M30 rear diff ratios

Post Wed Jul 22, 2009 8:04 pm

I'm starting to wonder if I would like a different rear diff ratio in my M30 to make better use of it on track. I have done the graphs below to try and help and just wanted to check the information with others!
I have used the following ratios for the gearbox which I believe are right for an e34 5 speed box:
1st: 3.82
2nd: 2.2
3rd: 1.4
4th 1
5th 0.81

Tyre sizes as follows: 205/50/15 Toyo R888s

Do these seem correct?

These are the graphs I came up with, the 3.25 one seems correct to me as that is what I have at the moment, if others with a 3.46 and 3.64 could check I would be most grateful!

Post Thu Jul 23, 2009 12:53 am

I suppose it depends on what you want to achieve on the track, high speed or acceleration.
iirc Robbo did my calcs for me.
The g/box ratios ring a bell as the box is a 1:1 ratio on 4th gear.
It looks right, but i'm on a 3.46 diff with a 5 box.

Excuse my naivity, but what is the bottom of the graph, the other is the revs?

Post Thu Jul 23, 2009 1:08 am

Entertain me for a moment...

I doubt on any UK track you will see more than 120mph. So even on the 3,64 you are driving effectively a 3 speed box on track, discount first and you are not going to need fifth for anything other than snetterton I wouldn't have thought. Everyone knows I am a short diff hooligan so entertain my thought processes and post up the figures for a 3,91.

And if anyone has a decent m30 power graph that they could post I reckon you could work it out quite well. For example mine makes max power at 7100rpm, so I want to be up there the whole time, the shorter the diff the better, but if your m30 is making power a lot lower down it might not be so simple. I have a great calculator for french tat gearbox's that shows:

-mph/1000rpm

-mph@redline

-rpm@Cruising(for a given speed)

-ratio differences

-upshift RPM

In fact I will see if I can input your figures into it and post back 3,91 results.

Post Thu Jul 23, 2009 1:16 am

So I have done that, sorry Henry I am too lazy to host a graph.

Max speed in any gear given 6500rpm limit and the gear ratios and FD you provided with a 205 50 15 tyre:

1st: 30
2nd: 52
3rd: 82
4th: 114
5th: 141

And quite importantly the upshift RPM(the rpm you will be from a 6500 shift from 1st to 2nd for example)

1st-2nd = 3743
2nd-3rd = 4136
3rd-4th = 4643
4th-5th = 5265

Interesting, cruise(70mph) rpm in fifth is an acceptable 3230. I wish mine was like that!!

The upshift figures look about right to me for a car that will probably be fairly good after 4000rpm up to 6000. Someone post an m30 dyno graph!!!

Post Thu Jul 23, 2009 1:19 am

Being sad I just looked at a 4,10 FD. Now that would be good fun in an m30.

Post Thu Jul 23, 2009 1:19 am

Rav335uk wrote:Excuse my naivity, but what is the bottom of the graph, the other is the revs?

Miles per hour.

Post Thu Jul 23, 2009 1:23 am

Right then, your graph is slightlt out on the 3.46 graph, i am doing 100mph at about 3300 rpm then.

Post Thu Jul 23, 2009 1:10 pm

Winx,

I get the same results as you do.

FYI, I have included results for the 3.91:1 and 4.1:1 differentials.

If your sole intention is to race then you would do well to follow Jon's advice and fit a 3.91:1 diff, or even a 4.1:1. These will at least give you four gears to work with and keep your engine RPM up.

However, if you are anything like so many other zoners who have asked this same question before, you probably want to keep your engine RPM down around 3000RPM at 70mph (in 5th) for motorway cruising, in which case you have to compromise and opt for a lower ratio diff (possibly the 3.64:1), in which case you will likely be limited to three gears on the track. It's a decision that only you can make based upon your intended application.

Regards
Geoff

Post Thu Jul 23, 2009 2:15 pm

Thanks for the graphs Geoff, I was being too lazy in excel last night, in my defense it was after 12.

The more I look at the 4,10 the more I think it is right.

