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MG MGA - 3.9 differential

For anyone wanting to fit the 3.9 Diff in their car, there is one on ebay at the moment. They don't come up that often and usually go for a good price. I saw 2 at Stoneleigh last Sunday with an asking price of 150 each.

Not my advert. No interest either. I already have one fitted.

Item: 320491287387

or

http://cgi.ebay.co.uk/MGB-Front-loading-3-9-Diff-MG-MAGNETTE-MGA_W0QQitemZ320491287387QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?hash=item4a9ec4f35b#ht_500wt_1182

Steve
Steve Gyles

Steve

Shame it's not a 4:1

John
John Bray

Thanks for the heads up Steve - do you find yours slows your accleration much?
Cam Cunningham

Cam

It's a bit difficult for me to give you an honest answer as you have the 1600 engine and MGA box. I have the 1800 with 5-speed.

Yes, it is higher geared and you can feel it as such when you start from rest. However, you can take the car to a higher speed with each gear change. The main advantage for me is the cruising with lower revs than with the pure MGA set-up. The figures are somewhere in the archives but I seem to recall sitting at 70mph with about 300 to 500 revs less than standard, but remember that is also with the 5-speed box. There must be someone out there with the standard set-up like you and the 3.9 who can better comment.

Barney covers the differential swap over extremely well in his technical pages. I followed it to the letter and it is a straight forward job. If you go down this route I am happy to come over and assist you.

Steve
Steve Gyles

Cam, I went from my 4.1 diff (Mk 2) to 3.9 (MGB) and was concerned at the time but found little obvious drop in acceleration but lower cruising revs as Steve says. The difference might be more noticeable if one went from a 1500 with 4.3 to 3.9 diffs.
J H Cole

I went 4.3 to 3.9 in my 57 1500 and did not notice any major change except a reduction of about 400 RPM while crusiing the highway. City driving and over heating are not a problem.
Gordon Harrison

Thanks JH and Gordon. Steve - I may take you up on your offer if I get it!!
I assume it will change the speedo readings as well.
Cam Cunningham

You will notice the difference in the 3.9 the first time you back the A out of the garage. Make sure any kids or dogs are clear!
David Werblow

Cam

Yes, it will require recalibration. I did mine through Speedy Cables. Their website shows the simple process of getting the data they require.

Any progress with your 5-speed intentions? If done at the same time, Peter Gamble at Hi-Gear can provide you with the correct ratio speedo right angle drive off-take that will obviate the need for recalibration.

Steve
Steve Gyles

Put a 3.9 in my mostly stock MGA a few years back. No problems at all. As far as recalibrating the gauges, if you're not ralleying or such that you need the accuracy, I'd wait until they needed a rebuild. Just my opinion. Marvin
Marvin Stuart

Hi Steve, My bid of 120 didn't succeed as it went for just over 150 - OK own up - which one of you got it! cheers Cam
Cam Cunningham

Marvin, here in the UK the Government has greatly increased the number of speed cameras for 30 and 40 mph limits- there everywhere now so you need the speedo to be accurately calibrated to avoid getting zapped and receiving a fine with points on your driving licence - or you drive with a margin below the limit that is not always easy to do. 3 points for speeding, get 12 and you loose your licence. I hope its easier in the USA.
J H Cole

John

I frequently cross-check my speedos (2007 Merc and MGA) against my sat nav. Surprisingly, the MGA is more accurate and usually within 1 mph, whereas the Merc generally overreads by about 5% to 6% above 30 mph.

Steve
Steve Gyles

Steve as I am sure you know ALL modern cars have speedos calibrated to give us all some tollerance for the reasons above.

I have 2 wonderful grandchildren now and I am pleased that we try to protect them from lunitics on the roads who believe they are capable of stopping if these children run into the road for no apparent reason. At 40 MPH they will have a good chance (80%) of being killed. However at 30 MPH they have a good chance (80%) of surviving, it is a no brainer as the advert says it is 30 for a reason.
Bob Turbo Midget England

Steve, like you my sat nav reads faster, for my Skoda its up to 8 mph at 50mph but I've never found a statement on the accuracy of sat nav in terms of movement so I'm not sure if it can be 100% trusted. How often does it refresh itself?
J H Cole

Flippin heck 8 MPH at 50 is a staggering difference

At a constant 50 MPH on my speedos (BMW Mini, Mercedes Benz) My Sat Nav returns a constant 47 MPH. The only way I can see that the sat nav would be out is if you are trveling up a steep hill in which case you will be travelling more miles than the Sat Nav will be measuring.

What kind of Sat Nav are you using John?
Bob Turbo Midget England

Bob, I agree that 8 mph on 50 mph does seem a lot. A 10% speedo margin would be 5mph. But where is it stated what the accuracy of a sat nav is? I would be a little wary of following a sat nav for speed control an an area of speed cameras.
J H Cole

Here in my home town the legal limit is .....30 KILOMETERS an hour.
Gordon Harrison

I understand your concern John but I look at it this way.

The Sat Nav knows exactly to the metre or so how far you have travelled. It also knows how long it has taken. So therefore as long as the arithmatic within the unit is correct, then the speed display must be infinitely more accurate than a speedo making the same calculations from wheel diameter and revs plus an allowance for error to ensure you do not sue the motor manufacturer when you get a speeding fine. :)
Bob Turbo Midget England

This thread was discussed between 25/02/2010 and 01/03/2010

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