| What is Piston Deck Height?? Also known as
Piston Deck Clearance.
How do we determine Piston Deck Height??
Why is it important when designing performance items for an engine??
Is there a benefit to having a smaller or larger Piston Deck Height??
Can the Piston Deck Height be easily changed??
Above are some of the questions I will try and answer in this technical article.
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What is Piston Deck Height??
or Piston to Deck Clearance?
The Piston Deck Height is the distance the piston EDGE is in relation
to the top of the cylinder deck. NOTE: This
distance
can be negative (the piston edge is recessed in the cylinder deck) or
positive (the piston edge is sitting above the cylinder deck). Please
remember... we are talking about the relationship of the EDGE of the
piston not the center. Of course, if the piston is of flat-top design
then the edge and the center will be the same.
How do we determine Piston Deck Height??
1. Remove the cylinder head.
2. Grab your Dial Indicator and Magnetic Base (very accurate), Flat-Blade
Depth Micrometer (accurate), Dial Caliper(less accurate), or straight
edge and feeler gauges (much less accurate) OR your Step Micrometer
(needed for pistons with a positive deck height .
3. Place piston at TRUE Top Dead Center (TDC) NOTE: Since every
crank has dwell at TDC, determining TRUE TDC can be a bit tricky. Using a
custom piston stop is the most accurate method for finding true TDC.
Since some of you may not have a custom piston stop, finding true TDC
can be determined using a dial indicator and a mag base. WHAT!! you
don't have a spare mag base and dial indicator laying around??
Hmmmm...
OK, Here is a fast way to determine true TDC without using a piston
stop or dial indicator. This method, if done with finesse and time, will
yield an accurate, or fairly accurate, measurement. This will require a
very thin, but rigid, piece of material with a very fine point (if the
piston is crowned).
a. Rotate the crank to what you believe is true TDC.
b. Take your rigid piece if material and place the sharp end in
the cylinder to where it touches the piston. Be sure to hold this
material in such a manner where it can be held in place without the
backing of the piston.
c. While holding the material on the piston, very carefully rotate
the crank (by hand) forward and backward while keeping your material
STATIONARY. If at any point the piston raises the material, even just a
little bit, then you were not at true TDC.
With this in mind
continue this method until the piston no longer displaces the rigid
material.
Once, this occurs, you will be very close to true TDC.
OK..
now, you have found true TDC.. you are half way there!
Since you know when you are at true TDC, all you have to do is measure
the distance from the piston edge to the cylinder deck.
How this measurement is done will be based on the measuring equipment you have chosen to use.
Please
keep in mind... when measuring the
piston deck height on a crowned piston, it is nearly impossible to get
to the actual edge of the piston. This is because your measuring device
does have mass and will hit the piston crown before it hits the actual
edge. The finer the measuring device.. the more accurate the
measurement. So.. if you are using the butt end of a dial caliper on a
crowned piston, please keep in mind that this butt end is very wide and
flat and will surely hit the crown way before the edge. Since the dial
caliper is the instrument most commonly available for this measurement, I
have determined an approximate offset factor for measurements obtained
with the dial caliper. Of course, the accuracy of this offset will be
determined by the accuracy of the initial measurement given. So, PLEASE
take your time in measuring.
For pistons with a positive
piston deck height, a step micrometer seems to be the most accurate tool for this measurement. It is nearly impossible to accurately measure positive deck without a step micrometer.
WITH
ALL THAT SAID...MATH, along with measuring tools, can be used
effectively to help determine
piston deck height.
If you know the piston's crown height, then this can
be used to aid in determining deck height.
For example.. if you can
determine how far above the deck the center of the crown is at true TDC,
you can subtract the known crown drop from this number to determine
your
piston deck height.
Why is it important when designing performance items for an engine??
Piston Deck Height is a very important piece of information when
determining engine parameters and designing combustion chambers.
Let's start with its role in determining engine parameters.
Many
performance shops utilize software and / or math equations as an aid in
determining engine parameters and upgrades. For example, it is
important to know the position of the piston, in inches or millimeters,
at different points along the stroke ie. exhaust opening, transfer
opening etc.. This information can be calculated using software or by
punching the numbers in the mathematical equation (software is much
easier).
OK,
you may be asking yourself, "What does the
piston deck height have to do with the piston position?" Well, not much
from an initial design point, but for the person porting your engine, it
can be very important. For example... many people send their cylinders
out to be ported. Once the shop gets the cylinders they have the
pleasure of determining how to modify it in order to yield performance
gains. Many performance shops determine what to change based on the port
timing of the engine. Now, if they only have your cylinders, not the
entire engine, how can they determine what your port timing actually
is??? Well, you may tell yourself that they have seen that engine before
and know what the porting arrangements are and your engine will be
exactly the same. Well, if you are convinced this is true then, you have
nothing to worry about. But if you are like me, and KNOW that this is
not always true, then you should be concerned if the shop you are
sending your cylinders to does not ask for a
piston deck height measurement or a base gasket thickness. There are
often variances in cylinder castings, piston heights, connecting rod
lengths, and base gasket thicknesses. ALL these things effect port
timing and
piston deck height. Knowing these parameters will assure that the shop
has the necessary engine measurements to do the best job they can in
modifying your cylinders.
How about combustion chamber design? How does
piston deck height effect the design of the combustion chamber?
Piston deck height is a VERY important measurement to consider when designing a combustion chamber.
Piston deck height is a major player in determining the squish
clearance of an engine. While it is beyond the scope of this article to
discuss squish band design (maybe later), let's just say the squish
action within a combustion chamber is very important in the combustion
process and power making process.
