mg axle ratios

greenmachine · Post by **greenmachine** » Mon Dec 17, 2007 1:54 pm

I need to change the ratio of my standard MGB axle for ether the MGC or the MGB V8 gearing ,my engine is a mildly tuned 3.5 rover unit so I don't wan't to over gear it and lose the acceleration . At the moment it do's about 70mph at just under 3000rpm which is fine on A road's but to thirsty on the motorways,can anyone give me an idear on what to expect from the C or the V8 ratio's
Keith Webb

David Large · Post by **David Large** » Mon Dec 17, 2007 6:54 pm

When we built our car we had a 3.7:1 MGC Automatic (I think) diff. This was ok but with the Rover 5 speed box I rarely used 1st gear.

I changed it for a 3.07:1 MGB GT V8 diff a few years ago. I had to learn how to drive the car again it was so different. It does take a bit of edge off the acceleration but is still fine. I have a new 4 litre engine which dynoed at 240bhp and 250 ft lbs torque.

3000 rpm is just short of 90mph.

By comparison my BMW 330d Touring is doing 80mph at 2000rpm in 6th gear.

Cheersw

Post by **clive** » Mon Dec 17, 2007 8:46 pm

David Large wrote:
By comparison my BMW 330d Touring is doing 80mph at 2000rpm in 6th gear.

Cheersw

And if it's the same as mine, around 48 mpg!

David Large · Post by **David Large** » Tue Dec 18, 2007 6:57 am

clive wrote:
David Large wrote:
By comparison my BMW 330d Touring is doing 80mph at 2000rpm in 6th gear.

Cheersw
And if it's the same as mine, around 48 mpg!

Absolutely.

Over the last 40k miles it has averaged overall 40mpg...an amazing engine. Next year I am thinking of swapping it for a 335; but then the XF looks nice.

cheers

petercharman · Post by **petercharman** » Tue Dec 18, 2007 5:55 pm

I am running a standard MGB axel in my 289FIA, had a 3.5 Rover engine now I have a 5.00litre TVR chimera unit installed and have been toying with IRS etc rear ends.

Decided I love the handling characteristics of a solid rear end but need to somthing about the final drive ratios.

MGB V8 diff sounds like a good solution, does it fit into a Banjo type axel (1967) or do you have to change the whole axel??

Peter

David Large · Post by **David Large** » Tue Dec 18, 2007 8:37 pm

I don't think it fits the banjo axle......but not certain.

Cheers

DL

greenmachine · Post by **greenmachine** » Tue Dec 18, 2007 11:32 pm

Thanks for the information,I had a look on the internet to see what I could find about the different axle ratios, the standard
MGB ... .................................3.909:1,
MGC manual with overdrive ... 3.7:1,
MGC Auto............................. 3.3:1
MGB V8................................ 3.07:1.
So Dave's 3.7:1 diff would'nt have been much higher than mine, as he says you don't need 1st gear.I think older cars with auto boxes generally have higher rear axle gearing.So I phoned Hardy engineering(the people Gerry uses for his work) and thay have one 3.3:1 set left,that will suit my engine better.

cheers
keith

kaiserbilly1 · Post by **kaiserbilly1** » Wed Dec 19, 2007 12:14 am

Hi Keith

Can you tell me is the new crown wheel that you are going to fit a straight swap ?
Does anything need to be machined, or uprated. I am assuming that you are fitting to a tube axle ?
I have been thinking about this mod too and would be interested to hear your thoughts.

regards

Bill

David Large · Post by **David Large** » Wed Dec 19, 2007 5:35 am

I think the 3.3 would be a good compromise.

You should not need to machine anything (although having said that I think the guy who fitted mine did machine the carrier to take the V8 diff), unless you are fitting a limited slip diff.
It does need to be fitted by a specialist because you have to "spread" the casing and shim the installation to get the right preload. Seen it done but can't do it!
Cheers

David Large · Post by **David Large** » Wed Dec 19, 2007 10:00 pm

I was thinking about this a bit more today while travelling on the train.

If you are running a 5 litre TVR engine, then you have 360BHP and a lot of torque........don't know how much. Given that your car probably weighs about the same as a TVR then you would not go far wrong if you ended up with a diff ratio similar to a 5 litre Chimaera / Griffith......assumng the gearbox has similar ratios. Worth doing a few sums before you leap into this. You may find the 3.07 is nearer the mark.

Don't know what diff people like Clive with a SBC are running but may be taller than 3.07:1. I think some of the Jag diffs are of the order of 2.88:1.

Regards