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MG Midget and Sprite Technical - Which Diff ?

Short version....what diffs do peole run and how do you find them

Long version.....The diff, rear axle and gearbox are the only bits of mine I've never tinkered with beyond filling with fluid and removing etc to do welding behind....so its a grey area for me. (Gearbox is std 1275 ribcase)

Current spec is std 1275 bottom/head + Kent 266 cam + HIF44 + LCB +1.5 roller rockers + Aldon yellow - and it currently pulls quite happily to a GPS verified 94mph @ 6200 (where legal). That seems to tie nicely into the maths for a 4.2 diff and 165/70 tyres on 13" wheels.

I now have a 1380 sat in the garage waiting to go in with Kent 286 and what appears to be a better head (35.6 Inlets with work done on the inlet - exhaust appears std). That will get the HIF,LCB & Roller rockers.

Assuming that delivers the 90-100 bhp it could/should (famous last words) I cant help feeling the car will be a bit limited by its gearing. I dont want to go 5 speed just yet....so was playing with the maths for changing the diff.

I do a fair bit of dual carriageway so The 3.7 seems to offer more relaxed cruising 70ish@4000 rpm....but how spritely will my midget feel (!) with that. ?

Dean Smith ('73 RWA)

Dean, my guess is that cruising with a 3.7 will reduce the revs and be a lot more comfortable but around town in lower gears will feel a little sluggish. The 3.9 then later fitted with a 5 speed box would be a better bet I think.
Bob T

I'm with Bob, but if you don't want to go the 5 speed route a 3.7 would probably be OK so long as you don't have a lot of stop-start especially on hills.
One of ours has a similar spec engine and manages quite well on a 3.9 - even on our 1 in 2.5 driveway!
(Though it does at least start around 1 in 12)
Paul Walbran

Dean,

It is my view that you will have to accept compromises when uprating engines. The Kent 286 has a power band 2-7.5K which in reality means not a lot of power below 3K. A higher ratio diff will allow the engine to run nearer to the upper limit of the cam and therefore a higher top speed and more relaxed medium speed cruising. The downside is that there will be a lot of clutch slipping when getting away and the peakiness of the cam will make town driving a bit fraught. Lower ratio diffs will make slow speed driving easier but at higher speeds the engine will be running faster. As I said, its a compromise.

JohnT
John Turner (Midget & MGB)

I wanted comfortable cruising with my Midget, so I put a 3.5 diff in. Spec is 1293cc lightened and balanced + metro big valve head + 266 cam + hif/itg + lcb + aldon ignitor.
There is a (small to my mind) loss in acceleration, but the car is still very nippy and cruises at 3500rpm @ 70mph. Nice and relaxing, and converting the back axle takes 1 hour.
Converting to 5 speed will though give you the best of both worlds.
Geoff Mears (1970 Midget)

I've cut so much rusty metal away over the years....I'm not sure I can bring myself to cut away a bit of perfectly good x-member to get the T9 in :-(

I guess the sensible thing would be to get the engine in and setup 1st...then I'll know how usable it is lower down the rev range.
Dean Smith ('73 RWA)

My 72 has a 1310 with 266 and Datsun 5-speed pulls the stock 3.9 with such ease that I have contemplated installing a 3.7. OTOH, I autocross enough that I have also contemplated dropping in a 4.2 ;-) So, I am compromising and staying with the 3.9. Other than sudden, unexpected hills (like the one on Rt94 last Wednesday), it does not need to drop out of fifth when cruising.

Our 60 has a 1098 with Datsun and a 4.2. It is a bit more pressured to keep up (good thing it is usually Trish driving it) and I would not contemplate a 3.9 unless it suddenly woke up for some reason.
David "never enough power" Lieb
David Lieb

Where did you find a 3.5:1?
Ron Koenig

I can't find a source for the 3.5 ratio gears here in the US in either the Moss, VB, or Minimania catalogs. Here's a link to a company in England who lists the 3.5 ratio gears. http://www.autogear.co.uk/product/?groupid=5&model=&manufacturer=5
Bill Young

My Sprite is geared such that 1st is 5.96mph at 1000rpm. The cam is now an APT but was previously a Kent 286.

I don't need to slip the clutch a lot as careful attention to bottom end tuning including a CV exhaust manifold means I do have power from a 1,200 rpm idle.

The only downside is that to win a challenging traffic light grand prix you need a near full power start and feather back the throttle to straighten out the back end by reducing wheel spin before feeding it back in again before snatching second. It's not the kind of thing you'll want to do very often, especially in built-up areas.
Daniel Thirteen-Twelve

I got my 3.5 diff from Heathrow Transmissions a number of years ago. I can look up the phone number on monday if anyone requires it.
Geoff Mears (1970 Midget)

I have a 3.9 and a 5 speed box. No problems with a tuned 1275 (SW5 cam). With the extra gear for cruising, a 3.9 is perfectly tall enough.

Who cares about that tiny bit of crossmember?! You'll forget all about it when you get a modern box in - a complete lack of transmission noise is a beaut.
Tarquin

Or source yourself a Datsun tranny, rare as hens teeth but what a wonderfull mod. And no cutting!
Bas Timmermans

This thread was discussed between 30/06/2008 and 07/07/2008

MG Midget and Sprite Technical index

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