The cylinder head largely controls the all-important process of burning the air/ fuel mixture. If your engine is to operate at the optimum, the heads must be properly inspected, measured, and corrected if necessary. Blueprinting heads involves attention to several areas, including deck surface, intake and exhaust port shape, valve selection, valve seating depth, and combustion chamber volume.
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Many of today’s OEM heads offer impressive performance and have the potential to be modified to further enhance performance. The GM LS engine family is a good example. LS heads flow some pretty impressive numbers straight out of Detroit. The L92/LS3 head, for example, can be further enhanced via CNC porting to increase intake and exhaust runner volume, multi-angle valve seat machining for superior valve seating and flow, and lightweight valvetrain components to enhance engine speed/acceleration.
Cast Iron versus Aluminum
In an apples-to-apples comparison, aluminum heads are lighter than castiron heads. So, if your goal is to reduce weight, aluminum is a no-brainer choice. Another variable relates to potential detonation. Since aluminum releases heat faster than cast iron, the theory is that aluminum heads are less likely to promote detonation. As a result, you can run a slightly higher compression ratio on the same fuel. Generally speaking, you can run upwards of 11:1 compression ratio with aluminum heads on premium 92-octane pump gas, whereas you may be limited to around 10.5:1 with iron heads.
In the old days, when OEM aluminum heads were installed, there were common problems with head warping. However, with the higher quality of metallurgy in today’s aftermarket aluminum heads, this problem has been greatly minimized.
Another thing to consider (and it’s only a consideration, not a problem or negative) is that aluminum is a softer material than iron. So more attention must be paid to areas such as female threads and specific pressure points such as valvespring seat areas. With a quality aluminum casting, as long as you don’t cross-thread or overtorque fasteners, you shouldn’t have any problems with thread stripping. And the same holds true for cast-iron heads.
However, some builders feel more confident by installing harder stainless steel thread inserts in areas such as spark plug holes, intake manifold bolt holes, valve cover bolt holes, and accessorymounting bolt holes (for alternators, power steering brackets, etc.). I’ve built many engines with aftermarket aluminum heads and have never stripped-out a single bolt-hole thread. I see no reason to install thread inserts unless it’s necessary to perform a repair.
Valvesprings are a different story. A cast-iron head may offer enough hardness in the parent casting to allow valvesprings to mate directly to the spring pocket. But with aluminum heads, always install hardened-steel spring seats or cups between the aluminum and the spring. This prevents the springs from digging into the aluminum. This not only prevents damage to the aluminum surface but prevents the spring installed height from changing.
Also, when installing any aluminum cylinder head, you must use hardened washers and lubrication under all bolt heads or nuts (if using head studs) because of the softer material. The importance of torquing the cylinder head fasteners is just as important with aluminum as with cast iron. Always follow the head manufacturer’s specs for torque value, tightening steps, and tightening sequence.
Other advantages of aluminum heads include easier repairs. It’s much easier to TIG-weld aluminum than to weld repair cast iron. Aluminum heads also can be heat-straightened if warped, with much less concern about stress cracking.
Today’s cylinder head technology has been elevated by quantum leaps compared to OEM and even aftermarket heads from only a decade ago. Outstanding performance heads are available from industry leaders such as Dart, Trick Flow, Brodix, AFR, Edelbrock, RHS, Kaufman Racing (Pontiac), Indy, Koffel (Mopar), etc. Cylinder heads are available for most applications, in a wide variety of options, including bare/unfinished, finished but bare, CNC ported, as-cast with CNCprofiled chambers, and fully loaded bolt-on and go versions.
Deck Surface Finish
The cylinder-head deck surface finish is measured in Ra (roughness average). The lower the number, the finer the finish. Generally speaking, for cast-iron heads, the accepted range is around 30 to 110 Ra. With aluminum heads, the finish needs to be finer, ranging from 30 to 60 Ra, and with some heads even finer at 20 to 30 Ra. A precision measuring instrument, referred to as a profil-o-meter, is used to measure surface finish. Today’s precision resurfacing machines can be calibrated for the desired finish.