If the car does less than 5000miles per year(bare in mind all those rocker snaps

) i'd fit it. The difficulty you might have is getting a 4,10 medium case slipper. Andyboy made reference to how you can mutilate a £20 e28 4,10 diff and an e30 LSD unit to make one good 4,10 medium case slipper. Search for a wanted post by 'on_a_roll.'

Interestingly I think a 4,10 medium case diff would be worth more than even the rare'ish e30 LSD ratios because of its high FD and the fact it will fit small and large diffed cars(with some prop adjustments). The rally boyos would pay good money for a high FD slipper if it ever came to breaking it into bits.

To sum it up: 3,91 for the sensible'ish choice - it is still geared to do 140mph remember

4,10 would be more effort but much more rewarding on track I reckon. Slightly unpleasant on long drives though.

Anyone got a power graph for an m30 yet?

Post Thu Jul 23, 2009 3:02 pm

M30s are torquey enough not to need constant red-line bashing!

Even with a 3.73:1 diff I always used to pull away in second except on steep hills, so a 4.1:1 will make first gear almost completely useless!

Post Thu Jul 23, 2009 3:57 pm

This is closest to stock I can find on google for an M30/B35...

Very interesting stuff here, I am hoping that my new car will be here by September at which stage the M30 really does become just a track tool. As it will live in London for now it will have to be driven to the tracks/ germany(ring) so I think I'll go with a 3.91... Or possibly get a 4.1 for UK work and then keep the 3.25 for relaxing trips to Germany!!

Thanks for the help guys

Post Thu Jul 23, 2009 4:51 pm

Being sad I just looked at a 4,10 FD. Now that would be good fun in an m30

Going that way with a dogleg box, lucky to see 100mph at knockhill circuit.

4.1 if you prepared to search ebay.com they turn up I bought a diff and paid the guy extra to strip the cwp and ship it, worked out about £175 for the cwp to my door.

Jason

Post Thu Jul 23, 2009 7:25 pm

out of interest, could someone work out the figures with a 3.38:1 diff for me.
I have a mutant diff here that i'm slooooowly building.

the other things to remember with a M30, mine has a 5750rpm limiter afaik (auto ecu) and to be honest, shifting at anything over 5000rpm is becoming pointless as the torque has dropped off by then.

Post Thu Jul 23, 2009 10:01 pm

Post Sat Jul 25, 2009 8:39 am

160+ mph

probably 'only' about 145 before i hit my current limiter though

Post Sat Jul 25, 2009 10:26 am

Gareth, a 3.46 will get you upto 160mph, theoretically,

Post Sun Jul 26, 2009 8:46 am

Gareth, best to be careful reading these sort of graphs. They tell you how fast your car will go (for a given tyre size, final drive ratio and set of gear ratios) as a function of your engines RPM.

Whether or not your engine will make it to say 6500RPM in 5th gear at close on 160mph is entirely up to you and your engine. It's generally a question of whether your engine has the power at that engine RPM to overcome wind resistance (drag) at that speed.

FYI, most road going cars are geared so that, in top gear, it appears that they can achieve a very high top speed. They of course seldom have the power to go this fast and the reason for choosing a ratio like this for the highest gear is to achieve low RPM while at cruising speed, not to achive high speed at high RPM.

Track cars (if not fitted with an entirely custom set of gears) will generally use a higher ratio differential than standard resulting in a fairly low top speed (hopefully a touch higher than the longest straightaway requires).

Post Mon Jul 27, 2009 7:51 pm

GeoffBob wrote:Gareth, best to be careful reading these sort of graphs. They tell you how fast your car will go (for a given tyre size, final drive ratio and set of gear ratios) as a function of your engines RPM.

Whether or not your engine will make it to say 6500RPM in 5th gear at close on 160mph is entirely up to you and your engine. It's generally a question of whether your engine has the power at that engine RPM to overcome wind resistance (drag) at that speed.

FYI, most road going cars are geared so that, in top gear, it appears that they can achieve a very high top speed. They of course seldom have the power to go this fast and the reason for choosing a ratio like this for the highest gear is to achieve low RPM while at cruising speed, not to achive high speed at high RPM.

