"Taller diff ratio = slower acceleration"- really?

[djk]

i think at the end of the day you can look at charts and numbers all day long, the thing is it's not going to matter utill you try one. If you put in a longer one you could say its great, but so could a shorter one too. And unless you plan to race your car ??? The other thing to look at is is your diff broken?? Is your gearbox tight, have you changed oils ,done a service lately, wheel bearings good? Lightened flywheel? Then engine mods bbtb,fiters,ecu(chip) etc. I'm not being pissy about this i just think basically its now all been said about diffs ???

The trouble is, so much talk about axle ratios goes on based on various different opinions of those with experience of what is better or worse in which circumstances. So often, there's a placebo effect, or a prejudice towards a certain outcome. It's just the nature of human reactions.

I'm doing a whole load of maths now on excel, not so I can argue a point based on my findings, but so I can really relate the physics with the experience. In this thread, I've got you saying 3.64 will be no good, and StuBeeDoo saying it will be fine - who do I believe? I think I understand gear ratios, but a little research into the topic finds some of my assumptions were incorrect and so I need to do a bit more homework to understand it properly. I will have to 'suck it and see' at some point, but I want to be able to understand what I experience when I do.

[LuckyP]

I mentioned in the original post that I had purchased a diff to replace the 3.9:1 unit in my 325i touring, mainly because I feel the car is under-geared to the extent that I rarely use first gear.

The diff I have bought has a 3.45:1 ratio. It may turn out to be slightly too tall (by that I mean the response in top gear at motorway speeds may be too flat). But my E30 is in the UK, and I only spend 90 days there each year, so it's not like I have to live with it every day.
Also, I am currenty collecting the parts to build a 2.8-litre M20, so I'm hoping the 3.45 unit will be a good choice.

Like djk, I too did a few caluclations on Excel. But as hongkongfuey said, all the number-crunching in the world means nothing until you actually try one.
The question of which ratio you prefer is very subjective. The E30 benefits from a huge range of available ratios, so you can pick and choose and find your perfect solution. Of course, this plethora of available ratios also promotes discussions such as this thread.


A couple of posts in this thread have mentioned that a longer gear would "feel" slower, due to the reduced acceleration in any given gear. I absolutely agree. But personally I can live with that, safe in the knowledge that my speedometer is, on average, going round just as fast.


Thanks for the excellent posts, guys, both for and against. I had suspected that I might have been out on a limb with my opinion, but it's nice to see I'm not alone.

Pete.

[daimlerman]

I tried a 3.45 diff in my M50'd touring.
Felt that the ability to cruise at 85mph at 3000rpm would suit me and the car...
In use the car felt overgeared,to the point I was spending more time in 4th!
I now have a 3.64,and I am using slightly less fuel.

[LuckyP]

Hmmm, maybe I've over-egged it!

But by my calculations 3,45:1 @ 3000rpm gives 76mph, as opposed to 67mph with the original 3.91 diff (205 55 15 tyres).

If my figures are wrong, I'd like to know for sure. I'm using the same calculation methods to determine where I need the torque to come in on my new engine - this determines which camshaft profiles I need to consider.


Either way, the main purpose of the exercise is to make 1st gear useable. I don't spend much time on the motorway, but starting off in second gear can't be doing the clutch any favours.

Pete.

[djk]

Hmmm, maybe I've over-egged it!

But by my calculations 3,45:1 @ 3000rpm gives 76mph, as opposed to 67mph with the original 3.91 diff (205 55 15 tyres).

If my figures are wrong, I'd like to know for sure. I'm using the same calculation methods to determine where I need the torque to come in on my new engine - this determines which camshaft profiles I need to consider.


Either way, the main purpose of the exercise is to make 1st gear useable. I don't spend much time on the motorway, but starting off in second gear can't be doing the clutch any favours.

Pete.

E30 speedos generally overread by about 8-9%, so Malcolm might be talking about speedo reading rather than 'true' speed as arrived at by gearing calculations or observed on GPS. Also it depends what factor you have included for tyre deflection in the rolling circumference for your calculations.

[hongkongfuey]

Hmmm, maybe I've over-egged it!

But by my calculations 3,45:1 @ 3000rpm gives 76mph, as opposed to 67mph with the original 3.91 diff (205 55 15 tyres).

If my figures are wrong, I'd like to know for sure. I'm using the same calculation methods to determine where I need the torque to come in on my new engine - this determines which camshaft profiles I need to consider.


Either way, the main purpose of the exercise is to make 1st gear useable. I don't spend much time on the motorway, but starting off in second gear can't be doing the clutch any favours.

Pete.

also one point i would ask, what gear (gearbox) did you use for calculating this??? if it is 4th then generally its 1/1 ratio but if in 5th its an overdrive gear so that would mean at 3000revs the prop would be going faster than the engine???

[LuckyP]

Good point, I should have made it clear.
I used the standard 325i gearbox ratios, and my calculations of road speed at 3000rpm used fifth gear.

Fourth gear is direct drive (1:1), fifth is overdrive (0.81:1), so yes the prop does go faster than the engine in top gear.
3000 engine rpm = 3700 prop rpm.

Pete.

[LuckyP]

Hello Will,
Good to hear you like the 3.46 ratio. Whilst my planned engine upgrade won't be quite the same as yours, I'm encouraged following your comments regarding first gear.
When planning my 2.8-litre E30, I thought long and hard if I wanted to end up with M52-power. In the end I decided on just using the crankshaft, as rebuilding my existing engine forms part of the satisfaction for me. Rusty 328i E36s are really cheap, though!

For the UK readers, 5.8l/100km @ 100kmh translates to 48.7mpg @ 62mph.

Whilst that's quite impressive, economy isn't really my thing - I tried it once though, just out of curiosity.
Twice a year I drive back home to England in my Golf R32 (that's the other car in which I rarely use first gear). Once, on the way to the ferry port at Dunkerque, I set the cruise control at 100kmh (measured on the sat nav). I had even pumped up the tyres to reduce rolling resistance.
8.7l/100km (32.4mpg) wasn't enough of a reward to offset the frustration of driving slowly on the Autobahn. And to think that figure is probably the best that car would ever do...

Pete.