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[feeutfo]

I want to lower the gearing on my 3.2 auto.

Why? It has 18"wheels with 245 40 18 tyres which may or may not be oversized, not sure. But it also has an LPG tank in the boot which adds a little weight. So it feels a fraction sluggish. Only a fraction mind, it's a seat of the pants thing so I may well be imagining that bit or I have just got used to it....

But more than that, as said before, as stock it's geared for top speed which I'll never see round here, too much traffic, and it's a free tune up so to speak. Better acceleration with out altering or stressing the engine tune as stock.

As some know I have already swapped the stock dif thinking it was lower geared, but it's the same as the stock one. This was intended to be an experiment to decide if the gearing was suitable for a rebuilt LSD to that gearing, but as it's the same we're back to square one.

So, anyone know what gearing resides in a plod spec auto dif...? Needs to be lower than 4.2. And should give 3.500 rpm at 80 mph as opposed to 3000 rpm at 81 or whatever it is as standard.

[Kevin Wood]

Here are the standard ratios:



According to that your car should have had a 3.9 diff from the factory. I'm sure we compared the two on the bench at Aldershot and the ratios were stamped on the crownwheel, and different. Unless we put the old one... No. I'm not even going to say it.

IIRC, plod LSDs were 3.9 but the manual cars would ordinarily have had 3.7 diffs, hence a manual plod car would be lower geared than a manual retail car (back in the pre-3.2 days when manuals were available).

A glance at that chart shows me that if your car originally had a 3.9 the upgrade would be to a 4.22 and there are plenty of potential donors (any 2.6, or a 2.2 auto).

I think we need to know what you've currently got, for definite, though. To be sure, we need an accurate RPM at a given accurate speed (from a GPS) and a WRPM figure for the tyres you have on it.

[Marks DTM Calib]

I have suspected for some time that there are two different ratios of plod spec (e.g. LSD) diff.

I am pretty sure my mini facelfit 3.0 estate auto have a 3.9:1 ratio diff which I know was LSD (as this is what was used for the oil change guide!)

[serek]

I want to lower the gearing on my 3.2 auto.

Why? It has 18"wheels with 245 40 18 tyres which may or may not be oversized, not sure. But it also has an LPG tank in the boot which adds a little weight. So it feels a fraction sluggish. Only a fraction mind, it's a seat of the pants thing so I may well be imagining that bit or I have just got used to it....

But more than that, as said before, as stock it's geared for top speed which I'll never see round here, too much traffic, and it's a free tune up so to speak. Better acceleration with out altering or stressing the engine tune as stock.

As some know I have already swapped the stock dif thinking it was lower geared, but it's the same as the stock one. This was intended to be an experiment to decide if the gearing was suitable for a rebuilt LSD to that gearing, but as it's the same we're back to square one.

So, anyone know what gearing resides in a plod spec auto dif...? Needs to be lower than 4.2. And should give 3.500 rpm at 80 mph as opposed to 3000 rpm at 81 or whatever it is as standard.

its that diff what you got of me last year??

[feeutfo]

Ah yes, memory fails again. 4.2 was the target spec. Not the stock spec of 3.9.

So if that is a 4.2 installed currently it's not low enough. Or it's brought the current gearing back to standard with those wheels...?

So yes, need to evaluate what's currently fitted correctly.

[feeutfo]

I want to lower the gearing on my 3.2 auto.

Why? It has 18"wheels with 245 40 18 tyres which may or may not be oversized, not sure. But it also has an LPG tank in the boot which adds a little weight. So it feels a fraction sluggish. Only a fraction mind, it's a seat of the pants thing so I may well be imagining that bit or I have just got used to it....

But more than that, as said before, as stock it's geared for top speed which I'll never see round here, too much traffic, and it's a free tune up so to speak. Better acceleration with out altering or stressing the engine tune as stock.

As some know I have already swapped the stock dif thinking it was lower geared, but it's the same as the stock one. This was intended to be an experiment to decide if the gearing was suitable for a rebuilt LSD to that gearing, but as it's the same we're back to square one.

So, anyone know what gearing resides in a plod spec auto dif...? Needs to be lower than 4.2. And should give 3.500 rpm at 80 mph as opposed to 3000 rpm at 81 or whatever it is as standard.

its that diff what you got of me last year??

It is indeed. Thank you Serek.

Edit, forgot the thumbs up smiley 

[Kevin Wood]

Continental SC3 in 235/45/17 has a WRPM figure of 821.

