what power will a straight cut box take?
Turbo Mini Forums

hi guys one for all of you with over 110 at the flywheel
looking at boxes for my supercharged mini which judging by the spec should give 120 plus at the flywheel now my question is will a straight cut jkd 5 speed handle the power or will it go bang
would i be better going for a 4 speed dog box or would just a straight cut 4 speed be better

using straight cut roller drops, straight cut crown and pinion and 4 pin dif

Torque is the problem with Mini gearboxes, which is why forced induction engines with their large amount of torque at low revs struggle more than high revving NA engines. The problem with the 5 speed boxes is that the 5th gear hangs out on a shaft with no bearing at the other side of it, and so that shaft is prone to flex and the gear moves when a high torque and hence a high gear separation force exists. I have not personally used a 5 speed mini box, but from the adverts and write ups I have seen I have got the impression they are only really suitable for standard injection coopers or very mildly tweeked cars say no more than 85bhp.
The dog engagement box is designed for rally cars that are used very roughly and is a form of gear engagement. For the road I don't personally see the point in spending the extra money on a dog box.
Finally not sure why you want a straight cut final drive, or even if such a thing is available? The standard helical item performs fine up to pretty serious states of tune.

if you go for sc drops plus a sccr set with a well rebuilt box and a 4 pin diff then the worlds are your oyster.

miniwilliams is pumping excess of 200bhp through this set up with no probs.

as tom says all forced induction gives a shed load of tourque low down which will kill any helical set up.

i used this set up with my 100 bhp 1380 set up and after 4 years of hard motoring it was as good as the day it went in. i didnt even have to replace anything so it was ready for the turbo.

always looking for them bigger bunches of bannanas

i'd probably go with a syncro s/c box as its a fair old wack more to get a dog box, and remember buying 2nd hand can be tricky what ever the engagement type

i was advised not to bother with roller drops, and as for the crown wheel i guess you mean semi-helical?? which i think most non-production cwp's are, not sure though

Edited by dan on 9th May, 2005.

Tom are you sure you’re correct on the JKD 5-speed info?

Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe.

point taken on the 5 speed but not planing on using that gear for acceleration more crusing
i was advised by mini speed that for the best reliability the roller drops and straight cut final drive would be the best option but after this may revise on the final drive
any more info would be graetly appreiciated

doodmeister
yep that is my baby do you use that box in anger?

Edited by sandfly on 9th May, 2005.

1450cc 100Bhp @ the wheels if you call that anger.
And the box is around 12 years old now i think and done it's fair share of track days.

Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe.

yeah i call that anger
does that include much use of 5th motorway miles and track miles etc

Maybe it was the Minispares (?) 5 sp box I looked at. I definately remember though that there was poor support at the far end of the 5th gear shaft? And in the catalogue I had it stated that the gearbox was not suitable for tuned engines.

More than anything else I was 18 at the time and could have probably flown to the moon more easily than afforded a 5 sp gearbox......

The Box has seen thousands of motorway miles running to London and back from Derbyshire. and a fair few track day miles but i find it very hard to hit 5th on the track.
It get's into 5th everyday going to work and back.

Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe.

i think jk 5 speeds use a brand new casing designed specifically for the job so they may be better.

mini speed cant even put on an engine steady properly, hogster builds matts gearboxes and a few others enough said!

Yes Hogster sorts out my boxes for me (plug, plug)

I Can't afford a 5 speed s/c so I wouldn't know but I have seen them and I see what Dave is saying about it hanging at the side.

As for Diff I run a standard 2.9 final drive and pinion wheels, can't see a problem.

Best 1/4 mile 13.2 seconds @116 mph
First 5 port miniturbo to make over 200 bhp on a carb?
First 5 port miniturbo to make over 200 bhp on Injection?

http://www.mattwoodsphotography.com

cheers for all the info lads just wanted to get the know from the people who have actually had this kind of power

mini speed and jackknight recommended the straight/semi helecal fd as belt and braces and they say it reduces strain on the fifth gear
i dont pretend to know nowt about gearboxes so have taken their advice i did want to change the fd anyway so no real loss any more advice or experiance i would love to hear but it has to be said i bow down to the hogster if he is rep for miniwilliams box

how much is the JK SCCR 5speed box now ?

£400 second hand or £2000 new
i got lucky with mine as i knew the car it came off a trusted engineer has been given the rebuild job as the previous comments on mini speed did ring slightly true