SLANT SIX HEAD PORTING
This article is a basic review on Slant Six cylinder head selection, preparation and porting. The focus is to provide information to allow a ‘do-it-yourself’ engine builder to do his own porting work, a job that can costs hundreds, if not thousands of dollars to have a professional porting service do. This article will not get into flow bench testing because most at home engine builders do not have access to one but if you are working on a max horsepower engine, running your head on a flow bunch would be good idea.
The four general areas we will discuss are:
Ø Porting tools
Ø Cylinder head selection and prep.
Ø Pocket porting and runner shaping
Ø Combustion chamber work
Ø Gasket matching
Ø Valve back-cutting
Porting tools
The best tool for head porting is a ¼ inch air powered die grinder using carbide-grinding burrs. The grinding burrs come in many shapes, sizes and shank lengths. Get the longer shank length (4 to 6 inch) and get an oval shape as well as a straight-sided cylinder to start. Other shapes like the ball, cone and “flame” are also helpful so keep you eye out for a deal on a set. A cut-off disc (wizzer wheel) and small sanding pads are also good to have but sanding rolls are not that helpful. Other special cutters can be made by brazing a small piece of carbide or tool steel to a junk valve and then sharpening the carbide’s edge, so it cuts a certain area. The tool is inserted thru the valve guide, spun with a drill motor and brought into contact with the head. These tools do not remove a lot of material but they do cut good reference marks that can help guide your hand porting.
Another tool that makes the porting job easier is a headstand. The “V” type stand works best because it allows you to position the head in any orientation. The ones I have are homemade using scrap steel welded together and with the arms covered with rubber hose.
Cylinder head selection
The Slant Six head can be put into to large groups, the early spark plug tube head and the later no tube head. There are a number of sub groups and different part numbers but with the early heads, the biggest change came in 1968 with a reshaped combustion chamber that allowed for a better burn of the mixture. Some other notable head changes were the elimination of spark plug tubes in 1976 and the wider valve cover head that was used on hydraulic lifer engines after 1980. It is good to note that in 1972, some head casting had an added passageway for air injection, this added appx 8 lbs of weight to the head but that also made the head more ridged.
When planning a performance cylinder head, it is nice if you can get 2 or 3 heads to clean, inspect and choose from. It is nice to have different heads to compare to and I always seem to find small flaws or other issues that lead me to the “best” head of the group. Once I have made your selection, take some time to look inside the water jacket for old rusty core wires. These wires were used to keep the water jacket sand cores in place when the head was cast. The wires can be removed by grabbing them with pliers and pulling. One trick is to thoroughly clean the water jacket with acid by bolting on a up-side down thermostat cover, place the head up-side down and level, then fill the water jackets with meriatic acid. (swimming pool acid) One final prep thing I like to do is grind some small oil return channels under the bottom row of head bolts. Doing this will direct oil back into the lifter galley while the valve cover is off and reduces the mess when adjusting the lifters. (solid lifter engines)
At this point it is good to test fit the head you are reworking to the engine block you plan to use it on. Bolt the empty head to the empty cylinder block the look up the bores to the head chambers and you will quickly see any mis-match. First try to correct any mis-match by changing the dowel pin locations. (use off-set dowel pins) If the mis-alignment is not consistent between the chambers, reach up the bores and scribe lines so you can grind away the over hanging edges.
Pocket porting and runner shaping
The Slant Six head is at a dis-advantage because of the long bend in the runners. (ports) As a general rule, a straight “line of sight” to the top of the valve is what you want. This means you will be grinding material off the roof of the runner as it enters the head, then remove material from the runner’s floor at it’s midpoint, then do additional grinding work around the valve guide boss and the valve pocket’s roof. Use a long, stiff rod to help pinpoint the areas needing attention and gauge your progress as you move port to port. Different diameter rods are helpful to have as gauges for the intake and exhaust ports.
