Thanks Thanks:  0
Likes Likes:  0
Page 1 of 2 12 LastLast
Results 1 to 10 of 18

Thread: Compression variables

  1. #1
    Team Member
    Join Date
    Nov 2009
    Location
    Kingston, WA
    Posts
    28
    Post Thanks / Like

    Default Compression variables

    What is a good compression for the 15 Mercury A and the 15 OMC A motors? I have ben told that many guys do not consider compression very important. I disagree and think that compression does have an affect on acceleration not necesarily top end. Anyway I am trying to establish a base line to follow when considering a re-ring or bore job on a block. Any advice would be helpful. Also does anyone have any idea what the rpm range should be on the straights and corners to achieve optimum performance?

    What prompts this post is I took a compression on an OMC and had 80psi. A leak down test showed a 40psi drop above the ports and a quick seal before the ports opened. Mesuring the no 2 cylinder showed .003 out of round, .003 taper and .009 clearance. WOW. No wonder I was 1.st, 2nd or 3rd across the line then still ended up last in every race.


    There is now a new block and pistons on top of the lower unit.

    Bob Smiley

  2. #2
    Team Member zul8tr's Avatar
    Join Date
    Oct 2008
    Location
    Orlando, Fla
    Posts
    509
    Post Thanks / Like

    Default Compression calc

    Obtain the cubic centimeters (cc) volume of the Combustion Chamber with the piston at TDC and convert to cubic inches by dividing by 16.39. This can be measured or maybe from manufacture specs. Obtain the cubic inches volume of 1 cylindere from manufacture specs or calculate as cylindere bore Area x piston stroke. Calc an approx cold compression gage pressure in psi from:

    Compression ratio =
    (Vol of 1 cylindere + Combustion chamber vol) / Combustion Chamber vol

    Compression pressure = (Compression ratio - 1) x 14.7

    This will give a generally lower value for a hand crank measuring with no account of the heated compressed air.

    Note all compression gages are different so use the same gage therefore relative readings can be monitored over time.

    Cylinderes should not differ by more than 10%

    Note the readings at each pull and how many pulls to reach a max pressure.

    Redo measurements by adding some oil to the cylinders to give an idea of ring leakage.

    Best to measure the engine stroke and bore diameter and combustion chamber volume if you want engine specific results

    Example: 1973 Merc 25ss per 1978 APBA specs has a minimum allowed combustion chamber vol of 22 cc and a cylindere volume of 11 cubic inches for a compression ratio of 9.21 and a calculated hand crank pressure of 121 psi.





























    0

  3. #3
    Sam Cullis Mark75H's Avatar
    Join Date
    Nov 2004
    Location
    Annapolis, MD USA
    Posts
    1,795
    Post Thanks / Like

    Default

    As stated, gages vary too much for a specific number to be quoted.


    Compression will not work out as described because 2 stroke exhaust ports are open for much of the first half of the compression stroke.
    Since 1925, about 150 different racing outboards have been made.


  4. #4
    Team Member
    Join Date
    Nov 2009
    Location
    Kingston, WA
    Posts
    28
    Post Thanks / Like

    Default

    Thank you for the replies.

    Sam regards to two stroke compression. If I calculated cylinder volume above the ports that should correct the problem you stated?

    Team member, does the K factor 14.7 correct for the concern that Sam stated?

    After a bore job, new pistons, 40 minutes in a barrell, 6 test runs and two race heats the OMC compression is 115 and 110. Much better than 80 psi. The leakdown tests are fine, no leaks throughout piston travel. RPMS and speed is up. Adding airtraps added another 200 rpm and increased the speed some more. Going in the right direction.

    Bob

  5. #5
    Sam Cullis Mark75H's Avatar
    Join Date
    Nov 2004
    Location
    Annapolis, MD USA
    Posts
    1,795
    Post Thanks / Like

    Default

    14.7 corrects atmospheric zero to absolute pressure
    Since 1925, about 150 different racing outboards have been made.


  6. #6
    Team Member zul8tr's Avatar
    Join Date
    Oct 2008
    Location
    Orlando, Fla
    Posts
    509
    Post Thanks / Like

    Default compression

    Quote Originally Posted by Mark75H View Post
    As stated, gages vary too much for a specific number to be quoted.


