Gearboxes and Diff Ratios

I was planning on using Frank's gearbox from his current Birkin, as he was replacing it with a dog-box, however best laid plans and all.....Frank bought himself a dedicated race-car, so the his Birkin will keep the nice Quaife semi-helical, meaning I now have some decisions to make regarding gearboxes.

The main concern with gearboxes are the various ratios. These, coupled with the diff, have an immediate impact upon:

Acceleration - how fast the thing gets off the line in various gears
Speed in gear - at what speed do you need to change up a gear
Top speed - at what point does the scenery blur stabilise?
Driveability - how hard do you have to thrash it at the lights to stop stalling?

For example, if you have a nice low ratio like Franks for first gear (2.39) and a low diff ratio (3.9), then you'll get to a ridiculous 80kph in first (on 15 inch rims). Which is great for tearing off the line and getting good 0-100kph figures. However, you'll also note that at idle (let's say 1000 revs), the car will attempt to do nearly 15kph. Now, when you're trying to pull off at the lights, this means you have to give it a bit of stick to stop it from stalling, which means you tend to squeal the tyres every time.

At the moment, the suggestion is to go for a Subaru 4.4 diff with a decoupling LSD (decouples on lift-off, reducing oversteer) and a rebuilt Sierra gearbox from a 6 cylinder. According to this setup, I should be able to get the following at 6500rpm

1st 48
2nd 89
3rd 128
4th 160
5th 195

While the top speed doens't look massive, clubmans tend to hit an aerodynamic wall at anything above this anyway.

The other thing to consider is - where does a shift leave you? So, let's say I'm doing 48kph in first at 6500rpm, and I shift to second. 48kph in second is about 3500 rpm, which is just before the torque kicks in at around 3750rpm on the sort of engine I'm looking at. So not 100% ideal, but not too bad. But as you go up the gears, it drops you into higher and higher rpm - so the shift from 4th to 5th at 160 kph puts you at nearly 5000 rpm.

Anyway, there are a few options, such as a copy of Franks gearbox, or a 3.9 diff ratio, but it's all a bit of fun to try and model what the car will be like using various numbers.

In other news - the chassis is now in Melbourne, and will be shipped this week to arrive early next week, so it's a little later than Christmas but oh well.

For those with any sort of interest, the following website was used for the speed calculations:

http://www.kabamus.com/garage/gears.html