The best way to approach the subject of optimal
cylinder wall surface is to understand some basics:
1. We attempt to seal compression and suction with
piston rings.
2. Ring, piston and cylinder wall design and material
play a big role in how and what type of surface you
generate.
3. The conditions under which you race (dirt, salt,
pixy dust, etc.).
4. The type of working fluid(s) you use (fuel,
gasoline, gas, etc.).
5. Metrology.
First, when we discuss the condition of a race engine
we reference a compression ratio or leak down value.
In each case, an attempt is made to find the culprit
that robs us of expensive and valuable compression.
But keep in mind, we only compress what air/fuel we
take into the cylinder during filling. The age-old
example of calling our normally aspirated gas and
diesel burning engines air pumps is not far from the
truth. If rings fail to seal under suction (filling),
we will not draw the maximum air/fuel volume into the
cylinder and performance suffers. This holds true for
positive displacement induction systems as well. If
the rings won't seal on suction, they sure as hell
won't seal when you kick a few additional atmousphers
into each cylinder.
Second, The combinations of ring, piston and cylinder
wall material is staggering. You literally have a
choice for any contingency. In our world, cast iron
cylinder walls, hypereutectic, hypoeutectic and forged
aluminum alloy pistons are commonly used for racing.
Hypoeutectic not so much but I still see them where
rules dictate. Four common racing piston rings are:
ductile and grey cast iron, tool steel and stainless
steel. Throw in nitriding, chrome and both plasma moly
and moly-filled coatings and you get the picture. No
single surface will work for them all.
While on the subject of rings, it should be noted that
many ring face designs exist. There is the proverbial
beveled face, radius (barrel) face and straight face.
We also can not forget reverse twist rings and where
they are most essential. In all cases, pre-lapped
and/or double lapped ring faces are very important to
qualify. Most quality ring packs come with lapped
faces so this should not be a worry.
Thirdly, certain ring and coating materials lend
themselves to specific track surfaces. At Bonneville
and El Mirage, where caustic and abrasive track
elements can find there way into the engine, it may be
better to consider nitrided or hard chrome face rings.
These harder surfaces resist wear and hold sliding
seal properties longer. Contrary to popular opinion,
they take the same surface finish as other common
racing ring packs.
Moly-coated/filled rings are used for high ring face
confirmability and are best suited where cleaner air
exists. The plasma moly rings are more forgiving and
do handle heat well. No wonder why these are the #1
choice among many engine builders. However, if the
surface finish is not right, these rings can degrade
rapidly and may not deliver optimal performance over
the period of a week of racing.
Fourth, the working fluid. I don't have to tell you
how many choices you have here. If we have learned
anything it is there is nothing simple about gasoline.
What should be considered is how your chosen fluid
works in the engine. Does it have high detergency?
Will it cause unusual tuning problems? Is it
compatible with all engine components it comes in
contact with? Indirectly, answers to these questions
play a part in which ring pack you select and in turn,
what surface finish you put on the cylinder wall.
Finally, metrology. Here is where it all falls apart.
Form most of us, a single surface parameter is used
(if used at all) to determine surface quality. That
parameter is Ra. Ra or Rms is nothing more than an
arithmetic average (or root mean square) of all peaks
and valleys recorded in a single sample length from a
measuring tool called a profileometer. Ra attempts to
tell you how rough a surface is. Too rough and the
surface will destroy rings. Too smooth and the rings
may not seat. I am here to tell you that Ra alone is
of little practical use and no more valuable than
rubbing a penny across the cylinder wall to determine
surface roughness and/or quality. At this point,
things get interesting. I will pick this up again
latter as work has once again, gotten in the way.
Suffice it to say, we will be discussing surface
measurement in detail and how this relates to
generating the correct surface for the correct rings
and for the desired racing venue.
John
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