Hi Guys,

Does anyone have an idea of what diff ratio I have in my Formula 27. It has 15" wheels, Zx9 bike engine and bounces off the Rev limiter in 6th at 96mph!
The accelaration is very good and I like it the way it is but how easy is it to change a rear diff and what would be a sensible ratio to go too? Would I be losing my accelaration?

Thanks,
Kerr

Probably a 3.92.
3.14s are sought after for BECs but rarity and demand boost there prices. You could try a 3.62 and look at changing 5th gear ratio if you want a cruising gear.

Easy way to find out is-

Put it in neutral, Jack up one back wheel
Mark a point on the wheel circumference and on the diff nose/Prop shaft
Turn the wheel that is off the ground, counting the revs to turn the diff input once
Double the wheel revs and you have the ratio.

For better accuracy turn the wheel until the diff has turned 10 times and then divide the wheel revs by 5 ( divide by ten muktiply by two)

If it is a live axle then ratios typically are 3.54, 3.77, 3.89, 4.125 and 4.4.

Hope this helps, for bike engines the Lower diffs are better I think, beter top speed but the 3.54 are not cheap, 3.77 are expensive but 3.89 is the most common.

If it is a sierra independent then it will be 3.38, most likely a 3.62 or a 3.92.

Also, this assumed is is an open diff, above does not work with an lsd.

Hope this helps.


Edit - should have said, to do the calc you need tyre size, rev limit rpm and 6th gear ratio.

[Edited on 18/10/14 by Ugg10]

Thanks for the replies.

It was originally built using mk2 escort parts and doesn't have an LSD. Maybe it doesn't need changed as the accelaration is wild but thought I'd ask.
It currently has 15 Sierra Cosworth wheels.....if I changed to 13" wheels would this make it even lower geared?

sounds like a 392
as said

a 3:62 will give you 122 mph with a 195-50-15 tyre

hope this helps

or go on the bec calculator

just re read is it a live axel car if so poss a 389 diff

[Edited on 18/10/14 by INDY BIRD]

[Edited on 18/10/14 by INDY BIRD]

Depends on the tyres, but smaller wheel diametre equals lower top speed. Escort mk2 is a live axle. Might be worth checking anyway as the 3.77 diffs go for around £250-300 on ebay, that would get you quite a way to a second hand 3.89 (less the £100) a new cwp(£250) in your choice of ratio and a re build. A top speed of 96mph with a rev limit of 12k seems to point out a short diff.

Edit - posts crossed, if you have a 3.89, likely, or a 4.125, then the 3.54/3.77 will need a couple of hundred pound to change second hand. I am in a similar position with my Anglia, I have a 4.125, would like a 3.77 but will need to save some pennies first.

Edit again, based on indy bird calc, I would say you have a 4.4, this gives 100mph with the same settings.

A lot of the irs bec guys have used the freelander diff which is 3.14 iirc.

[Edited on 18/10/14 by Ugg10]

[Edited on 18/10/14 by Ugg10]

Thanks for the help.

Is a 4.4 very rare? It is a GPS verified 96mph and has nothing else to give, bouncing off the limiter!

I'm very interested to know what it is buy will probably just keep it. I fancy some lightweight 13" wheels as I think my original cosworth wheels could be heavy.

Kerr

Freelander diff is 3.21:1
Ford do a 3.14 but he zx9 won't pull that!
Drop to 13"s with a 205/60/13 and A 3.38 diff if IRS
3.54 would be an option if live axle!

3.54 is your best bet gives top speed of approx 120 to 125mph
Top speed dependant on your choice of tyre and wheel combination

[Edited on 18/10/14 by ian locostzx9rc2]

4.4 is not super rare but the short track oval, sprint and rally boys like them. Have a look on completed items on a well known auction site for prices, in general, 3.89 is the cheapest followed by 4.125, 4.4, 3.54 and finally the 3.77. It all comes down to condition though.

Mines zx9 3.62 13" wheels , is happy ;-)