    Compression will not work out as described because 2 stroke exhaust ports are open for much of the first half of the compression stroke.
    ----------------------------------------------------------------------------------------------------------------------
    Sam as you correctly state , and I am aware of, the exhaust timing will effect the compression psi and the formula was stated for the full cylindere volume. As you know the compression ratio will be lower if based on cylindere volume from TDC to the top of the exhaust port thus a lower compression psi will be the result. The formula derives from the general gas law and is used in many forms by others.

    No formulas that I am aware of are completly accurate because they rely on some degree of emperical measurement. The intent of the formula was to state a means to calculate a relative expected measure of compression psi to be compared to a gage. Certainly different compression ratios can be used in the formula.

    Based on my experience with stock 2 cycle outboards the formula yields fairly close results after several hand pulls compared to compression gage readings. Also I have noted that the first pull compression gage reading comes fairly close to the calculated compression pressure based on the exhaust port compression ratio. Which is why I always note the gage readings at each pull.

    The 14.7 is local atmospheric pressure and the minus one ( -1) in the formula adjusts the calculated pressure from absolute pressure to gage pressure to match what a pressure gage is scaled to read.

  7. #7
    Team Member
    Join Date
    Nov 2009
    Location
    Kingston, WA
    Posts
    28
    Post Thanks / Like

    Default

    Based on my experience with stock 2 cycle outboards the formula yields fairly close results after several hand pulls compared to compression gage readings. Also I have noted that the first pull compression gage reading comes fairly close to the calculated compression pressure based on the exhaust port compression ratio. Which is why I always note the gage readings at each pull..

    FROM THE ABOVE PARAGRAPH, THERE ARE TWO PRESSURES TO OBSERVE. WHAT USEFUL ANALYSIS OR CONCLUSION CAN YOU GLEEN FROM THE TWO DIFFERENT RESULTS?

    Bob Smiley

  8. #8
    Team Member zul8tr's Avatar
    Join Date
    Oct 2008
    Location
    Orlando, Fla
    Posts
    509
    Post Thanks / Like

    Default compression readings

    Bob

    When engines are new and when broke in I take the readings noted to use as a reference to compare to compression testing at a later date. The compression readings at each pull and number of pulls provide an indication of how compression builds in the combustion chamber. At future compression testing a lower first reading compared to the initial reading would indicate more ring leakage and reduced ability to build compression more quickly.

    In race engines it is important that each cylindere have fairly close compression readings at each pull and at max readings with the same number of pulls. I find 4 pulls to be adaquate to reach a max value if engine is in good condition. I do cold engine compression testing because that is easier to do and it keeps the variable of ambient temperature equal to engine temperature an easily measured value.

    Your latest compression readings are more inline with good values but without specs on your engine hard to determine expected values.

    Note that carbon buildup can greatly effect results so get the carbon out. Also note that carbon will sometimes build on the exhaust ports at the exit side of the ports and effect port timing and performance so that needs to be cleaned as well.


































    .

    As stated compression gages are not absolutly accurate but relative devices so keep the same gage and protect it.

  9. #9
    Sam Cullis Mark75H's Avatar
    Join Date
    Nov 2004
    Location
    Annapolis, MD USA
    Posts
    1,795
    Post Thanks / Like

    Thumbs up

    I agree on all points. Very good advice.
    Since 1925, about 150 different racing outboards have been made.


  10. #10
    Team Member
    Join Date
    Sep 2008
    Location
    Singapore/Melbourne/Italy
    Posts
    780
    Post Thanks / Like

    Default

    You can always cc the combustion chamber and do the piston crown as well then the stroke above the exhaust ports and use the ratio as a working guide.
    This eliminates all other errors
    ( as explained to me by an OMC race engineer)

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Ignition timing vs compression.
    By Blind_Mel in forum Technical Discussion
    Replies: 4
    Last Post: 07-05-2010, 05:03 PM
  2. Help - Need advice Re crossflow compression
    By Bruce G in forum Technical Discussion
    Replies: 4
    Last Post: 06-17-2009, 03:30 AM
  3. Compression/Squish Clearance
    By quick2008 in forum Technical Discussion
    Replies: 16
    Last Post: 12-31-2008, 08:44 AM
  4. head compression vs fuel octane
    By tsrt in forum Technical Discussion
    Replies: 13
    Last Post: 08-17-2008, 10:55 AM
  5. This aint no Hemi
    By Mark75H in forum Outboard Racing History
    Replies: 6
    Last Post: 04-15-2007, 06:02 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •