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MG Midget and Sprite Technical - Does the diff ratio affect power?

Hi, I'm considering purchasing a taller diff as my current one is too low and the engine is screaming whenever I approach motorway speeds. (1968, 1275. Approx 4500-5000rpm at an indicated 70mph)

I should have a 4.2:1 (the numbers stamped on the diff casing indicate this) and my speedo/rev counter readings back this up. BUT it seems that I'm doing approx 10mph lower than the speedo states so either the speedo's wildly out or I have a very low diff fitted.

Anyhow, I know that swapping the diff will change the acceleration/top speed balance, but does it change the BHP that an engine delivers, given that it would be put under more strain due to a taller diff?

It's probably a silly question, and I'm not fussed either way but you know how these things play on your mind!!!!

Thanks
Mark

Mark Whitmore

Short answer is no, the max bhp of the engine will not be influenced by the dif ratio,

But the rev rate for a 4.2 dif is high; its very possible that your tachometer is reading high.
Guy W

what Guy says. Do you have a GPS thingy (satnav/phone) that can tell you speed?

Different diff will change the torque at the wheels, but not from the engine.
Rob Armstrong

Mark,

I had a 1275 with a 4.22 way back in the early '70's as my daily car (F reg)- changed that to 3.9 - it was such a difference - much better for general driving - thought it improved the car a lot and that 1275's should all have left the factory with the 3.9.
Didn't really notice acceleration loss as gearbox ratios seemed better spaced for the engine.

Now I'm even thinking about fitting the 3.7 to avoid a five speeder.

You will need to change/recalibrate speedo.

R.

richard boobier

the speedo and rev counter can be out by various amounts across their ranges so as Rob has put GPS readings (allowing for satellite rebound delay, if they still get it)

there will be a slight difference in where you are on your power and torque bands with the two diffs at set speeds

wear and tear on the different diffs could make a slight difference and having clean and different type and grade of oil in the axle could make a slight improvement with less loss of the power that arrives at the rear axle (that leaves the engine, through the gearbox and propshaft)

wheels and tyres can contribute too

but unless something is/was in poor condition you're unlikely to notice much differences other than in the revs and hopefully mpg

I wouldn't adjust the speedo until the diff is fitted and after changing tyres if you put on different size tyres
Nigel Atkins

YES - any gear is a torque multiplier and thereby aids acceleration. However, I don't think your car is that short on torque it will make a difference if you fit a taller diff. It's possible yours isn't a 4.2 but a 4.55 or something lower still and on occasion will sell at a premium.

NO - gearing doesn't change brake horse power.


There are books with this stuff in.
Guulsfrq

During an axle rebuild, the diff was found to be 4.22 instead of the expected 3.9. However, it was replaced by a 3.7 which the modified engine [80-85 bhp] pulls fine. Only noticable differences are lower revs and improved mpg - every little bit helps.
Doug Plumb

Mark

The diff ratio was changed in December 68 from the 4.22 to 3.91 (HAN9-77591 and GAN4-66266), so your car may or may not have had one originally. THe higher ratio should give you 16.5 mph/1000 revs, which means 70 would be about 4250 rpm.

I had a very optimistic speedometer because the correct one had been fitted for a 3.9 dif (as suits a '71 sprite) but the 4.2 dif had actually been installed. While I seemed to be bombing along according to the speedo, I was always suprised how long the queue was behind me. Good margin of safety for speed cameras, but a bit off-putting when at an indicated 40 mph, cyclists were passing me (slight exaggeration, but almost true!)

I changed the speedo for the correct one (rather than changing the diff) as round the lanes the lower diff ratio gives better acceleration and less need to change down but on a motorway it wouldn't be so good! If you look at the "rotations per mile" marker on the speedometer that should tell you whether you have the correct one for the diff gearing. This is the number printed on the right below the milometer. A 3.9 dif speedo should have 1376 marked on it I believe, and a part no on the face ending in "06". The 4.2 diff needs a speedo marked as "1472" (part no ending "00" for UK).

Presumably the diff casing is the same for all ratios so new gearing could have been fitted into a wrongly-marked case possibly?
Graeme W

Hi all, thanks for the replies.

Sorry, I should have put BHP AND torque at the wheels in the opening post.

I did check my speedo with a sat nav somne time ago and seem to remember that it was at least 5-10 mph too fast.

The rev counter was well out when I bought the car so it was adjusted and tested with both a multimeter with tacho function and a gunsons auto tester with a similar function so that should be pretty accurate.

My car is an earlier 1275 so should have left the factory with a 4.2 diff (and the stamp on the diff casing backs this up) but the car did have a full rebuild prior to my ownership so who knows!

Am I right in thinking that as the speedo takes its reading from the gearbox the relationship between the speedo reading and tacho reading would remain the same regardless of what diff was fitted (although true road speed would be affected to varying degrees)?

The reason I ask this is that at indicated speeds my speedo tally's up with the 4.2's 15.5mph per 1000rpm but the speedo does run high.

I also have the correct speedo for the 4.2 diff fitted.

I will be sticking to standard sized tyres so that shouldn't complicate the issue.

Thanks for indulging my brains ramblings!!

Mark
Mark Whitmore

Mark, do you know if the 'box is original? I'm thinking it's just possible it's been swapped for a Morris Minor box with a different speedo drive gear...
David Smith

Hi David, I'm pretty sure its a midget box (if not the original one as I had it reconditioned a couple of years back by a chap in Wolverhampton (recommended on this forum) and I'm sure that he was told me that it was a Midget one as it had some subtle differences and was stronger than the Minor one.

Thanks
Mark
Mark Whitmore

Ah good, if Ken's done your gearbox it'll be 100%.
David Smith

Mark, you are correct in thinking that the speedo and tacho should correlate, irrespective of diff ratio.
Dave O'Neill2

This thread was discussed between 06/11/2012 and 07/11/2012

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