does seem as if it might benefit from the lower gearing.
We'll see!

Post Mon May 11, 2009 9:29 am

MunichMotorsport wrote:195/50/15's GeoffBob.
It's quite possible I am expecting too much from our 1.8, but it does seem as if it might benefit from the lower gearing.
We'll see!

Thanks Rob, I'll see what I can work out for you later today. For the time being I will assume your car still has its original diff.

If your modifications to the engine have indeed resulted in an increase in torque at lower RPM, then I quite understand your predicament - you need to get your RPM lower in order to find the torque that you require, and when you are in 5th already, it's not as if you can shift up to bring the RPM down, so I can sympathise.

Be warned, however, that by lowering your diff ratio you will reduce your acceleration off the line, since you will effectively be reducing your wheel torque. But if you are at the wrong RPM to find the torque you need while circling the track, then what you lose off the line you just may regain on the straights, so this operation may work for you.

I'll post the results in this thread and on my FTP site as well. That way you (and anyone else who is interested) can download and play with the spreadsheet values (gearbox and diff ratios, and tyre sizes) as you see fit.

Post Mon May 11, 2009 11:46 am

Sorry Rob, but I'm drawing a blank here. Can you please confirm for me, is your car an M10 or M40 engined 318i, or an M42 engined 318is. From Cragles comment it sounds as if we are discussing a 318is with the M42 motor?

According to the e30zone wiki, the M10 engined 318i was available with three possible diffs, and the M40 engined 318i with two.

If its an M42 engined 318is, then it was only available with the 41:10 (4.1:1) diff.

I'm afraid I'll need for you to answer the above and check that tag on your diff for me (unless its a stock 318is) before I can go further. The year of manufacture would also be a help.

Post Mon May 11, 2009 5:06 pm

Its an M40 318i, 1987 5 speed. I'll be under the car later in the week, and can check the tag then. Thanks for your interest in this!

Post Mon May 11, 2009 6:45 pm

Rob, this is beginning to makes sense. The M40'd 318i was sold with the two highest ratio diffs in the E30 range (the 49:11 and the 47:11). This explains why she'd be a real screamer on the track. Look out for either of these two markings stamped on the diff tag. Chances are you are in the unlucky position of having one with a 49:11 (4.45:1) ratio diff, which would make her the worst screamer of the lot.

These high ratio diffs were intended to give the smaller engined cars acceleration (by increasing the wheel torque) that was comparable to other models, but at the expense of top speed. However, with your modified engine you probably have enough torque to warrant a lower ratio diff in favour of a higher top speed.

You're welcome. I'll check back once you have the data off that tag.

Post Fri Jun 19, 2009 12:24 am

Sorry for the delay in the answer/update, been busy adding seats and harnesses to the car prior to track evening at Snetterton on Tuesday.
Is the tag mentioned visible without getting the car up on ramps?
The revs definitely need dropping, on the straights it's getting to 105 ish and running out of steam

I'm not expecting too much, I could nt even keep up with an S plate Vectra SRi down the straight, and thats still got seats etc in it.
Once I've got the tag code, I'll come back on and update.
Thanks

Post Fri Jun 19, 2009 2:49 am

my 318 has a 3.91 diff as standard, the ones with the 4.45 diff were all automatics so you probably have the 4.10, but could well be 3.91 (unless its been swapped for a 4.45 one).

My 3.91 one with my very (very very) unhealthy m40 could do 100 if pushed and that's in a non stripped car so if yours is topping out at 105 it sounds to me like you have had your diff swapped for a 4.45 one.

(edit)oh and I'm on 14" wheels with 195/65 tyres(/edit)

Wont know for sure until you can get the code though. Will be interesting to find out, as I'm debating whether or not to go for a 4.27 or 4.45 diff as my car is used for lane work and so high acceleration is a big plus. But I'm worried it will get negated by the increase in gear changes.

Post Fri Jun 19, 2009 7:49 am

So any ideas if the code is visible without jacking the car up and where it's located on the diff?

Post Fri Jun 19, 2009 9:11 am

MunichMotorsport wrote:So any ideas if the code is visible without jacking the car up and where it's located on the diff?

You might be able to get at it without jacking it up, but so much easier if you at least raise the rear right wheel. The tag (metal strip, roughly 15mm x 50mm square from memory) is usually attached to the diff at one of the right-hand bolts that hold the aluminium diff cover in place (unless the diff has been worked on and the tag moved or discarded). On later model diffs, to the best of my knowledge, the tag was replaced with a sticker stuck more-or-less upper-centre of the aluminium diff cover. A bit of rubbing with a rag is usually required to make either visible/readable.

Post Fri Jun 19, 2009 10:26 am

Thanks GeoffBob, I'll get on it over the weekend.

Post Fri Jun 19, 2009 6:19 pm

Dont forget that Snett has the longest straight in the country - it wont be a problem ANYWHERE else!
You'll lose out big time accelerating out of the tighter corners on most UK tracks.

Kim.

OK yes, I know - if you're on the full circuit at Bedford you might be pulling those kind of revs.
But Bedford doesn't count as it's not a RACE circuit.