3.91 LSD or 4.10 LSD???

[jon-m42]

Morning All,

I have read up somewhere on here and on a other few forums that a 3.91 LSD is desired and will improve acceleration on the old little pug.

I was thinking about going out hunting for one however before I do, I would like to know a little more about it.

It will be intended for my absolute standard 318iS m42 H Reg.

If possible I would like to improve the acceleration off the line and constant speeds on the motorway runs.

A few have mentioned to go for the 4.10LSD but more have mentioned to go ahead with the 3.91LSD which I presume comes of a 325i Motorsport.

Has anybody had this done and which do you prefer?

what are the advantages ?

Are there any real benefits?

Thanks in advanced
Jon

[daimlerman]

4:1 gives 65mph at 3000rpm
3:64 gives 73mph at 3000rpm

So a 3:91 will fall between the two,say 68mph at 3000rpm.

In theory you will gain a touch of mpg,and lose a little acceleration off the line.
May be a tad quieter on the motorway.

[jon-m42]

4:1 gives 65mph at 3000rpm
3:64 gives 73mph at 3000rpm

So a 3:91 will fall between the two,say 68mph at 3000rpm.

In theory you will gain a touch of mpg,and lose a little acceleration off the line.
May be a tad quieter on the motorway.

Ah would any of these improve acceleration?

A few months back I did look into briefly the 325 clutch and lightened flywheel. This seemed to show great benefit to a few of the individuals who had gone ahead with this.

About the LSD's which one would improve the acceleration fo such instances on the line at santa pod?

[Brianmoooore]

LSDs have nothing to do with acceleration!

[jon-m42]

LSDs have nothing to do with acceleration!

Please explain, It seems I have been told different. I have read up that installing a 3.91 lsd will improve acceleration.

If a LSD will take no effect at all then what will improve acceleration??

Thanks

[daimlerman]

Fitting a LOWER ratio diff will improve acceleration!

Try hunting up a 4:27 or a 4:45,but you will not find an LSD version.

[jon-m42]

Fitting a LOWER ratio diff will improve acceleration!

Try hunting up a 4:27 or a 4:45,but you will not find an LSD version.

How will this improve acceleration?

[daimlerman]

Fitting a LOWER ratio diff will improve acceleration!

Try hunting up a 4:27 or a 4:45,but you will not find an LSD version.

How will this improve acceleration?

Are you trying to wind me up?

[jon-m42]

Fitting a LOWER ratio diff will improve acceleration!

Try hunting up a 4:27 or a 4:45,but you will not find an LSD version.

How will this improve acceleration?

Are you trying to wind me up?

No not at all. I was just trying to understand a little bit more about what you said.

You mention it improves acceleration and I asked how? Please explain your answer?

Thanks

[daimlerman]

4:27= 60mph at 3000rpm

Compare that to my earlier post!

A quick clue,the LOWER the road speed,the FASTER you get there...

[ross_jsy]

Right a lower ratio differential (be it LSD or open) improves acceleration as it's effectively like having a lower gear ratio in the gearbox. Imagine pulling away in 1st as opposed to second, or changing down gears on a push bike.

[jon-m42]

Thanks Guys.

I can understand my lack of knowledge and questions can be annoying to sum of you e30 gods out there. Please do note I do not mean to annoy anybody and or upset anybody. I am just simply trying to question and learn.

I understand that the lower the ration i.e. 3.91 & 3.64 dif's will improve acceleration in comparison to the 4.10 & 4.27.

Thanks again guys.

[Willeh]

Fitting a LOWER ratio diff will improve acceleration!

You mean higher ratio? To the OP, the general rule with anything to do with gearing, including the rolling radius of your wheels, is that higher ratio differentials, gearboxes, smaller wheels, etc. give more acceleration with the trade off in top speed. That's why you don't pull away in 4th. And to extremely simplify the purpose of an LSD, its to ensure you are putting power to both wheels when you need it.

http://en.wikipedia.org/wiki/Limited-slip_differential

The only way it'll help you accelerate is if you're breaking traction.

[daimlerman]

Fitting a LOWER ratio diff will improve acceleration!

