This cutaway illustrates the...
This cutaway illustrates the short versus long side of the valve. Air behaves as a fluid when flowing, so imagine a garden hose blowing water down the port. The water would tend to rush toward the far side of the valve and only begrudgingly exit through the short side once the far side is at capacity.
Porting gets all the attention when it comes to preparing cylinder heads to rocket more air fuel into the chamber. Granted, a well planned and executed port job can turn stifling heads into free-flowing ones that allow an engine to be a more efficient air pump, but there's one small, but oh-so important detail that never gets the credit it should: the valve job.
No matter how good or bad a cylinder head's runners flow, the biggest impediment is always the valve. All that air rushing in from the intake has to slip through a tiny curtain of space that's equivalent to the area between the bottom of the valve at peak lift and the valve seat. That's why camshaft lift and duration are the two factors that can most alter an engine's performance and operating range; the larger the curtain and the longer it stays open, the more air and fuel can flow into the combustion chamber.
But there's more to it than that. Air hates to change direction, especially fast-moving air, and forcing it to do so will decrease airflow. To help ease the transition, valves and valve seats have angles cut into them to make the air curve. This is especially important on what's known as the "short side" of the valve, which is the side of the valve seat that rolls from the floor of the port. Since air abhors a curve, it wants to take the straighter path toward the far side of the valve. Consequently, that side of the valve is generally jam-packed with all the air it can handle and then some. That's where a good valve job can show the largest gains-by convincing more air to take the side less traveled and mitigating the bottleneck.
Troy Bowen at Ford Performance Solutions (FPS) in Anaheim, California, says a proper performance valve job is critical-even more so than porting in some ways-and he volunteered to show us how a basic performance valve job is performed on FPS's own Avenger aluminum cylinder heads. We may be starting with a high-performance head, but the concepts will work for any other aftermarket or OEM head as well.
There's one other massive benefit to a performance valve job. Porting is time intensive and therefore expensive, but a good performance valve job can get you nearly 50 percent of the way there for a fraction of the cost of a full porting job. Whereas fully porting a set of heads can run from $500-$1,000 or more, a performance valve job is a bargain at less than half the price. FPS charges $179 for their own heads, $225 for most inline valve heads, and $260 for canted-valve heads.
Granted, unless you're working...
Granted, unless you're working on prewar antiques, you're unlikely to ever see a transition as stark as image 1, but it does create an excellent visual aid for how flow increases with less dramatic valve seat angles. Image 2 is similar to many early heads with a single 45-degree valve seat, image 3 is a three-angle cut, and image 4 represents a four-angle similar to what FPS uses with 30-, 45-, 60-, and 75-degree cuts from top to bottom.
The rationale out there is that if angles work to smooth out the flow, then cutting more should be even better. It sounds good in theory, but in reality there is a point of diminishing returns-especially on mild performance engines. Troy Bowen prefers four cuts on the seat of varying degrees, depending on the head for most combinations. As for more angles affecting longevity, Bowen says it's a myth. As long as the valve seat is the appropriate width of around .045-.060, longevity shouldn't be an issue.
Angles vs. Radius
Why does the exhaust get a radius rather than angle cuts on the seat? Troy Bowen says it's due to the sheer speed and pressure of the exiting exhaust gas. "We use a flow bench to measure the air going into and out of the head at 28 inches of water with the velocity at approximately 100 mph. These machines are trying to simulate a running engine, but this is not the environment that a head really functions in. After the explosion in the combustion chamber, extreme pressure is created and when the exhaust valve opens, the air is pushed out at speeds well above what the bench can create, making it impossible to simulate on a flow bench. There are guys working on computer models to find out how to optimize the exhaust." Basically, angles don't really help in this case since the gas is forcefully flowing and a well-formed radius is employed to simply funnel gas out of the chamber.
Our valve job victim is one...
Our valve job victim is one of FPS's Avenger 190 heads for small-block Chevy. If working with used heads, it's a good idea to have them magnafluxed to check for cracks, and the valves, seats, and guides inspected and replaced if excessively worn. It's especially critical to address the valve seats if they've been cut previously, as a second cutting could result in sunken valves, which would nullify the valve job gains.
In as-cast form when they...
In as-cast form when they arrive at FPS, the heads already have a basic valve job. They work well in this form, and occasionally racers buy them like this to perform their own top-secret valve jobs, but typically all heads receive a performance valve job before leaving FPS.
The results of the stock valve...
The results of the stock valve job were respectable with a peak of 244 cfm at .600-inch lift on the intake, and 188 cfm at .700-inch lift on the exhaust.
To establish a baseline, the...
To establish a baseline, the Avengers were strapped to FPS's in-house SuperFlow flow bench with 2.02 and 1.60 REV valves, and flowed at 28 inches of water.
Beyond the expensive machinery...
Beyond the expensive machinery involved, here's why experience is a must; this Goodson chart shows their line of angle cutting blades. Do you know which ones you need?
We've seen plenty of valve...
We've seen plenty of valve jobs cut by hand with grinding stones on a drill, but accuracy can be questionable for all but the most experienced hands. For precision, all heads are strapped to FPS's trusty Sunnen. As for accuracy of the machine, FPS keeps their equipment well maintained to ensure tolerances are tight.
To begin, Troy Bowen uses...
To begin, Troy Bowen uses this blade to sweep out the chamber a bit and unshroud the valves to increase airflow. This is especially necessary when using larger valves, but there is a fine point between unshrouding for performance gains and taking too much out and going backward.
Here's the blade of choice...
Here's the blade of choice for the small-block Chevy Avenger. It's hard to see even in person, but trust us, there are four angles on the blade-30, 45, 60, and 75 degrees in this case.
While the intake seat receives...
While the intake seat receives four angles, Bowen only gives the valve itself two angles-the 45-degree seat and a 30-degree back-cut. Why? In testing, Bowen has found that extra cuts rarely help peak power, and can even adversely affect performance on the top end.
Bowen follows up with a bowl...
Bowen follows up with a bowl cutter to ensure the transition into the first angle is smooth. Care must be taken here though, Bowen warns. Opening up the bowl too much can cause low-lift flow to suffer.
Rather than progressive angles,...
Rather than progressive angles, the exhaust gets a much simpler profile with a 45-degree valve seat followed by a radius to allow the pressurized exhaust gas to exit quickly.
In finished form, it's difficult...
In finished form, it's difficult to see the minute angles, but the small amount of material removed will make all the difference for airflow.
If you look closely you can...
If you look closely you can spot the back-cut on the foremost intake valve. Thirty degrees is good for most street engines since it improves low-lift flow, but some racers opt for slightly different angles since the back-cut can affect the flow at each level of valve lift.
Undercutting involves reducing...
Undercutting involves reducing the diameter of the valve stem in an attempt to lessen the intrusion the air has to flow around. Bowen says it's more hype than help in most cases, and he typically doesn't undercut the valves unless asked by the customer.
Back on the flow bench, peak...
Back on the flow bench, peak intake flow rose to 253 cfm at .600-inch lift and 203 cfm at .700-inch lift on the exhaust. There are a lot of variables to consider, including differences in engine combinations and cylinder head design, but Bowen says he's seen similar flow increases net an easy 20-30 hp on relatively mild naturally aspirated small-blocks.