Track cars (if not fitted with an entirely custom set of gears) will generally use a higher ratio differential than standard resulting in a fairly low top speed (hopefully a touch higher than the longest straightaway requires).

Understood entirely, however with my current low limiter and 3.91 diff, it steams into the limiter in 5th at a fair rate. it't still pulling very well so i guess has a fair bit to go yet.

another factor to the top speed is where the power / torque curve of the engine are in relation to the speed you're travelling. my mates 1.4 nova SR (back in the day) used to achieve a higher top speed in 4th than it would in 5th as it didn't have the torque to pull through the air in 5th!

Post Tue Jul 28, 2009 9:33 am

gareth wrote:my mates 1.4 nova SR (back in the day) used to achieve a higher top speed in 4th than it would in 5th as it didn't have the torque to pull through the air in 5th!

Yip. A good example of where 5th has been geared for economy, not performance (and admittedly more of a problem with smaller engined cars). Increasing the final drive ratio might have made his 5th gear more usable as he would have been able to get his revs up to where he could get to the torque needed to overcome wind resistance. Problem is that this would have come at the cost of increased fuel economy, something that manufacturers don't want their cars to have.

But doesn't sound like you have this problem with your car

If she is comfortably running into the rev limiter at 145mph then a) you're braver than I am and b) sounds like you might do well with a lower ratrio diff?

BTW, how did you manage to assemble a 3:38:1 diff? IE: How many teeth on the crownwheel, and how many on the pinion (possibly 44:13, but where on earth did you get a 44 tooth crown-wheel, E36?), and most importantly, do they mesh properly? Sounds like a really special diff you have there.

Post Tue Jul 28, 2009 7:29 pm

ahaa, i could tell you, but then i'd have to kill you!

ok, it's the CWP from a E34 525ix rear diff. from my initial measurements, the crownwheel and pinion from it seem the same size as the 188mm E30 large (bmw medium) case diff.

originally this E34 diff is a strange bit of kit, it's got a fluid port on it where the traction control can then lock it (via pressure from the PAS pump i guess) as it's got a set of LSD clutches and a hollow hydraulic cylinder to apply the force to them instead of the traditional ramps.

here it is:
http://www.realoem.com/bmw/showparts.do ... g=33&fg=05
http://www.realoem.com/bmw/partgrp.do?m ... g=33&fg=10
http://www.realoem.com/bmw/partgrp.do?m ... g=33&fg=20

Post Wed Jul 29, 2009 5:12 pm

Now that sounds like a nice bit of kit to have Gareth. Wouldn't mind get my hands on something like that myself one day.

Post Wed Jul 29, 2009 11:03 pm

Henry, not pissing on the fire but if you go 3.91 you could well hit the limiter in 5th in 3 or more places in 5th round the ring! Its the opposite to the short sprint tracks we've got over here. f**k me it'd go with a 4.1 in it

3.64 job jobbied! Also if you went turbo, the jon bmw ratio becomes an even worse idea

Interesting reading looking at the graphs guys

Post Thu Jul 30, 2009 1:18 am

Simon13 wrote:Henry, not pissing on the fire but if you go 3.91 you could well hit the limiter in 5th in 3 or more places in 5th round the ring!

i did that when i was there last! 145mph indicated, 132mph on the tomtom.

Post Thu Jul 30, 2009 9:08 am

The solution is to fit a massless eight-speed gearbox made from Unobtanium with an ideal:1 ratio diff. Then you have a car that's matched to all applicattions. If only such things existed ..

Alternatively, wouldn't it be great if you could effectively change your final drive ratio at the flick of a switch on the gear knob! Wait a minute, British Leyland used to fit those (gearbox overdrives) to Triumphs in the seventies, some good they were.

Now I know why I have three diffs in my collection and made an access panel in my boot so I can swop diffs much easier.

Post Thu Jul 30, 2009 10:08 am

Ring n00b here, but is it really important to do 145mph rather than 138mph in a few places??

Agreed about the turbo though, the massive increase in torque you would PROBABLY(turbo size) get would mean you could be a bit more cack handed with gearing choice and get away with it.

