I've seen people on this form guess how much horse power a car would produce by mods. I would guess that this is from previously seeing dynos with similar mods, and just knowing the product. My question is, how do you guys figure out what the difference between a 10.1.1 piston and a 12.3.1 piston is? Obviously the compression is higher, but what does that do to your horsepower? At that, what do those 3 numbers mean for the compression ratio? I've read some articles and searched alot and read the posts. Now I need some more indepth explaination I think. If anyone knows any links or can explain this to me it'd be great.

 

generally higher compression means more torque and power, but you need the fuel to handle it also
the numbers 10.1:1 and 12.3:1 is the ratio at which the air/fuel mixture is compressed, statically.
this means that 10.1:1 is compressed 10.1 times its original density, or 12.3:1.
to get these numbers you need to know, bore, stroke, piston to deck height, combustion chamber volume, and piston dome displacement (could be either positive or negative).

 

that is an incorrect way of writing compression ratio. it is written 10.1:1 or 12.3:1 using your examples. they are read like this: "10.1 to 1 compression ratio." this is a ratio of volume to volume from bottom dead center to top dead center. this means that 1 cylinder can hold a volume of 10.1 units when the piston is at bottom dead center (BDC) compared to 1 unit when at top dead center (TDC).

so lets say you have 2 sets of pistons. one of them is 9:1 and the other is 12:1. the 9:1 piston has MORE volume above it when at TDC compared to the 12:1 piston. this is usually done by "dishing" the piston. when the piston has MORE volume above it during compression, it is forced down into the bore with LESS force than the 12:1 piston. therefore the higher compression piston (12:1 in this case) will yield more power than the lower compression piston.

stock hondas have between 8.8:1 - 11.5:1 comp ratios.

 

Okay, knowing that it is in the format of number-point-number-to 1 makes much more sense now. The other stuff then becomes more complicated to figure out. I have a fairly decent understanding of low vs high cr when running a forced induction setup. With an N/A setup how does the higher compression piston generate more force when there is less mixture volume capable in the cylinder?

Please excuse my ignorance. I am really trying to learn.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Supra &raquo;</TD></TR><TR><TD CLASS="quote">With an N/A setup how does the higher compression piston generate more force when there is less mixture volume capable in the cylinder?
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that's why forced induction motors make more power than N/A motors. turbochargers stuff so much air into the combustion chambers hence making sick power. in an N/A motor, only so much air can be put into the chambers. since that is the case, u might as well pack it as tightly as possible, so that when the combustion occurs, it expands with such a great force. this is why high compression pistons are used in N/A motors. Pressure is inversely proportional to Area; (P=F/A). F=force.

 

the displacement of the engine does not change unless the bore and/or stroke of the engne have been changed. When the piston has a large dome (and therefore a higher compression), the a/f mixture (constant original volume as determined by the displacement) will be sqeezed into a smaller space. This causes it to heat up, which will help the mixture to combust more efficiently, and with more force. There is more to this, but just know that the displcement is constant at all and every compression ratio... i think that is what is confusing you.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Supra &raquo;</TD></TR><TR><TD CLASS="quote">... With an N/A setup how does the higher compression piston generate more force when there is less mixture volume capable in the cylinder?</TD></TR></TABLE>There's not really less mixture, it's just being compressed harder after it's taken into the engine. Why this makes more power, is just how the power cycle works. You'll have to trust us on this one until you take a thermodynamics course...