In Chapter 1, I brought attention to the fact that the number-one impediment to flow on both the intake and exhaust side of things is the valve seat. You can know how influential it is, with reasonable accuracy, by determining the mean velocity at the gap between the seats of the valve and head and then comparing it with the mean port velocity. If these two velocities are put in graph form, we can see the relative importance of both the seat geometry and the port configuration.
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Figure 8.4 shows how this works out for a stock production head in as new condition. Where the two lines cross is the point at which valve seat priority gives way to port priority.
On this particular head, a 2.02inch intake valve was used and the cross-over point was at 0.390 lift. This is probably a lot higher than you may have previously thought. As a rule, the intake seat is the number-one influence until valve lift has reached about 0.18 of the valve diameter. On the exhaust side, the seat has influence until higher lift values. In fact, for an exhaust valve, the seat and its immediate throat form can have a significant influence right up to 0.35 of the valve’s diameter.
Valve Seat Forms
The more rounded and streamlined a seat and the approach and departure areas are, the more efficiently it’s likely to flow. Figure 8.5 shows about how much the average flow efficiency of the first 0.250-inch lift of an approximately 2-inch-diameter valve varies with seat design. Although a hypothetical knife-edge valve seat as on cylinder number-1 gives the largest throat area, note that its efficiency is pathetically low at 45 percent. Number-2 is really the first seat that can actually be used, and at best this is 56 percent efficient.
By streamlining the underside of the 45-degree seat, the situation is further improved when a 30-degree top cut is applied as on number-4. For number-5, a surface is present to constrain at least one side of the jet of air/fuel as it enters the cylinder (or exits for the exhaust). With suitable attention to detail, the flow efficiency of an intake valve in the first 0.100 to 0.150 inch can closely approach 100 percent while the exhaust, at higher lift values, can actually exceed 100 percent by virtue of nozzling. That is, the seat and port is starting to emulate a nozzle similar to a venturi of a rocket nozzle. All this only comes about by virtue of a well-developed form before and after the seat.
Working Valve Seat Shapes
All the forgoing begs the question: What shape should you use for best results? Ultimately, each application may need something a little different to suit the particular head. That said, the seat form in Figure 8.6 can be made to work very well. SAE efficiency figures at as much as 0.250 lift can be as high as 84 percent. With a three-, four-, or even five angle valve job, efficiency figures mostly top out at about 76 percent by the time the valve has reached 0.250 lift.
For an exhaust valve to flow well, it is quite imperative that a reasonably generous radius follows the seat. If the seat progresses into a generous radius, having the throat of the exhaust port at as much as 12.5 percent smaller than the exhaust valve itself can be beneficial to flow. Flow efficiency right off the seat is not so critical with an exhaust valve. At low lift, the flow is supersonic, and flow is dictated by area rather than flow efficiency seen on a flow bench. Where the flow of the exhaust valve is more critical is from about 0.100 inch of valve lift on up. That said, be aware that the seat form and the following 0.250 inch can measurably effect flow over the entire lift range for an exhaust port, so getting the seat form right is key factor toward success.
The Top Cut
Making a top cut, as seen in Figure 8.6, on the intake seat helps low-lift flow in almost every case. However, there is a downside to such a top cut. In practice most intake ports flow better in reverse than in the forward direction. A top cut increases this tendency. What we really want is a port that flows well in the right direction but poorly in the wrong direction. If low-speed output is important to you, foregoing a top cut is a move in the right direction.
If high-speed power in the top half of the RPM range is a greater priority, the form following the actual seat needs to allow a slower expansion into the chamber. As of 2012, there has been a growing tendency to make the top cut at 38 degrees on both the intake and the exhaust. Note the dimensions in Figure 8.6, especially the seat width. It is often tempting to reduce the seat width to as narrow as possible to achieve the largest throat diameter possible. The reality is that a 45-degree seat of anything significantly less than 0.055inch width flows less than a wider seat, as does anything significantly more than 0.065. Targeting 0.060 for most applications is about as surefire as it gets.
Alternative Seat Angles
Although a 45-degree seat is the most common, there are good reasons for adopting other angles. For instance, the heads for the 358-ci V-8 engines used in NASCAR’s premier series typically use 50and 55-degree seats. Where an engine is extremely air starved, such as for a typical stroker big-block Chevy or Ford, I sometimes use a 30-degree seat. To understand why, let us first look at the flow capability of a 30versus a 45-degree seat, so we can see what happens as far as area presented to the cylinder during the initial phase of opening.
If you are working with simple plain-angle seat cutters, this design of seat works well. If you have the option, the lower angles can be substituted with a radius that emulates the form shown in Figure 8.8.
