Still looking into doing a build (dependant on available budget) and looking at Diffs.

What's the pro's/con's of the 2 ratios 3.6 v 3.9 of the sierra diff? (These 2 ratio's seem to be the most common that I can find).

Car will be Zetec with a type 9 box.

Basically, a difference of about 8% in revs, at any given speed in any gear.

The 3.9 diff. will give slightly better acceleration, but will be slightly less relaxed in road use (particularly when cruising on motorways or dual carriageways), and will probably give slightly worse overall fuel consumption for this reason, too.

It probably wont have any real effect on top speed, as the 5th gear will be well overdriven in any case (ie. you'll run out of power to overcome the rising aerodynamic drag, long before you run out of revs).

Personally I have a 3.6 and actually long for a longer 5th when cruising at national speed limit.

I have a 3.9. with Type 9 gearbox, At 70 mph the Zetec is doing approx. 3500 RPM so its certainly not relaxed cruising. Think I would prefer 3.6 or even lower for cruising.

Acceleration is good though and I accelerate more often than I do 70 mph. If that make sense.

15" wheels by the way.
Tyre Size: 195/50 x 15

[Edited on 10-5-17 by Slater]

use a gear calculator to see what effect it will have on your setup

don't worry about the theoretical top speeds (its not very likely you'll have enough power to get to them )
but instead look at the engine revs you'll need for a given road speed and gear - that will tell you what it'll be like to live with.
eg. will you need to do 4500rpm in 5th at 70mph or are you going to at full speed on your favourite track without ever needing to use 4th or 5th gear.....

personally I like this calc (its got the type 9 ratio's built in), but there are loads to chose from:

https://locost7.info/gearcalc.php

quote:

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Originally posted by Slater
15" wheels by the way.

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I changed my 3.6 for a 3.9 and it did change the character of the motor very noticeable..

Do you have a long first gear? Then you could go for the 3.9 otherwise a 3.6 might be better.

From my calculations, a 3.9 diff with a long first gear would be quite nice for a tuned/modified Zetec (if it could revs to 7k).

But for a 'normal' zetec with a rev limit of 6000rpm. The normal gearbox ratios with an 3.6 diff tend to work out better.

Your kit car is a toy, if you want relaxed cruising and better fuel consumption drive your run too
3.9 all the way

My car was fun road driving and track days. These days it is sorn and track days only.

I had a 3.9 and it was fine. 2 x trips to the ring and many breakfast runs. There is no relaxed with these, not with an aero screen, GRP seats and a lead foot anyway!

For this reason I have recently purchased and planned to go to a 4.27. Faster acceleration and more bang for my buck, THAT is what these cars are about IMO.

I am NEVER going to cruise long distance with my car. Its got a 3.9 diff and 195/45 x 15 tyres. It's brilliant! Couldn't care less about fuel consumption or motorways, don't even have a motorway in Norfolk anyway! 3.9 or 3.6 will be brilliant whatever you go for

3.62 is cheaper and easier to find in LSD flavour (Ford viscous). It's not just the gearing of course, engine characteristics can be a big factor.

I changed my 3.92 open for 3.62 LSD in a Scimitar SS1 with Nissan turbo and can barely notice the difference in normal use because the power is pretty linear.

The 1600 CVH SS1 (sprint/hillclimb) however has noticeably quicker acceleration since I swapped the 3.62 for a 3.92 (the fact it's now a quaife makes it a lot faster off the line too).