Closed Chamber or Open Chamber Heads?

[Kevin Shasteen]

In my attempt to fully and completely understand Dynamic Comp. & Static Comp. I have always wondered what the essential distictive differences are in two difference approaches in the exact same Chamber Volume...(Volume above the top of the piston when it is at Top Dead Center aka:TDC) and how it effects performance.

 

Specifically, if one were to build an engine w/75cc chamber volumes: they could do so with a 60cc Cylinder Head Combustion Chamber & make up the difference of 15cc's in a dish, crushed head gasket thickness, crevis and Quench cc's. They could take that approach or:

 

One could have that same 75cc chamber volume w/a

70cc Cylinder Head Comb.Chamber, flat top pistons, and .005 minimum Quench/Decked block.

 

I guess what I'm asking, is this, If two engines were build w/the exact same chamber volume (as in the above two examples) yet the builders approached that exact same chamber volume differently; would the two engines behave the same or would they have different personalities?

 

As usual I am interested in everyone's thoughts and oppinions, so dont be shy & if you feel you have something to add, please dont hesitate.

 

I have an inquiring mind & it wants to know.

 

Kevin,

(Yea,Still an Inliner)

[Lockjaw]

I think a closed combustion chamber engine will make more power than an open one anywhere you want to measure it.

 

If you look at most aftermarket HiPo heads, most are gravitating towards the heart shaped closed combustion chamber.

[Mike C]

This is another one of those "it depends" questions. In my readings, it seems that basically cylinder head design of one engine prefers one type of chamber. A small block Chevy will make 20 hp more with the closed chamber head than the open if all else stays the same (ie compression ratio/total volume) However, it is the opposite with a big block Chevy. They always seem to make more power with the open chamber head. It all comes down th the quench area in the end. Quench is probably the most important aspect to the equation, followed by chamber shape and valve flow and flow direction in the chamber IMO.

[pparaska]

I get so many car magazines, I don't know where I read this lately, and no, I don't believe everything I read, but here's a theory I've seen presented:

 

1) Smaller combustion chamber means more quench area on head. This seems better for swirl, etc. Having a matched dished piston (one that matches the chamber footprint at the deck surface) like how the Trick flow twisted wedge pistons mate with the twisted wedge heads makes for a better way to get enough chamber volume for large stroke engines than using a flat top and larger chamber heads.

 

2) Heat transfer from the combustion chamber surfaces. Up to the point that you have detonation and/or pre-ignition, keeping the heat in the combustion chamber makes more torque (and hence power). This is simple Otto-cycle thermodynamics (highest efficiency with largest difference in combustion and intake temps).

 

This is also supposedly one reason why newer OHV engines like the LS1 have higher specific power. In essence, less surface area to volume ratio (a sphere-ish chamber being optimal, considering flame propagation and flows as well) means less heat transfer and higher combustion/post-combustion temperatures and pressures, and more torque/power. The smaller closed chamber heads with "D"-dished of conforming dish like the TW piston/head have a less surface area than the larger open chamber/flat-top piston combo with the same volume. And heat transfer in the head is generally larger than the piston, since it's cooled with high coolant flow right near the chamber walls.

 

All theory, and I probably got some stuff mixed up, but that's how I see it.

[Kevin Shasteen]

I agree about the recent onslaught of aftermarket performance cylinder head trends. It appears they are leaning more towards smaller combustion chambers and pistons with slight dishes to match the cyl.head's chambers.

 

It was this trend that initiated my posing this question on this thread.

 

Just wanted an overall consensus.

 

Kevin,

(Yea,Still an Inliner)