Here is good bit of info (for all brands)

Unleashing An Engine's Performance Potential

By Larry Carley, Contributing Editor

Cylinder Heads are the Key to Real Horsepower Gains When Building a Performance Engine

If you want to win races, you have to get ahead. As every serious drag racer knows, cylinder heads are the key to real horsepower gains when building a performance engine. Whether an engine is being built for the street, street/strip combination or strip only, prepping the heads is the foundation to unleashing an engine's performance potential.

Every ounce of air and fuel that enters and exits the engine's combustion chambers flows through the heads. The size, length and shape of the intake and exhaust ports, the size and angle of the valves, the restrictions in the ports created by the valve stems and guides, and even the surface finish of the ports all influence how well the heads breathe.

Air flow determines an engine's personality and power curve. In theory, the more air an engine breathes in cubic feet per minute, the more power it makes. But in the real world, things aren't so simple. Air velocity is just as important as air volume, especially for street-driven vehicles and off-the-line torque in larger cars and trucks with automatic transmissions.

At low rpm, an engine actually breathes better if air velocity in the ports is kept high. Air entering at high speed fills the cylinders more quickly and efficiently. That's why stock engines have relatively small intake ports and valves. It boosts low rpm air velocity and torque. Hogging out the ports on a relatively stock engine that's going to be driven on the street would therefore be counterproductive and impair low-end performance.

Stock street engines operate at relatively low rpm and need more low-end torque. A typical street engine's power curve might go from 1,500 to 5,500 rpm with horsepower and torque maxing out at around 4,500 rpm. By comparison, a performance engine built for the strip might have a power curve that starts at 3,000 to 3,500 rpm and goes up to 7,500 rpm or higher, depending on the rpm potential of the valvetrain. The engine's peak power output might occur from 6,500 to 7,500 rpm or higher, again depending on how the engine is built.

So for high-rpm power, which is what drag racing is all about, big ports and valves are needed to handle the increased air flow. Air velocity at high rpm is maintained by the extra volume of air that's being sucked into the engine.

The type of heads that work best on a given engine application will depend on the other modifications that are being made to the engine. For street performance, a short to moderate duration, high- lift cam, headers with small diameter primary tubes, an intake manifold with long individual runners and a split plenum are a good combination with a set of stock or slightly modified heads. For the strip, a long duration, high-lift cam, large tube headers and an open-plenum intake manifold with a set of reworked heads or high-flow aftermarket performance heads will yield the winning combination.

The key point to remember about head port and valve sizing is that small ports and small valves increase air velocity for low-to-medium range torque, while big ports and big valves increase air flow for high-rpm power.

BASIC HEAD PREP
Before you spend a dime on cylinder head modifications, you first have to make sure the castings are free from any defects that could cause problems later on. After the heads have been
disassembled and cleaned, they should be checked for cracks and leaks.

Magnetic powder and a powerful magnet should be used to check cast iron heads for cracks, especially in the combustion chambers, under the valve seats, below the spring seats and around the intake and exhaust ports. The heads should also be pressure tested with air to check for leaks that might not be visible in exposed areas. Aluminum heads should be inspected for cracks using penetrating dye, and then pressure tested to check for hidden leaks and porosity leaks.

Small cracks and leaks in cast iron heads can usually be sealed by pinning. The same technique can also be used to repair aluminum heads, but larger cracks may have to be welded using a TIG welder. Serious cracks or other damage in cast iron heads can sometimes be repaired by furnace welding, but it's hard to find shops with the necessary equipment and expertise to handle the job. So if repairs are not possible, replacing the head(s) is an only option.

HEAD PORTING If you're building a street engine with stock heads on a limited budget, your engine's power potential is going to be limited by the breathing characteristics of the stock heads. Even so, there's a lot that can be done to improve air flow without spending big bucks.

One of the simplest modifications is to use a die grinder to blend the bowl area under each valve with its port. The castings on some engines can be rather rough, so smoothing out the area where the port and bowl come together can improve air flow across all rpm ranges.

Another change that can improve air flow is to reduce the amount of restriction in the bowl area by narrowing the guide boss, shortening the amount of guide that protrudes into the port, and replacing the stock valves with performance valves that have necked down stems just above the valve head.

Matching the ports to the intake and exhaust manifolds can also eliminate sharp edges and disruptions to smooth air flow. This can be done by using the manifold gaskets as templates to check for proper alignment. A die grinder can then be used to remove any metal that protrudes into the port area as indicated by the opening in the gasket, and to gently blend the port opening into the rest of the port.

One thing to avoid is any abrupt change in the cross section or diameter of the port. There should be a smooth transition from the port entry all the way to the valve opening. Changes in cross section will cause a corresponding change in air velocity, so the idea is to start out with a large opening and gradually taper it toward the valve so air velocity increases as it flows past the valve.