You mean higher ratio? To the OP, the general rule with anything to do with gearing, including the rolling radius of your wheels, is that higher ratio differentials, gearboxes, smaller wheels, etc. give more acceleration with the trade off in top speed. That's why you don't pull away in 4th. And to extremely simplify the purpose of an LSD, its to ensure you are putting power to both wheels when you need it.

http://en.wikipedia.org/wiki/Limited-slip_differential

The only way it'll help you accelerate is if you're breaking traction.

LOWER ratio improves acceleration

4:27 is LOWER than 3:91 because the bloody engine is running FASTER for the same sodding road speed!

Did you lot learn nothing at school?

[pacerpete]

Thanks Guys.

I can understand my lack of knowledge and questions can be annoying to sum of you e30 gods out there. Please do note I do not mean to annoy anybody and or upset anybody. I am just simply trying to question and learn.

I understand that the lower the ration i.e. 3.91 & 3.64 dif's will improve acceleration in comparison to the 4.10 & 4.27.

Thanks again guys.

WRONG ! The reverse is true. A 3.64 in a torqueless turd like a M42 would be rather tragic from an acceleration perspective but theoretically relaxed and very economical at constant motorway speeds. If you must have a LSD, your easiest option is a medium case 325 3.91 and fit 205/50 tyres (assuming you have 15" BBS) to get the gearing similar to stock.

Personally, I would save my money. An IS does not really have the power / torque to benefit from a LSD

[Willeh]

LOWER ratio improves acceleration

4:27 is LOWER than 3:91 because the bloody engine is running FASTER for the same sodding road speed!

Did you lot learn nothing at school?

I have a feeling we're arguing semantics here, but 4.27 is HIGHER than a 3.91 because the ratio of number of teeth between the crown wheel and pinion is HIGHER. That's the way I was taught in college during my transmission lessons, but maybe I wasn't paying attention.

[pacerpete]

Higher numerically = lower geared.

[Willeh]

Well I guess if that's the general consensus, then I'm wrong. Lesson learned.

[daimlerman]

LOWER ratio improves acceleration

4:27 is LOWER than 3:91 because the bloody engine is running FASTER for the same sodding road speed!

Did you lot learn nothing at school?

I have a feeling we're arguing semantics here, but 4.27 is HIGHER than a 3.91 because the ratio of number of teeth between the crown wheel and pinion is HIGHER. That's the way I was taught in college during my transmission lessons, but maybe I wasn't paying attention.

No you were not listening correctly,I guess a short skirt distracted you?

[jon-m42]

Thanks Guys.

I can understand my lack of knowledge and questions can be annoying to sum of you e30 gods out there. Please do note I do not mean to annoy anybody and or upset anybody. I am just simply trying to question and learn.

I understand that the lower the ration i.e. 3.91 & 3.64 dif's will improve acceleration in comparison to the 4.10 & 4.27.

Thanks again guys.

WRONG ! The reverse is true. A 3.64 in a torqueless turd like a M42 would be rather tragic from an acceleration perspective but theoretically relaxed and very economical at constant motorway speeds. If you must have a LSD, your easiest option is a medium case 325 3.91 and fit 205/50 tyres (assuming you have 15" BBS) to get the gearing similar to stock.

Personally, I would save my money. An IS does not really have the power / torque to benefit from a LSD

Right I think I have finally understood the whole concept of the ratios higher and lower. I clearly at first thought the 3.91 was a lower ratio as numerically its lower then 4.10 but in gearing terms its the opposite.

3.91 may well improve acceleration but will then provide a lower top-end (not that the iS has much of a top end) but that's totally fine as I never really push past 85mph on long motorway journeys.

the 4.10 may well provide a higher top end but will be slower off the line.

I was thinking to drive up to santa pod and just for a little fun take the old girl up the stretch.

Thanks to All.

[Brianmoooore]

That's the way I was taught in college during my transmission lessons, but maybe I wasn't paying attention.

It's also the way I would have taught it, if I'd been giving the lecture! To me a 4.27 is a higher ratio than a 3.76, I'd increase diff. ratio to increase acceleration, and reduce it to increase economy (within reason).
I am aware that plenty see it the other way around, however, so always include the word 'numerically' when talking about a higher ratio diff.

[Willeh]

3.91 may well improve acceleration but will then provide a lower top-end (not that the iS has much of a top end) but that's totally fine as I never really push past 85mph on long motorway journeys.

the 4.10 may well provide a higher top end but will be slower off the line.

