Compression vs. Compression Ratio

[FreeBeeTony]

Can someone define/tell the difference bewteen compression and compression ratio?

I have seen many posts stating that their compression is ~210 psi and I think they think this relates to a high compression ratio........however I do not.

My understanding is as follows: compression is related to the "health" of an engine, how well the rings and valves are sealing and compression ratio relates to combustion chamber size and will affect timing/detonation/octane requirements, etc.....

Am I close or way off??

 

[Bluestream]

Re: Compression vs. Compression Ratio

Any compression reading of 210 PSI on a stock marine engine are highly suspect. Compression gauges can vary widely in the readings they offer, especially those that sit in a tool box for 20 years and are never calibrated.

Compression ratio is the ratio of the volume from when the piston is at the bottom of its stroke to when it hits TDC. Eg. 9:1

 

[FreeBeeTony]

Re: Compression vs. Compression Ratio

I understand 1 is a reading in psi and 1 is a ratio, but is my description accurate?

 

[gbeltran]

Re: Compression vs. Compression Ratio

I think you have a good understanding of the two. That is as good a description as any.

 

[coheej]

Re: Compression vs. Compression Ratio

You're close. As you said compression is a measurement of cylinder pressure in psi and it is used as indication of the health of the motor; especially when comparing on cylinder reading with the others.

The compression ratio is a static calculation and is set at the time the motor is built and as previously stated is a ratio of the total volume compared to the compressed volume.

 

[FreeBeeTony]

Re: Compression vs. Compression Ratio

I think you have a good understanding of the two. That is as good a description as any.

Thanks............

So my point is, just because you have good compression (210psi) doesn't mean your compression ratio is high!!

 

[Bluestream]

Re: Compression vs. Compression Ratio

Thanks............

So my point is, just because you have good compression (210psi) doesn't mean your compression ratio is high!!

I would say there is a correlation between high compression ratios and high compression readings. Just what that exact correlation is I don't know

Here: http://mgaguru.com/mgtech/power/pp105.htm

 

[Alpheus]

Re: Compression vs. Compression Ratio

Atmospheric pressure, nominally 1-BAR, is about 14.7-PSI. (at sea level) So if you divide your cylinder pressure readings by 14.7, you will get the compression ratio.(Roughly)

Cylinder pressure (psi) / 14.7 = Ratio

210/14.7 = 14.29:1

I also see in all those posts of people that are stating they have a compression reading that high (4 stroke Mercruiser/Volvo Penta) are being told to get a new gauge, replace the heads or get dished pistons...

 

[John_S]

Re: Compression vs. Compression Ratio

Answers.com:

A 1:1 ratio is equal to 0 PSI. 14.7 PSI is equal to a 2:1 ratio. Just multiply your ratio by 14.7 to get PSI, or divide PSI by 14.7 to get ratio.

This is only in a perfect cylinder where valves close exactly as the piston reaches the bottom and stays closed the whole way, and if no air bleeds out from the valves, or between the piston and cylinder wall. Not to mention the difference between if it's hot or cold. Plus, according to the math problem, if you had a compression ratio of 1:1, you would be pushing 14.7 psi. So there isn't any REAL way of telling an engines compression ratio without getting the specs for everything.

 

[Silvertip]

Re: Compression vs. Compression Ratio

The term "compression ratio" tends to get shorted in conversation which is where your confusion probably enters the picture. Lets use some examples.

Engine #1 is "designed" with a 12:1 COMPRESSION RATIO. That is generally considered "high compression ratio" although people tend to shorten the term to just "high compression" (as in high compression ratio engine). As such the volume of cylinder at the top of the stroke is 12 times as much as it is at the bottom of the stroke.

Engine #2 is "designed" with 8.5:1 COMPRESSION RATIO. This is considered a relatively low compression ratio. Again, people tend to shorten the "low compression ratio" to just "low compression engine".

Engine #3 is a two stroke outboard and has a 6:1 COMPRESSION RATIO. This is considered a very low compression ratio.

Now then, since low compression or high compression "engine" generally relates to compression ratio, one must understand that a worn engine, regardless if it has a higher or low compression ratio. may have very low compression which means the rings and valves are no longer sealing so when the piston travels upward during the compression stroke, much of that compression is lost past the rings or out the valves. That does not change the compression ratio which is a design factor. Low compression therefore simply means the engine has lost its ability to compress the fuel/air mixture regardless what the compression ratio is. If an engine truly has higher than normal compression as measured with a compression gauge, then something is wrong either with the gauge or there is something else going on in the engine.

 

[kewlkatdady]

Re: Compression vs. Compression Ratio

well put...

The term "compression ratio" tends to get shorted in conversation which is where your confusion probably enters the picture. Lets use some examples.

Engine #1 is "designed" with a 12:1 COMPRESSION RATIO. That is generally considered "high compression ratio" although people tend to shorten the term to just "high compression" (as in high compression ratio engine). As such the volume of cylinder at the top of the stroke is 12 times as much as it is at the bottom of the stroke.

Engine #2 is "designed" with 8.5:1 COMPRESSION RATIO. This is considered a relatively low compression ratio. Again, people tend to shorten the "low compression ratio" to just "low compression engine".

Engine #3 is a two stroke outboard and has a 6:1 COMPRESSION RATIO. This is considered a very low compression ratio.

Now then, since low compression or high compression "engine" generally relates to compression ratio, one must understand that a worn engine, regardless if it has a higher or low compression ratio. may have very low compression which means the rings and valves are no longer sealing so when the piston travels upward during the compression stroke, much of that compression is lost past the rings or out the valves. That does not change the compression ratio which is a design factor. Low compression therefore simply means the engine has lost its ability to compress the fuel/air mixture regardless what the compression ratio is. If an engine truly has higher than normal compression as measured with a compression gauge, then something is wrong either with the gauge or there is something else going on in the engine.