Squish
clearance is the distance from the edge of the piston, at TDC, to the
outer edge of the combustion chamber's squish band. So, one can easily
see how the
piston deck height effects the squish clearance measurement.
So, you may ask yourself..." How can one design a proper combustion chamber without knowing the
piston deck height??" Well.... the answer is simple..
ONE CAN'T!! Sure,
they can get close. All I am stating is that they can get a lot closer
if they have a list of engine measurements, like the
piston deck height.
It
has already been determined that there are many factors that effect the
piston deck height.. cylinder casting, piston casting (or forging), con
rod length, and cylinder base gasket to name a few. So, with this in
mind, how can a head designer
properly modify your head, or design a new head, for your engine without knowing the
piston deck height of your engine?
THEY CAN'T!
You might be saying to yourself " So, what if I am off on my
piston deck height measurement .008", how big of a deal can that be?"
Well...
While I will not go into the difference .008" has on squish action
within a combustion chamber, let's look at what .008" does to the
compression ratio and volume of an engine.
Volume of a cylinder: PI * R^2 (radius = 1/2 bore) * H (Stroke)
So,
let's take the 800 Rotax twin engine with an 82mm bore and a 76mm
stroke. We will convert the .008" to mm so the numbers in the equation
coincide. .008" = .2mm
What effect will .008" have on the compression ratio?
Let's do the math:
3.14 * (82mm/2)^2 * .2mm = 1.05cc
of
volume change. So, what does 1.05cc of volume increase or decrease on
this particular engine? Well.. 1.05cc equates to a 0.4 change in
un-trapped compression ratio. OK.. 0.4 change in un-trapped compression
ratio will change a 12:1 engine to a 12.4:1 or a 11.6:1 engine.
Well..may be that is not so bad. so, lets change the engine by .015" or
.38mm
3.14 * (82mm/2)^2 * .38mm = 2.0cc
of
volume change. So, what does that do to the un-trapped compression
ratio? Well it changes the un-trapped compression ratio on a 12:1 engine
to 12.79:1 or to 11.21:1.
So,
you can see that the compression ratio is effected but what is also
effected is the squish action within the head. Squish action is
important in determining power characteristics of an engine. The squish
band acts as a cooling layer to help cool the end gases as they are
being rapidly compressed. By keeping these gases below their combustible
temperature, one can prevent undesired combustion of these end gases in
the squish band area. If these end gases are allowed to combust before
the oncoming, spark initiated, flame front chooses to combust them, then
you have the receipt for detonation and engine damage.
The
squish action also creates turbulence within the combustion chamber.
This turbulence has a direct effect on the flame front speed so ,in
actuality, it effects ignition timing.
OK, another measurement for the squish action is the Maximum Squish
Velocity (MSV). In short, this is the max velocity of the end gases as
they are be compressed. It is actually a lot more complicated than that,
but I will leave it at that for now. It is measured in meters/sec
(m/s).
Squish
velocity has a very large effect on the heat release and rate of
burning in a two stroke engine. Hence power output and engine
reliability.
Software exists to give a close approximation of this velocity
but.
Let's take our above examples and see how squish velocity is changed by a small variance in squish clearance.
The
first example showed a change in squish clearance of .2mm. Using 2
stroke software this .2mm change in total squish clearance will increase
the squish velocity in a 13.5:1 head by 7.4m/s if this .2mm is removed
from the total squish clearance. If this .2mm is added to the total
squish clearance, then the squish velocity is decreased by 5m/s.
The 2nd example, showed a squish clearance difference of .38mm
This
.38m change in total squish clearance will change the squish velocity
by 18m/s when this .38mm is removed from the total squish
clearance. If this .38mm is added to the total squish clearance, then
the squish velocity is decreased by 8.4m/s.
NOTE:
The above MSV calculations were taken from a specific head design.
Since MSV has many determining factors, the changes in MSV could be much
less or much greater than the ones listed above.
The overall head
design and cylinder port timing determines the magnitude of the MSV
changes.
One
can see that the relationship between adding and subtracting squish
clearance is not linear and does have pronounced effects on squish
action.
So, you can see how one needs to be careful when purchasing a new
aftermarket
head or modifying a stock head. Next time you are talking to a head
maker or a
shop that may be modifying your head, ask them what the piston deck
height is
for that engine. If they do not know, how can they design a head for
that engine
that will have acceptable squish velocity, squish clearance, and
compression
ratio? OR when speaking with these people, tell then that you have added
an
extra base gasket to your engine. Ask them how that will effect the "on
the
shelf" head they want to sell you. I have already shown you what a .008"
and a .015" difference in deck height will do to compression ratio and
maximum squish velocity.
So...
the old adage of shaving off .015" of a stock head can get you into
excessive squish velocities in a very big hurry. Excessive squish
velocities can lead to piston breakage and severe engine damage.
Is there a benefit to having a smaller or larger Piston Deck Height??
There maybe some benefits to having a large or small
piston deck height.
The
one that comes to mind first is in the cooling effects of the engine.
For example.. if the
piston deck height is large in the cylinder, then there may be an
argument for the end gases retaining more heat due to them being trapped
in the cylinder vs. the head. One may argue that end gases trapped in
the head portion of the squish band would be subject to the better
cooling properties of the head. This would be a hard theory to prove,
but it does have merit.
Can the Piston Deck Height be easily changed??
Yes, it can be easily changed.
Below are several methods of altering
piston deck height, which, as I have shown, also alters MANY other operating factors.
1. Changing base gasket thickness
2. Decking cylinder base
3. Decking cylinder top
4. Changing piston
5. Changing crank
6. Changing rod length
7. Changing stroke
8. Altering piston crown
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