Pistons and Compression Ratio

[Mark52]

I don't know much (anything) about engine internals. Can someone explain what the physical difference is between a piston with lower CR vs. a piston with higher CR? Also, I've noticed that different year Miata engines have different CR-why?

[Miatamaniac92]

CR, if I even understand it a bit, if the amount that the air/fuel ratio is compressed.

"Compression
The piston moves up and compresses the trapped air fuel mixture that was brought in by the intake stroke. The amount that the mixture is compressed is determined by the compression ratio of the engine. The compression ratio on the average engine is in the range of 8:1 to 10:1.
This means that when the piston reaches the top of the cylinder, the air-fuel mixture is squeezed to about one tenth of its original volume."
<http://www.familycar.com/engine.htm>

Longer piston equals more compression. More compression equals more power amongst other things. Madza has been upping the power and decreasing emmisions by upping the compression. I also think they installed slightly weaker pistons on the 99+ Miatas (excluding the MS probaly)

Someone correct me if I am wrong.

Compression also equals an engine more realdily responsive. I've heard the BPT is such a pig with really low compression, but you can run 15 - 20 PSI reasonably safe.

Chris

[Mark52]

So a higher compression piston would be "taller" so that the air/fuel is compressed into a smaller space?

[Miatamaniac92]

Correct.

[OUTRACE]

If this newbie can pipe in here.......

Compression ratio is calculated by taking a cylinder's volume at Bottom Dead Center (BDC) and comparing it to the same cylinder's volume at Top Dead Center (TDC). Measurements are done with the complete engine assembled. Thus, the volume at BDC and TDC will include the space used by the head gasket, the piston shape, and the combustion chamber (including its components).

In a 1600cc 4 cylinder engine, each cylinder displaces 400cc. If the compression ratio were 10:1 then we can "assume" that the combustion chamber volume would be 40cc.

A change in compression ratio can be accomplished by changing one or more of the components involved. e.g. you can change the volume of the combustion chamber, you can change the size or shape of the top of the piston, you can change the thickness of the head gasket (which essentially changes the combustion chamber volume), and/or you can change the size or shape of the valves (in as much as the valve head protrudes into / forms part of the combustion chamber).

So, if we were to change ONLY the piston in order to change the compression ratio, it would be practical to add or subtract material above the wristpin in order to accomplish this. HOWEVER, this is seldom done by simply changing the distance between the wristpin and the piston top. It is normally done by changing the shape of the piston top.

I know that might sound like an overly complex answer to a simple question, but piston design is an amazingly difficult science.

I have included pics of two different piston designs used for the same motor. Same bore, same stroke, but two significantly different compression ratios. As you can see, there is no appreciable difference between the two pistons with regard to the distance between the wrist pin and the squish band (the circumferential flat area above the top compression ring land).

[Miatamaniac92]

Much better stated.

Chris

[Mark52]

Thanks guys