Megagrads used CR internals but that class is obsolete. SigMax uses standard Type 9 ratios and 150 class (which is a slightly more flexible version of the Caterham 310R spec) uses standard Mazda ratios.
The Mega gearbox was an improvement but the difference wasn't massive. The only place I really felt it was Anglesey, where there are several corners that sat inconveniently between second and third on the old Supergrad box.
To change tack slightly (not that you can tack slightly), what is it about the 6 speed box that is "wrong" for touring?
Back in the day, I happily did gazillions of road miles with a 6 speed box and never wanted to change it.
Identifying what you want to change will help with finding the right solution.
#23: Agree entirely, and I've had a 6-speed for around 20 years.
My view on the 5 speed Julian:
I have a standard 5 speed and a very revvy K series. 1st is too short, no doubt. I don't do racing starts so it's rarely a problem though. 2nd and 3rd are ok for me, In 3rd it does an indicated 90mph at 8k, probably closer to 80 in reality, but this does allow much of the overtaking to be completed in one gear.
I would like 4th to be a little shorter. It's often the gear I sit in when I'm waiting, anticipating an overtake; so if I can nip past in 4th, I do. In these cases, the overtake would obviously be quicker if the ratio were a little lower.
I quite like having the over drive 5th. I knock it into this on flowing roads at a constant speed, as well as motorways, but also sometimes on the straights on track days, to give the engine an easier time. This would still be possible with a lower 5th though.
I have driven a 6 speed SLR, which was brilliant, and I don't doubt I would prefer that box on my car. It's likely I would aim to alter my 5 speed ratios though, because it's so much more affordable. A longer first and shorter 4th maybe, but I would definitely take advice from club members and transmission specialists before committing what would still be a decent chunk.
You can't generally change 4th - it's always 1:1. If you want different revs for a given speed in 4th you have to change the diff ratio.
That's useful to know.
Your correct, the tunnel is wider so even a metric chassis doesn't guarantee the space will be available for the Mazda gearbox to fit. I think the only real clue is the removable gearbox mounting plate in the middle of the tunnel.
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