The next step up would be to do a "port and polish" job on the cylinder heads. Porting the heads enlarges the size of the ports all the way through to increase air flow. Polishing removes all the casting roughness in the ports, bowls and combustion chamber to create a smooth mirrorlike finish. But on street engines, a little surface roughness in the intake ports actually improves air flow and air/fuel mixing.

Before CNC (computer-controlled numeric) porting machines became popular, head porting was a time-consuming, laborious and expensive process because all the work had to be done by hand. To make matters worse, there was always the danger of grinding the casting too thin and grinding through into a water jacket. The job also required a lot of know-how, such as where to grind and where not to grind to improve the flow characteristics of the head.

CNC changed all of that. With CNC porting, a port profile is first developed the old-fashioned way with a hand grinder on a flow bench for one cylinder. Once the flow characteristics have been optimized, special measuring equipment is used to convert the port profile into a three-dimensional electronic map that a CNC machine can follow to duplicate the profile again and again. The head is then placed in the CNC machine, and the ports are reshaped by milling and grinding away metal. CNC has transformed head porting from an art into a precise science and eliminated much of the time-consuming labor from the job. It's also brought the price down to where almost anyone can afford a set of professionally ported heads.

Another method of porting is an extrusion process that pushes an abrasive putty through the ports to smooth and enlarge them. The "extrude hone" process, as it is called, is performed by special equipment that controls the extrusion pressure, flow rate and volume of the media that is pumped through the ports. The neat thing about this process is that it can easily reach places that conventional grinding equipment cannot - like the inside of long runner intake manifolds.

Acid porting is another method that can enlarge head ports. Muriatic acid is used for aluminum heads, and hydrochloric acid is used for cast iron heads. This technique requires experience, great care and is potentially dangerous because of the corrosive nature of acid (in other words, don't try it yourself!).

With this process, the port exits are blocked with a glass plate. The head is then turned upside down and acid is poured into the ports through the valve opening. The level of the acid is kept below the bowl so it doesn't remove metal in this area or eat into the seats. The amount of metal eaten away by the acid will vary, and depends on the length of time the metal is exposed to the acid. After about 10 minutes, the ports may be enlarged up to 10 percent or more. Once the desired amount of metal has been removed, the acid is dumped and the ports are washed out with water to stop the etching process. If the acid is left in the ports too long, it may eat through into the water jackets and ruin the head, so timing is critical.

COMBUSTION CHAMBERS The shape of the combustion chamber is critical because it affects the mixing and burning characteristics of the air and fuel. Opening up the chamber around the valves ("unshrouding" the valves) generally improves breathing but the amount of metal that can be removed is limited by the thickness of the casting. The size of the combustion chamber affects the engine's compression ratio, and for drag racing, more compression equals more power (up to a point). Too much compression increases the risk of detonation even with high octane fuel, which can ruin a high-rpm engine in short order. For gasoline fueled engines, 14:1 compression is about the limit for high-octane gas. Alcohol can generally handle a couple of points higher because of its higher octane rating.

One way to run more compression is to increase swirl in the combustion chamber, achieved by either modifying the shape of the chamber and intake ports, and/or by using pistons with a crown design that promotes swirl.

A simple modification anyone with a die grinder can make to reduce the danger of detonation and preignition is to smooth and blend any sharp edges within the combustion chamber itself. Sharp edges can become hot spots that make fuel explode prematurely and spontaneously, causing engine-damaging and power-robbing detonation (spark knock).

Something else that should be done is to equalize the volume of the combustion chambers. This is called "cc-ing" the head because it involves measuring the cubic centimeters (cc) of water each chamber holds, then grinding away metal so all the chambers have the same displacement.

BIGGER VALVES OR AFTERMARKET HEADS? Though a machine shop may be able to install larger diameter valves and seats in some heads, most late-model castings are too thin to accommodate such modifications. If more valve area is needed, the only option may be to replace existing heads with OEM performance heads or buy a set of aftermarket performance heads such as those manufactured by Brodix, Dart, Edelbrock and others. Aftermarket heads are available in cast iron and aluminum, and are available in street-legal trim with exhaust crossover ports and "race only" configurations.

The nice thing about many aftermarket performance heads is that the heads need little or no additional prepping - at least not for the typical street or street/strip application. The ports have already been enlarged and reshaped to optimize flow, and the combustion chambers sized for a specific compression ratio.

One thing to keep in mind about some aftermarket heads is that they may not mate with a stock manifold or an aftermarket manifold designed for a stock head. The location of the ports and bolt holes may be such that a special manifold and/or headers may be required.

VALVES & SEATS For higher rpm, stronger one-piece stainless steel valves are a must. Some stock valves are a two-piece welded construction that may pull apart at high rpm under the stresses imposed by drag racing. High-performance valves are usually swirl polished to enhance flow, and some have a necked-down stem just above the valve head for the same reason.

Reducing the weight of the valvetrain by installing titanium valves can also increase an engine's rpm potential, but for the average racer who's on a budget, titanium valves are a luxury, not a necessity.

