Thread: How to determine deck height.
08-21-2006, 09:52 AM #1
How to determine deck height.
This was copied from an unknown author. Thanks for the excellent info.
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
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.
Now.. you know when you are at true TDC, so, all you have to do is measure the distance from the piston edge to the cylinder deck.
Please keep in mind... when measuring the 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.
For pistons with a positive 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.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 deck height.
Deck Height is very important when determining engine parameters and designing combustion chambers, as there are often variances in cylinder castings, piston heights, connecting rod lengths, and base gasket thicknesses. ALL these things effect port timing and deck height.Deck height is a major player in determining the squish clearance of an engine.
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 deck height effects the squish clearance measurement.
You might be saying to yourself " So, what if I am off on my deck height measurement .008", how big of a deal can that be?"
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)
Example, 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
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 untrapped compression ratio. OK.. 0.4 change in untrapped compression ratio will change a 12:1 engine to a 12.4:1 or a 11.6:1 engine
So, you can see that the compression ratio is affected but what is also affected 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 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 affects ignition timing.
One important 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).
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. 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.
Is there a benefit to having a smaller or larger Piston Deck Height??
The one that comes to mind first is in the cooling effects of the engine. For example,if the 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??
Below are several methods of altering 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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