Diff Ratios Again

[jaguar68]

Hi Guys,

I have a quick query about diff ratios.

What would be the best diff ratio to use with a tuned 1098cc engine to give better acceleration than standard?

The engine will be 20 thou overbored, standard cam, 9.5:1 compression ratio, 12G295 head, 1 1/2" SU Carburettor, Mini Manifold and a Stainless exhaust. It will also probably be running a Ford Type 9 Gearbox too.

I'm not too worried about top end speed. At the moment the car will touch an indicated 90ish mph, but I would like to be a little quicker away from traffic lights etc.

Regards

John

[bmcecosse]

Well - if it's really acceleration you are after - the 4.55 final drive from a 948 engined car would help - the standard in your car would be 4.22. If you really want to accelerate - at the expense of economy (although the overdrive 5th speed will offset it slightly!) then the 5.33 from an 803cc car would be ideal! And 13" wheels may help too. However acceleration is all about overcoming the inertia of mass - so cut the weight of the car as much as you can - and consider a lightened flywheel! Try not to break the half-shafts........

[mike.perry]

I have tried a 5.375 diff on a 1098 engine, great for accelerating up 1 in 6 hills in top gear and overtaking at the same time but it revs the nuts off the engine. The best compromise is a 4.55 diff.

[jaguar68]

I'm going to look into 13" Midget steel wheels with a 4.55 ratio diff, also a mildly tuned 1098cc engine, probably with the Type 9 Ford gearbox.

Any thoughts?

Cheers

John

[mike.perry]

The Midget 13" wheels will lower the gearing by having smaller rolling radius tyres, 175/80 x 13 tyres will give you very slightly higher gearing than 155 x 14 tyres.
The Ford box will give you four normal ratio gears and an overdrive 5th gear.
Have you checked on the cost of the modifications?

[jaguar68]

I've picked up the Midget steel wheels for £20 from Ebay, I have the Ford gearbox spare from another project. The only thing I don't have is the 4.55 ratio diff.

I'm lucky in the respect that I have quite a few of the parts lying around the garage

Regards

John

[mike.perry]

If you have a 4.22 diff I would try it and then make up your mind about a 4.55