Back to Figure 8.7 for a moment. Here is a comparison of the opening area delivered by a 45-degree seat versus a 30-degree seat. The flatter the angle, the quicker through-flow area is presented to the cylinder.
On the face of things, it looks like an open-and-shut case for using 30-degree seats, but things are not that simple. As the angle becomes shallower, the “wedging” action that helps seal the valve to the head is reduced. And the flatter the seat, the greater the tendency for the valve to bounce on closing. Flatter seats are better at low lift, but without some serious geometry studies, they tend to flow worse at higher lifts.
On the other hand, the 50and 55-degree seats used on Cup Car motors tend to deliver better high-lift flow (in conjunction with the steeper downdraft angle these heads have), and the increased wedging action at seating tends to act as a bounce dampener to create a more stable valve-train. At the end of the day, you can see it’s very much a “horses for courses” deal.
No doubt, a 30-degree seat delivers high flow at low lift, and for many, it’s a compelling argument to use it. Factory engineers have used 30-degree seats in many engines. A prime example is the flathead Ford V-8 introduced in 1932 and produced until 1953. Also Pontiac used 30-degree seats extensively in the 326to 455-inch engines built from the late 1950s to the late 1970s.
Granted, 30-degree seats offer considerably more low-lift flow. That extra opening area translates, for a typical intake in the 2.000to 2.400-inch range, to an increase of about 20 percent at lifts just off the seat (declining to zero at about 0.300 lift). Unless suitable care is taken, a 30-degree seat can be down on flow at the higher-lift figures commonly used by a big two-valve engine, such as a typical V-8. Above about 0.600 lift, making the 30-degree seated intake port work can consume quite a lot of flow bench time. However, it can be made to rival a 45or even a 50-degree seat on most occasions if you spend the time perfecting it in detail. The dimensions of a 30-degree seat are as shown in Figure 8.8.
Even though a functional seat (as shown in Figure 8.8) produces excellent flow bench results, an improved performance deal is still far from sealed. There are other issues with a 30-degree seat (or any seat significantly flatter than 45) that you must address to see positive results. First, there can be a sealing problem, especially at RPM much above 5,000. Because of the reduced wedging action as the valve sits down on the seat, leakage can (and most often does) occur at higher RPM. This is especially relevant to heads that have any instability in seat form at the temperatures seen in a high-performance engine at sustained wide-open-throttle (WOT) conditions.
To combat this, I have used what I call a conformation groove in the top face of the valve. For what it’s worth, I have seen this also improve the high RPM sealing capability of regular seats where big temperature differences across the chamber have been suspected. Figure 8.12 shows a conformation groove for a 30-degree seat.
Per the blueprint in Figure 8.15, a conformation groove is almost a given. Also be sure that a 30-degree seat seals; it does not hurt to increase the valve-spring preload by 10 percent or so more than what was needed for a 45-degree seat. Another move is to coat the seats with a high adherence dry-lube film. Tech Line Coatings provides a suitable coating (CermaLube or C-Lube) for this job. By reducing the friction between the seats, the valve can more easily centralize itself and thereby develop a 100-percent seal.
Seats on Valves
The valve seats in the head are only half the equation. Though it is not practical to streamline the exit from the intake valve itself, it is not only practical but very simple to streamline the approach to the intake seat. On the exhaust side of things, the outward-bound gases are usually aided by having a radius between the front face of the valve and the valve margin, plus a 30-degree back cut— same as the intake.
If you use a 30-degree intake (it is not really any advantage on an exhaust in terms of potential power increases), you need to ensure the valve seals at high RPM. This involves the conformation groove mentioned earlier. This, for intake valves in the 1.6to 2.8-inch range, can be cut per the blueprint in Figure 8.15.
For what it’s worth, the conformation groove also acts as a means of anti-reversion. Using even a moderate-performance cam, I have on numerous occasions seen an increase in torque output in the lower-RPM ranges of engines.
One last point on 30-degree seats: It is not that important in most instances to back cut the valve at a shallower angle, such as 20 degrees.
Just as the ports affect the flow, so do the shapes of the valves, but to a much lesser extent. In the 1920s, when building racing engines was in its infancy, it was assumed that a tulip valve was the best because it looks so much more streamlined in form than the lighter and what was to be the more commonly used “penny-on-astick” valve. Here, again, we find that nothing can be taken for granted. For a cylinder head in which the approach is relatively flat (not much down-draft angle) a flat valve not only allows more flow, but it is also considerably lighter.
I have experimented with valve forms, from a spherical back, through the tulip shapes, and on to the nearly flat back. The flow bench can come up with some surprises, but for the most part tulipor spherical-shaped backs on intake valves need to be restricted to use in ports with steep (45 degrees or more) down-draft angles. For all other cases, a relatively flat back of 15 degrees to as nearly flat as 10 degrees gets the job done best.
