After going to Cadwell Park yesterday it got me thinking about diff ratios for track cars.

A 3 doors standard gearing is far too long for most tracks if not all in this country.

What would you think is the best ratio ? and what is involved in installing them ?

Typically my mates 3 door has just had a new diff but would the whole thing need replacing or would it just need a part strip down ?

Some info and prices would be handy

 

what ratio has he got in there at the mment?
the most common ones are 3.62 which you can replace for the 3.92 as found in most twin cam sierras, same 7.5 inch diff and, as far as i am aware, it's on the pinion gear thats different as it has one more tooth on it, 14 instead of 13 or soemthing, the crown wheel still has 47 teeth on it

 

A lot of people choose the 3.9:1 over the standard 3.6:1
You can either have a 3.9 crownwheel & pinion fitted to your diff, of fit your lsd and flanges to a complete 3.9 diff that you find in the breakers.
Cost wise, depends which way you choose to go, but give us a call in the week and I'll explain

 

Jonnybravo is asking with my car in mind here.

Another question - if we change to a 3.9:1, then I understand my top speed will drop, but by how much?

I also understand my acceleration will increase as my gears will be shorter?

 

Now this is a popular anomoly:
It really depends on how much power you have, and at where in the rev range.
With top speed governed by power to aerodynamic drag/frontal area ratio, a car with only so much power can end up FASTER on a lower diff!

If you currently, with the power you have, can reach the rev limiter in top gear, then you have buckets of power - enough to break the aerodynamic drag envelope that matches your gearing.

However, if you don't currently make the limiter in top, then there is a chance that by lowering the gearing you will get your engine into a sweeter rev spot that is making more power, which will in turn help you to punch better through the air resistance, and actually achieve a higher top speed

 

Cool, I thought this would lower my top speed, not increase it!

The idea is to make my gears shorter by doing this to make it more track friendly through the bends.

I currently run about 400bhp at the flywheel, and the power seems strong right through the rev range

 

Tuning of ratios is something I prefer to discuss on the phone, as we can normally, by asking the right questions, arrive at a recommendation quickly. Email/web discussions can take too long...

The sort of questions I would be asking are:

WHAT IS WRONG WITH WHAT YOU HAVE?
Many people think just bolting on this or that goody will make their car faster. Not always the case!

WHAT ARE YOU CURRENTLY RUNNING?
We need this info to do the maths

IS THE PROBLEM THE SAME AT ALL CIRCUITS?
Gearing for Cadwell may well be different to Snetterton

IS THERE ONE SPECIFIC CORNER ON A CIRCUIT THAT YOU DON'T HAVE A GEAR FOR?
Some corners are more important to gear correctly for than others.

WHAT IS AVAILABLE?
Unless you're ready to shell out on custom ratios, we will be looking to come up with a remedy based on the rest of manufacturer's range.

Don't forget our ratio calculator is available at
http://www.gearboxman.co.uk/inforatios.html
and it will help you to have a picture in your head of a lap of the particular circuit, noting what sort of rpm you're doing in what gear through problem corners. From that info, you/we can work out the speed you are capable of taking through that corner, and just why the revs are all wrong!
HTH

 

Excellent, we will be in touch during the week, cheeRS