The other way around. The higher the number, the better the acceleration/ lower top speed.

No you were not listening correctly,I guess a short skirt distracted you?

I hope not, engineering lecture is practically the dictionary definition of sausagefest.

It's also the way I would have taught it, if I'd been giving the lecture! To me a 4.27 is a higher ratio than a 3.76, I'd increase diff. ratio to increase acceleration, and reduce it to increase economy (within reason).
I am aware that plenty see it the other way around, however, so always include the word 'numerically' when talking about a higher ratio diff.

That's how I've always seen it.

[jmc330i]

3.91 may well improve acceleration but will then provide a lower top-end (not that the iS has much of a top end) but that's totally fine as I never really push past 85mph on long motorway journeys.

the 4.10 may well provide a higher top end but will be slower off the line.

Nope. Fitting a 3.91:1 diff to an iS (with the standard 4.10:1) will provide a slightly higher top end with slightly slower acceleration.
If you fitted a 4.27:1 diff, you would get a lower top end with quicker acceleration.

[jon-m42]

3.91 may well improve acceleration but will then provide a lower top-end (not that the iS has much of a top end) but that's totally fine as I never really push past 85mph on long motorway journeys.

the 4.10 may well provide a higher top end but will be slower off the line.

Nope. Fitting a 3.91:1 diff to an iS (with the standard 4.10:1) will provide a slightly higher top end with slightly slower acceleration.
If you fitted a 4.27:1 diff, you would get a lower top end with quicker acceleration.

Right I think Ill go take a tea break and let my baffled old mind steer away from all this for a moment. I thought the 3.91 would provide a quicker off the line acceleration and a lower top end and the 4.10 to provide a slower off the line with a higher top end.

[jmc330i]

It will be intended for my absolute standard 318iS m42 H Reg.

If possible I would like to improve the acceleration off the line and constant speeds on the motorway runs.

As Brian said, LSDs have nothing to do with acceleration, so look at the ratio for your desired needs. Then look for an LSD in that ratio if you want - for a bit of track use or wet roundabouts...

If you fit a diff with a ratio to improve acceleration (a 4.27:1 in your case) you will have slightly higher revs while cruising on the motorway. If you fitted a 3.91:1, you get the opposite. Unfortunately, there is no diff that can improve both, just a happy medium.

If you want an LSD, a 3.91:1 will be a good choice from a cost and availability point of view compared to a 4.10:1 LSDs which are quite rare.

I fitted a 3.91:1 open diff to my 318i touring (which had a 4.27:1 iirc) and it made it more civilised on a run. It was possibly slightly slower 'off the line', but not overly noticeable with the weasy M40!
I am fitting a 3.91:1 LSD to my touring once I've fitted the 16v engine.

[maggspower]

A 3.91:1 diff means:

3.91 turns of the prop shaft, to 1 turn of the rear wheels. The rest you should be able to work out for yourself.

[jon-m42]

A 3.91:1 diff means:

3.91 turns of the prop shaft, to 1 turn of the rear wheels. The rest you should be able to work out for yourself.

3.91 turns from the prop = to 1 full turn of the wheels

4.10 turns from the prop = to 1 full turn on the wheels

this should mean if im correct then that the 4.10 turns to 1 full turn of the wheel is faster than the 3.91 turns to the 1 full turn of the prop.

should this mean that the 4.10 ratio would have a faster acceleration.

Im very much a school kid still learning when it comes to all this technical stuff.

[daimlerman]

Correct!

So a 4:27(out of a 316i) will give you faster acceleration...

[jon-m42]

Correct!

So a 4:27(out of a 316i) will give you faster acceleration...

Thank you for the help so far I must admit I get it but then if I keep on thinking about it I get confused again. I guess that's what happens with old age.

4.10 turns to 1 turn of the wheel means that it takes more turns on the prop to complete 1 full turn of the wheel

now 3.91 turns to 1 turn of the wheels means less turns on the prop to make the 1 complete turn of the wheels... therefore is this now in the thoughts of understanding it takes less turns to complete 1 turn of the wheel so this must mean its quicker or is this not the case?

the 4.10 takes more turns which in turn will take more time to spin 1 complete turn of the wheels no?

[Grrrmachine]

you are thinking right jon, but you're confusing speed with acceleration - they're not the same thing.

