Hi guys newcomer to this,so please be patient for some of my questions.

What Reasonable HP can you put though a 5 speedbox , assuming its a KAD, or Minisport, (ie) running a supercharger.

If using a 2.9 or 3.11 can the revs get down to about 3000 rpm sitting on about 100 kph.

Thanks

A "normal" road mini 5 speed where the 5th gear is an overdrive can not handle high power, it warns about it in the add.
If you went for a straight cut 5 speed set then yes they can handle big power BUT the 5th in them is only a 1:1 same as the 4th in all minis, straight cut 5 speeds are meant for the race track.

You may be able to use a 5 speed road gearbox but you'll always have to be careful and baby the car when in 5th, and probably drive sensiable in all other gears = no traffic light races, no fast driving, no fun .

Spend the $5000 elsewhere.

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I wonder why a straight cut 5 speed can handle more power than a helicle cut 5 speed box? It's the opposite in a 4 speed box. Is it purely about the ratios?

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(I believe) The 5 speed gear sets are made to fit in a std or slightly modified mini gearbox. The issue is the 5th gear is thin and their for cannot take the load. 4th gear is still strong so do all the accelerating and highway overtaking/hills etc in 4th (which is still 1:1) and when your crusing at high speeds slip it in 5th and enjoy lower revs.

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The SCCR 5 speed has 5 gears crammed in where the normal box has its 4 speeds. Pretty strong, as good as the 4 speed SCCR (however I agree with TK, it's a fallacy that straight cut gears (same size) are stronger than helical, which have more than 1 tooth in mesh at all times).

However the common 5 speed helical box has 5th gear added on, over hung out in the speedo drive space, it is a pretty dodgy way of adding a 5th gear in a box that isn't designed for it. They admit 5th gear is a problem, they say it's not suitable for S/C or turbo.
I think 90ft/lb crank torque is all it can take. That's not enough safety margin, these days for other than a stock motor.

If you are going to turbo, I would stick with 4 speeds and put a tall diff in it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

pretty sure helical gears have more contact patch and therefore strength than the equivalent sized straight cut gear.
straight cut gears reduce friction and side loading.
just saying

Yeah there's two strengths to Helical, as noted above. Wider teeth, and more than one in mesh at any time, and they're quiet because of a gradual engagement along the width of the tooth. Downside to helical is the sideload as Belinda said.

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The other down side to helical gears is thrust loading.

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If you will have enough power to pull a 2.9 or 3.11 then recondition the gearbox and away ya go...should be a sweet ride

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I put a 3.1 in my car and I'm thoroughly enjoying it, still scares the crap out of me

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I am running a 3.647 [edit, with S gears] in the 1360, it just about climbs trees in 4th gear. But I can't get traction in 1st gear after 5000rpm so have to short shift it.

The spare Mk2 S box with 3.44 diff is going back in it next time the motor is out.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

That's where the herring bone gears come in:) there are other (better?) ways of achieving a reasonable cruising rpm. Diff ratio for one, especially if you're going forced induction, secondly you could just increase the rolling diameter of your tyres. Will need to get the speedo calibrated and accept the odometer reading incorrectly.

3.44 is good with that much torque, and with the standard 4 synchro ratios. Best compromise for acceleration and top speed. I don't worry too much about nice cruising revs because there is no cruising on the track.

With the torque from the RE282 you can still hold 3rd gear through the fish hook at Wakefield and put in a decent time.

cheers
michael

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Thanks gents, think l will stick with the 4 speed with taller final drive. seems more tried and tested.

Cheers

Sorry to hijack this thread

I'm also looking at a 5 speed g'box. I'm currently running a 4 speed with a 3.2 FD, I like the extra top end speed but it's noticeably slower off the mark.

I just have a road car (no racing at all) but do like to take for an enthusiastic drive every so often. Am I better off getting straight cut or helical for a 1310 A+ with twin 1 1/2" carbies and a GR RE83 camshaft

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