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Thread: Determining compression ratio

  1. #11
    gnarley
    [QUOTE]Originally posted by 058:
    Static C/R + Boost [lb/sq/in]= Dynamic C/R.
    Something just does sound right about this. If your static & say 150 at cranking compression & you added the amount of boost which may be say 10 PSI then 160 would be the dynamic compression. This does not sound right as cranking comp will be lower than running comp at a given RPM not to mention the difference in volumetric efficiency of the assembly. ???

  2. #12
    Clown
    "Rexone" Awesome turbo site thanks!!!!!

  3. #13
    058
    Charley, No, It isn't as simple as that. There is a mathamatical formula for calculating dynamic C/R and I don't have it handy right now. But if my feeble old memory serves a 8 to 1 engine boosted to 1 atmosphere [app. 14.5 lbs] will make about 15 to 1 dynamic C/R. or almost double the static C/R because you are stuffing approximently twice the voluume of a/f into the cylinder. There is more info regarding this in books for turbochargers and superchargers.

  4. #14
    gnarley
    058,Yeah I think it's time to do a lot of reading! I'll get the book out again, I haven't read it in years but it would be good to read it again. I don't think it should be so hard to understand but adding an air compressor to the motor changes EVRYTHING. I don't need the biggest or baddest, but what I have should be as dialed in & efficient as humanly possible to maximize the available power & when not wide open getting the best GPH I can tune in, itÂ’s a fine balancing act.

  5. #15
    058
    Charley, What are you building and what supercharger [or turbo] are you using?

  6. #16
    gnarley
    058, I sent you a private message.

  7. #17
    058
    Charley, Got it. Anything I can help with?

  8. #18
    Fiat48
    Remove the cylinder head and tell us what you see. We'll walk you through it and come up very close. Or do it this way: Remove the spark plug of one cylinder. Put the piston at Bottom dead center. Turn the motor upside down (easier if it is removed from the boat). Fill the cylinder to the bottom of the spark plug threads with a liquid. Solvent works well, maybe with a little atf for color. Record how much fluid you could get into the cylinder. Now drain the cylinder by rolling the motor over (still easier if motor removed from boat). Now put the piston at Top dead center with both valves closed. Fill the cylinder and record the how much fluid it takes. Give me the numbers and I will tell you: A: your compression ratio. or B: you spilled some.
    Or just do like most everyone else that has a "blueprinted" motor. 13.5 sounds good so tell 'em that. Also do not rely on compression gauges as many are not accurate and camshaft duration, where it's a degreed, and atmospheric pressure all have to do with compression readings. Remember the engine is just an air pump. More air available, it pumps more air. Air changes more than you think on each day. We used to tune our Pro Gas Flat by a compression gauge. We took compression readings at each track. Best air density gauge we ever had. I better quit. My age is showing!

  9. #19
    gnarley
    Well I think it'll be hard to rotate the boat to check the compression. I know of several ways to ck compression ratios but I wanted to see what this engine is really supposed to be at static. After all I wouldn't really want to blow 8 psi on to a stock 9.5:1 motor with cast slugs & std marine gaskets. But an 8.0:1 motor with cast would be a whole different story. The bottom line is knowing what your compression is to supply it with the right boost, timing & octane ect..... I think that I found out what it is but need to verify with secondary sources.

  10. #20
    058
    Charley, For a secondary source try a race track such as Petaluma, they usually have one of those devices that verify comp. ratio. They are quite accurate.

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