At 245/40/18 it's 811. Not really a significant difference.


At 80MPH with the 235/45/17 Wheel RPM is 821*(80/60) = 1095

Multiply by diff ratio 1095*3.90 = 4269 at the propshaft

Multiply by gearbox ratio 4269*.72 = 3074 at the engine.

With the new tyre size 3074 * (811/821) = 3036 RPM

With the 4.22 diff 3036 * (4.22/3.90) = 3285 RPM

So, bearing in mind that the speedo will be over-reading, if you are seeing 3,000 RPM at an indicated 80 you might well have the 4.22. I know mine shows almost 85 at 3,000 RPM.


EDIT: Hmm. That might not be right, actually. Need to have a look on the way home.
Either way, at a true 80MPH you should be doing 3285.

[2woody]

formula is.....


mph per 1000 revs = 60000 divided by ( wrpm x final drive ratio x gear ratio )

your car has a 1:1 ratio in 3rd gear, so that's the one to measure in, but here's the output anyhow....

821 wrpm is spot on ( well, very close to the "standard of 817 )

so you have 18.74 mph per 1000 revs in 3rd and 16.02 in 4th if you have a 3.9

or 17.31 mph per 1000 revs in 3rd and 24.04 in 4th if you have a 4.22

the LSD I have for you is 4.22.

[feeutfo]

Continental SC3 in 235/45/17 has a WRPM figure of 821.

At 245/40/18 it's 811. Not really a significant difference.


At 80MPH with the 235/45/17 Wheel RPM is 821*(80/60) = 1095

Multiply by diff ratio 1095*3.90 = 4269 at the propshaft

Multiply by gearbox ratio 4269*.72 = 3074 at the engine.

With the new tyre size 3074 * (811/821) = 3036 RPM

With the 4.22 diff 3036 * (4.22/3.90) = 3285 RPM

So, bearing in mind that the speedo will be over-reading, if you are seeing 3,000 RPM at an indicated 80 you might well have the 4.22. I know mine shows almost 85 at 3,000 RPM.


EDIT: Hmm. That might not be right, actually. Need to have a look on the way home.
Either way, at a true 80MPH you should be doing 3285.

Oh ok. So "my seat of the pants" is quite accurate. And given I haven't really understood most of those calculations, what does that tell us... 

I'll set my idiot nav on the way home and see what rpm we have at a gps indicated 80mph in top and third gear. See if we arrive at the same figure. 

[feeutfo]

formula is.....


mph per 1000 revs = 60000 divided by ( wrpm x final drive ratio x gear ratio )

your car has a 1:1 ratio in 3rd gear, so that's the one to measure in, but here's the output anyhow....

821 wrpm is spot on ( well, very close to the "standard of 817 )

so you have 18.74 mph per 1000 revs in 3rd and 16.02 in 4th if you have a 3.9

or 17.31 mph per 1000 revs in 3rd and 24.04 in 4th if you have a 4.22

the LSD I have for you is 4.22.

Ooh ooh LSD  is there scope to lower the gearing if needed 

[feeutfo]

Having found a nice roundabout out Brackley way last weekend I can see a fitting end coming up for those Linglongs my wheels came with.

Anyone got any Falkens they want to kill. 

[Kevin Wood]

Right. Been out and tried it. True 80MPH = 84 MPH on the speedo and a shade over 3000 RPM.

Oh ok. So "my seat of the pants" is quite accurate. And given I haven't really understood most of those calculations, what does that tell us...

If yours indicates 84MPH at just over 3,000 RPM you have the same diff as me (tyres make no odds to the speedo reading).

If yours indicates 84 MPH at 3285 RPM you have a 4.22 diff.

so you have 18.74 mph per 1000 revs in 3rd and 16.02 in 4th if you have a 3.9

If you meant 26 then I agree.

[feeutfo]

Just got home. True 80mph on nav (flicking between 79 and 80 over a good 10 miles)with cruise set shows 3250 rpm. Press sport mode or engage 3rd gives 4550.

[Kevin Wood]

Sounds like a 4.22 diff to me.

[feeutfo]

Ok thanks Kev. 

So that proves that, which is good. But it also proves plod spec is lower geared/higher rpm at 80.

Keeping things simple for a second  an idicated 80 in an auto plod spec car gives 3500. An indicated 80 in mine gives lass than that. Needs another 250 rpm at 80. As I understand it.