there seems to be two different ford diffs available can someone tell me which is best for a bec the difference and what is the best ratio for a blackbird engine as far as accelaration and top end,also there seems to be two different half shafts some that bolt on and some with splines again which is best regards wayno!!!
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Have a look for any post by ChrisGamlin, he has a link to a spreadsheet at the bottom of his signature, that allows you to select the bike engine, diff ratio and wheel / tyre size. This will give you a vmax in each gear.

Just to add to the choices, the Sierra diff comes in two types, slide in shafts and bolt on. Not sure of the relevant benefits of one sort over the other though.

Then there are 4 different ratios, 3.14, 3.38, 3.62 and I think 3.94?

I'm using a ZZR1100 and a 3.62 diff on 14" wheels. Gives me a "theoretical" 125mph top speed, allows me to hit 45 mph in first, 65mph in 2nd and cruise at 80 mph in top at around 7 grand.

HTH


Mike

Someone will be along in a minute with more specifc information but to start things off...

SIZE:
Your typical ford differential is available in two different sizes. 7 inch and 7.5 inch.

Most kit car chassis have bolt points to accept the 7.5 inch, I'm not sure which kit manufacturers only accept the 7 inch?

The smaller have the advantage of saving a little weight.

If you want to fit the smaller diff to a chassis that accepts the larger you'll need to fabricate or purchase a spacer kit.

RATIO:
There are various ratios available (ie. the number of turns the prop need to make to turn the driveshafts by one revolution).

Straight out of the scrapyard you may stumble across the following ratios

1. 3.92 : 1
2. 3.62 : 1
3. 3.38 : 1
4. 3.14 : 1 (extremely rare and command a good price)

All will work with a BEC but (1.) will give you faster acceleration and low top speed, and (4.) will give you slower accelation but will bring out the potential of a higher top speed.

Motorbike engines rev high but the actual output is generally 'geared down' meaning that it doesn't actually spin as fast as the BEC guys would like.

When I bought my car it had a 3.92 fitted. Acceleration was phenomenal but the top speed was only around 105mph.

I've since upgraded to a 3.62. No significant loss in acceleration but the top speed's increased to around 117mph mark

I'm ideally after a 3.32 as this would in my opinion be the best compromise.

A 3.14 diff takes you closer to the 'terminal velocity' (for want of a better expression) of your vehicle (ie. unless you do some serious work to the output figures of your engine you'll struggle to get past 120-125mph).

It's more complicated than all the above but that's a simple man's approach to the world of diffs.

As Guinness mentions, wheel size also plays a big part.

Just found this link too which might help further?...

Linky

[Edited on 20/1/08 by The Great Fandango]

thanks guiness and fandango cleared that one up a treat,what a great source of information these forums are,sure i'll be posting more questions shortly,as i,m a mere noobie thanks again!!!!!

If you want to know more about them and where to find them have a look here:

http://forum.fordsierraclub.co.uk/viewtopic.php?t=5843&highlight=diffs

thanks mcerd1

there seems to be quite a bit of play in the two medium size gears is this normal cheers wayne
p.s is there anything that can be done about this or do i have to get new ones!!!!!

Just been out to measure my diffs on the indy's just to be sure I wasn't loosing my mind and as I thought both are 7 inch
Tony