Compression pressure

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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stella51
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Compression pressure

Post by stella51 »

Compression pressure, factory new G80 mid 1950:is.
Without going into the thermodynamic and adiabatic details of the compression pressure in a cylinder, with a compression ratio of 7,3:1, mathematically it is about 234 psi (16,17 bar).

If you are interested in the mathematics of if it you should look at: https://en.wikipedia.org/wiki/Adiabatic_process

Does anyone know if Matchless ever made public what the compression pressure was in their new G80 7,3:1 machines? If they did, that figure would interest me.

The figure is interesting because, in all discussion on the Club Forum, the compression pressures discussed are mostly in the range of 120 to 180 psi. Of course these figures come from used machines, but still they are very low.

Your views would be greatly appreciated.
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1608
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Re: Compression pressure

Post by 1608 »

Forget the maths. 234 psi would blow the head off, thats if anyone could manage spinning the motor over. I would imagine the average comp would be about 140psi. IMHO
stella51
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Re: Compression pressure

Post by stella51 »

Re: Compression pressure
The compression pressure would never reach the theoretical value in a real engine. Thats why it would be interesting to know the compression value of a brand new machine. Then you would know what to aim for when you renovate an engine.
Matchless probably measured the compression value many times on new machines. But maybe no figures have survived.
SPRIDDLER
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Re: Compression pressure

Post by SPRIDDLER »

stella51 wrote: Fri Aug 12, 2022 11:14 am
The compression pressure would never reach the theoretical value in a real engine. That's why it would be interesting to know the compression value of a brand new machine. Then you would know what to aim for when you renovate an engine.
I've no particular interest in the subject but whilst I'm passing time hiding from the 34 deg heat today............

My brief and probably naive observations are that (unsurprisingly) the cylinder pressure is dependant upon the comp ratio and increases with revs and depends upon whether the tests are carried out with a self-powered engine or being being driven by an external source as it would be impossible to kick it over at sufficient revs for a comprehensive test.
(My chum 'Thrifty Peter' is away on his P11 or (G15 CSR?) camping but he may comment later as he has been involved in high performance engine development and testing for very many years).

Whilst interesting I don't think that knowing the max cylinder pressure would be of any practical advantage whatsoever to us shed-fettlers when assessing a renovated engine. Few of us, if any, have the necessary equipment to test nor are seeking racing performance but happy just as long as it starts and runs well, doesn't belch smoke and the chaincase doesn't drip. ;)



Here are a couple of graphs from very many on the web......
Cylinder-pressure 3.jpg
Cylinder-pressure 4.JPG
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Harry44
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Re: Compression pressure

Post by Harry44 »

There are a few factors that change the results. Firstly, we never really fill the cylinder. Restrictions in the inlet tract including the valve itself restrict flow. The adiabatic process in reverse means this air is now cooler than ambient temperature. Secondly our compression ratio is a nominal value. We should measure from where the inlet valve closes. This will give a lower ratio than that stated.

Also how many of us remember to open the throttle fully when doing a compression test ?
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Re: Compression pressure

Post by SPRIDDLER »

Harry44 wrote: Thu Sep 08, 2022 9:39 pm There are a few factors that change the results. Firstly, we never really fill the cylinder. Restrictions in the inlet tract including the valve itself restrict flow. The adiabatic process in reverse means this air is now cooler than ambient temperature. Secondly our compression ratio is a nominal value. We should measure from where the inlet valve closes. This will give a lower ratio than that stated.

Also how many of us remember to open the throttle fully when doing a compression test ?
Indeed so, Harry, it's an interesting topic and well worth a pint or two at the bar on club nights. I'm sure there are thousands of pages on the subject if one wants chapter and verse, but I doubt that many of us have the need, inclination, skills or equipment to apply such advanced thinking.

I have always opened the throttle wide when compression testing because that's how I was taught in my apprenticeship (60 years ago) and so I've always done it that way. I don't have my own tester to hand now but it would be interesting if someone who has the kit produced some results comparing the wide open and fully closed throttle positions. :?
I poke badgers with spoons......
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