FINAL DRIVE RATIO

[Thorp]

Hi All,

Im building up a 1293 unit with a 286 cam, big valve head that is basically for a weekend toy (road based rather than track). The crank/rods etc are all worked so its going to be able to rev quite well.

The box it's going on is a mk2 S 4 syncro one, currently fitted with a 3.44:1 diff. Is this final drive ratio ok with a 286 cam? or would I be better off going slightly shorter one (3.765 perhaps)

This topic has probably been covered elsewhere so apologies if Im covering old ground.

Thanks

[Smiffy]

With the 3.44 you end up with 16mph per thousand rpms and 15mph with the 3.7. I found 3.44 a bit too highly geared with the 286, I changed to a 3.9, but that was a little to low. So the a 3.7 will be perfect with that cam.

[Spider]

On a similar set up, I tried a few from 2.9, 3.1, 3.44 and 3.76.

I found the 2.9 was sluggish. The 3.1 was acceptable, but not quite quick enough off the line and it also gave short clutch life (as would have the 2.9). The 3.76 was probably a bit too quick off the mark and gave wheel spin (even in 3rd) rather easily. I felt the 3.44 was on the money.

[nick@dunsdale]

I wouldn't go higher than 3.44 for road use.

In years gone by I have tried them all on 10" wheels from 2.9 to 3.9, personally I found that once you got past 3.44, a acceleration was good, very good infact, but top end suffered.

I suppose it all down to presence if you want rapid acceleration and a lower top speed.
Or do you want something you can drive a 100 miles in relevant comfort, with still good pulling power in third gear.

[Smiffy]

Bloody hell nic how much top end do you want, a 3.7 at 7500rpms is 120mph. Easily enough for a ban.

[mk1]

I agree, for a roadgoing car that is reasonably well developed a 3.44 would be the best choice as an all rounder.

[Thorp]

Thanks all for the opinions. Very useful.

[rich@minispares.com]

if its just a local use road car then a 3.4 will be ideal - its what my 1430 (with a 649) runs, it does get a bit hard work when its been flogged on the ringroad, but on back roads its great.

a 3.7 would drive you round the bend as you really need to be working the crank to get the best from it and it would soon drive you round the bend - they are really suited to the smoother 1071 engine - I run a 3.6 in my 1071 and its great, but its got probably another 1500 useable rpm over a 1275 motor.

a 3.1 would be great if its a long distance cruiser - they lope along quite nice with them, but do have a fairly blunt acceleration

[almondgreen]

I use a 3,65 since 20 years and will not change....

[In the shed]

3.65 as that's the tallest you can fit a salisbury to without resorting to machining.

[nick@dunsdale]

Bloody hell nic how much top end do you want, a 3.7 at 7500rpms is 120mph. Easily enough for a ban.

7500 I might need my ear defenders on , just personal taste I suppose, used like mine geared so I could drive for long periods at a bare able engine level of engine noise lol

I did try a 2.9 for awhile as well, changed the car completely, I wonder what they were like in the city e, witch where only 1000cc.

[Smiffy]

2.95 in a thou, is a bit like having over drive.
Not really any use except on flat motorways, down hill, or with the wind behind you. Changed the g/f's to 3.6 from a 2.95 and she reckons it's the best mod I've done to it.