At a fixed engine speed (3000rpm), a 3.91 diff will make the car go faster than a 4.10 diff. But that presumes the car is already moving and the engine is already turning. What you're concerned about is how to go from 0 to 100mph, and more importantly, how much the car DOESN'T want to do that.

Think about it from the wheel's perspective. The wheel doesn't want to turn; it's connected to the car, which is really heavy and stubborn and reluctant. And because the weight of the car is much larger than the "weight" (torque) the engine can produce, then the car is going to win in a one-to-one fight.

So we multiply the "weight" of the engine with the diff, and for every one turn of the wheel, a 3.91 diff can multiply engine "weight" by 3.91 times. However, a 4.10 diff can multiply the engine power even more, which gets the wheels moving quicker, which means you accelerate faster. In contrast, a 3.25 diff hardly offers anything, so if your engine has low "weight" to start with then the weight of the car will outweigh whatever the engine/diff can provide, and the engine will just stall and die.

If you're still not sure what I mean, think about a spanner turning a rusty bolt. If the spanner is a usual 9" long type, then the nut will not move; it's just too rusty. But it we make the spanner 3.25 times longer (so 29") then we stand a chance of turning the nut. Make the spanner 4.1 times longer, and it's now a 3' breaker bar - that nut has no chance. That's how a shorter diff (higher number) makes a car ACCELERATE quicker, but gives it a lower top SPEED.

Does that make sense now?

[jon-m42]

you are thinking right jon, but you're confusing speed with acceleration - they're not the same thing.

At a fixed engine speed (3000rpm), a 3.91 diff will make the car go faster than a 4.10 diff. But that presumes the car is already moving and the engine is already turning. What you're concerned about is how to go from 0 to 100mph, and more importantly, how much the car DOESN'T want to do that.

Think about it from the wheel's perspective. The wheel doesn't want to turn; it's connected to the car, which is really heavy and stubborn and reluctant. And because the weight of the car is much larger than the "weight" (torque) the engine can produce, then the car is going to win in a one-to-one fight.

So we multiply the "weight" of the engine with the diff, and for every one turn of the wheel, a 3.91 diff can multiply engine "weight" by 3.91 times. However, a 4.10 diff can multiply the engine power even more, which gets the wheels moving quicker, which means you accelerate faster. In contrast, a 3.25 diff hardly offers anything, so if your engine has low "weight" to start with then the weight of the car will outweigh whatever the engine/diff can provide, and the engine will just stall and die.

Does that make sense now?

Thank you very much

I am finally clear with this all. To follow on from this what have others done to improve acceleration to their little m42's?

I do remember reading up on the zone somewhere where people have opted for the 3.91 slipper but I have not read or come across much conversation about the 4.27 diff.

[Grrrmachine]

Generally, the best way to speed up a 318is is to replace it with an M20B25

You don't hear much about fitting 4.27 and 4.45 diffs, because the cons outweigh the pros; your top speed is lower and your fuel economy is worse than with a 4.10, and it's noisier inside the cabin. And all because you want to get away from the traffic lights quicker than an E46 320d? Not worth it.

[jmc330i]

Ignore the outdated M20 comments... most people go for 24v

To improve acceleration, you will need a diff that's numerically higher than the 4.10:1 your car should have fitted. This means a 4.27:1 or a 4.45:1, but this will mean the cruising revs will be higher, so as Grrr said, more noise and fuel used. I also don't think you'll find an LSD in those ratios if you want one.

People go for the 3.91:1 LSD because they are fairly easy to come by compared to the 4.10:1 LSD.

[Brianmoooore]

The lowest ratio diff. you will find as standard on any E30 (with an overdrive gearbox) is 3.64. The highest is 4.45. Changing from a 3.64 to a 4.45 would make your fifth gear become almost the same as fourth gear was before the swap.
In a car with sufficient power to run a 3.64 (a 325i), you would be making your first gear almost useless, and effectively have changed your fifth to fourth, leaving you with just a four speed gearbox with a very tall (replacement) first gear.

[DHFiS]

To follow on from this what have others done to improve acceleration to their little m42's?

1. Have you had the car on a dyno to check what power the engine is producing at the moment? If the engine is healthy you might be able to get it remapped to produce up to 150bhp.

2. Get the fuel injection nozzles refurbed before the remap.

3. Live with the fact that modern cars are faster.