Porting cylinder heads
The purpose of this article is to acquaint engine builders to the possible benefits of cylinder head porting and making more power through the porting process.
 Port volumes vs. HP
Port volume (or actually cross sectional area) is an important consideration when it comes to how big to make ports. Below is a table by David Vizard showing the relationship between port volumes and HP on a SBC 23 degree head.
 Areas to work on
Proper porting procedures will result in getting the largest volume of air/fuel mix to the combustion chamber in the shortest amount of time. Porting works hand in hand with valve grinding and seat cutting.
During the manufacturing and machining processes of a cylinder head, there are areas that are left unfinished. This is due to cost/time restraints imposed on the manufacturers.
When a port is machined for a seat, often a lip is left below the seat. This lip is sometimes semi-removed using a type of tapered reamer that helps smooth the transition from the seat into the pocket portion of the port. But because this is done quickly and by machine and not carefully and by hand, there is still almost always places where improvements can be made.
Another example of areas that need attention. This is a BBC head that has had some work done to it already:
 Layout dyes
One common type of metal dye is machinist lay out paste, dykum blue, prussian blue, etc. Comes in a tube, is thick enough to be spread out with a piece of paper towel, etc. After it sets up for a few minutes, the scribing can be done.
Another type comes as a liquid in an alchol base that can be sprayed or wiped on. When the alcohol evaporates, the dye is left behind for scribing.
In a pinch a Sharpie/Magic Marker can be used.
 Step by step porting methods
Before you go out and grab your tools and start grinding on those $1000 Edelbrock heads you just bought, STOP. Porting requires practice, so do exactly that, practice on a set of old nondescript heads that you picked up at the local auto recycler. Try out your tools and see what they do. Remember to use the correct tool for cast and aluminum heads so you do not ruin the tool and hurt yourself. After you feel confident with working with your tools, we will go on to explaining what we want to accomplish in each segment.
 Gasket matching
This is a simple process whereby we will match thermostat housing to an intake manifold for better flow of coolant from the engine to the radiator. Begin by cleaning off the surface of both housing and intake mating surfaces. Remove all gasket material and glue with acetone and a gasket scraper. True the surface of slight imperfections by using a mill/bastard file place flat on the gasket surface working the file back and forth. The shiny spots are the highs and the dull ones are the lows, true these surfaces. When a flat surface is obtained,paint the surface with Dykem dye, lay a new gasket on the surface and insert the bolts in the holes of the intake to center the gasket over the openings. With a scribe, scratch along the gasket edge around the opening. When complete, remove bolts and gasket. This will reveal a scribed line in the purple dye that you will be enlarging up the opening to.
- If this is an aluminum intake, chuck up a open cut burr that is used for aluminum, and spray it with WD40 or similar lubricant. This will make cleaning the tool easier, reduce friction and heat.
- Put your safety glasses and respirator on and start up your die grinder. Note which way the grinder turns, clockwise.
- Gently ease the burr against the upper portion and draw it along the intake opening working in a clockwise motion towards the bottom. Just lightly feather the burr and adjust pressure against the surface as you grow more accustom to the process. Trim more and more, as you come closer to the scribed line, stopping when you are one line's width away from it.
- Blow away any of the metal splinters that were made by the burr. Keep your work area clean.
- Begin removing material again from a different area until the entire open area is within a line's width of your scribed line.
- Remove your burr, and chuck up a sandpaper roll of 80 grit.
- Gently move it around the perimeter of the opening once or twice. Check the surface for texture and note how close you are to your scribed line.
- Chuck up a 180 grit roll and move around the perimeter again one more time.
- Finish off by tilting the roll 30-45 degrees and go around the outer edge and remove the sharpness off that edge.
- Blow off the sanding dust and clean the gasket surface.
- Place the gasket on, insert your bolts and check to see how the gasket fits the opening now.
- Match the housing to the gasket to the surface the same way as you did the intake and you should have a perfect match.
- Apply this process to the other gaskets on your heads for optimum flow.
 Valve job
A three or more angle valve seat is best for flow. Most shops today use a multi-angle cutter that does at least three angles all at once. This is an acceptable method for most performance engines, but a skilled machinist can use individual stones to give a superb result, placing the angles exactly where they need to be. This method also allows the seat widths to be set by the machinist and not the tool.
