General Q: Why does cutting the heads INCREASE compression (basic nooby question)

[pjbrownva]

I'm clueless when it comes to some of the popular performance mods, so please indulge this very basic question.

I'm trying to understand how and why cutting the heads, INCREASES compression.

I'm not doubting this well-accepted fact. I just don't understand it.

If anyone has a diagram, animation or explanation to help me understand, I would appreciate it.

Below I've created a crude diagram, that shows my (mistaken) belief that cutting the heads might increase the volume of the combustion
area, and thus could actually decrease compression.



Please help educate me. I'm clearly missing something.

I've never been shy about asking questions, as i firmly believe that if you don't ask, you won't ever learn.

Maybe I'm misunderstanding what "cutting" the heads means, or maybe I'm confused in some other way.

It's not that I'm about to cut my heads and *need* to know, it's more that I'm curious by nature and would love to understand how and why cutting the heads

increases compression.

Thanks in advance.

-Philip

[nitro_rat]

You are thinking that the “head” is the head of the piston. The part labeled in your diagram “head” is the piston. Machining that surface not only increases compression but alters port timing as the port in the cylinder wall is now “taller” in relation to the top of the piston. We call this cutting the piston.

When we say cut the “head” we are talking about machining the mounting surface of the cylider head which reduces the volume of the combustion chamber. This increases compression.

[nitro_rat]

James Perry cutting a cylider head (stolen from chattanooga fishing forums)

[pjbrownva]

You are thinking that the “head” is the head of the piston. The part labeled in your diagram “head” is the piston. Machining that surface not only increases compression but alters port timing as the port in the cylinder wall is now “taller” in relation to the top of the piston. We call this cutting the piston.

When we say cut the “head” we are talking about machining the mounting surface of the cylinder head which reduces the volume of the combustion chamber. This increases compression.

James Perry cutting a cylinder head (stolen from chattanooga fishing forums)

Thank you so much. That clears it up immediately! :-)

[nitro_rat]

Many times on a 2-stroke if the block deck surface has to be machined, the top of the piston is machined the same amount so that the port timing remains unchanged.

[pjbrownva]

That makes sense. Thank you.

[Chaz]

Many times on a 2-stroke if the block deck surface has to be machined, the top of the piston is machined the same amount so that the port timing remains unchanged.

You might want to "retink" that one a bit ..

[nitro_rat]

You might want to "retink" that one a bit ..

Right, I wasn’t awake yet. Cutting the deck and the piston top equally raises port height without changing compression...

[FORBESAUTO]

What uncle Chaz is trying to say is deck height doesn’t effect port timing, but shaving piston does. Port timing is in relation to the crankshaft, thus the reason I don’t like the method of checking port hieghts from the deck since the deck height can vary all over the place, but using a degree wheel will let ya know what got regardless of deck shaving and boo whopped piston set ups

[Chaz]

I had ta get me a great big in cuz it gotz so many lil numberz on it ...