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The Inside Angle on Valve Seats: What you need to know to go with the flow
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Another factor that has a huge impact on airflow through the valve port and seat is the ratio of the size of the valve opening to the size of the throat area just below the base of the valve seat (as measured from the largest area at the bottom of the valve seat). The number one rule here, says Mondello, is having the optimum ratio that maximizes air velocity through the “primary choke” area in the valve bowl just under the seat.
“A lot of the aftermarket performance heads today are typically made with a 91 to 92 percent throat area, which, in my opinion, is too big,” says Mondello. “To get maximum velocity and airflow, the ratio needs to be a little smaller. On many heads, a throat dimension-to-valve size percentage of 86-1/2 to 89 flows best. On Harley motorcycle heads, we use 89 percent. On big displacement high flow V8 racing heads with valve sizes from 2.100˝ to 2.25˝, we may go as high as 90 to 91 percent. We try to keep the choke area as small as we can so it will flow efficiently and improve velocity in the port runner.”
He explains, “If you have big ports in a cylinder head but only a so-so valve seat, the head won’t flow as well as it could. Engines run on velocity, not cfm airflow numbers. The right valve angles will have a supercharging effect that really helps ram more air and fuel into the cylinder. That’s what gives you instant throttle response instead of a bog when you stomp down on the gas pedal.”
Mondello says a bowl cutter should be used to achieve the proper percentage for the valve-to-throat dimension size. Even if a performance engine has to run under a “no porting” rule, the bowl area under the seat can be cleaned up to improve airflow. Reworking this area can often improve airflow an additional 8 to 14 cfm.
Is Good Quality Valve Work Possible With Outdated Equipment?
Mondello chastises shops that are still using old, worn-out equipment to do what they call “performance” valve jobs. “It isn’t really performance work if the equipment can’t hold tight tolerances,” he says. “A tolerance of a couple thousandths of an inch is not close enough when performance valve work requires holding tolerances to tenths of a thousandth. The valve-and-seat machine bearings and pilots have to be in good condition.”
The concentricity of the valve seats to the valve guides is critical not only for proper valve seating and sealing, but also for the longevity of the valves. Misalignment between the valves and seats forces the valve stem to flex every time the valve seats. Eventually, this can lead to metal fatigue and valve failure. So the seats have to be as concentric to the guides as possible.
Mondello said the valve guide pilots that some shops use have too much play for accurate valve work. A valve-to-guide tolerance of .0004˝ is too much for performance work. It should be down around .0002˝ or less. One way to achieve that is to use a high-pressure lubricant on the pilot. Mondello prefers a lubricant called CMD-3, which can handle up to 50,000 psi. The same lubricant can also be used on a dead pilot to take slop out of the valve cutting system.
Chatter is another problem that can ruin a performance valve job. Chatter can be caused by three things: too much play in the pilot (or to the guide if the valve-and-seat machine uses a live pilot versus a dead pilot) or the speed of the cutter. However another culprit may be the casue and have nothing to do with the pilot or the cutter: examine how level your machine actually is compared to what you think it is.
“If your valve-and-seat machine is off-kilter just a bit, the valve seat cutter can chatter when it cuts the seat,” Mondello explains. “When was the last time you leveled your machine? More importantly, when was the last time you leveled your level?”
The Living Legend continues: “Many people don’t know that a level is not flat on the bottom. It has a bow in it. Most people also don’t know how to check their level. You can’t accurately level your valve-and-seat machine if the level you are using is off. When you set your level, make sure the little level is always to your left. If you turn it around, you will get a different reading.”
Mondello says using a lubricant when cutting hard seats will also reduce chatter. He uses a 2020 Mondello Signature Series cutting fluid for this purpose, and 2030 Mondello Signature Series fluidwhen refacing titanium and stainless steel valves.
The type of seat material used will depend on the application. Racers running titanium valves typically use a beryllium-copper alloy seat, or one of the new beryllium-free copper-nickel alloys. Beryllium-copper and copper-nickel seats have a high rate of thermal conductivity, and are a must for high revving, high power engines with titanium valves.
Mondello says one of his favorite materials for valve seats is powder metal. He says he uses powder metal seats from a leading U.S. manufacturer in many of the heads he rebuilds. He likes the powder metal seats for a variety of reasons: they have great machinability when they are new, they produce very little chatter when you cut them, they have built-in lubricity for the valves, they hold up well, and they are made in the USA.
Mondello urges engine builders to be very vigilant about their suppliers, because in his opinion, not all manufacturers pay close enough attention to their alloys. Consequently, the hardness of the seats may vary greatly even within the same size seat.
Keeping the seats in is just as important as the angles that are cut on the seats. Mondello says one of the most common problems he sees are engine builders not using the proper press fit when they install valve seats. “You should always preheat aluminum heads and freeze the seats prior to installing them (Mondello does not recommend preheating iron heads).
I recommend .005˝ to .0065˝ of interference when installing seats in a cast iron head, and .0065˝ to .007˝ in an aluminum head, unless the seats are Beryllium Copper, in which case I recommend .004˝ to .0045˝ of interference fit. Some guys tell me .007˝ to .0065˝ of interference in an aluminum head is too much. But you know what? I’ve never had a seat drop out, not even after an engine got too hot and overheated.”
Mondello says to achieve a good fit, you need a good hole (correct dimensions, smooth finish and no distortion or damage to the seat counterbore). You also have to use a lubricant when driving in the seat.
“I don’t believe in using locking compound on valve seats. It interferes with the heat transfer. All you need is a little assembly lube,” he says.
Mondello says shape is important too. “The valve seat should also have a chamfer on the bottom edge. If you’re finding your seats are straight cut with a flat bottom. You need to put a bevel on those seats before you install them. And regardless of what type of seat you are installing, always use a pilot and driver. Some guys will just beat the seats in using a driver and a puck. That’s asking for trouble. Use a pilot to align the seat so it goes in straight.”
Mondello cautions against using excessive heat when preheating a cylinder head to install new seats. “You only need to heat the head up to about 160 to 180 degrees F. If you get it too hot, say 200 to 250 degrees F, the guides can move around and change the alignment between the guide and seat. When we do Harley heads, we preheat the head on an electric stove. You can also use a hot plate or a torch for the same purpose.”
To be sure, Mondello urges precision. “Use an infrared thermometer to check the temperature. We put the seats in a freezer while the head is being warmed up. That makes it very easy to install the seats. You can almost drop them into place.”
Mondello says you have to install the valve guides before you do the seats. The position of the guides determines the position of the seats, so once the guides are in place, they can be used to pilot the position of the seats.
“On Harley heads, we drive the valve guides into the head from the top (from the spring seats). On most other V8 heads, we also drive the guides in from the top (spring seat side). On early 396/427 Chevys where the guide exhaust guide is stepped, you have to drive out the old exhaust guide from the combustion chamber. New intake and exhaust guides can then be driven in from the top side.”
The key to performance valve work, therefore, is knowing the angles, having a valve-and-seat machine that’s in good condition and can maintain tight tolerances, and paying attention to details.
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