No one watch the GP race on Sunday, you choose the right equipment for the current conditions. Don't do a stoner and choose a 3,25!

Ok, i'll comprimise, 3,91 is the diff to have.

P.S I have always got a bit confused with this next bit as I don't have a physics background...at all.

The car is geared to do 145mph in fifth for example.

But you have resistance in the form of traction and air speed. My presumption is that you will need more revs and power to exceed the two resistances than if you were travelling in a vaccum on sheet ice. So can you still do 145mph because the car has the power to get up to 6500rpm and beat the two resistances or does the car slow down and not make the full 145mph?

I am still not sure.

Probably a very simple question.

Post Thu Jul 30, 2009 10:40 am

Jon the ring is a very fast track, i never use 2nd. It's all 3rd and 4th and once you learn the circut you grab 5th in 4 or 5 places. This is in M20 engined car, but i've no doubt most other cars do the same just alot faster than me E30!

Post Thu Jul 30, 2009 10:50 am

Fair does.

Post Thu Jul 30, 2009 11:09 am

Jon_Bmw wrote:I have always got a bit confused with this next bit as I don't have a physics background...at all.

The car is geared to do 145mph in fifth for example.

But you have resistance in the form of traction and air speed. My presumption is that you will need more revs and power to exceed the two resistances than if you were travelling in a vaccum on sheet ice. So can you still do 145mph because the car has the power to get up to 6500rpm and beat the two resistances or does the car slow down and not make the full 145mph?

I am still not sure.

Probably a very simple question.

If the car is geared to do say 145mph in 5th on it's RPM limit then it will theoretically do exactly this regardless of wind resistance (assuming traction is maintained). This fact is goverened purely by the drive train that joins the engine to the road.

Now, if the car in fact has the power to do 145mph at its RPM limit in 5th on a flat stretch of tarmac then in vacuum (with no wind resistance) it would do exactly the same speed at the same RPM, but at a smaller throttle opening since the engine now doesn't have to work so hard due to the absence of wind resistance. Opening up the throttle won't make the car go any faster at the same RPM since this is goverened by the drive-train, not by the wind resistance (or abence there of). In vacuum, however, the engine may well have the power to make the car faster provided you are prepared to allow the engine RPM to rise higher.

I wouldn't recommend you try this though as both the engine and driver won't function in a vacuum

Putting the car on ice is an altogther different matter since this destroys the traction by introducing slippage. But I can see where you are going with this. Ideally you don't want a zero friction surface, but you do want zero rolling resistance - In other words your wheels just become inert objects that join your hubs to the road surface without the wheels themselves consuming any of the engines output power.

Regards,

Post Thu Jul 30, 2009 11:37 am

Cheers Geoff, it is what I thought, but I didn't want to guess.

Yeah right!

Post Thu Jul 30, 2009 1:59 pm

Simon13 wrote:Henry, not pissing on the fire but ....

No need mate, the car does that all for it's self...

All this talk of graphs may well be academic in 48 hours, trying to arrange a part exchange with a chap at work for a 270bhp, 560kg kit car, which will hopefully actually get used on track unlike this one

Post Thu Jul 30, 2009 5:32 pm

the end is nigh then

Post Thu Jul 30, 2009 9:31 pm

winx wrote:
Simon13 wrote:Henry, not pissing on the fire but ....
No need mate, the car does that all for it's self...

All this talk of graphs may well be academic in 48 hours, trying to arrange a part exchange with a chap at work for a 270bhp, 560kg kit car, which will hopefully actually get used on track unlike this one

Oh dear, sorry to hear this.

On the other hand, lets have some details on the kit car please!

Post Thu Jul 30, 2009 10:54 pm

Was supposed to be seeing it tonight but had to work late so the guy is bringing into work tomorrow. It's an MK i believe with and sr20det you will be pleased to hear Theo!

Post Fri Jul 31, 2009 1:31 am

A mate of mine built quite a serious and well sorted (don't laugh) MGB Roadster with an SR20DET, and it went very well indeed!

I'd like to see pics of this kit car too, sounds great!