Compression/Squish Clearance

[quick2008]

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

[Fast Fred]

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?

[Mark75H]

Two Stroke Tuners Handbook by Gordon Jennings

[quick2008]

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

[Mark75H]

Bell's book is my second reference. I haven't compared the two, but the info is likely the same.

[Seagull 170]

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.

[john miffco]

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

[Tomtall]

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/