With modern tools like CFD software and a Superflow bench, cylinder head porting has becom
The first angle in a high-performance valve job is the throat cut, which is typically around 60 to 70 degrees. This helps ease the air's transition to the seat cut. The second cut is the seat cut, which is generally done at 45 degrees and is the surface that the valve actually seals against. The third and final cut is called the top cut, which is normally in the 20-30 degree range and is made immediately after the seat. This cut helps reduce valve shrouding of the airflow past the valve (or before the valve on the exhaust side) as the valve starts to lift off of the seat.
The bowl area and the short turn radius on the port floor are also critical to airflow in most import cylinder heads. In fact, on the Honda D-series, B-series and K-series cylinder heads I've had ported and flow bench tested, as much as 30-40 percent of the improvement to airflow has come from work done in the bowl area alone. This is also the area of the head where a head porting specialist's experience and experimentation comes into play, since changing the radius and overall shapes in the bowl can have a big impact on the angle the airflow enters the combustion chamber at, which can have a big impact on the combustion event itself.
Hand porting is still the norm in the industry, especially with custom jobs equipped with
When chasing that last 10-20 percent, factors including the shape, cross section, surface finish and overall volume of the ports come into play. Larger volume ports may seem like a good idea, but more volume can often mean losing torque due to lower air/fuel velocity, especially at lower engine speeds. If taken too far, volume increases can even lead to airflow reversion or backup, where air literally backs out of the combustion chamber toward the intake manifold. Straightening the ports while increasing volume as little as possible is therefore the most common approach in this area. Combine that with a textured surface on the intake side to encourage fuel atomization and boundary layer activation and you should find most of the remaining airflow gains.
There's no question cylinder head porting can be a very effective way to improve volumetric efficiency and overall airflow capabilities of any cylinder head. But when's the right time to spend the $1,000+ it takes to hire a reputable head work specialist? That depends on what you're trying to achieve with your engine build, but in general the time is right when you upgrade to a bigger turbo or higher lift/duration camshafts and the airflow capabilities of your intake and exhaust side bolt-ons exceed that of the head itself. In other words, if your cylinder head starts to become a bottleneck in the system, it's time for some head work.