Compression readings on new rebuild

[dougie]

I arrived at 138PSI with carbs removed on both sides of a cold 8.4 1000cc engine
Stephen

How can that be Stephen?
8.4 x 14.7psi = 123.48psi.

[Frog]

Thanks Blaise, I might. I keep meaning to buy one, but... There is one at Harbor freight on sale now for $35, anybody use this one?

Don

That is the one that I have. I have not tried it yet.

[DonW]

He he- please read the instructions carefully before you do. I bought one several months ago, blew it up using it like the aircraft units I was used to, and returned it... When I looked for ones like what I was used to, they cost $100+, so now I'm going to get another one of the harbor freight units.

D

[George Ryals]

It has been posted on here before that there are places that can replate the nikacel (sp) cylinders. I think I saved a link to one of them. I will post it tonight if I did.

[StephenB]

I arrived at 138PSI with carbs removed on both sides of a cold 8.4 1000cc engine
Stephen

How can that be Stephen?
8.4 x 14.7psi = 123.48psi.

What kind of equation is that? Must me measurement accuracy then?

[Major Softie]

I arrived at 138PSI with carbs removed on both sides of a cold 8.4 1000cc engine
Stephen

How can that be Stephen?
8.4 x 14.7psi = 123.48psi.

Stolen from Wikipedia:

Measuring the compression pressure of an engine, with a pressure gauge connected to the spark plug opening, gives an indication of the engine's state and quality. There is, however, no formula to calculate compression ratio based on cylinder pressure.

If the nominal compression ratio of an engine is given, the pre-ignition cylinder pressure can be estimated using the following relationship:

p = p_0 \times \text{CR}^\gamma

where p_0\; is the cylinder pressure at bottom dead center (BDC) which is usually at 1 atm, CR is the compression ratio, and \gamma\; is the specific heat ratio for the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture.

For example, if an engine running on gasoline has a compression ratio of 10:1, the cylinder pressure at top dead center (TDC) is

p_\text{TDC} = 1 \text{ bar} \times 10^{1.4} = 25.1 \text{ bar}


So, you are ignoring the heat issue. You can see the formula correctly displayed at Wikipedia: http://en.wikipedia.org/wiki/Compression_ratio

[StephenB]

Engage ...

[Garnet]

Measuring the compression pressure of an engine, with a pressure gauge connected to the spark plug opening, gives an indication of the engine's state and quality. There is, however, no formula to calculate compression ratio based on cylinder pressure.

If the nominal compression ratio of an engine is given, the pre-ignition cylinder pressure can be estimated using the following relationship:

p = p_0 \times \text{CR}^\gamma

where p_0\; is the cylinder pressure at bottom dead center (BDC) which is usually at 1 atm, CR is the compression ratio, and \gamma\; is the specific heat ratio for the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture.

For example, if an engine running on gasoline has a compression ratio of 10:1, the cylinder pressure at top dead center (TDC) is

p_\text{TDC} = 1 \text{ bar} \times 10^{1.4} = 25.1 \text{ bar}

That's easy for you to say.

[Roy Gavin]

I think Boyle's law is the one that applies here, in degrees Kelvin.