Gearbox update

5-Speed vs. 6-Speed box

The bottom line is that I am unsure as to which gearbox I will end up with. I have placed the order based on the standard 5-speed gearbox which is now a Mazda sourced unit. The 6-speed unit is a Caterham box, but also comes with a pretty steep £2,500 optional extra charge.

The consensus is that the 6-speed box is better suited to track days, and the 5-speed more appropriate for touring, with more widely spaced ratios and more of an ‘overdrive’ 5th gear.

My problem is that I enjoy track days and road driving, and with a fairly torque rich Duratec 2 litre engine (certainly compared to my old K-series engined Caterham), does it matter that much, seeing as I’m on track for fun, not to chase an additional tenth of a second?

Gearbox analysis

I downloaded an Excel tool from Gearboxman.co.uk available here  to try and see if I could work out the best option just based on simple maths and gear ratios / tyre sizes and final drive (the differential gear ratio).

I also think, but am not 100% sure, that the current differentials shipping from Caterham are from BMW (used on 1-series cars) and have a final drive ratio of 3.64:1, irrespective of gearbox option purchased.

So I plugged all of the ratios and wheel sizes into this spreadsheet and got the following results:

Mazda 5 Speed data

5 speed

Caterham 6 Speed data

6 speed

Analysis of results

A few things strike me with these results, and to be honest, I’m not an expert here, and probably need guidance from someone who is. Feel free to comment on this post, all advice and thoughts welcomed!

I like the fact that with the 5-speed box, the engine revs are around 3,000 at 70 mph, as this makes for more relaxed driving on more tedious A roads and motorways (compare this to 4,000 @ 70mph for the 6-speed box).

With this ‘overdrive’ though, comes the problem that on track, I will only be using 2nd,3rd and 4th gear, and there are some fairly big gaps between those gears and larger rev drop-off’s, so it will be harder to keep the car on the boil. But having said that, my understanding is that the engine torque characteristics will cover much of this problem.

The 6-speed box is obviously more suited to track work, with 2nd, 3rd, 4th and 5th coming into play, with all of these ratios closer to one-another, giving better options of being in the perfect gear for all corners. Its a bit of a shame that the 6th gear isn’t considerably longer.

I’m still not sure what to do here. Perhaps the most obvious thing would be to test drive cars with both configurations and simply get a ‘feel’ for the right decision that way.

Currently the order has been placed for a 5-speed box, and I could always swap this out at a later stage.

2 thoughts on “Gearbox update

  1. Really good analysis on the gear ratios. I have a 420R on order and I am not sure what gearbox to get. I am mostly aggressive road driving with possible track days. I like the option of an overdrive 5th gear, but I am most concerned that the 5 speed doesn’t neuter the engine power. What I mean is, I want the car to be able to break the rear wheels loose in at least 1st and 2nd gears and possibly 3rd, strictly for fun . What transmission did you go WITH? Can you share your feelings on it? Thanks!

    Like

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