by David Vizard
To extract maximum flow and therefore maximum performance from a given set of heads, you need to recognize certain principles that affect air flow and therefore performance. If you want to get the most out of your heads, heed the following advice when porting and flow testing heads.
Rule Number 1:
Locate the point of greatest restriction and work on that first.
As obvious as Rule Number-1 seems, the big problem for the novice is almost always a question of recognizing exactly where the greatest restriction is in the induction/exhaust tract. One pleasing note for the novice porter is that tackling the most restrictive part of the system and freeing up some flow potential delivers the best power return for the time invested. For the record, pocket porting heads (reworking and blending the seats into the first 2 or so inches of the port) is all about focusing on Rule Number-1, to the exclusion of almost all else. At the end of the day, pocket porting may not produce the fastest-looking set of heads or the most photogenic, but the results can be very satisfying.
Rule Number 2:
Let the air move the way it wants to, not the way you think it should.
Any time you force air to flow along a particular path, the total flow almost certainly drops. If you investigate where the air in a port is flowing, you find that there are two distinct situations that determine its path. In the first situation, a substantial amount of air is flowing in a certain part of the port because the route along which it is flowing has minimal flow resistance. In the second situation, a lot of air is flowing at a certain point/area because of the shapes upstream, downstream, or both of that high-flow or “busy” area.
It is important to be able to recognize the difference between these two types of busy areas. In the first situation, there is a strong indication that the area needs to be enlarged to make room for more air to flow along what can be seen to be a flow-efficient path. The roof of a typical port is a good example here.
In the second situation, the fix for more airflow is to add material at and around the point of fastest flow. A prime example here is the very-high-speed flow that can occur on, or just in front of, the short-side turn of a relatively low-angle intake port (small-block Chevys and Fords are prime examples).
The trick here is to distinguish one source of high-speed flow from the other because they require totally opposite responses.
Rule Number 3:
Air is heavier than you think. Keep up port velocities and avoid redundant cross-sectional areas.
When we get to the stage of flow testing ports, we find that not only is there a need to know how much air is flowing, but there is an equal need to know where it is flowing and how fast it is flowing. After a head porter or head designer appreciates just how heavy air is, he tends to have a whole different perspective on the importance of port velocities and cross-sectional areas.
All this, in one form or another, comes under the heading of velocity probing, and the cost of the equipment necessary to do that falls into the “peanuts” category. We have looked at how to build a flow bench, and down the road we look at what it takes to make and calibrate a velocity probe for just a few bucks.
One last thing before moving on: an explanation of what redundant port area is. As the term “redundant” suggests, it is an area of the port where little flow is taking place. If this is the case, it is redundant to requirements. The best action to take here is to fill it in. Redundancy in a port makes for a lazy port, and that results in a less-than-optimal torque output everywhere in the RPM range.
Rule Number 4:
Mixture motion is important. Do not ignore the need for it.
A charge that has little motion not only burns slower but also burns less effectively. This is most noticeable at low engine speeds. Lack of adequate mixture motion can cut torque output at, say, 1,000 to 2,000 rpm by as much as 25 percent. When engine speeds are 5,000 to 6,000 rpm, the need for port/chamber-induced mixture motion is far less. Mixture motion from quench action between the piston crown and the cylinder head face can be instrumental toward increased torque at all engine speeds. At part throttle, lack of mixture motion can also have a direct negative impact on mileage. Another desirable engine characteristic to suffer when you have low mixture motion is throttle response.
Rule Number 5:
Shape is all-important, a shiny finish is not!
The flow capability of a head absolutely cannot be judged by its reflectivity! This is a big one. Heads with a rough-finished right shape always out-power heads with a shiny-finished wrong shape! So don’t be in too much of a hurry to start work with those 180-grit or finer emery rolls.