By Doug Dutra
Many performance minded SL6 owners know that there is a lot of extra performance "hidden" inside the engine's cylinder head. The simple fact is that the SL6 head was designed to support 170 cubic inches (port layout & valve sizing). This tells us that air flow will be less then adequate for the larger displacement engines (198 & 225). Even the "small" 170 will turn into a real RPM "monster" if more cam, bigger carb. and some head porting work is done.
Knowing that a "head swap" is a big job and that lots of time and money can be invested into a good "performance built" SL6 head, I wanted to review and remind everyone of the combustion chamber change made to the SL6 head in 1968. I feel that the "revised" chamber design is a better design for both performance and cylinder efficiency (fuel economy) so it is worth the effort to locate and use the 1968 and later head in order to get the most out of your efforts and your SL6. The new chamber shape can be found in heads with and without the spark plug tubes.
The combustion chamber shape was changed to get rid of some mixture "dead spots". These are places in the combustion chamber where the mixture does not burn completely. The main motivation for the auto makers to do this is to reduce exhaust emissions through a more complete combustion process.
Take a look at this "new technology" combustion chamber for a "wedge type" engine. This shows the trend to a "close chamber" design where there are quench / squash areas. Another design feature is to try to concentrate / direct the mixture around the exhaust valve and have the spark plug's tip in the middle of the mixture "charge". All this helps create a more complete "burn", which will yield more power.
This side by side view of the two SL6 head chamber styles shows the more open and symmetrical shape of the early chamber ( right side - click picture for a bigger view). The area to look at is the surface where the spark plug comes through. The early head has some dips on either side where this surface transitions into the rest of the chamber wall and roof. These are the "nooks & crannies" where the mixture will not burn completely. (Another big "hiding place" is the space between the cylinder wall & piston crown, down to the first piston ring. This is the reason why we see aftermarket piston ring grooves moving higher & higher up the piston crown.)
The pointer in this chamber picture shows the extra metal added to the exhaust valve side of the chamber, this helps to keep the air / fuel charge close to the valve. A distinguishing feature missing from these later chambers is the "half moon", dime size " dot" cast into the early head's chamber, opposite the spark plug opening. (see previous photo) You don't want to put a bunch of work into a SL6 head with the "dot". (most carry casting number 2206035, under the valve cover) You do want heads with the extra metal angling in towards the exhaust valve as shown here. There are many different casting numbers for these "later" revised chamber heads, none have that casted-in chamber "dot". All "no spark plug tube" heads, using the small B-series spark plugs (peanut plugs) have the later revised chamber design.
There are some additional feature changes to the later heads. This picture shows how the revised chamber has a more defined 3.400 bore diameter, which extends up into the chamber for the first 3/16th of an inch. (appx. .200) This creates the possibility for quench/ squash areas on both sides of the chamber.
With careful "blueprinting", machining and parts matching, these areas can be used to create a high turbulence, "closed" combustion chamber design. The chambers in the photo above are "smiling" because this type of chamber design makes lot's of power! - DD