13 replies to this topic

[richieg]

What is the max power / torque a standard helical gearbox with x-pin diff handle?
At what point do you think you need to switch to straight cut gears and which is the way to go, SC drop gears or SC everything?

I have tried different searches but can't find what I'm looking for, maybe I missed it?

[mini13]

I'd say around 130 hp.

some people get away with more, some break stuff with less, mechanical sympathy has a big influence.

[samsfern]

straight cut gears arent as strong as helical gears, straight cuts just reduce sideloading, and friction, and they make a fantastic noise.

[mini13]

true for gears of equal material....

however.

if you try to put 200hp through a std helical box the teeth will fall off rather rapidly, if you use a good straight cut box it will last.

[Wil_h]

Doesn't Robrt run 180bhp with a helical box?

As said mechanical sympathy goes a long way. I would say that the standard ratios are badly spaced and this is exagerated as you get more power, as the power tends to get more peaky. So once over 130bhp it's worth it anyway.

I'm currently planning a 120bhp build using standard dropsand box with a cross pin diff.

[richieg]

I'm thinking about supercharging so power will be low down and I'm expecting 120-130 BHP. By the sounds of things the std box with x-pin diff should be ok.

[Dan]

The weak point in the standard gearbox is nothing to do with the gears themselves, they are fine and strong enough really. The main weakpoints are the diff (rectified with a cross pin or even a competition grade single pin and good assembly), and the sychromesh. Nothing can really be done about synchro problems other than changing to dog engagement. 85 lbft is really the acceptable limit for the second gear synchro. Other problems include the pinion trying to climb up the final drive (which can be corrected by fitting a main shaft end support housing from MED or KAD), and the laygear shaft and bearings (simply down to part quality I believe).

The main reason that straight cut gears are prevalent in competition gear kits is simple, they are much much cheaper to design and make.

[Cables]

Doesn't Robrt run 180bhp with a helical box?

As said mechanical sympathy goes a long way. I would say that the standard ratios are badly spaced and this is exagerated as you get more power, as the power tends to get more peaky. So once over 130bhp it's worth it anyway.

I'm currently planning a 120bhp build using standard dropsand box with a cross pin diff.

agreed, however if you can source a s/gt close ratio box and match it with a good final drive then your ok

[mini13]

Will,

I know robert destroyed a helical box at 150ish hp, it was rebuilt as a straight cut in the thread below, i thougght ihe was still S/C.



http://www.turbomini...p...68594&fr=50

[Wil_h]

You are right, he no longer has a helical box, lasted a long time at 150 though. so 130 with similar torque is a best guess limit then.

[Guess-Works.com]

The usual demise of the gearbox with forced induction motors is changing down from 4th to 3rd on boost, this put's a sudden load on the laygear, and you loose teeth very rapidly... and I've seen this on both helical and straight cut...

Once you're into the mid 100's in torque or power then you should be upgrading your gearbox with a X-pin and SC drop gears... The SC drops prevent the idler gear from disappearing out of the transfer case, the need to change from the std helical gears is more a personal preference than a requirement...

GT gears are an alternative, however, unless you are really lucky and have one of the last sets made which actually run the A+ mainshaft and laygear, then you are hampered slightly by having to use A series main shaft and laygear which are weaker in their use, smaller bearings.

[Cooperman]

Going right back to the early 60's, the straight-cut gears were introduced using superior grade material and optimum heat-treatment. They were made straight-cut to keep machining costs down and to provide alternative closer ratios to keep the engines 'on-the-cam' in competition. Straight-cut drops were introduced for the same reasons. The early 60's engines, even in 1275 form, only had about 100 to 110 bhp bhp in competition state, so helical gears would have been fine. A standard helical box with, say, a 3.9:1 diff for forest rallying, has a 1st gear which is so low as to be almost useless.
Unless you can find some early GT helial gear sets, you are stuck, in helical form gearing, with wide ratio gears, even though they may unsuitable for 'hot' and cammy engines, which is why SC gears are now widely used. Thus SC gears are now used to keep hot engines 'on-the-cam', even though the materials from which they are made may not be as good as the original 'works' SC gears.

[John H]

Wil

Robert has an SC box but still has helical drops.

[Wil_h]

Wil

Robert has an SC box but still has helical drops.

That makes sense, I knew he still used some helical bits.