Compression/Squish Clearance

[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/

[Fast Fred]

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

[john miffco]

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

john

[Seagull 170]

Nice work John
I tried the same on a domed piston on a lathe, with the tool ground to the combustion chamber radius, so the step was less pronounced.
This never worked as well as CNC milling the head which shrank the combustion chamber and allowed the head to match the piston to a thou, before it got hot.
I get the carbon deposit from the transfer port right into the combustion chamber area on the piston crown, maybe I run my pistons hotter than you do.
I was thinking of ceramicing the top of the piston to prevent some of the heat absorbtion but with very little signs of excessive heat on the underside of the piston thought it wasn't super critical, & anything that raised the crankcase compression was beneficial, on a motor with 2 smallish transfer ports & no room for any more.

[quick2008]

Thanks guys, John thanks for the pics
Have a huge library of info now!, but may need some more money yet.

But just for now, till i start going blueprinting, balancing, decking. etc
If i machine my HEAD to be within the 0.8mm squish clearance,
Im guessing that i will see mild power gains from doing so.?
keeping in mind its a 30hp engine.

Thanks!

[john miffco]

the reason i o ringed the cylinders and machined the pistons was to remove
the head gasket to get tighter tolerances
and also to be able to use universal heads for different compressions

this way no special machining was needed to be done to the heads
for that 1 particular motor,,,,,,,
i could use any head and compression ,,or head design for testing to find what the motor liked best,,,,,
simplier method for fine tuning

when you cut the head to gain compression which will see signifigant gain in hp over the squish tolerances
but if use same comp heads with tighter squish then there is more signifigant gain

my theory is to make any motor efficient
this squish area and compression is where combustion effiecieny is achieved
and combustion efficiency is only 1 item of 100
to make a motor stronger than another
wheather it is a spec motor or unlimited
john