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Formula To Predict Compression
Jay,
There are always posts that say something like "cut .100 off your heads and
you'll have 150 lb. of compression." My limited experience tells me that
exhaust port height has a lot to do with compression. Is there some
kind of formula that will predict what your compression will be at a given
cc for a specific engine? Say, 33 cc heads would give me 130 lb. of
compression with my motor; Is there a way of calculating how many cc's I
should go to get 150 lb. with that same motor and head configuration?
Thanks,
Dave Raymoure
Michigan
Dave,
To answer your question about the compression standards as to the amount to
cut off a head to achieve a specific compression number; I'm afraid to tell
you in my opinion each situation is an individual case.
Each of the outboards companies have different combustion chamber shapes and
sizes and to cut . 050 off say, a 1995 200 HP OMC to yield 145 lb. of
compression would maybe not work on a 1993 200 HP OMC. The Mercury's have
many combustion chamber shapes - a lot also has to do with the port height
as the port height is lower on fishing motors that need torque to carry
loads and the window of opportunity or duration time for a piston to build
compression is greater, and likewise the higher the port, the less time the
piston has time to build compression.
We work with a lot of 2.5 liter, one-piece, lost foam heads that come 37,
38, and 41cc. I have cut hundreds of these heads, and we know that with a
motor that has a stock port height 1."510 -512 deck height stock with a
thin Hi Per head gasket ( .040 approx.) that 35.5 cc will put the motor at
absolute-safe pump gas compression - about 145 lb.
The formula I use to cut these "umbrella" shaped combustion chambered heads
is that for every .010 of an inch is removed, it will diminish the cc of the
head by . 7 of a cc (example, if you cut off .020 of an inch you will
diminish the cc by 1.4).
But this works only on the above mentioned heads. Drag motors have a shallow
bowl combustion chamber and it will "lose" a lot more cc with a smaller
amount of material removed than the above mentioned head. Some Mercury 2.4
liter engines have a small "Hemi" style chamber and some have a larger,
wider shape. So like I said, there are several things that must be
considered before one just begins the machining process.
On another note, it has to
be said that in addition to all heads being different, head gaskets with the
same part number can vary as much as .030! Careful measuring is crucial for
absolute accuracy of results.
Hope I've answered your question, and thanks for asking JSRE.
Jay @ JSRE
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