Check every cylinder head, new or used, for deck flatness. This is particularly important for heads that have been previously heat cycled on an engine. With the deck surface clean, use a precision machinist’s straightedge (a hardware store ruler won’t do) and a feeler gauge. Always check with the manufacturer’s specifications for maximum allowable deviation, but generally shoot for a maximum deviation of .002 inch.
Flatness must be checked along the length of the head from front to rear, on intake and exhaust sides of the main deck, and diagonally from corner to corner in an X pattern. MLS (multi-layer steel) head gaskets are more sensitive to deck flatness, in some cases requiring even a tighter maximum allowable flatness. Although some OEM specs may suggest a slightly more acceptable out-of-flat condition on V-6 or inline-6 heads, for performance use, it’s best to use about .002 as the maximum.
When a cylinder head deck (or block deck) is milled, the intake deck drops, which can create problems with port alignment and manifold bolt hole alignment. A rule for 90-degree Chevys (smalland big-block) is that for every .005 inch removed from the block or head deck, remove about .006 inch from the cylinder head’s intake deck, and about .008 inch from the manifold’s end rails.
As you can imagine, trigonometry formulas are involved here, and I simply don’t have the space to go into detail on this calculation. The easy way is to take advantage of specialty machining fixtures available from sources such as BHJ.
Intake valves are normally larger in diameter than the exhaust valves for a reason. As the piston moves away from the deck, this creates a vacuum pull for the intake air/fuel. The larger the intake valve, the more air/fuel can be pulled into the combustion area. As the piston moves up on the exhaust stroke it pushes exhaust out, so the exhaust valve doesn’t need to be as large (pushing air is more efficient than pulling air). The specific size of the valves is dictated by displacement and (with regard to the intakes) valve lift. Of course, the real estate available in the combustion chamber limits how large the valve heads can be. (For more details about valves, see Chapter 12.)
Valve Angles and Canted Valves
Intake and exhaust valves are placed at angles relative to the piston centerline for reasons of flow and fit. If the valves are “straight up” (in-line with the piston centerline), the flow area is limited and the head needs to be much taller to accommodate the valves.
When I refer to valve angle, I’m referring to how the valve is angled toward the intake side (with the valve tip angled closer to the intake side and farther away from the exhaust side). This angle is called the valve’s “roll” angle. Commonly used valve roll angles range from 9 to 23 degrees.
One of the factors considered when designing a head (and its valve angles) is port cross-section. The shallower the valve angle, the larger the cross-section can be. For instance, with a shallower valve angle, the cylinder head can have a taller intake port cross-section.
An example is the LS cathedral-port head (LS1, LS6, LS2, LS3, L92), which has a 15-degree valve angle. Typical early-generation small-block Chevy heads had a 23-degree valve angle. The closer you bring the valve to vertical (zero degree reference), the more you can increase intake port height. Shallow valve angles also allow small-volume combustion chambers.
Some cylinder head designs, such as Trick Flow’s twisted-wedge Ford heads, utilize different angles for intake (15 degrees) and exhaust valves (17 degrees), which differs from the original Ford 20-degree in-line valve angles. These modified valve angles allow the head designer to place the valve in a better position for flow and to move the spark plug location for improved flame travel.
Canted valves have a layout where, in addition to the roll angle, the intake and exhaust valves for the same cylinder are angled relative to each other (viewing the head from overhead, the valve angles differ from left to right), resulting in compound angles. Canting is basically used in order to accommodate valve fit in the head when needed.
In-line valve layouts have all valves at the same angle (identical roll angle). On a twisted-wedge head, for example, roll angles differ between intake and exhaust. Canted valves are common on big-block Chevy designs. Hemi applications (with hemispherical piston domes) feature opposing valve angles.
When deciding among valve angles and airflow volumes, it’s really best to consult the cylinder head manufacturer to select the head design that best suits your application. Greater port volume usually accommodates engines with larger displacements and smaller port volume is suited for smaller displacements. Depending on port cross-section, other engine variables, and the intended use of the engine, it’s difficult to make blanket statements.
