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MG Midget and Sprite Technical - 3.9 vs 3.7 diff with Datsun 5 speed


With an uprated 1275, it seems to me that a 3.7 diff would be a wise choice vs the 3.9 since the first and second gear ratios in the Datsun 5 speed are a little lower (numerically higher) than those of the standard midget 4 speed. In other words, I could have still have pep and performance with the 3.7 and lower cruising revs in the top gears.

Any thoughts?

Thanks!
Jack

I have wondered about the 3.7 with the Datsun too. I have the 3.9 and a stock (and a bit tired) 1275. It can't really hold 80 mph on the steeper hills and often I end up in 4th gear grinding along at 65 mph. I have wondered engine modifications would be appropriate to be able to run it along with the 3.7 diff comfortably.
Glenn

Mine with a tired 1275 and a Datsun/Rivergate 5 speed with 4.22 rear axle holds 80 fine (if a bit smokey). Certainly not going down to 65mph under any circumstances unless I lift off.
I am however about to put in a 3.9 rear axle
Same old Gary

I'm running a .040 over, basicly stock 1275 with headers, Datsun 5 speed and 3.9 gears in my Frogeye without any problems either accelerating or cruising. The only advantage with the 3.7s that I can see is that you would be at slightly lower RPMs at a given speed.

Martin
Martin

I've got a pretty quick 1275 with the Rivergate and a 4.22. Though 1st is geared lower than I would prefer, it's great when driving through the winding roads and hills....there's always power available. And, I've had it over 100 mph. Unless you just want to cruise the highway, I would definitly go with the 3.9

Ken
Ken

I have been kicking this around as well, eventully Im putting a hot 1275 and currently have a 3.9 diff but I want to put in a LSD posi-track and ssome heavy dutty Stainless steel axles, so I am really considering going the 3.7, but not sure what the end result will be.....BUt that modification wont happen till late fall or this time next year...

I guess what Im looking for is something that is going to cruise down the highway nicely ...save a little gas, but still have the low end get up and go to out run the cops, when I need to...lol. or at the vaary least out run "david lieb" so he will have to deal with the FUZZ, instead of me...lol

prop
Prop

I am running a supercharged 1293 with the Datsun gearbox, I must say the 1-st gear is too 'overgeared', it spins all over the place. But in 5-th it runs 3200 rpm 75 mph (max speed over here), and it still accelarates like a rocket. I have 165-55 tyres (if I remember well). A 3.7 would be nice only if I would like to have more speed (which I don't want)
Arnold

I tried a 3.7 with my Toyota 5 speed for a few months last year. My engine is fairly peppy and I felt the car felt over geared and I've reverted to a 3.9.
Jeremy Cogman

With my Toyota box and the 1275 I had to drop down too a 4:2:1 the engine wouldn't pull with a 3:7:1 or the 3:9:1, with the 4:2 its a flier with a top end about 105mph (its a rever not a puller)
K Harris

I run a 3.9 with the Toyota 5-spd and 185/70x13 in our Minor, with a well modified 1380. It pulls very well in 5th, and is a heavier and less aerodynamic car, but a 3.7 would be too much, and the Datsun 5-spd has a taller 5th than the Toyota box as well.

I think I would use the 3.9, at least to begin with, to find out if you could run a 3.7?

Richard Wale

I have had a Toyota 5 speed in for many many years. My 1275 engines were always cammed with a 286 and high CR so were aways a little beauty of an engine and always pulled a 3.9 without difficulty.

Like Arnold I have now gone to forced induction with a turbo. With a turbo or supercharger the engine is far more powerful at substantially lower revs and so lends itself to a higher gear ratio and so I am putting in a 3.7 without any worries. I think to pull a 3.7 with a 5 speed box and a normally aspirated engine, the engine would need to be pretty well sorted.

I am suprised how many 1275s there are around which seem to struggle with a 3.9?
Bob (robert) Midget Turbo


Thanks, fellas. I'm running the Kent fast road cam, 10:1 CR, HIF 44 with Titan intake, and LCB header. Newly rebuilt engine with plenty of power. But at 75 mph I'm running about 4200 rpm with 3.9 diff and midget 4 speed. Getting ready to install the Rivergate conversion and figured I might as well put in the 3.7 while under the car. Car is also lighter than stock ('74) with fiberglass tilt front end and no rear bumper.

Based on your responses, guess I'll wait on the 3.7 til I see how the 5 speed feels with 3.9.

But my feeling is that with all the power I've got and a lighter car that a 3.7 won't hurt.

Like Bob, I'm surprised at 1275s that struggle with a 3.9. Right before I rebuilt my 1275, I was running stock everything and able to pull long steep grades around Chattanooga with no problem in 4th. And my engine was crapped out--burning oil.

Cheers!
Jack

This sruggling must be caused by a lack of Forced Induction ;-)
Arnold

Arnold,

Are you going to gaydon this may? If so what's your trip planning?

Bas
Bas Timmermans

I have a peppy 1275 with 5-speed and 3.9 in my 72 Midget. I have also wondered about a 3.7, but their relative scarcity has kept it at the wondering stage. I now have a 1960 with a "peppy" 1098 and 5-speed and 4.22. I would not dream of going to a 3.9 with that engine. My advise would echo the above comment of staying with the 3.9 for a while until a 3.7 just happens to fall into your lap. Don't sell the 3.9 until you are very sure!
David "Maybe a new cam and some tweaking will alter my opinion..." Lieb
David Lieb

Bas, sorry what is gaydon?
Arnold

Arnold,

In what Turbo cave did you spent your time last year:-))

The Spridget50 event in Gaydon the 24th of may!!!

Already 6 Midgets on the thursday night boat.

Bas
Bas Timmermans

No sorry, I am not in for 'events'. I prefer my cave
Arnold

It's not an Event

It's a PARTY

(OK it is an event too, but it's for fun...)
Bill

Jack
You mention you have a fiberglass front end. After wrecking mine, I'm putting on a fiberglass bugeye nose. I need to perform some fiberglassing to mount it pivoting at front. Would like to exchange some info with you. Where in Chatt. do you live....I'm in Hixson?

Ken
Ken

Ken--you're not going to believe this--I'm in Hixson too. My close friend (the bloke that helped me bring my Midget back to life) is Fred Mohr on Signal Mtn. He's got a bugeye with a fiberglass front end but it opens in the standard manner, same as stock.

Mine has a crudish angle iron frame that bolts to the front frame horns, and has hinges attached that also attach to the bonnet allowing a forward tilt. The mounting frame was part of a kit that came with the bonnet--made up by Speedwell--originally intended for a bugeye.

I bought the bonnet and mounting kit years ago from another fella in Chattanooga but just this last year have gotten it installed.

We'll have to get together. Mine is about 90% complete, but has 3 (that's right) disastrous attempts at paint jobs, so far(not my doings). Getting ready to try for a 4th, after i get the 5 speed installed.
Jack

Jack
Send me an email and I'll give you my phone #/address. I would like to take a look at your front end arrangement.
Ken

This thread was discussed between 24/02/2008 and 02/03/2008

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