For exhaust, something more closely resembling a tulip shape often provides not only a flow advantage but also a better heat path to the valve-stem, and as a result heat is removed more effectively. It’s worth emphasizing that keeping the exhaust as cool as possible staves off detonation and, ultimately, detonation fixes the limit on the power a cylinder can make.
Clearances and Temperatures
It is important to keep the valves as cool as possible. The exhaust valve is likely to be the most problematic, so I’ll start with that. First, a great deal of heat from the valve is passed to the head and on to the cooling system via the valve seat when the valve is closed. For this reason, apart from flow, the exhaust valve width needs to be not less than 0.060 inch wide.
Also, as you can see from Figure 8.17, a substantial amount of heat is dissipated through the valve-stem to the guide. Just how well it does that depends on the clearance and the oil film thickness at the guide/ stem interface. Too much clearance and the valve overheats and fails to seal properly. Too little and the valve seizes in the guide. A good working clearance for the exhaust is 0.0018 to 0.0021 inch. For the intake valve, this can be reduced to 0.0014 to 0.0016 inch. Although cast-iron guides are fully functional, I prefer to use a good-quality bronze guide.
The effect of guide clearance on power output is an issue that is well worth bringing up here. Many years ago, while developing the Ford 2-liter SOHC Pinto engine for a book, I had run some tests of stem clearance versus output. The results were surprising. Switching from 0.002 to 0.004 clearance on just the intake dropped the output of a nominal 165-hp mule engine by about 3 hp. Increasing that clearance to 0.006 inch reduced power by no less than 9 hp! This should make the point that valve-stem clearance is an important factor, but what if we can find a way to tighten it up more?
Guess what? I brought this up because I have something to say on the matter. As it happens, the K-Line guide liners (very functional to fix worn guides and highly recommended if done properly) have ultra-high anti-seize properties. It is possible to run an intake valve with a 0.0001 clearance and not have it seize.
On that same Pinto mule engine, I ran a test with the intakes as close to zero clearance as I could make them. The result was a loss of about 4 hp! That begs the question: Why? My thoughts here are that it was almost certainly due to the valve’s reduced ability to center itself on the seat because of the reduced clearance. The real clincher to this theory is that when a conformation groove was machined into the front face of the valve, that lost power returned.
Thermal coatings can be used to manage and mitigate valve heat issues. Coating the front face of an exhaust valve is very effective for the minimal effort involved and coating the entire surface (other than the seat itself) is better yet. I do not have definitive figures of the valve’s bulk temperature reduction, but from prolonged tests I estimate it to be in the region of 150 to 200 degrees F for the exhaust.
On the intake valve, there is little chance of overheating, but we do have a power-influencing factor at work here. First, the intake valve temperatures in a high-output engine under WOT conditions is far higher than you might think. While looking down the stacks of a Cosworth BDP Midget engine with the dyno cell lights out, I noticed that the intakes were glowing dull red.
That indicates something around the 900-degree-F mark. The only reason they did not get even hotter is that the incoming charge was cooling the valves. Putting combustion heat into the incoming charge is not what we want for power. When the front face of the valve receives a good thermal barrier coating, the intake valve heat is reduced by at least 100 degrees. Doing the whole exposed valve surface is even better. On that same Cosworth BDP, treating the entire valve surface resulted in an increase of 2 to 4 hp.
Cutting Valve Seats
I have focused on the need to optimize valve seat form, so it’s worth saying a few words about seat cutting equipment. At one time or another, I have used almost all the seat-cutting machine/equipment out there. At the time of this writing, my number-one choice for getting a really superb job done is Newen’s CNC single-point cutter machine.
The beauty of this machine is that it allows you to design a seat right on the machine, and the accuracy is phenomenal. For the record, most of my work on seats was done using a Serdi 100 seat and guide machine. I was very happy with the results of this machine, but it was necessary to get whatever seat form was required ground on a cutter. These were not that expensive, considering they were precision carbide pieces, but when you are testing seat forms, the bill for multiple cutters can become a sizable amount very quickly.
As it happens, Serdi-style cutters are about standard for the industry. Here, I can recommend high-flow form cutters at a good price from Goodson. If you want to cut your own seats, entry-level seat cutting equipment can be had from Soiux, Neway, and a few others. If you want to step up a little in cost, but still be far from the $40,000 budget for high-end equipment, check out the head-mounted Hall-Toledo orbital grinder. Once the seat you require has been formed, it does an outstandingly good job of replicating it on the head.
Written by David Vizard and Posted with Permission of CarTechBooks