I been doing research for building a new engine. I understand the difference between domed, dished and flat top pistons and their effect on compression. I am currently reading an article that talks about swapping a dished out piston for a flat top to increase the compression but, it talks about providing valve notches. The pictures I am looking at make the depth of the material removed from the top of the piston look the same. Article indicates the compression will go from 10:1 to 11.5 to 1.
What is the difference between the dish out piston and the piston with the material removed for valve clearance? Doesn't the removal of material in either case reduce the compression in the cylinder head? The photo graphs I am reviewing make the depression in each piston top appear to be approx. equal. If that was true, the compression should be equal as well.
Maybe I'm splitting hairs but, I trying to make sense of it.
Being that I have no clue what article or pictures you are talking about. The volume of material removed in a dish piston would be much less than that of valve notches. Visually you can see the area of the notches are much smaller than the "dish" of a dished piston. Therefore less volume and more compression.
The only way for a dished piston to have the same compression as a flat top piston with valve reliefs ("notches") would be for the depth of the notches to be so great that they are equal in volume to the volume of the dish in the dished pistons. This would be quite deep.
*This is of course assuming that all other variables remain constant (piston height for example.)
Also in reference to:
"Doesn't the removal of material in either case reduce the compression in the cylinder head?"
I think this might be a confusing way to look at it. Picture a piston at top dead center. For simplicities sake we'll forget quench and say the piston is touching the cylinder head. The volume of the total "combustion chamber" is:
The volume of the combustion chamber (say 56cc's, 1 cc is equal to 1mL if that makes it easier to visualize)
+
The volume of the volume of the dish, reliefs, or (the negative volume) of a dome
64cc's CC
+
Flat top with reliefs (1cc each x 2) 2cc
=
66cc "CC"
64cc's CC
+
12 CC Dome (visualize, this is actual removing volume from the heads CC because it is protruding into that space, therefore subtract this volume)
=
52cc "CC"
The relative compressions:
Dish: 9:1
Flat top: 10:1
Dome: 11:1
So you see now that the larger your "combustion chamber" volume is, the lower your compression ratio. (Why? More volume means that the same amount of air is compressed into a larger space, therefore the lower pressure (PV=nRT, Pressure is inversely proportionate to Volume, increase one decrease the other) and lower compression ratio. )
So know that we have refreshed your basic understanding. Your question in particular seems pretty simple.
You are comparing a dished piston with a flat top piston, if the dish is around 12cc's. And the flat top has two valve reliefs at 2cc's each, the total volume is 4cc's for that flat top. That is 8 less cc's than the small dish and therefore it will have a higher compression ratio.
Also, remember Volume= surface area X depth. So two little valve reliefs about a 1/2 a square inch of surface are total, are going to need a lot of depth to equal in volume to a large dish with a surface are of 4 inches and 1/2 depth as well. In fact, in this example, the valve reliefs would need to be 4 inches deep to have the same compression as the dished piston, not too plausible eh?
Last edited by USAsOnlyWay; 02-25-2007 at 08:32 PM.
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Piston Question dished vs Valve Notches
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