1400 K Series De-Dion: Standard Diff Ratio

[Laurence7]

Numpty questions
Anyone any idea what the standard diff ratio is for a '94 1400 K Series De-Dion car?
What are the alternative ratios and to what were they fitted?
Reason for asking - the standard ratio is way too high for track day use, so am looking for something lower.
Am also considering closer ratio 'box - possibly using a Tran-X gear set - they can provide a vast range of different gear sets:-
http://www.tran-x.com/gearkits/f5spt9.html
Anyone any experience of Tran-X gear sets for the type 9 box - good or bad?

Edited by Laurence7 on Thursday 14th May 01:04

[Ecosseven]

I think it is 3.9 or thereabouts. My car has the 3.62 diff but i'm running the 6 speed box.

All the best.

[Incorrigible]

We use the Tran-x gears in MegaGrad race cars. As you'd expect, the straight cut gearrs are fairly noisy. As long as you build the 'box with the appropriate bits they seem to be reliable

[Murph7355]

So I'm clear, I think you mean that the car is geared to pull too high a top speed (in 5th?). I therefore *think* it's likely to be a 3.62 (lower the number, the higher the road speed). But could be wrong.

If you're leaving the engine as is (i.e. a 1.4K), you can do no better than buy a Caterham 6spd. The box was designed for that engine and it really works well with it.

2nd hand ones do come up, and you could flog your type9 to make up the difference. Well worth it, and I'd do that in preference to changing the diff initially. 6th in the 6speed is like 4th in the std 5spd for reference.

[Laurence7]

Thanks for your inputs chaps.
Ben – as my car is very much a road car doing the occasional track day, rather than a full-on racer, I’ll be going for helical rather than straight-cut. I’ve not driven a car with straight-cut gears on the road – reckon it could get quite wearing!
Murph – you’re spot on with your assessment – there’s nothing like enough grunt to be able to usefully use 5th on track. Last time out at Cadwell I was just about getting into 4th before the braking point for Coppice, and don’t even use 5th for the faster sections of Combe or Goodwood (I’m from doon sooth, but occasionally make the pilgimage north to the hallowed Cadwell ).
I take your point about a 6-speed box, though I am about to replace the tired 1400 unit with a mildly-tuned 1600K Supersport (its been sulking under the bench for about two years now!)
I believe that the standard type-9 ratios are 3.65-1.97-1.37-1.00-0.815. Tran-X do a close ratio gear set that’s 2.16-1.46-1.16-1.00-0.87, ie, a much taller first and a shorter 5th, with nice even spacing between – I’m not sure how this compares with the 6-speed box.
I’m assuming that a 1600K with a close ratio 5-speed or the 6-speed box would work well on-track with a diff of around 4ish:1? But where to get a 4ish:1 CWP?

[Murph7355]

I would still go with a Caterham 6sp if I were you. I have one on my SLR-ish spec car (which started life as a 1400 Supersport 6spd) and love it.

I think Caterham can supply 3.92 diffs. And I'm sure Road Race Transmissions could too.

I wouldn't go any higher than that personally. With 13" wheels (a must) you'll end limited to under 110 which the car would pull easily.

All "I think"

[Red Seven]

Your car will have a 3.92 diff, with five speed box.
It's more normal to run a 3.62 diff with a six speed, but I run my lightly modified 1600 SuperSport with a 3.92 and six speed.
First is tall enough to be usefull (unlike a 5 speed!), but low enough that it does not bog down from the line, and the other gears just fall into place.
I was just short of the rev limiter in sixth at Goodwood a few weeks ago.
The cost of a different gear set AND a different diff ratio, make the difference in cost between the two quite small.
The ratios in the Caterham six speed are spot on for a 1600SS.

[Laurence7]

Murph, Red-Seven - thanks for your inputs - REALLY helpful.

I was bemused by the "It's more normal to run a 3.62 diff with a six speed", 'til I looked up the ratios for Caterham 6-speed box and found that (unlike the 'overdrive' top gear in the 5-speed Type-9 box), the top gear in the 6-speed box is direct (1.00:1), so, in effect, top gear in the 6-speeder is the same as 4th in the 5-speeder (4th in the 5-speeder is also 1.00:1).

Here are the ratios for the 6-speed box
1st: 2.69
2nd: 2.01
3rd: 1.59
4th: 1.32
5th: 1.13
6th: 1.00

If you look at the ratios between successive gears of the 6-speed box you get the 'inter-gear' ratios, which look like this:-
1st – 2nd 1.34
2nd – 3rd 1.26
3rd – 4th 1.20
4th – 5th 1.17
5th – 6th 1.13
Beautifully spaced !

The upshot is (I think) I'll stick with the standard 3.92 diff, source a 6-speed box, go to 13" wheels (in lieu of current 14") and haul out the the mildly-modded 1600K Supersport engine from under the bench, and fit that.

[Murph7355]

Laurence7 said:

...
The upshot is (I think) I'll stick with the standard 3.92 diff, source a 6-speed box, go to 13" wheels (in lieu of current 14") and haul out the the mildly-modded 1600K Supersport engine from under the bench, and fit that.

I think as a first set of moves, this would be wise.

Welcome to the slippery slope that is upgrade-itis!

Next - exhaust, decent ECU and a whole raft of engine bits and pieces.

[The Wookie]

Incorrigible said:

We use the Tran-x gears in MegaGrad race cars. As you'd expect, the straight cut gearrs are fairly noisy. As long as you build the 'box with the appropriate bits they seem to be reliable

Just to expand on what Ben's said, they can be extremely unreliable if you don't get the right bits.

Also first gear is very long for road use, and makes the car difficult to launch if you're intending to do sprints/hillclimbs. It also seems to put more strain on the clutch, as the springs and friction plates have a greater propensity to fail.

Personally... If your car is a track day car it's an improvement on the standard 5-speed, but having driven both I'd rather save up for a 6-speed box.

[Vlad the Imp]

I used to have a standard 5 speed box on a 3.92 diff and never used 5th on track. I've kept the 3.92 diff and got a six speed box, it's ideal on track and will hit the limiter in 6th just before Avon Rise at Coombe.

The 6 speed box transforms the car, I'd agree with the others, go out and buy one!

Vlad