Before going any further, I want to make a point about this chapter. Usually the front end of a tech book starts with stuff that is easily assimilated. Although this chapter may at first seem a little more complex than might be expected, bare with me because here I set and simplify what comes later. Take a look at my “power box” (Figure 1.1). Right in the middle of everything, you see in large red letters “Optimize Cylinder Head Airflow.” That is our focal point. If we don’t get good results in terms of airflow here, the horsepower output per cubic inch (or liter) suffers, and there is nothing we can do to compensate.
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In Chapter 7, I talk about what I loosely describe as “Five Golden Rules to Successful Porting.” To make my point here, I need to jump the gun and introduce you to Rule Number-1: Locate the point of greatest restriction and attempt to improve it as far as is possible. Applying this rule means identifying the point of greatest restriction and minimizing it. So without any Alfred Hitchcock suspense, I can tell you that the ultimate roadblock to making super-high horsepower per cube is the intake valve. The path on which the air has to flow to make it around the intake valve is tortuous at best. As you can see from Figure 1.2, flow is valve-limited to such an extent that it is only a couple of steps removed from having a leaky cork in the system. The bottom line is: Everything we do is directly influenced by the valve seats and valve form in close proximity to the seat.
Point of Maximum Flow
Initially, it might seem that having flow-efficient valve seats is of only minor consequence because the piston motion on the induction stroke is very slow when the valve is at low lift. But it also seems that the high lift flow must be important because, when the valve is at high-lift, demand by a piston that is now rapidly moving down the bore is high. I go into the subject of cylinder demand versus valve lift in detail in Chapter 10. Suffice to say at this point, a well developed high-performance engine actually has two induction phases. The first is caused by the scavenging of the tuned exhaust and takes place during the overlap period, and the second is the suction caused by the piston going down the bore. For engines with cams of about 280 degrees or more of seat-to-seat opening period, the scavenging brought about by the exhaust is greater than the suction caused by the piston on its way down the bore.
Valve-and-seat combinations that flow poorly during the overlap period can cost dearly in terms of high-RPM power. To demonstrate the point, in Figures 1.3, 1.4, and 1.5 I have used a single-pattern cam of 285 degrees of off-the-seat duration (which from here on I refer to as “seat-to-seat duration”) as an example. Figure 1.3 shows where in the valve opening events the overlap occurs. Figure 1.4 shows the overlap period on a typical card as issued with the cam when purchased from the cam manufacturer. In this diagram, we can easily see the shared time (overlap), in terms of degrees, that the intake and exhaust are both open. What is not apparent from such a diagram is how far off the seats the valves are. To better appreciate this, refer to Figure 1.5.
Using a hydraulic cam for a pushrod motor, we can see from Figure 1.5 that the tappet lift at the top dead center (TDC) point of the overlap phase is approximately 0.060 above the lash point. Selecting a typical 1.6:1 rocker ratio means (assuming split overlap) that the intake and exhaust valves are both open by some 0.096 inch. That is close to 1/10 inch, and here we are considering nothing more than a fairly hot street cam. If this were an all-out race engine, the amount of lift seen at the valves at TDC on the commencement of the induction stroke could exceed 1/4 inch!
For the record, cam manufacturers are always striving for faster lift rates off the seat. This provides the cylinders with as much flow as possible in an attempt to keep pace with air demand at increasingly higher RPM. The reality is that the cylinder does not actually see lift; it sees flow. This means the more efficient the valves are throughout the lift range, the less we need to rely on aggressive and, consequently, wear-prone valve-trains to get the job done.
Seat and Port Priorities
At low valve lift, we can see that the flow is almost totally dependent on the forms just before and after the actual seat, and this determines the flow efficiency. If the chamber scavenging is well sorted, we find that the gas velocity between the valve and the cylinder head seats is really high. However, because the flow area between the seats is small, the velocity in the main body of the port is low. In other words, we could see 300 ft/sec at the seats and only 10 ft/sec in the main part of the port. At such low port-flow velocities, the shape of the port and its surface finish has almost zero influence on the overall flow while the seat design is all important.
This leads to the question: When does the port form become more important than the seat form? By making a study of the velocities involved, this can be determined and such tests indicate that almost regardless of the port type, two or four-valve layouts and the like, seat priority is the most important up to about 0.18 inch of the valve’s diameter. Above that figure, the port shape starts to become the dominant factor toward good flow. For most production and even high-performance modified street motors, valve lift rarely exceeds about 0.28 inch of the valve’s diameter.
What this means is that the form of the valve seat plays a dominant role for more than half the valve lift involved, and it does this twice during the open/close cycle. The only conclusion that can be drawn here is that the valve seat form is of great importance to the overall success of the port—intake or exhaust. In practice, the flow bench and dyno prove this to be the case.
Do You Need a Flow Bench?
So, I have already introduced a degree of complexity that you may not have thought of. From this, you may be thinking that having a flow bench is almost a necessity. Well, a flow bench provides important data, and therefore is a real benefit. However, you can do a basic porting job based on sound principles and expect a good return for the effort put in. As long as your aspirations don’t include preparing the winning heads for a car running in a big-time international event, you are okay.
But if you want better-than average results, a flow bench is about as necessary as the die grinders and cutters you need to do the job itself. Yes, we can port without a flow bench. But not only is a bench a valuable tool to take you well into the professional arena, it’s also fun to use. Finding that extra air and trying to beat Mother Nature can become an exciting challenge. If you are concerned about the cost of an effective bench, fear not—you can, as you will see in the next few chapters, be in business with a highly functional bench for as little as $150.
Written by David Vizard and Posted with Permission of CarTechBooks
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