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MG Midget and Sprite Technical - Gear ratios

Hi all,

I have a 79 1500 midget. Cracking fun, and a few mates keep on at me to take it to a track. Id love to have a go and see how I can drive, and how the midget performs!

But, I can't help but feel it is far too low geared..

I love blasting around the country roads near me, but 50 is as fast as I'd like to go due to the 1500's rev range. I find that, with my set up, there is much more power and torque lower in the range. There is still plenty higher up, but I'm scared of high revs on this engine!

I know not many race the 1500 engines, but are there any other rear diff that can be fitted that are slightly longer?

Karl Bielby

Firstly, have you checked the actual ratio of the diff fitted to your car?

It's not uncommon for people to replace worn/broken diffs with whatever is easily and cheaply available.

I recently found a 4.2 diff fitted in a late 1500 axle.

You *should* have a 3.7. If you find that you have the correct ratio fitted, there is/was a 3.5 CWP available.
Dave O'Neill 2

As your car left the factory, with 145/80 tyres and a 3.7 diff, 50 mph would be at less than 3000 RPM.

Even with a 4.2 diff, 50 mph is about 3250 RPM, which isn't excessive.

I obviously can't comment on your car, but on the motorway mine is much happier at 65 - 70 than 55 - 60 mph. Although an oil cooler is a recommended add on.

There isn't anything you can do about the standard gearbox, 4th gear is a 1:1 ratio. The diff makes a small difference. Or you could convert to 5 speed or overdrive gearbox.

Just man up and put your foot down! :-D

Hope this helps.

Malcolm Le Chevalier

Also, the Midget 1500 has a crap exhaust manifold and cam. Sort these to get it breathing and therefore revving more freely.
Malcolm Le Chevalier

But don't go into the red! 1500 tends to disintegrate over 6000 rpm.

I used mine as a day car for 3 years to last Christmas and was able to keep up with the quick traffic no problem in Nottm and Derby. No mods; original suspension. Make sure your brakes work OK.
Dave Squire

Hmm how do find what is fitted??

Yeh well I have an oil cooler, high lift cam (no idea what specs, bet it definitely has one judging by the cammy idle!), Air filter, manifold and exhaust. I can't quite remember the tyre size but if changed to minilites.

Suspension is slowly getting poly bushed, but I know my rear axle is leaking into my rear drums... changed the O-ring and paper gasket but it's still leaking, must be the inner hub seal that is a pain to do... hence its still leaking!

Although, Malc you are probably right! I'm always scared of things reving high and wearing out.. was the same with my landrover that was my daily. worst case, I just rebuild the engine!
Karl Bielby

Even a 1500 can handle 5000rpm without a hitch if there are no problems with the engine.
Lower revs often are more damaging than high revs, when in low revs you lenghten the period where max force is applied on the bearing shells.
At high revs the problem can be the centrifugal forces in the crank working against the oil pressure.
But with a healthy oil pressure this should not be a problem
Onno K

To find out which diff you have, you need to either read what is stamped onto the top of the casing (see photo), or follow this procedure:-

Jack up the rear of the car.
Block one wheel so it can't rotate.
Mark the position of the other wheel and the diff input flange.
Turn the (unblocked) wheel two complete revolutions and count the number of turns of the input flange.

Dave O'Neill 2

Karl, when you are happy to give it some stick get yourself over to Cadwell Park for a track day or evening. Perfect for the Midget, lots of tight bits and and all the straights start off downhill so you can get a run up!

Or for somewhere less intimidating Blyton Park would be a good place to Start. Big run off areas if you get it wrong!!

I use Javelin trackdays, nice and relaxed and a good mix of cars.
john payne

Il have a look next time I'm under there Dave. I might do both as they may have kept the same housing and changed inners.. you never know!

I'd love to have a go around Cadwell! Great track. But it would take me a day to get there! Il definitely have a look at javelin though, cheers!

I'd also be tempted to fit a short shifter, but it might make it impossible to change gear!
Karl Bielby

This thread was discussed between 12/06/2016 and 13/06/2016

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