gearbox pressure

[mic]

Been having a ponder over this. I read that a standard gearbox won't last long if you turbo/supercharged a standard engine when realistically, its only gonna give it a certain % power - even if it doubled the power, its still not gonna be as high as a standard cinq sporting engine at 50bhp+ (depending on where you read). So how come the standard 'box is ok for an engine that has twice the power, but not ok for a power increase by forced air?

[bis13]

its not okay for any other engine as it is weak however it can be used and will suffer in the long run it just supplies an easier route to convert and inevitably will fail so a good stock of boxes is advised. if you wish to supercharge or turbo charge the stock engine then go ahead and do it but think about how much extra heat this is going to build up by doing so also i hope you have a buldging wallet ...another point is if you go down this route then ensure you lose the silent blokes and replace with tripods.

[stuey]

I have had a good think about this and if the problem is caused by sudden of "jolt" when a turbo kicks in or if you floor a supercharged engine- I suspect it you where to drive gently at low speed and fed the power in gradually wether it would be ok?

but to be fair if you had a turbo'ed or charged 126 would you drive gently?

perhaps a low pressure turbo like saabs have it it would give the 126 an increased top end without all that gearbox destroying bottom curve power?

the above is possbibly all b*llsh*t but I just enjoy the discussion ...you thoughts are welcome!

[hudsonhenry]

The answer lies in torque not power.

The 126 box is not particularly strong but the standard 126 engine does not produce very much torque or power, but it is the way it delivers the power/torque which makes a difference, a 360 degree twin produces its torque and power in two big pulses for every full cycle of the engine. If you use a n/a four cylinder engine then you have four smaller pulses for every full cycle so you can get away with a little more power. If you tried running a 1100 twin cylinder engine the box would not last very long at all (indeed I have broken gearboxes with a tuned 650)

Turbos and superchargers deliver big chunks of torque. if you want more power then its best to aim for a high power lower torque engine which would be less stressful on the box and remember not to do any drag racing starts but to feed the power in gently. My 1108 never broke a gearbox despite covering 6000 plus miles including a trip to Italy (except the driveshaft donuts which only last about 2000 mile per set). The 1284 engine I have just completed is a screamer tuned for power not torque so we shall see what breaks on the box in due course.

Nigel

[Eklipze3k]

That's good to know Nigel, thanks for that info. I was a bit worried that the Sei engine might kill the box if driven hard but (if I ever get around to starting the conversion!) I wouldn't be doing that anyway.

[hudsonhenry]

The box I use at the moment is from a 500R (with a bis crownwheel and pinion) not a 126, it does not have syncromesh so it needs to be double de-clutched but as I understand it it is the strongest of the 500 or 126 boxes. I was told that when Fiat added syncomesh to 2nd 3rd and 4th they narrowed the physical width of the gears to make room. I have not had this confirmed but sounds logical.

The other thing to consider is tyre width. With skinny wheels its easier to break traction therefore creating less resistance in the box – breaking traction rather than the gearbox, with wider tyres and more grip the forces are concentrated on the gearbox internals. Also make sure that you use a sprung clutch (panda/punto etc) plate as this has a damping effect when changing gear – the standard 500/126 clutch is solid.

Nigel

[Rusty's Uncle]

  As Stuey says , all very interesting stuff . I have got the top off a 500 box at the moment so a quick measure of the 4th gear on the input shaft shows it to be about 11 mms wide. Went to measure a 126 one from my box of bits but went off the idea when I saw how deep it was buried in my shed so maybe some one has a 126 one in bits to compare the width.

[Darrell]

I'll measure one this weekend.

I'm kind of suspicious of the fable that the 126 box is weaker than the 500 as I pelt about in mine like a demon possesed at times.

I learned a costly lesson with first and that seems to be the weak point in my mind however I've also learned to check the oil level and change the oil from time to time, not as often as engine oil.

I put it in first before the car came to a stop, and it took all the teeth off the first gear.

Now even if it requires a bit of torture to the clutch, second is used unless the car comes to a complete hault.

I'd try double de-clutching but never done it before and cannot risk mucking-up another first.

I have noticed that after a long quick run, the box does seem to be a little stiffer to change gear, perhaps it is a sign of something else but I can report this new box has the noisyest fourth I have ever heard, I'll get off my butt soon and take it out and have a look inside.