Along with stronger, lighter valves, stronger springs and retainers are a must. If the rocker arm studs are not the screw-in variety, the studs should be crossdrilled and pinned to keep them from pulling out of the head at high rpm.

Valve seats must be as concentric as possible for a tight compression seal and proper valve cooling. The rounder the seat, the better. Seat runout should not exceed .001-inch per inch of seat diameter. Some shops aim for .0005 inches or less of runout. The best way to check concentricity is with a runout gauge. Pulling vacuum on the valve port with the valve in place is another method for checking the mating of the seat and valve. But the ability to hold vacuum is no guarantee of concentricity in itself. That's why both methods should be used to check concentricity.

It goes without saying that both the valves and seats require a three-angle valve job (60-, 45- and 30-degree cut) to improve air flow. For serious racing, blending the edges of the 60/45 and 45/30 corners on the valve face and seat to create a five-angle job will enhance airflow a little more.

Seat depth and installed valve height are two more dimensions that must be watched carefully to maintain correct valvetrain geometry. On OHC heads that have mini-hydraulic adjusters, installed valve height is critical for proper valve lash. When valves and seats are refaced, the valves sit deeper in the head than before. This causes the stems to stick up higher which changes the rocker arm geometry and may lead to a loss of valve lash when the engine gets hot. If the proper geometry can't be restored by grinding the tips of the valve stems (no more than about .010 maximum or you run the risk of grinding through the case-hardened layer), the seats should be replaced to correct installed height.

VALVE GUIDES & SEALS At 6,000 rpm, the valves are opening and closing 50 times a second. Each valve is only on its seat for about one hundredth of a second between openings. Since most of a valve's heat is conducted away through the seat, it's obvious the valve isn't going to get much cooling at high rpm - especially in an engine with a long duration cam and narrow valve seats. So valve cooling through the valve stem takes on added importance.

One of the functions of a valve guide, besides positioning and supporting the valve stem, is to help cool the valve. Somewhere between 15 to 30 percent of the valve's heat is conducted up through the stem to the guide. At higher rpm, this cooling is critical.

The amount of clearance between the valve stem and guide will obviously affect the rate at which heat can conduct away from the valve stem. The tighter the fit, the more rapidly heat can pass from stem to guide. But here's the catch. Performance engines generally need looser clearances because of their higher operating temperatures and speeds. If the guides are too tight, there's the danger of sticking. If they're too loose, the valves can wobble on their seats, drastically shortening the life of both valves and seats. Poor heat transfer between stem and guide can also make the exhaust valves run hot and cause preignition or detonation. Too much clearance between the stems and guides will allow oil to be sucked past the guides. This can become a source of spark plug fouling and misfiring.

The trick, of course, is to run the guides just a little looser than stock but not too loose. Generally speaking, factory specs call for .001 to .003 inches between valve and stem on the intakes, with an additional .0005 of clearance for the exhausts. Running on the high side of the specs usually provides sufficient clearance for high-performance street applications without sacrificing too much heat transfer or increasing oil consumption.

To control oil, many performance engine builders machine the tops of the guides to accept positive valve seals. Some use them only on the intakes while others use them on both intakes and exhausts.

Many engine builders say the best thing to do on cast iron heads with integral guides is to ream out the guides and install bronze wall liners whether the original guides are worn or not. On heads with replaceable guides (such as big block Chevys and aluminum heads), they recommend replacing the cast iron guides with silicone-bronze.

Performance engine builders prefer bronze valve guides and guide liners because the metal's anti-seize characteristics and its low coefficient of friction eliminate most of the worry about high-speed galling and valve sticking. Bronze liners can handle tighter clearances for improved valve stem cooling, reduced oil consumption and more accurate valve seating. Bronze also conducts heat better than cast iron which can actually improve valve cooling through the stem.

Whether you choose to go with bronze guides or not, it's important to check the old guides for wear and misalignment. Worn exhaust guides are pretty common, but so too are worn intake guides. Worn guides suck air which can upset the air/fuel mixture, and they'll also suck oil (a lot of oil) which can foul spark plugs. Worn guides also allow unwanted valve movement, which can lead to stem flexing and breakage.

An oval-shaped guide can result from incorrectly installed valve height (either too high or too low) after grinding the valve face and seat. If the rocker arm geometry isn't right, it will push the valve sideways when the valve opens, creating an egg-shaped wear pattern.

HEAD REFACING The final step when prepping a race head is refacing. The head must be smooth and flat for a good seal with the head gasket. On some high-horsepower engines (more than 450 to 500 hp), it may be necessary to go with an O-ring style head gasket to improve sealing reliability. A small groove is machined around each combustion chamber to accommodate an O-ring style head gasket.

One thing to keep in mind is that milling the head also increases compression. This needs to be taken into account so you don't end up with too much compression, resulting in detonation trouble.