closed v.s. open chamber head



While checking out some of the post in reguards to porting a cyl. head, I see a lot of talk in reguards to open or closed chamber heads. I have been around engines for a while but have never looked into the differance. Whats the differance and what performance gains can be had?

Jes 8)

Generally speaking closed chamber heads are for performance and open chamber are for low performance.

Used to be most closed chamber heads were machined and open chambers were cast. The closed chambers were rough cast and machined for shape. Machining cost money. Nowadays I think they are all cast.

Generally speaking closed chamber heads are better for performance because the closed chamber controls the burn rate of the flame front. This is instrumental at controlling pinging and detonation.

On the typically low compression open chamber head it does not come into play. The compression is not high enough to have to worry about uncontrolled flame front causing pinging or detonation.

When you build a high performance high compression engine with open chamber heads you can kinda build around the open chamber head problem by carefully choosing your cam. If you have the money you can get pistons custom made that control the flame front like a closed chamber head would but it is not cheap. You can ship the head to the piston manufacturer so they can match the piston to the head design.
So on a '77 what does a person look for when trying to figure out what they have? Are there any significant markings on the closed or open Chambered heads?

Jes 8)
Dang It! :banghead: I wish I would have asked these questions before I had my head milled .040, had all new seats and guides put in, and three new valves.
So....Since I haven't put a cam in this thing yet , what would be a god one to run. I was thinking of the Comp Cams 270 grind.

FYI: I am running a cliffy intake, motorcraft 350cfm 2bbl carb (an Edelbrock 500cfm 4bbl after the cam),late model efi exhaust manifold,3" single exhaust to a flowmaster, 4spd,with 411:1 gears.I have not recurved the dist. yet. Waitng till after I buy my cam.

Any suggestions?

Jes 8)
I was just concerced by Anlushac11's comment about buildig around the open chamber heads problems by carefully choosing your cam. Thank you for all the info . I guess I will just continue as planed. :LOL:

Jes 8)
The only thing that has been done internaly is a .030 overbore, and .040. off the head. Do you have any idea about were the comp. ratio would be? I am planning on moving the cam gear ahead one tooth. The guy that does all my machine work said he did it to his buddys 300 and he got a lot more mid-top end power. He also said it is equivilant to 4 deg.... I think we already talked about that though.
When you talk about having a piston that mirrors the cumbustion chamber, you are refering to the shape and contours of the comb. chamber....right?

Jes 8)
Hey Ted!
Did you ever get the Private message from me in reguards to the water pump pulley that I need?

Jes 8)
Combustion Chamber Shape:

There is one other item that should be mentioned to describe the "differences" between so-called "open" and "closed" chamber heads.

In the sixties, the "wedge" or "closed" chamber head was popular because it provided two significant features to combustion; a) higher CPR and b) a "quench" zone to "stabilize & cool" the combustion wave during the combustion process;

Feature (a) yeilded higher operating pressures which translated into more power

Feature (b) yielded combustion stabilization which precluded detonation/pinging and thus permitted even higher use of CPR

In the early '70's, the SMOG NAZI's wielded their mighty swords. Turns out that CPR and "quench" zones promote the formation of , yep, you guessed it, NOx!!! OUCH!!! NOx is also generated "around" the cylinder wall near the head. Thus, CPR's, heads and cylinder diameters had to be "redesigned" to reduce NOx emissions.

All hail the SMOG NAZI's for their contribution to smaller engines operating at very high rpm to make back the power lost as a result of smaller diameter cylinders and lower CPR's.

I love this country! .. ;)
High compression ratio causes more NOx. Quench area promotes swirl which is a good thing. They just had to adjust properly, which they eventually did (hence the EFI 300 heads are a closed design). Now of course its all about pent-roof quad valve heads with whatever quench they can get in there.

Ok.. I admit it, I realy don't know a whole lot about head design and the effects they have on performance. that would pretty much explain the reason for my post. So, with all that out in the open, What the hell is a quench done? :roll:

Jes 8)
"quench" is one of many terms used to describe the "chemical dynamics" of a combustion wave/zone. In this case, the combustion wave is stabilized by allowing the "flame" to stabilize through the process of "attaching" to a stationary (or at least nearly stationary; referring to the time it takes to combust) boundary layer and dissipate some heat into that boundary layer/wall. This feature, of one possible combustion process, aids the compression process prior to combustion by absorbing "compression heat" and by establishing a stabil "flame holder" for propogation of the deflagration wave front (rather than a detonation wave forming; i.e. supersonic combustion waves, pinging, ect.) This is also why additives are used in fuels to raise "OCTANE" numbers; the avoidance of detonation, pre-ignition, pinging, etc. These unwanted combustion processes reduced power generation. If they become severe, they can also damage/ruin an engine.