 Replacement valves
 Installing screw in rocker arm studs
When screw in studs are going to be installed, the stud boss will usually need to be machined shorter to make up for the thickness of the guide plate (if used) and for the hex portion of the stud and the radius between the hex and the stud. In the case of the SBC, 0.400" is usually a safe amount to remove, but this can vary some depending on the exact parts used.
 Milling the deck
When the decks of the block or heads are milled to raise compression, or to true the surfaces, or to correct the quench measurement, often the ends of the intake and/or the intake flanges need to be milled to correct the port and bolt hole misalignment this can cause. Whether this is necessary depends on the amount of material removed and the angles involved. In the case of the Chrysler B/RB and Pontiac engines, nothing needs to be removed from the ends.
It should be taken into consideration that when the heads are milled, the geometry changes and piston to valve clearances will be closer.
See Milling cylinder heads for more info.
 Reality check
While porting production heads was the thing to do in years past, nowadays there are relatively cheap, great performing aftermarket heads that will outperform all but the very best (read not ported by a beginner, and uber expensive) ported production heads.
Porting is relatively easy to do as far as removing metal goes, BUT knowing where and how much to remove makes all the difference between a good performing head and a boat anchor.
Photos of ported heads can look very impressive. Unfortunately that means nothing for the most part, unless the work was done by a seasoned porter, and there was a lot of track testing done to confirm the results, and/or there was access to a flow bench- along with the knowledge needed to digest the bench's results and turn the data into viable, meaningful, positive changes. So looking at photos can show generalities, but as far as being a useful tool, photos fall far short of giving any really meaningful info. If templates made from flat stock, and ground down valves used as sizing indicators are used to help a porter, this can make a big difference in the outcome. Unfortunately, these templates are hard to come by, as they were the same thing as a CNC program before there were CNC machinery.
Doing a full-on porting job is very labor intensive and is a dirty, time consuming project- made doubly so when working with cast iron. To bring a set of 305 heads (which are often touted as a cheap, easy head to modify for use on the SBC 350) up to what even a set of stock untouched L31 Vortec heads can do, is hard to do regardless of who you are, let alone someone with no experience. Just doing one port is a chore. By the time all 16 ports are done along with the cost of the machine shop and parts, most guys will see that buying aftermarket heads or even using a better production casting to start with (like the Vortec) is the better way to go.
If the decision has been made to use the 305 heads regardless, staying with the "low hanging fruit" like removing/blending the lip that's almost always present where the bowl meets the bottom seat cut (shown in images above), and removing casting flash and irregularities, and careful port matching (NOT gasket matching) can result in an improvement of several percent with only moderate time and effort. Often backcutting the valves gives an improvement for little cost. A true quality valve job can be worth a few percent more and should be considered a 'must-do' on any production head- and even aftermarket heads need to be checked over carefully.
As has been said already, the difference in cost between a properly rebuilt, ported and prepped set of production heads and a set of aftermarket heads becomes less the more parts, time and work they consume. But if a production head is going to be used, it makes more sense to start out with the best performing head available rather than using a head because it's cheap (or even free). Remember- the same machine shop time and money will be spent regardless if you're starting with a Vortec head or a 305 head.
- If you like the hands-on video approach, this is it: CYLINDER HEAD REBUILD - VALVE JOB AND PORT & POLISH ON DVD
 Compression calculators
- Static compression ratio
- Dynamic compression calculator by Kelly
- Dynamic compression calculator by KB
 Other calculators
- [http://users.erols.com/srweiss/calcplv.htm Port Limiting Velocity
or Minimum Port Cross Sectional Velocity]
- Port CSA
- Average Port CSA
- Port FPS / CFM / CSA
- Convert Airflow to a Different Depression
- Estimate Horsepower from Intake Airflow
- Estimate Intake Airflow needed from HP wanted
- Estimate Volumetric Efficiency
- Inlet Runner and Peak Torque Calculators
- CNC cylinder head porting and block machines
- Cylinder head porting abrasives
- Cylinder Head Porting 101
- Valve shrouding from diyPorting.com
- How to calculate the optimum intake port volume