Compression ratio Q!!!

[vallely]

Hello, I am new here and I have a question about the compression ratio on my recently acquired 1981 cr450r. Now I know that this bike has been labeled one of the worst bikes, but im just using it for fields and as ,and to get aquainted with bikes. Now let me get to the point.

My CLYMER manual says the compression ratio is (7.1:1). Does this mean that it is 7.1 bar to 1 bar or 7.1 atmospheres to 1 atmopshere of pressure. If it does then that would make the psi inside the cylinder at around 100-110 psi on a good engine.

I put the compression tester on my bike and kicked it at WOT and The readings were right around 110 or a little above 110 psi. Does this mean that my engine is properly sealing for comprssion?

If I have all of this totally screwed up then please tell me..and set me straight. Also if you know of what a cr450r should be reading for pressure please post it here.

I thank you for your time and hope that you can help me out here!!!

-Rob

 

[trial_07]

Your results surprise me! My bike scores 150 psi and maintains it. There must be something wrong! I can't help you though because I ain't the one who performed the test. Best of lucks!

 

[photomd]

My CLYMER manual says the compression ratio is (7.1:1). Does this mean that it is 7.1 bar to 1 bar or 7.1 atmospheres to 1 atmopshere of pressure. If it does then that would make the psi inside the cylinder at around 100-110 psi on a good engine.

I put the compression tester on my bike and kicked it at WOT and The readings were right around 110 or a little above 110 psi. Does this mean that my engine is properly sealing for comprssion?


-Rob

First post for me, so if we get flamed...blame it one me. :)

Compression ratio is a simple ratio by volume of your cylinder with the piston at BDC and TDC. So there is 7.1 times more volume in your cylinder with the piston at BDC than at TDC.

To me 110 sounds about right. In my mind, compression by a compression tester will depend on the compression ratio. My CR has a ratio of about 8.5:1 and it's been around 135 since I bought it. My airhead BMW with 8.5:1 is 134 and 138 with 2-3% by leakdown testing and I've got a boat with 11:1 that read 156-164 when I bought it.

HTH's :cool:

 

[robwbright]

About every post I've seen on here says 150 psi minimum on a full size bike.

 

[mongial]

I am not familar with that bike, but I did have a 1980 YZ465. I never did a comperssion test on it but kickin my RM250 is a dream compared to kicking the hell out of the old BEAST. Without riding boots you were begging from a bruized foot.

 

[trial_07]

Mechanics say 150 psi is ideal, not minimum. I know of kx 125s that score 170, but it ain't necessarily good.

 

[SHSPVR]

photomd your close it more like
Cylinder Bore Size
Piston Stroke Length
Head Gasket Bore Diameter & Compressed Head Gasket Thickness
Combustion Chamber Volume In CCs
Piston Dome Volume In CCs Negative
Piston Deck Clearance Negative If ABOVE Deck

2 stroke motor have to be done like this CR = ( 1 + 2 ) / 2
CR is compression ratio
1 is cylinder volume at exhaust closing
2 is combustion chamber volume

vallely compression ratio has very little to do with psi pressure
compression ratio = mechanical energy an engine that can squeeze its air-fuel mixture.
Your rings play the key role in both psi pressure & compression ratio also 2 stroke don't have some type compression ratio as 4 stroke becuases of the Open Port and short squeeze time for its air-fuel mixture.

And ask me I belive the 110 psi it is about rigth for properly sealing on compression ratio of 7.1

 

[76GMC1500]

1987 CR250 made 180 psi on a compression test. I do not know how old the top end is.

 

[motometal]

photomd your close it more like
Cylinder Bore Size
Piston Stroke Length
Head Gasket Bore Diameter & Compressed Head Gasket Thickness
Combustion Chamber Volume In CCs
Piston Dome Volume In CCs Negative
Piston Deck Clearance Negative If ABOVE Deck

2 stroke motor have to be done like this CR = ( 1 + 2 ) / 2
CR is compression ratio
1 is cylinder volume at exhaust closing
2 is combustion chamber volume

vallely compression ratio has very little to do with psi pressure
compression ratio = mechanical energy an engine that can squeeze its air-fuel mixture.
Your rings play the key role in both psi pressure & compression ratio also 2 stroke don't have some type compression ratio as 4 stroke becuases of the Open Port and short squeeze time for its air-fuel mixture.

And ask me I belive the 110 psi it is about rigth for properly sealing on compression ratio of 7.1

no offense to the poster, but there is some real misinformation in here. For example, the rings have nothing to do with the compression ratio, it is just a number based on engine parameters, in other words the volume of gas being compressed vs. the compressed volume.

 

[SHSPVR]

motometal yes your rigth in a way but in way compression ratio need squeeze the air-fuel mixture volume so in way rings is needed for a ture number based even if it hold no psi

 

[photomd]

I can't post a link, but from what I remember, compression ratio is the difference between BDC and TDC. I think what's stated above is trapped compression ratio. That is the ratio of the volume just as the exhaust port is blocked to the TDC. Is that what's in our bikes? Dunno.

I did dig out my physics book from college, blew the dust off and started looking at pressure calculations.

For an ideal gas (P1*V1)/T1= (P2*V2)/T2. P= pressure in atmospheres, V=volume in liters and T=temperature in kelvin. 1 is the BDC and 2 is TDC. I assumed T stayed the same. I know: false assumption, but it makes calculations easier.

I went with the definition of compression ratio of BDC:TDC. I also assumed the volume could be anything as long as it stayed in same ratio as the compression. In my CR250 it is 8.7:1. For the original example it will be 7.1:1

So the formula for mine is (1atm)(8.7L)=P2*1L. 14.696PSI=1 atmosphere so I get 127.8 psi for mine.

For vallely, I get 104.3 psi.

I know it's not exact, but for all the engines I've checked, it's within 5-10%. I apologize for the :blah: , but I thought this might shed some light on it.

 

[motometal]

that's more like it...good stuff!