Super-slick ports like this...
Super-slick ports like this high-flowing Dart Pro 1 intake port can deliver high flow throughout the lift range for two reasons. The first is obvious--port shape. The second is more subtle and comes under the heading of "seat science."
Ever heard the expression "all that glitters is not gold"? When it comes to heads, maybe we should-at least for the racer-change that expression to "all that glitters does not necessarily produce power."
Although the high-flow port of a Dart Ford head made big power numbers, it would not have been nearly as effective if a lot had not gone into the design of the seat and the area about half an inch before the seat to about the same amount after it.
Many racing classes call for unported heads but allow the valve seats to be remachined. This may appear to be of little value, but having the best seat job within the realms of what the particular port may like can be worth a significant amount of power. On top of this, an increase in low-lift flow can provide a very useful extension of the rpm range. This can be so to the extent it may allow the car to pull one rearend ratio lower to come off the turns that much faster. The bottom line here is that a little valve seat science could move a car's average finishing position up the field by one or two spots.
Many classes call for unported...
Many classes call for unported heads, so you could be starting with a production casting (shown) or an aftermarket equivalent. The only avenue open to modification is the valve seats, and how they are cut can be a deciding factor in overall engine performance.
When it's time to cut the seats in your heads, the very first move to make is to see that the guides are not worn. If they are outside acceptable limits (0.0020 for the intake and 0.0025 for the exhaust), then your first job is to fix that problem, as any seat work will be of little value. Worn guides can cost a lot more power than you might suppose. By refurbishing to "as new," I have seen 15hp increases from guides that had 0.0035- and 0.0050-inch clearances in their worn state.
Once the correct guide-to-valve stem clearances have been established, we can consider the seat forms.
It is fairly obvious that, at very low lift, the seat form totally defines the flow from the port into the cylinder. That much everybody knows. The question here is, at what point does the port become the dominant factor toward high flow numbers? The answer may surprise you and will also reveal the reason that the seat form can be so important. Intuitively, you might think that past about 0.100-or-so lift, the seat design's influence is about all done. In reality, it's way more than that. Here, by doing a mathematical analysis of the flow velocities that exist in the port and between the valve seats of the valve and head, it is possible to establish the relative balance of importance between the two. The nearby chart (Fig. 1) shows just such a test on a stock Chevy casting. What is surprising here is that the seat priority is higher than the port priority right up to 0.390 lift. If the rules limit valve lift to about 0.450 to 0.500, as many cost-conscious classes do, then the real importance of the valve seat's ability to flow well begins to be apparent.
Here is a simple way to check...
Here is a simple way to check valve stem-to-guide clearance with a cheap mag base and dial indicator. With a valve lift of about an inch, the clearance between stem and guide is about one third of the movement shown on the dial indicator.
On a Cup car head, a lot of time is spent determining what may work best with a particular port in terms of a seat and the area immediately before and after the seat. The question here is, can we use any of the Cup technology in engines used for our predominantly low-cost amateur classes? The answer is a cautious yes.
That cautious yes means you can just about rule out the use of a 50-degree seat, but much of what else goes down can be applied. Valve seat design is influenced by the port's angle of approach, so what works on a steeply angled race port doesn't always work when the flatter angle of a production-style head is involved. That said, let's go through some basic rules that apply about 95 percent of the time without recourse to a flow bench to check them out.
A basic set of rules for plain angle-cut seats (straight-edge cutters) looks like this:
1. Don't make the seat width too narrow. This is as bad as having it too wide. For a typical production-style V-8 head, a seat width of 0.055 to 0.060 on both the intake and exhaust works well.
This mean flow velocity test...
This mean flow velocity test on a stock high-performance Chevy head shows the significance of the valve seat's role toward flow is higher than the port's up to the circled area (0.390 inch lift). This establishes that the seat's role in overall airflow is higher than generally believed.
The arrows indicate the seats,...
The arrows indicate the seats, which are as often as not the starting point of the development of a no-compromise high-performance port.
AFR's heads are noted for...
AFR's heads are noted for their good seat science, whether it be on as-cast heads or CNC heads (shown). The radius on the exhaust is an important part of the equation toward allowing this production-style exhaust port to flow an outstanding 240 cfm at 0.700-inch lift.