A lot will depend on what you are doing with the engine. If just ticking over on a couple of psi of air for display it will not make a lot of difference. On the other hand if you are using that engine under load which will require a higher pressure and at higher speeds then it may give smoother running.

You will also need to alter the actual slide valve to give that lap not just the eccentric which moves it.

Now if you were just to adjust the position of the eccentric that would alter the lead and when the air is admitted to the cylinder, this is what will allow air into the cylinder before the piston reaches the end and have a cushioning effect.

I think it is lead that you are asking about not lap.

A Jason says, increasing the lead helps engines run smoother on air.

Neil

….and he also suggests that adding lap may also make the engine run smoother on air.

It puzzled me why a model engine maker would add lap to the slide valve if they only intended to run the engine on compressed air.

Lap is used, as I understand it, to exploit the expansive properties of steam. Without lap a whole cylinderful of steam is used for each stroke of the piston. Adding lap, without going into detail, utilises the expansive properties of the steam resulting in the engine using less steam and thus being more efficient.

I think I can see what Jason is thinking, at high speeds and with considerable momentum does the engine need a whole cylinderful of air for each stroke?

Just my thoughts guys

Geoff