Compression Ratio verses Compression test results

Could someone, anyone, please, explain to me the connection between compression ratio and compression test results using a compression gauge? I would be so grateful for a straight answer! I understand gas laws but can't make the connection. p.s. guess what? I'm a newbie at this game!

You mean how do we get 160psig with 9:1 and 14.7psi ambient?
We do work in compressing the gas and add energy to the system! Ideal gas law and kinetic theory of gasses

edited to fix link

Post edited by: Duck, at: 2006/05/05 19:01

Thanx for the input Duck, but I don't get it.To stupid I guess.

Post edited by: tellietubbie, at: 2006/05/05 19:09

Post edited by: tellietubbie, at: 2006/05/05 19:15:blink:

Post edited by: tellietubbie, at: 2006/05/05 19:27

I picked up some info trying to figure out my rebuilt engine measuring 100-120 psi cylinder pressures.
1: The compression ratio is the ratio between the volume of the cylinder at BDC and the volume at TDC. This is fixed when the engine is built. Not too much rocket science there.
2: When measuring cylinder pressure with a gauge, as the piston comes up compressing the gas in the cylinder, the temperature of the gas increases and is trying to expand, increasing the measured pressure over and above what would be theoretically calculated based on the compression ratio and atmospheric pressure. This temperature effect supposedly increases the gas pressure to about twice the theoretical amount.
3: However, the intake valve typically closes after BDC. At cranking speed (low engine RPM), some amount of gas is allowed to be pushed back out of the cylinder before the valve closes so this reduces the actual amount of gas being compressed. Obviously the valve timing will affect this to some degree.

Normal engines will have a measured cylinder pressure of roughly 17 - 20 times the compression ratio at cranking speed.

Hope this helps.

check out the comp ratios thread I started awhile back, it really cleared up the issue for me:) , lots of good info posted by other guys
Scott

PV=nRT

or P=nRT/V

The volume goes down so the pressure goes up.
But, you also heat the mix when it acquires the energy used to compress it. This makes 'T' bigger so the quotient is more than
you would see with a constant 'T'.

for example

if 'T' were constant: 10:1 compression and 14.7PSI ambient gives 147PSI.

If the work of compressing the gas doubles 'T' you will see 294PSI.

Physics is fun because it provides a framework for how everything works.

Post edited by: Duck, at: 2006/05/05 23:41

Usually I'd say "Thank God". On this occasion, thankyou to the guys on KZRider.This was confusing the hell out of me and I couldn't decide whether to buy new pistons/rings or not 'cos I didn't understand the implications of the crazy readings the compression tests gave. e.g cylinder 1 dry =155 but wet was 287 !!
Now I understand, thanks a lot guys!.
While I'm here, when [when, when???] I finish this 500 can I have a category in the piccy section of my own 'cos I havn't seen any other members with a 500 and so no piccys? Just a thought.

PV=nRT.. Oh God, I am back in Thermodynamics class. NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

tellietubbie,

I'm guessing that you have a Kawi Z/KZ500-4 cylinder?
That's a pretty rare model, and one that never made it's way over here to the US market. I'd imagine it's because we already had the KZ550 model.
If it is indeed the Z/KZ500-4 you own, there was a member of the now archived KZr site, SkynyrdMan (John), who also owned one.

He did quite a bit of work on that bike, and a great portion of the parts he had to manufacture himself since he was unable to find even a parts listing for his bike. If you do a search under his name in the archives, it will pull up a lot of great info pertaining to it.
Check out THIS THREAD in the archives, it shows some really nice pics of his bike.

I know that a few members here have/had contacts with him.
ibsen22000 (Ivar) is one that I know of that even had an opportunity to meet up with John some time back.
If you were wanting some specs and/or info on what John did on his Z/KZ500-4, I imagine if you asked nicely enough that one of the other members here could either relay a message for you, or possible even put you in contact with John directly.

I didn't understand the implications of the crazy readings the compression tests gave. e.g cylinder 1 dry =155 but wet was 287 !!

Are you saying that when you put some oil in the cylinder and retest that you get a reading some 85% higher, IE: 287/155 ?. I would wonder about this large increase.

kawtoy wrote:

PV=nRT.. Oh God, I am back in Thermodynamics class. NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

It's all TT's fault ; His initial question contained the statement,

I understand gas laws but can't make the connection.

hwms,the compression tester came off ebay is about 20 years old.........first time Ive done one..ho hum,the whole reason was to get an indication of the integrity of the 26 year old piston rings. I accept the differences between cylinders as the most important aspect, the actual observed readings simply confirmed my suspicions that they'd need replacement.So far the best I've found cost £252 for four new second oversize pistons and ringsets so bugger the bills, I'm having them.Regards.