confusion

Cobras4life · #1 (**permalink**) 05-18-2005, 04:11 PM

I was just talking to a guy at a machine shop over an engine and it really threw me off.... he said that the heads were made so the engine could be a bit higher in compression.. 10:5:1.. i'm not sure what he meant by that and most likely misunderstood so i am going to ask you guys to help me out...

My questions is... do the heads make the compression or does the block make the compression?

I'm pretty sure it's the pistons make the compression but hey i could be wrong

marcocsx3121 · #2 (**permalink**) 05-18-2005, 04:27 PM

In reality, the answer is both. The volume of the combustion chamber combined with the configuration of the dome of the piston ( flat top, raised dome, dished) determine compression ratio.

blykins · #3 (**permalink**) 05-18-2005, 04:29 PM

Is this like which came first....the chicken or the egg?

Yetiman · #4 (**permalink**) 05-18-2005, 05:51 PM

What they have done is mill metal from the mating surface of the heads (commonly known as decking the heads) which does raise the compression ratio if all else stays the same.

This is a very common practice.

Tongue Pirate · #5 (**permalink**) 05-18-2005, 06:41 PM

During the compression cycle the heads do not move so they do not make compression, the pistons do. So as stated above the heads can raise the amount of compression with a smaller combustion chamber(where the valves & spark plug are). You can see this on many aftermarket sites where they state the combustion chamber size in CC's of a specific head

Cobras4life · #6 (**permalink**) 05-18-2005, 07:26 PM

So if I wanted to switch to aluminum heads do I have to get them done specifically for my engine since it's a bit more on compression?

Cobras4life · #7 (**permalink**) 05-18-2005, 07:26 PM

So if I wanted to switch to aluminum heads do I have to get them done specifically for my engine since it's a bit more on compression?

Tongue Pirate · #8 (**permalink**) 05-18-2005, 08:39 PM

Why would aluminum heads have more compression?

Excaliber · #9 (**permalink**) 05-18-2005, 08:56 PM

Compression ratio is simple to calculate, but hard to get all the numbers to do the math!

FIRST: How many CC's is the size of the combustion chamber in question? If the the head has been "milled" or made "thinner" then the ORIGINAL CC of it's combustion chamber will be smaller. Small combustion chamber WILL equal higher compression than a larger combustion chamber assuming pistons stay the same.

SECOND: What is the shape/size of the TOP of the piston? Is it flat top? Does it have a bulge on the top? Is the top "scooped out" or "dished"? A dished piston will leave extra room for the compressed gases within the combustion chamber. A flat top will leave ONLY the room IN the combustion chamber head. A DOMED piston will make the combustion chamber area smaller, thus more compression.

THIRD: Compression height of the piston itself. Does that piston come to the top and stop exactly EVEN with the surface of the block? OR (usually) does it not quite come up far enough to be flush with the top of the block?

Assume a "zero deck height" piston, perfectly smooth with the top of the block. Assume a flat top piston, it will neither add nor subtract from the size of the combustion chamber. The last "variable" is the thickness of the head gasket! Thinner gasket, more compression. FAT gasket, less compression.

After you get all the above numbers then you calculate the bore size and the stroke to determine the compression ratio.