Discussion in 'Mechanic and Repair' started by GLC, Feb 25, 2003.

  1. GLC

    GLC LawnSite Member
    from Alabama
    Posts: 175

    What is the compression ratio on a 25hp Kohler twin cylinder? Thanks alot!!!
  2. kevin55rc

    kevin55rc LawnSite Member
    Posts: 23

    Hello the 25 kohler commands have a 9 to 1 comp ratio.
  3. allenandinga

    allenandinga LawnSite Member
    Posts: 33

    No matter where you live (elevation) commands have a much higher ratio than this. A 25 command with no problems should have 175-185 psi per cylinder. Anything less than 150 won't work.

    An easy way to figure compression ratio is take a reading with a compression tester and devide that number by the atmospheric pressure in your area (14.7 here) and that will give you the compression ratio. When I devided it a minute ago, it came up to just over 12-1. This is not the most accurate formula for a race car but it works just for lawnmowr engine.
  4. kevin55rc

    kevin55rc LawnSite Member
    Posts: 23

    Hi there. Comp ratio is fixed. preasures are not. 9 to 1 comp ratio means the combustion chamber is 9 times smaller than the volumetric capacity of the cylinder. This rule of thumb is the case on all internal combustion engines. Compression preasures change with altitude because of the air density/make up, ambiant temperature and throttle posistion, and of cousre any engine mods, etc.
  5. allenandinga

    allenandinga LawnSite Member
    Posts: 33

    Sorry dude, I'm going to have to call BS on that one. Compression ratio and chamber size/cylinder volume have nothing to do with each other, other than the fact that if the chamber gets smaller, the compression rises or if the cylinder gets smaller, the compression drops. If what you say were true, I could put a flattop piston in a 25 command and the compression would drop because the volume of the cylinder has been decreased causing the chamber size # to be multiplied by a lower # (the compression ratio according to you) to equal the cylinder volume.

    Anyone with a good knowledge of engines knows that if the chamber size stays the same and the top of the piston goes from a dish to a dome, the compression ratio will rise dramatically.

    Surely you know someone that has been around race cars (yes it's the same for a car engine as it is for a lawnmower engine) that could verify this for you. If you don't, go to the 5.0/5.8 engine tech forum on and start a new thread about your theories of compression ratio and see what happens.:cool:

    BTW if compression ratios were fixed, all engines would have the same ratio. Do you really think the ratio of a lawnmower engine is the same as that of say a top fuel funny car.
  6. kickin sum grass

    kickin sum grass LawnSite Senior Member
    Posts: 628


    Hey there. I am trying to understand your method of figuring comp. ratio. I am not trying to bash or anything. maybe i just misunderstood what you are saying.
    a ratio is just that. a ratio between two things. It doesn't matter if you are at the bottom of the ocean or on the moon the compresion ratio will not change by the atmospheric pressure. It is a realationship between cylinder size and compression chamber size. If you change one of or the other sizes your ratio will change. However you will change the compresion psi by the amount of atmospheric pressure.

    A domed piston will cause a higher ratio because when the piston reaches the top the chamber has less area due to the doming of the piston and there for increasing the ratio. also is true if you bore out the cylinder and enlarge the piston.

    ratio is the amount of air/fuel from the cylinder squuezed into the compression chamber.
    if this is not what you are saying please and try to explain in a different way.
  7. kickin sum grass

    kickin sum grass LawnSite Senior Member
    Posts: 628


    another point to your method of figuring with the psi method is that you could have a valve leaking and cause the psi to be lower. then when you divide by air psi your ratio will be lower. that is not possible. the air psi can not make the cylinder volume or compression chamber volume increase or decrease.
    If the motor comes from factory at 9:1 then it stays 9:1 unless someones alters this.
    wxample. you say the previuos mentioned engine should have 185 psi . your method 185 psi/ 14.7 air psi = 12.58:1
    if your gasket or valve leaks and compression psi is down to 100 then 100 psi/14.7 air psi = 6.8:1

    if the engine comes with 9:1 it stays there unless altered mechanically.
  8. Mr.Wrench

    Mr.Wrench LawnSite Member
    from N.H.
    Posts: 65

    Don't forget guys. This engine has a compression release. So your not going to see those figures while cranking it over wet.
  9. allenandinga

    allenandinga LawnSite Member
    Posts: 33

    You guys have never been around racing very much, have you?

    Compression ratio is the number of times the barometric pressure where you are (this varies with elevation as barometric pressure decreases with altitude) has to be multiplied to achieve the same reading you get when take a compression reading on an engine.

    COMPRESSION RATIOS ARE NOT FIXED!!! If you have an engine with a blown head gasket, the compression ratio it now has is lower than if did not have a blown gasket.

    Example: If the barometric pressure where you are is 15 psi and you have an engine that has a cylinder pressure of 150 psi, this engine has a compression ratio of 10-1.

    To put it in simple terms, if the cylinder pressure drops for whatever reason, the compression ratio also drops.

    Any compression ratio an engine comes listed as from the factory is meant for the engine running under ideal conditions with no problems.

    In the case of engines that have a compression release for cranking, the actual ratio the engine runs at is even higher than what it comes up to by dividing the reading on the compression gauge by the barometric pressure.

    Compression ratio is not and has never been figured by the number of times the combustion chamber volume has to be multiplied to achieve the same volume as the cylinder.

    As a Kohler dealer myself, I can't ever remember seeing a Kohler publication that states a 25 command has a compression ratio of 9-1. Infact I'd be willing to bet that Kohler does not publish the compression ratio of any of their engines (I know Briggs does not) but I'll be sure to call Rob Minster at Kohler tomorrow and ask him if they do.

    If this doesn't explain it good enough let me know and I'll try to elaborate a little more.
  10. MWHC

    MWHC LawnSite Member
    from Wyoming
    Posts: 202

    Hey allenandinga,

    When you were a Kohler dealer did you ever bother to look in a service manual? They list the compression ratio; at least in my book. It's a definition dude. -> Its the ratio of the maximum to the minimum volume within the cylinder, between the piston and cylinder head. Yes it can be altered, but it has to be altered physically.

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