Thanks Thanks:  0
Likes Likes:  0
Page 1 of 2 12 LastLast
Results 1 to 10 of 17

Thread: Compression/Squish Clearance

  1. #1
    Team Member
    Join Date
    Nov 2008
    Posts
    20
    Post Thanks / Like

    Default Compression/Squish Clearance

    Hi There Guys.
    My first post so hopfully this goes ok!

    Im trying to work out some compression figures for my outboard and how much meat to slice off my head, to raise compression. Am wondering if anyone can shed me some light on what the minimum squish clearance, i can safely run should be.?? . I have read in a book that per 175-250cc cylinder, the squish should be 0.8-1.0mm (30-40 thou") clearance. Is this correct?? can i go any more safely or are these figures true and around the ballpark???

    This is with an undecked, standard block with dome top pistions

    have done a soft solder test, cranking engnine over slowly by hand with correct head gasket, correct torque for head bolts, then measuring with micrometer?

    any avice appreciated

    Cheers

  2. #2
    FFX-61
    Join Date
    Aug 2005
    Location
    Stellwagan Ledges
    Posts
    1,389
    Post Thanks / Like

    Default

    so what are the the number of the solder test, what is the thickness of the head gasket, does the edge of the piston pull up past the deck of the block, if so how much? what moda we talkin about?

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

    Default

    Two Stroke Tuners Handbook by Gordon Jennings
    Since 1925, about 150 different racing outboards have been made.


  4. #4
    Team Member
    Join Date
    Nov 2008
    Posts
    20
    Post Thanks / Like

    Post Squish / Compression

    Thanks Guys

    Thanks for the book reference mark, i got my figures from a book by A.graham bell?? I have seen the one by gordon jennings, just dont own it

    Fred, the solder test i got around 1.45-1.47mm for my test
    Edge of piston does not pull up past deck height.
    Crushed gasket is around 1.25-1.27mm

    at my info, im only taking a guess that around 0.65mm is what i need to be taking?? Engine is 30yam

    Thanks guys

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

    Default

    Bell's book is my second reference. I haven't compared the two, but the info is likely the same.
    Since 1925, about 150 different racing outboards have been made.


  6. #6
    Team Member
    Join Date
    Nov 2004
    Posts
    76
    Post Thanks / Like

    Default Tested Squish Clearance

    I found on a 100cc, petrol looper, very over square, cylinder with .85mm squish clearance, running circa 155 lbs compression on a good rope pull, I was burning synthetic lube oil, causing a carbon build up on the piston crown (showing the transfer port direction/swirl, almost perfectly).
    After 4 or 5 hours running at full tilt there was enough build up on the piston, to touch the cylinder head, which I felt was not a very confortable situation.

    Perhaps you should try lowering in 10 thou steps until your happy.

  7. #7
    Team Member john miffco's Avatar
    Join Date
    May 2005
    Location
    houston ,texas
    Posts
    0
    Post Thanks / Like

    Default carbon build up

    the squish band is between the flat area on the head and the piston top
    should not build up carbon,,,,,,,
    this is where lots of power is made on a 2 stroke
    when a motor is properly blueprinted to a specification and tuned
    some carbon build up in the combustion area on the piston top(golden brown colored)
    but should be able to see the swirl pattern on a loop charged motor in that carbon
    and no carbon build up in the head combustion chamber,,,should just be fuel/oil and wipe clean
    the look of a clean burning efficient 2 stroke motor

    a precision built motor with a sqish band in the .030 -.040 window
    compresses the fuel in that area and does not burn when plug fires
    (all fuel,,lacking oxygen)

    from where timing advanced is set till past TDC when piston is in the downward power stroke,,,,this gives maximum power in the center of the piston top(burning fuel in combustion area only)

    for flame to start in the center and expand outward to the unburned fuel in the squish area as the piston travels downward in the power stroke to keep a continuoius burn (expansion)during the entire stroke to out the exhaust
    then when intake charge is taken in,,,the raw fuel cleans the outsude piston top and COOLS piston for next fire,,,,and this process also crates the swirl pattern seen in the light carbon

    this is the reason for the sqish band
    a squish band of .080 and larger burns the fuel at ignition all the way to outside of piston ,,,,
    hense causes the carbon build up your desribing

    also cutting the head to add compression does not decrease the squish band
    on 2 strokes that have a flat sqish band area between the outside of combustion chamber and gasket seal area

    on the outside of the combustion chamber on the flat of the head
    decreasing the squish band is achieved by thinner head gasket and decking the block to extrude the piston top more out of the cylinder

    if working with an o ringed head
    then the squish area must be mill up in that area to create a proper squish clearance,,,,,,
    all them factors depends on the motors piston to head design and gasket

    production motors are manufactured with large clearances for longevity
    race motors are built with tighter tolerances for maximum power
    a tight squish band creates power,,but there is a thin line to deal with
    when aciveing tight tolerences
    ,,,,,heat,,,,,pistons swell,,,,cast and forged different swell,,,,how often
    change pistons,,,bearings,,rods,,,,,tighter squish band adds strass to all components
    so must beak down often to inspect and replace as needed
    before failure occurs
    the price for power

    john

  8. #8
    Tomtall
    Guest

    Default Software

    This company offers some very sound software and can provide you with many options to your quest for the perfect combustion chamber. Johns explanation above is dead on. Good Luck!

    http://www.bimotion.se/
    Attached Images Attached Images  

  9. #9
    FFX-61
    Join Date
    Aug 2005
    Location
    Stellwagan Ledges
    Posts
    1,389
    Post Thanks / Like

    Default

    the short of it is, the squish band stays the same, but it not easy to shrink the chamber, so to get the effect were lookin for, ya deck it and put a nice squish band cut back in,even though the chamber is the same size, the over all vol. is smaller.

    cool soft ware stuff

  10. #10
    Team Member john miffco's Avatar
    Join Date
    May 2005
    Location
    houston ,texas
    Posts
    0
    Post Thanks / Like

    Default

    your correct fred
    but there also is another way to achieve the desired band for max power

    i have a lot of yrs manufuring my own parts for when i played with
    unlimited merc v6s in the 80s and didnt have an out of the box s3000
    so we made our own cyls,pistons,heads,reeds,intakes,ect

    this is what i found i liked the best on my merc
    it was a 2 fold enhancement

    i threw away the head gaskets
    used wide flanged sleeves to cut in viton o rings into cyls
    and decked the block to extrude the piston
    then milled the squish band clearance into the piston top
    this allowed the center of the piston to actually go inside the heads combustion chamber,,,,,
    more compression and maximum fuel/air in the combustion chamber
    and no way to ignite the cool fuel in the squish area

    2nd + to this set up is it enhanced the port timing
    by milling the top down
    less raising of the ports in the block
    and can taper the milled area to add or take away timimg for very fine tuning
    cause 1nce you grind the port higher in the block
    cannot go back,,,,,,,,,,,,,,,,,,

    here is a used piston out of 1 of my motors for referance of my method
    of creating desired port timing,comp,squish clearance,ect
    went through a lot of pistons to find my best working specs
    but cheaper than lots of blocks and heads
    also this is a used piston,,and can see no carbon ever built up in squish area
    just in center was cleaned

    domed pistons with domed squish area is harder to machine
    but if use this theory still can be achieved
    Name:  merc pistons 001.jpg
Views: 4798
Size:  81.4 KB
    john

Thread Information

Users Browsing this Thread

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

Tags for this Thread

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
  •