Compression ration and psi.


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CBX1000chris
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Compression ration and psi.

Post by CBX1000chris »

Hello all,

This is a general question about compression ratio and psi. The workshop manual states the CBX has a compression ratio of 9.3:1 and page 3-15 of the manual says the compression should be 170 +/- 14 psi. I must be missing something because if we assume air pressure at sea level is 14.7 psi the maximum compression in the cylinder should be 14.7 x 9.3 = 136.7 psi. If that's the peak pressure then no matter how many times the engine turns over a gauge in the plug hole can't go beyond 136.7 psi.

Clearly I'm wrong, I just don't know why! Please can a clever person put me out of my misery, (with an explanation, not a rifle).

Thank you in advance,

Chris.

daves79x
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Re: Compression ration and psi.

Post by daves79x »

I don't know about all the math, but with my two compression gauges, I've never seen over 140 on any CBX I've worked on with either gauge. They all ran just fine. Others will disagree.

Dave

Rick Pope
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Re: Compression ration and psi.

Post by Rick Pope »

Well, just a couple thoughts to confuse us more...... Your pressure gauge will read zero when we know the atmospheric pres. is 14.xx. So you've already lost some pressure. But the rise in pressure of a gas when squeezed isn't linear, making the pressure rise very quickly when it climbs the scale. Something to do with a guy named Boyle. And then there's valve overlap, letting some of the air leave the party.

So we can't calculate cranking pressure just by the listed compression ratio. Cam timing, valve overlap, and other factors come into play.
Rick Pope
Either garage is too small or we have too many bikes. Or Momma's car needs to go outside.

CBX1000chris
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Re: Compression ration and psi.

Post by CBX1000chris »

I've just seen the typo in title, oh well.
I remember Boyles law from school, or some of it anyway, the relationship between pressure, volume and temperature. I suppose in a petrol engine the pressure rises unevenly anyway, the vertical movement of the piston for 10 degrees of crank rotation is less at TDC and BDC than it is halfway between the two. Given good old Boyle might it be true that as the gas compresses it heats, further increasing pressure, and hence giving you more pressure that the maths suggest?
I like the point about the gauge reading zero, were now entering the world of relative and absolute values. Having entered that world I've immediately done a 180 degree turn and headed for safety.

Rick Pope
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Re: Compression ration and psi.

Post by Rick Pope »

CBX1000chris wrote: SNIP.... Having entered that world I've immediately done a 180 degree turn and headed for safety.
Discussing physics is a slippery enough subject, especially if, like me, you have no formal training. But I like trying to tag along with the more learned among us and putting the pieces together.

Many moons ago, I read an article by Fred Rau, of Motorcycle Consumer News, about compression ratio v. cranking pressure. Some of it stuck with me, perhaps not enough. :o
Rick Pope
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Re: Compression ration and psi.

Post by EMS »

Guys, it is NOT rocket science. the compression ratio is not a ratio of pressures, although the name may indicate so.
It is a geometrical figure!!
The compression ratio is the ratio between the swept volume of the cylinder and the volume of the combustion chamber.
It is a number that is often misunderstood. Especially when people sell oversize bore pistons.
We had this discussion here on this site a couple of times.
Any time you increase the bore/displacement by changing pistons and leave the combustion chamber alone, you increase the compression ratio.
What you are doing is compressing a larger volume into the same size chamber.
So, in simple terms:
The ratio between the volume in the cylinder above bottom dead center of piston and the volume of the combustion chamber is the compression ratio.
For a stock CBX motor with a bore of 64.5mm and a stroke of 53.4mm you have a displacement of 174.4cc. (Dig out your high school geometry book and verify!)
the combustion chamber is roughly 21cc.
174.4+21 to 21 = 9.3:1
Cranking pressure is something totally different.
It depends on the density and the pressure of the charge entering the cylinder.
Compression ratio will be the same for a natural aspirated or turbo-charged engine while cranking pressure for a the turbo motor will be higher.
Engine 101.
Last edited by EMS on Fri Feb 07, 2020 7:13 pm, edited 2 times in total.

CBX1000chris
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Re: Compression ration and psi.

Post by CBX1000chris »

Brilliant explanation, thank you.

jnnngs
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Re: Compression ration and psi.

Post by jnnngs »

daves79x wrote:I don't know about all the math, but with my two compression gauges, I've never seen over 140 on any CBX I've worked on with either gauge. They all ran just fine. Others will disagree.

Dave
Hi,

I am with Dave on this one, all my gauges read around 140 on a good engine. Never managed to get close to a reading of Honda's stated "normal" pressure of 170 psi.

Paul.

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Re: Compression ration and psi.

Post by Larry Zimmer »

CBX1000chris wrote:Hello all,

This is a general question about compression ratio and psi. The workshop manual states the CBX has a compression ratio of 9.3:1 and page 3-15 of the manual says the compression should be 170 +/- 14 psi. I must be missing something because if we assume air pressure at sea level is 14.7 psi the maximum compression in the cylinder should be 14.7 x 9.3 = 136.7 psi. If that's the peak pressure then no matter how many times the engine turns over a gauge in the plug hole can't go beyond 136.7 psi.

Clearly I'm wrong, I just don't know why! Please can a clever person put me out of my misery, (with an explanation, not a rifle).

Thank you in advance,

Chris.
Just a note regarding compression. As the manual says for checking compression, "warm the engine". Or, something to that effect. What that means is GET IT HOT! Ride a 20 minutes loop. Then, check the compression within about 10 minutes. To get the 170 requires a block and head temp of minimum 230F (110C). At ambient, 68F/20C, you will get the readings that you have. They are, as Dave said, good stuff. It's a thermodynamics thing that I won't bother to get into. Basically, multiply your room temp pressures by 1.3 to see how close you are to 170. Hope this helps.
Larry Zimmer
cbxlarry@sbcglobal.net

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