Couple questions about compression (kinda confused)

[lemming303]

I have a question. If you change your piston, it changes compression, right? But I don't quite understand why. Theoretically, if you make the height of a piston shorter in relation to the pin, it should lower the compression right? But, wouldn't it also change how much space is in the cylinder at the bottom of the stroke? What I mean is, at the top of the stroke, the space is larger. But at the bottom of the stroke, the space would still be larger too, right? So, how does this actually change the compression? Why wouldn't it stay the same if you are moving essentially the same amount of air? I'm probably just over thinking it. Or I don't understand how compression really works....

[ChallengerHK]

The amount of air in the cylinder at the bottom of the stroke would indeed be more or less the same if you only changed the piston. It might be a little greater or a little less.

However, the shape of the top of the piston might change. It could be flat, dished, or domed. Depending on the piston you removed and the shape of the top of that piston, you might have less volume, more volume, or the same volume when the piston is at TDC.

The other thing that can effect this is head gasket thickness.

[Kapteenikosmos]

Air volume you move in or out the cylinder isn't affected by the piston height. It's a matter of displacement and as the name states it's the amount of air the piston displaces when it moves in the cylinder. The displaced air is the piston bore size times the travel the piston moves so even if the compression height changes the piston travel stays the same. Thus the travel of the piston is set by the crankshaft stroke.

Now when we are talking about a compression, it tells you how much is the displacement change ratio from the bottom dead center to top dead center. So if you have a cylinder volume of 1000 cc while the piston is down and and 100 cc while it is at the top dead center, you have a compression ratio of 10.

[NoMope Greg]

As Challenger H-K stated, the only way the piston changes the compression is with the shape of the piston - domed, dished, stepped, whatever.  When you do that, it changes the volume of the combustion chamber, so all other things being the same, that alone changes the ratio.  For example, if you install a piston with a 10cc dome, you may go from the 100cc chamber in Kapteenikosmos' example to a 90 cc chamber.  This changes us from a compression ratio of 10:1 (1000/100) to a ratio of 11.1:1 (1000/90).  (This is simplified -the dome changes the swept volume a little too, but not much)

The reason for changing the height of the top of the piston in relation to the pin is usually to allow it to be used in a stroker engine.  Otherwise, at TDC, the extra throw will stick the piston too far above the deck, causing an interference problem.  So, you either have to go with a shorter rod and/or a lower piston height.  In some cases, the piston has to be shortened so much that the pin moves up into the oil ring land.

Here's an excellent article on selecting stroker pistons.  It's written around a BB Ford, but the principles are the same.  http://hotrod.automotive.com/22928/hrdp-0704-pitstop-ford-stroker-piston-selection/index.html

[lemming303]

OK, but here's what I don't understand, and I used changing the piston height as an example, but it seems like it would be the same for changing the size of the head gasket. Thats a better example. When you use a thicker gasket to decrease compression, wouldn't it also add volume at the bottom of the stroke? Wait a second, I think I just figured it out.

A thicker gasket at TDC is more of a change, where as at BDC, that thicker gasket really doesn't make a whole lot of difference, does it?

[NoMope Greg]

Exactly right.  Say going to a thinner head gasket makes the combustion chamber 4cc's smaller.  At the top, you've gone from 100cc's to 96 cc's.  At the bottom, you've gone from 1000 cc's to 996cc's.  So, the ratio goes from 10:1 to 996/96 = 10.4:1 (actually 10.375:1).  Going to a thicker gasket may add 4 cc's: 1004/104 = 9.7:1 (9.653:1).  When you've got an engine that's right on the edge of detonation, those 4 cc's may make the difference between survival and a holed piston.            

[ChallengerHK]

The ratio of the change in volume to the total volume is greater at TDC than the same ratio at BDC, so it results in a change in overall compression.

Let's say (in a totally bogus example) that you have a cylinder which, in stock form, displaces 50 cubic inches, and you have a 5 cu in combustion chamber with a current, flat-top piston. You would have a 10:1 compression ratio.

But if you were to switch to a domed piston whose dome was 1 cu in, the combustion chamber would effectively be only 4 cu in at TDC. The total displacement of the cylinder at BDC would then be 49 cu in, so the compression ratio would be 49/4, or 12.25:1.

[lemming303]

Alright. I get it now. I was sitting here thinking about it, and it didn't make sense at first. Thanks for the help yall.