 

[Bondo]

Re: Compression vs. Compression Ratio

My understanding is as follows: compression is related to the "health" of an engine, how well the rings and valves are sealing
and compression ratio relates to combustion chamber size and will affect timing/detonation/octane requirements, etc.....

Am I close or way off??

Ayuh,... You've got it Tony,...

To further complicate things,... There's the Static compression ratio everybody's talkin' about,...
And,...
There's the Dynamic compression ratio....

The Static compression ratio is the mathamagical equation noting the change of volume of the cylinder...

The Dynamic compression ratio is controlled by cam, 'n valve timing,+ can result in a vastly different number that the Static compression ratio...
Ei; your Runability issues...

 

[wca_tim]

Re: Compression vs. Compression Ratio

Good information here laread... and in general, static compresion ratio is going to always be higher than dynamic comression ratio. You can think of dynamic comression ratio as giving a more realistic idea of what the comression is actually going to be - and to be most closely related to the comprssion measured using a guage.

static comression ratio is an easy number to get at with simple information on the machuine specs of an engine, while dynamic comression ratio is a bit more involved. That's what when most people refer to "comression ratio" they mean static compression ratio.

note that 205-210 cylinder pressure is pretty much tha max you can get away with on 92 octane pump gas in a well designed engine on the street... (or water). and that is higher than any stock engine should yield.

As an example... the static compression ratio on a 383 I was running, was just about 10:1, and gave a cylinder compression reading of 200-205 psi on all cylinders. It was designed to run premium pump gas and did so very well in this configuration...

Hope this is helpful

 

[bruceb58]

Re: Compression vs. Compression Ratio

You can't just divide the PSI you get from a compression test by 14.7 to determine the compression ratio. New engines these days have 9.4:1 compression ratio. 9.41 x 14.7 = 138 PSI which is way below specs on a new engine.

The PSI you are measuring is actually PSIG( PSI gauge). If you compress a cylinder to half its volume(2:1 compression ration) you will only measure 14.7PSIG which shows the formula to divide by 14.7 to get the compression ratio is not accurate.

This doesn't even take into account the heating of the air caused by the energy which is supplied in compressing the air. This additional energy causes the air to heat which further increases the pressure. Anyone who has used a bicycle pump to pump up a tire and felt the pump heating up can attest to this fact.

 

[bruceb58]

Re: Compression vs. Compression Ratio

As such the volume of cylinder at the top of the stroke is 12 times as much as it is at the bottom of the stroke.

Actually...its the opposite of this.

 

[Simoniz]

Re: Compression vs. Compression Ratio

Thanks............

So my point is, just because you have good compression (210psi) doesn't mean your compression ratio is high!!

Actually this is the wrong way round.

Compression ratio is determined when the engine is designed and built, just like cubic capacity, and has nothing to do with the condition of the engine. As others have said, if the volume of the cylinder with pistion at BDC is 500 cc and the volume with the piston at TDC is 50cc then you have a compression ratio of 10:1. Even if there is a holethrough the top of the piston the CR is still 10:1.

Compression is to do with the condition of the engine and when measured with a warm engine with the throttle open at cranking speed, it might be 160 psi. 210 is certainly high for a petrol engine but would be hopelessly low for a diesel engine which might have a compression pressure of 700psi (it has to compress the air, and thus raise its temperature, so much that the diesel fuel will ignite when sprayed into the cylinder).

 

[wca_tim]

Re: Compression vs. Compression Ratio

You can't just divide the PSI you get from a compression test by 14.7 to determine the compression ratio. New engines these days have 9.4:1 compression ratio. 9.41 x 14.7 = 138 PSI which is way below specs on a new engine.

The PSI you are measuring is actually PSIG( PSI gauge). If you compress a cylinder to half its volume(2:1 compression ration) you will only measure 14.7PSIG which shows the formula to divide by 14.7 to get the compression ratio is not accurate.

This doesn't even take into account the heating of the air caused by the energy which is supplied in compressing the air. This additional energy causes the air to heat which further increases the pressure. Anyone who has used a bicycle pump to pump up a tire and felt the pump heating up can attest to this fact.

am I going to have to start dreaming about adiabatic compression cycles again???

 

[FreeBeeTony]

Re: Compression vs. Compression Ratio

I thought this might be a good topic..........seems quite a few people are confusing their compression readings (psi) and their compression ratio.

 

[Aloysius]

Re: Compression vs. Compression Ratio

Atmospheric pressure, nominally 1-BAR, is about 14.7-PSI. (at sea level) So if you divide your cylinder pressure readings by 14.7, you will get the compression ratio.(Roughly)

Cylinder pressure (psi) / 14.7 = Ratio

210/14.7 = 14.29:1

I also see in all those posts of people that are stating they have a compression reading that high (4 stroke Mercruiser/Volvo Penta) are being told to get a new gauge, replace the heads or get dished pistons...

Nope, not accurate. Remember, a diesel is "compression ignition" squeezes the mixture in the cylinder enough to raise the temperature to the point of combustion. There is no quick and easy way to associate compression PRESSURE to RATIO. What's the equation..PV=NRT?

 

[Alpheus]

Re: Compression vs. Compression Ratio

Nope, not accurate. Remember, a diesel is "compression ignition" squeezes the mixture in the cylinder enough to raise the temperature to the point of combustion. There is no quick and easy way to associate compression PRESSURE to RATIO. What's the equation..PV=NRT?

I don't think anyone was talking about diesel engines.

I stated in my post:

I also see in all those posts of people that are stating they have a compression reading that high (4 stroke Mercruiser/Volvo Penta) are being told to get a new gauge, replace the heads or get dished pistons...