Compression ratio is the ratio between the volume of the cylinder at bottom dead center and at top dead center.

And dish pistons allow more volume at top dead center.

But it wouldn't the volume at bottom dead center be increased also?

So then it would equal out and end up with the same ratio?

 

wiki:

Volumetric efficiency in internal combustion engine design refers to the efficiency with which the engine can move the charge into and out of the cylinders. More correctly, volumetric efficiency is a ratio (or percentage) of what volume of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiencies greater than 100%. Volumetric efficiencies can be improved in a number of ways, but most notably the size of the valve openings compared to the volume of the cylinder and streamlining the ports. Engines with higher volumetric efficiency will generally be able to run at higher speeds (commonly measured in RPM) and produce more overall power due to less parasitic power loss moving air in and out of the engine.

sooo...

the slightly larger (if any) volume of mixture drawn into the cylinder is basically so minute that it does not matter, it all comes down to the final squeezed product.

just think as well, an engine will intake x amount because the intake valves are open for x amount of time at lets say 1000 rpm. although the the velocity of the intake charge is higher, the volumetric efficiency of an engine is reduced by the shorter amount of time the intake valves are open although the piston is creating more vaccuum.

42.7cc of volume in an ls head for a cylinder the roughly displaces .45 litres

but a sbc performance head has around 65cc of combustion chamber volume in a cylinder that displaces roughly .71 l. (thats about a 10:1 head with flat tops)

honestly though, its a slightly tricky subject to get your head around.

 

the more volume in the cylinder,being taken up by the piston height,(if you keep the cylinder bore and cc chamber the same size) the less than mixture is going to be compressed,like, changing the piston dome height to a lower number will change the static c/r lower.

a piston with a +4.75mm dome height vs a -.090mm dished piston the higher then dome, the less space there is in the cylinder,to compress the mixture, hence the same amount of air is being introduced into the cylinder yet it is being compressed more with a 4.75 piston vs a dished piston.

 

basically in lamens terms, the engine only sucks in so much air...its not enough to completely fill the chamber 100% so piston height doesnt really come into play until you start to compress the air.....this is how I always understood it, but please correct me if Im wrong

 

The shape of the piston crown and the volume of the combustion chamber at TDC have nothing to do with the amount of air drawn into the cylinder on each induction stroke, that is a function of the 'swept volume'.

To envisage swept volume, imagine the piston at TDC with a hypothetical combustion chamber volume of zero (i.e. no space). As the pistion starts to descend the volume increases above zero, reaching it's max at BDC. This max value is the swept volume of the cylinder, and in reality it is the same no matter what the TDC combustion chamber volume is (i.e. zero or any larger volume).

Placing a dish in the piston crown, or a dome on top, makes zero difference to the swept volume (i.e. the amount of air drawn into the cylinder per induction stroke (for ease of illustration, assuming 100% cylinder filling)), so at each inlet stroke the cylinder will be filled with 'X' quantity of air, regardless of the shape of the piston crown (disregarding possible minor effects associated with the effeciency of air flow into the cylinder that may be associated with the shape of the crown, but having nothing directly to do with the 'volume' of the crown).

However, the dish (or dome) will affect the volume of the combustion chamber at TDC, (with a larger dish increasing the volume). This means that with a larger dish the intake charge ('X' volume, as above) will be compressed into a larger space at TDC, so compression ratio is lower.

 

Run-N-Go you seem to not fully understand what compression ratio is... i suggest you revisit that topic...

 

What am I not understanding?

 

Why doesn't the shape of the piston crown affect the amount of air drawn into the cylinder?

If you have a high dome, you would have less volume for air to fill it.
If you have a giant pot hole for a piston crown (theoretically) you'd have more space for the air to fill.

If you have a bucket and put something in it that takes up a large amount of space and fill it with water, it will hold less water than an empty bucket.

 

bump

 

so you can fit more, but what does that have with the ability of the engine to draw in air? A large suitcase can hold A LOT of money but getting it in there is another story

 

Then how can cylinder bore affect the amount of air drawn into the cylinder?

So what does affect the amount of air intaked?

 

I think what Run-N-Go is missing is that compression ratio is a ratio.

Even if you have more air in the cylinder from the dish piston (go with me here), the smaller volume at TDC of the higher compression pistons will result in a higher compression ratio.

Picking number out of thin air here. Say its a 500CC cylinder (BDC) with your high compression pistons. Now you have 510CCs with a dish piston. Not much difference there. But, when one is compressed to 60CCs and the other is compressed to 50CCs, you get a big difference in the total ratio. One is 10:1 and the other is 8.5:1

Forgive my poor explanation...

 

Was in the back of my mind

10 and 1 = 10:1 = 10
Add .5 (volume)
10.5 and 1.5 = 10.5:1.5 = 7

And 10 =/= 7
Therefore you have a different ratio. Percentage added to cylinder vs Combustion chamber.

What johnlear said, I still need clarified.

Static CR is calculated from volume @ BDC & TDC ... Not the measurement of air that actually enters in cylinder right?
A dished piston would increase the volume of the cylinder.


What would affect the amount of air drawn into the cylinder?

 

Do you have a goal here? This is starting to get annoying.

 

...
Compression ratio is a ratio of volume cylinder/combustion chamber, not the amount of air that enters and is compressed.

 

Here is a wiki explanation of Compression ratio for ya,