7 and 7 1/2 Inch diff

[Tarmac Terrorist]

Apart from the obvious size what are the differences between these two diffs?

If anyone can explain the ratios, 3.9 and 3.6 as well, that would be great  

[Martin Keene]

7.5" is the Cosworth diif and is a hell of a lot stronger (and heavier I believe) than the 7".

Unless your going to be running massive power, with stupid amounts if torque, the 7" is enough for most people.

Massive power would be ~280+ and stupid torque would be ~250+

[Tarmac Terrorist]

Only going to go for a measley 200hp  

Whats the difference in ratios?

[Martin Keene]

Oh Er... Not a clue. That's beyhond my knowledge of them.

[adamnreeves]

The ratio is the relationship between the number of turns of the input (propshaft) to the number of turns of the output (driveshaft). So 3.6 means the wheels will turn once for every 3.6 turns of the propshaft.

going for a higher number can increase accleration at the expense  of top speed.

I think generally car engined westfields use around 3.63. whereas bike engines generally the 3.92 as they are much higher revved and can take the higher ratio to improve accelaration.

The torquer engines such as my V8 which is also lower revved tan the 4 cylinder variety generally use the 3.36. I have even heard of people using the 3.14 (having converted into an LSD first!).

I am sure this have been posted here before but I can put my hands on it so here it is:

[Dave Eastwood (Gadgetman) - Club Chairman]

I think generally  car engined westfields use around 3.63. whereas bike engines generally the 3.92 as they are much higher revved and can take the higher ratio to improve accelaration.

Not quite as straight forward as that, I'm afraid. Gearbox and wheel choice play a big factor in determining the final drive ratio as well as engine type. FWIW 3.92 seems to be a good compromise with four cylinder 16v cars, with 4.09 and longer being beneficial, especially for tuned (car) engines that rev a bit higher than standard.

For many years 3.92 also seemed to be the standard ratio for Wf's Zetec engined models.

Bike engined cars are a different proposition if I remember rightly, unlike a car, which has a 1:1 connection from the engine to the gearbox, are they not already geared up/down or whatever in the primary drive? Presumably the reason why a lot of the bike engined cars seem to go for 3.36/3,21 final drives as standard.

If you want to get a good grip on what effect the different ratios have, try downloading one of the "gear calc" programs and having a play with some figures.

And just to return to the original question, the 7 and 7.5 refer to the size of crown wheel. Hence the larger crown wheel and pinions being stronger. (Some of the American Ford diffs go up to at least 9", that I've heard of, anyway).

[Tarmac Terrorist]

Cheers for the info there lads

[jak]

I think generally car engined westfields use around 3.63. whereas bike engines generally the 3.92 as they are much higher revved and can take the higher ratio to improve accelaration.

The torquer engines such as my V8 which is also lower revved tan the 4 cylinder variety generally use the 3.36. I have even heard of people using the 3.14 (having converted into an LSD first!).

I am sure this have been posted here before but I can put my hands on it so here it is:

Quick question folks. My 1.8 zetec, all parts supplied by Westfield has a 7 1/2 inch diff, but I believe it is a 3.92 and certainly is n't LSD. It does n't seem to match anything on the list that Adam posted. What is it from?

John.

[Tarmac Terrorist]

Possibly from a Granada according to my recent research

[adamnreeves]

I think generally  car engined westfields use around 3.63. whereas bike engines generally the 3.92 as they are much higher revved and can take the higher ratio to improve accelaration.

Not quite as straight forward as that, I'm afraid. Gearbox and wheel choice play a big factor in determining the final drive ratio as well as engine type. FWIW 3.92 seems to be a good compromise with four cylinder 16v cars, with 4.09 and longer being beneficial, especially for tuned (car) engines that rev a bit higher than standard.

For many years 3.92 also seemed to be the standard ratio for Wf's Zetec engined models.

Bike engined cars are a different proposition if I remember rightly, unlike a car, which has a 1:1 connection from the engine to the gearbox, are they not already geared up/down or whatever in the primary drive? Presumably the reason why a lot of the bike engined cars seem to go for 3.36/3,21 final drives as standard.

If you want to get a good grip on what effect the different ratios have, try downloading one of the "gear calc" programs and having a play with some figures.

And just to return to the original question, the 7 and 7.5 refer to the size of crown wheel. Hence the larger crown wheel and pinions being stronger. (Some of the American Ford diffs go up to at least 9", that I've heard of, anyway).

That's why I used the word "generally". Of course factors that effect the diff ratio are engine dynamics, wheels size, tyre size, gear ratios, etc.

I have computer simulation software in which to plug all this data into and analyse the results.

[tex]

I think generally car engined westfields use around 3.63. whereas bike engines generally the 3.92 as they are much higher revved and can take the higher ratio to improve accelaration.

The torquer engines such as my V8 which is also lower revved tan the 4 cylinder variety generally use the 3.36. I have even heard of people using the 3.14 (having converted into an LSD first!).

I am sure this have been posted here before but I can put my hands on it so here it is:

Quick question folks. My 1.8 zetec, all parts supplied by Westfield has a 7 1/2 inch diff, but I believe it is a 3.92 and certainly is n't LSD. It does n't seem to match anything on the list that Adam posted. What is it from?

John.

are you sure its a 7 1/2" diff? i have the one from the 2.9 granada - 3.36

obvious difference is the ratio - very long.. also the top mounting bolts - it uses individual bolts as opposed to one long bolt right thro the top of the casing.

[Kevin Wood]

I've got a 3.64 7.5" lsd from a Granada Cosworth. Having used this and a 3.92 on a Zetec I'd say if you've got standard gear ratios the 3.64 would be the one to go for. Otherwise, 1st gear is so low as to be useless. More LSDs available in 3.64 (3.62 if 7") too.

My wheels are relatively big too. 205/50/15 so this holds even more true for smaller diameters.

Kevin

[The Great Fandango]

So what does it take to fit a 7.0 inch differential in place of where the 7.5 one was?

Anyone actually done this?

Cheers

Kyle

[Peter M]

Half inch spacer is what westfield supply and the top and bottom mounts use threaded studs.

[cast iron]

And just to return to the original question, the 7 and 7.5 refer to the size of crown wheel. Hence the larger crown wheel and pinions being stronger. (Some of the American Ford diffs go up to at least 9", that I've heard of, anyway).

Its a common misconception that the size refers to the crown wheel, whilst they do differ, 7" ad 7.5" reflects the size across the diff casing where the main top bolt goes through.