Norton 16H WD

Is it the compression ratio x 14.5

Atmospheric pressure is 14.5 psi, so for 6 to compression ratio it would be 6 x 14.5 = 87 psi

just insert your compression ratio.

A compression test is fast enough to be 'adiabatic' - the pressure can go up to roughly 40% more than that given by the compression ratio times atmosphere due to the fact that the gas gets hotter as it is compressed - and hot gas in the same space makes more pressure.

I understand the WD 16Hcompression ratio is 4.9. So the perfect compression test might give 4.9 x 14.5 x 1.4 or almost exactly 100psi.

I might test mine - give me something better to do this evening than watching TV - if the shed is not too hot...I bet it won't reach 100...

...I was right about not reaching 100psi!

I put it down to kicking over a single cylinder engine after having to use the decompressor to get it over TDC. Anyway - I got about 70psi. I've no reason to believe there's anything amiss. Currently it's been going well.

I wonder if any other numbers are out there?

On reflection: the cr of my 16H is roughly haf that of my Dommie - and the pressure (70) is roughly half the best figure the Dommie has produced (about 150) - so I think all is well (or reasonably so).l

Hi Robert,

Was there a reason you needed to know the cylinder pressure ? I ask this because the compression varies through the speed range and temperature of the engine due to the valve timing and gas expansion. I think the inlet valve on your 16H closes at about 65 degrees up the compression stroke which means when you turn it on the kick-start 4.9 to 1 will only be about 3.5 to 1. As the engine speed increases the valve timing becomes effective and the incoming gas gains inertia and the exiting gas is evacuated. This is the power band and is dependant on the duration of the valve timing which in turn relates to the needs of the machine. So taking all this into account any vacuum induction engine only operates at maximum compression for part of it's rev range. All engines are quoted with a compression ratio but this is only the dimensional measurement due to the above factors. The explosive pressure with the engine at maximum power will be over 250 psi so make sure your engine is not mechanically defective.

Regards, Richard.

Not scientific at all, I'm afraid but a 16H in reasonable condition should support the weight of an eleven stone rider standing on the kickstart for two or three seconds before it slowly begins to sink.

Thanks, Richard (and Richard...). In the absence of a self-starter, I imagine the only way to do a realistic compression test would be by freewheeling in a high gear down a hill. Life's too short...

But I wonder what is the risk of damaging the magneto on a Single if kicking over withno plug attached? The later K2F mag has safety gap screws to allow a spark to limit the voltage in the secondary coil. Did the earlier mag have any such internal arrangement? (I left a plug attached and lying on the cylinder head - was that necessary I wonder?)

They must (I imagine) be safe or mags would forever fail when plugs don't fire - wouldn't they?