Directly under the valve is the area called the valve pocket and this is where most of the grinding work will be done. Clean this area and use dye to “blue it”. At that point, use an old valve and some lapping compound to define the valve seat so it is easy to see. Once this is done, you will see a lot of extra metal to the inside of the valve seat, this “overhanging” material needs to be removed by grinding straight down and to the inside edge of the valve seat. Be careful not to grind into or undermine the seat area, especially where the runner is curving under the seat where the cross-section is thin.
The valve guide boss is the other pocket feature that will need some work. Spend some time thinning-out the big lump of metal surrounding the valve guide. I try to leave the original guide length the same, in order to keep as much valve stem contact surface area but racers tend to grind down the protruding guide boss in order to get more flow thru the port. The goal is to remove excess metal and smooth-out “lumps and bumps” that can cause turbulence in the airflow. Do not get carried away with opening-up the port’s cross-section because the risk of breaking thru a wall outweighs the flow advantage on all but the most powerful SL6 engines.
Combustion chamber work
The main areas of concern are sunken valve seats, sharp edges in the combustion chamber and valve shrouding at the edge of the chambers.
Inspect the area around the valve seats to see if the seats have been recessed into the head. A “sunken” valve seat does not flow as well as one that is high up in the chamber. This condition can be corrected with the installation valve seat inserts but that process to install false seats into the intake and exhaust locations can add a lot of cost to a cylinder head rebuild. Inspect the chamber area for any sharp, raised edges around the seat area, these can cause turbulence and act as hot spots that can lead to detonation. Burrs around the rough-cut valve seats can be “knocked-down” with a flat valve grinding stone or small sanding discs.
Another important area for flow improvement is the side of the chamber walls near the valves. The factory’s rough valve seat cuts start to open-up this area in order to unshroud and promote flow around the valves. Additional material can be removed from these locations to increase and smooth-out the flow, especially at lower lifts. Using the head gasket as a template is the best way to see the areas next to the valve that require additional grinding and blending work. Be careful not to open-up the diameter of the chamber to the point of removing material needed to compress the fire ring on the head gasket.
Gasket matching
This is the process where you use the gaskets as a “template” to insure there is no overlapping material getting in the way of airflow. The intake / exhaust gasket and the head gasket should both be used to check for “over-hang” material and also for part-to-part alignment. Place the gasket on the surface with the locating studs (or dowel pins) installed and scribe (mark) any material that protrudes out beyond the gasket. Remove the gasket and grind as needed. Dummy studs or dowel pins can be used on the mating part to locate the gasket on it. This check will tell you if there are any major mis-alignment issues with the parts you plan to use. Note: As a general practice, I always check the manifold gasket to head port alignment with the head up-side down and the gasket shifted as far to the roof of the port opening as possible. (gravity does the shift for you when the head is up-side down) The corresponding grinding work will help raise the port’s opening and that increases the “ line of sight” access to the back of the valve. The same is done on the manifolds and I go as far as redrilling the intake manifold’s center hole in order to get the intake manifold ports a little higher. When you install the manifolds / headers, do that with the engine up-side down on the engine stand, so the weight of the manifolds everything into the higher position.
The head gasket can also be used to double-check the cylinder to combustion chamber alignment, especially if you could not do the empty block to head check as described earlier. The other check I do is to check the head gasket’s “centerness” on the cylinder block bores. Make any needed adjustments so no head gasket edges hang into the cylinder areas then peen or glue-in material around the locating holes so the head gasket fits tightly around the dowel pins and locates the gasket in a repeatable fashion.
Valve back cutting
Additional flow can be gained if the small ridge on the backside of the valve head is removed. This area can be ground smooth at the same time. The section of the valve stem that is in the port’s airflow path can also be thinned. All the valve prep work needs to be done before grinding the final seat angle on the valve.
With some basic grinding in the head ports and combustion chambers you can substantially improve the airflow through a SL6 head so it is well worth the extra work.