BMC Axle diff ratios etc.

[gordong]

I'm considering changing the axle in my Fisher Fury (currently Ford English) for a lighter one. The BMC as used in Caterhams etc. looks like a good alternative since it already has an alloy diff nose.

 

Main question: what diff ratios are available as standard, and what donors should I be looking at? I'm looking for a ratio between 3.4:1 to 3.6:1 (going in a Fireblade engined car). I've done some web searches but turned up nowt of any use.

 

To get the right (i.e. Ford ) PCD, can the hubs be redrilled for new studs at a 45 degree offset to the original holes (i.e. are the hubs round and wide enough)?

 

Many thanks for any help or pointers.

 

Gordon

[deanteam]

Would it not be easier to get an alloy diff nose for your axle. Burton advertise them.

[gordong]

Alloy diff noses are expensive (£400 ish IIRC) and I hear people have problems with them. If I swap for a different axle, I can sell on the LSD in my current axle (or the whole thing) and get it for much less.

 

From looking at the BMC axle, the casing itself also looks lighter than the English (though I would plan to brace it to add strength).

 

Still interested in ratios. Does anyone have any other axle types I should be looking at?

[Joe 90]

Gordon,

I got my BMC axle out of a Morris Ital (when they were common in scrap yards). I think you are correct when you day they are much lighter, but they don't have an alloy diff nose. James Whiting was working on one a few years ago, but I don't know how far he got. I don't think you can re-drill the hubs as the ford PCD is bigger than the BMC PCD, and there just isn't enough material.

 

99,000 miles so far

[cdg]

I have a whole rack of BMC diffs in my garage, and they most certainly do have alloy diff. noses. Perhaps the later 1700 Itals had iron diff. noses (I don't know)possibly an MGB(?) but the Marina was definately alloy.

[gordong]

A whole rack of BMC diffs? And what ratios do they come in?

[cdg]

Unfortunately most have broken teeth on the crown wheel or pinion. They are almost all fitted with Ford planet gears to enable the use of Ford Quaife half shafts.

 

The BMC route is a good idea if you're on a tight budget, but I would be inclined in your case to sell your LSD Ford, buy an alloy Ford nose from someone like Lotus Cortina Spares, Flowtech etc, and look for an RS2000 3.54 ratio CW/P. You already have the axle complete with all bracketry. Not sure the LSD is totally necessary for a BEC with fairly low torque output.

 

Chris.

[gordong]

Thanks for the thoughts, Chris

 

LSD is *definitely* required, especially since I race it. I won't go into the old argument about BECs and torque, but any car that can do 0-60 in little more than 4 seconds is likely to be needing a LSD to do that.

 

An alloy nose for the English axle is the simplest option, but doesnt have the attraction of the challenge, being different or low cost. Ho hum.

 

Someone out there must know what ratios were available in the Marina/Ital?

 

G

[cdg]

My hillclimb/sprint Mallock used to have a 180bhp X-Flow with 140lbs/ft of torque.It has a very successful competition history, yet it does not have an LSD, will not be getting one, and is shortly to be fitted with a GSXR1147 WP sidecar GP engine making 190hp.

There are many speed eventers in my car club using single seaters with both bike and car power, but I can't think of many that have found the need for an LSD. Perhaps with the sticky slicks that we use it's not a problem. If you're on road tyres I accept that it may be...

 

I'll go down into the bowels of my garage and check on BMC ratios.

 

Chris.

[gordong]

Chris

 

Yes, I can imagine if you were on hillclimb slicks in a single seater, traction wouldnt be so much of a problem, and any LSD induced understeer would be potentially terminal. I raced the car briefly on Ao32rs (205/60/13) without the LSD, and from apex to exit in any of the first 4 gears it lit the inner rear wheel.

 

The Quaife LSD is magic.

 

TIA for your garage excavation.

 

G

[cdg]

Crikey,

 

I've just found an absolutely perfect 3.7 BMC diff. hiding amongst the detrius. Just what I was looking for myself!! Result.

 

And no Gordon, who can't have it...

 

Chris.

 

ps. You do realise that the Ford CW/P is much stronger than the BMC??

(as evidenced by the fact that I have got 3 BMC diffs with teeth missing.)

 

Edited by - cdg on 20 Sep 2002 13:43:15

[DanB]

Just thought I'd join in the BEC Yahoo Groups chit chat. The longest standard BMC diff appears to be 3.7, then 3.9. 3.2ish CWPs are available from James Whiting and the company making the new ADR1000 Sports racer which will be competing in next seasons 750MC Sports 1000 series. The latter is about £280 IIRC.

 

Personally, I'm looking at getting a water-cut billet titanium axle casing made up....

 

Dan