Originally Posted by Farmstore34
I see alot of info about switching out chevy heads from a 58-64cc chamber to a 76cc. What is the point or reason? Why would you want lower compression in a SBC motor?
I have no idea where you're seeing this information, the 76 cc heads are old fashion SMOG heads they absolutely wipe out any chance of making efficient, large amounts of power. They can have a place with nitrous or blower motors where compression needs to be reduced to accommodate the added volumes and/or pressures of the materials entering the combustion chamber. But for a normally aspirated (that's one that just breathes atmospheric air pressure) engine smaller chambers are the way to greater efficiency and power.
In addition the above thought has to be added an understanding of the entire combustion chamber and how it works.
- The time it takes for the burn to cross the bore diameter is based on many factors which I'll identify when I bring them up. But first is spark plug location. The larger chambers move the plug to one distant side, this increases burn time by the distance involved. To compensate so the burn causes maximum cylinder pressure early in the rotation of the crank more advance than is desirable has to be used. This results in a sensitivity to detonation and often causes enough pressure too early such that much effort is lost trying to push the piston backwards from whence it came. Too early presure rise also encourages detonation. A major feature of small chamber heads is that the spark plug is placed as close to the center of the cylinder to reduce burn time, this allows the amount of advance used to be reduced which decreases detonation and improves power, mileage and emissions by not trying to force the piston backwards.
- It turns out that just letting the mixture into a cylinder without trying to control its direction and expansion reduces cylinder filling and tends to foster the separation of fuel from the air. Large chambers are notorious for this where tight chambers maintain control of the mixture by either preventing the separation in the first place or fostering a remix of fuel and air. Involved with this is a phenomenon known as swirl. This is a characteristic of an in-line valved wedge chamber that greatly improves combustion. To this end the Ricardo chamber as seen in modern heart shaped chambers like the Chevy Vortec head is most important. Compared to older large open chamber heads the Vortec is an easy 40 horse improvement by just bolting them on. These are an improvemet over the old small chamber performance heads as well by about 20 horse because of subtle design differences beyond getting the spark plug toward the center of the cylinder diameter. These include a spark plug moved toward the exhaust valve which with swirl initiates combustion over the exhaust valve to sweep around the cylinder. The space between the plug boss and the intake valve allows the mixture to sweep in forming the swirl and the boss trips liquid fuel from the long wall turn to remix with the entering air stream. The beak that projects from the squish/quench deck directs the secondary swirl off the back side of the valve so it doesn't interfere with the primary entry swirl which other wise reduces flow through the valve. For a typical well ported wedge head about 80% of the mixture will enter from the valve area between the cylinder wall side of the valve to the spark plug. the remaining 20% will worm its way in on the rest of the valve. The tight wall from the spark plug around the exhaust valve reduces the amount of flow on the short wall of the exhaust port which if not controlled obstructs the primary flow into the port from the center of the cylinder thus reducing total flow. On a flow bench you can see these flow obstructions affecting overall flow, on heads without the beak. This is commonly seen on intake flow as it just stops improving much or may even reduce once the lift gets beyond .5 inch.
- Squish and quench, these are functions of the flat step on the side of the head opposite the valve pocket. This builds what's called mechanical octane into the engine. To be most effective the piston crown surface needs to close between .040 to .060 inch to the head surface for a street engine with steel con rods and normal piston skirt clearance. For a competition engine that will see seasonal tear downs it's permissible to go closer but one needs to remember that as the cylinder and piston wear a greater amount of rocking around the pin can occur, this combined with competition bearing clearances on the rod can result in the piston kissing the head when you start getting to .035 and less clearance. So for a street engine where you don't expect to open it up for many thousands of miles and carbon will form to reduce that distance a bit more is safer and less costly than piston to head collisions. All this means is you need to run flat top or D dish pistons to get this clearance. Typical factory round dish pistons put too much of the crown too far away from the head reducing squish and quench.
- OK so what is squish and quench. Two functions from the same parts separated in micro time. Squish is the ejection of about half the mixture in the cylinder toward the valve pocket as the piston closes on TDC. This provides a powerful stir helping break up remaining fuel globs into vapor and thoroughly mixing it with the air for a fast and complete burn. This also increases the density of the mixture immediately before the spark plug making it easier to set on fire and causing it to burn faster. This allows a 4 to 5 degree reduction in spark advance and places the maximum cylinder pressure point more in line with the piston position slightly after TDC instead of before TDC getting more downward push in the direction of rotation. The second function is quench. As the flame front spreads pressure and temperature go up very quickly. So much so the unburnt mixture ahead of the flame front can be heated and compressed to the point where it spontaneously ignites with explosive force, this is detonation. Pre ignition is a similar thing in effect where something hot sets off the mixture before the spark plug does. The quench function is a heat sink that delays the onset of explosive self ignition by having an area of large surface area to low volume, this pulls the excess heat out of the mixture preventing combustion till the flame front gets there. Thus the mechanical octane nomenclature.
So the importance of these functions is buried in the design of the head and piston crown, they need to work together. That's not to say that other combinations can't be made to work, but they will produce less than the optimum performance and burn more fuel and require wilder cam timing to do it. If you look at NASCAR cup engines since the mid 1990s to present the design is to smaller and smaller chambers they seldom get over 50 ccs. Using these modern concepts it is very easy to assemble a Gen 1 Chevy 350 with D dish Keith Black hyper cast pistons with the a Comp XE262H or XE268H or someone elses equivalently timed cam, a set of vortec heads, a GMPP 4 barrel vortec intake, a 650 to 750 CFM Holley or Carter carb, a set of 1-5/8s long tube headers with 2-1/2 inch dual exhausts and have an engine that dynos around 420 hp. This is a nice calm street-able combo that will scream with anything short of a competition built engine.