Valve seats can be cut (or recut) in cast-iron heads, providing the seat area is not damaged. Otherwise, the seat area can be countersunk and a seat insert can be interference fit and installed, which can then be cut with the appropriate angles. Aluminum heads always require hardened seats (seat materials can include ductile iron, silica bronze, beryllium copper, powdered metal, and tungsten carbide).
Typical seat angles in a common three-angle seat has a bottom cut of 60 degrees, a seat cut of 45 degrees, and a top cut of 30 degrees. Top cut refers to the angle location closest to the flow exit (closest to the combustion chamber). Seat cut is between the top and cuts. Bottom cut refers to the angle cut that is positioned deeper, closer to the airflow inlet.
The reason for these three angles is to promote smoothness of the airflow without obstructions (picture a trumpet or tulip shape). The bottom cut (depending on how much material is available) generally has a width of about .100 inch. Seat width (the 45-degree angle) is critical for valve sealing in the closed position and heat transfer (the greater the contact width, the more the potential heat transfer).
For most street-performance, flattappet cam applications, the seat width is cut at the 45-degree angle of about .040 inch for intake valves and about .055 inch for exhaust. As valvespring rates increase, seat widths increase to around .060 inch for intakes and .080 for exhaust valves. With high-pressure springs, as found in all roller cam applications, a narrower seat flows better than a wide seat, but a narrow seat isn’t as durable. Wider seats offer more heat transfer (necessary to pull heat away from the valves), which is why exhaust seat widths need to be greater than intake seat widths.
In order to promote airflow even more, seat transitions can be slightly radiused to allow a smoother path for airflow. This should only be done by a skilled machinist and isn’t necessary at all for a street engine.
Performance valveguides are typically made of cast iron, brass, bronze, aluminum bronze, or powdered metal. Cast-iron heads require no separate guides; the cast-iron material itself is adequate, providing the guide hole is appropriately sized and is concentric with no wear. They can also be restored by oversizing the hole and installing a guide insert that is then reamed to size. However, aluminum heads must use guide inserts.
Valveguides wear (become oval shaped) due to the lateral operating pressures (side loads) of the valvestems against the guides (because of the force exerted by the rockers as they sweep across the valvestem tips). The higher the cam lift, the more angular force is applied.
Valveguide height needs to be checked in relation to spring retainer clearance when the valve is fully open (this is when the bottom of the spring retainer is closest to the top of the guide and valve seal). During test fitting, if the bottom of the retainer contacts the top of the guide or seal, clearance must be made by using a different seal or reducing the height of the valveguide. Ideally, you need a minimum of about .060-inch clearance between the retainer and valveguide when the valve is fully open.
A set of quality, high-performance valvesprings should have springs closely matched for height and pressure rating. However, each spring should be checked for open and closed pressure at specified heights, using a valvespring tester.
Quality control of today’s premium springs is excellent, but checking each spring in a set is always a good idea. A spring that falls out of the specified range can be replaced in order to achieve a closely matched set, and greater efficiency.
Ports and Runners
When I refer to ports and runners, the term “port” refers to the hole at the deck of the intake or exhaust path. The “runner” is the “tunnel” path through which the flow occurs. The size and shape of the cylinder head’s intake port directly affects the power curve and torque. It’s important to match the shape and size of the intake manifold port to the cylinder head’s intake port. This promotes a full flow and avoids interruptions and/or restrictions to the flow as it moves from the manifold runner into the head. This is referred to as port matching, where any deviation in the transition from where the intake manifold ports meet the cylinder head intake ports is corrected.
As a result of the casting process, slight shifts can occur where the intake manifold’s ports don’t align perfectly with the ports in the heads. The basic goal of port matching is to eliminate any overhang that interrupts airflow. Along with eliminating any obstructions in the airflow, you want to maximize the flow. For instance, if the intake manifold ports are slightly smaller than the ports in the cylinder head, excess material at the manifold ports can be removed. (See Chapter 15 for more information on port matching.)
Common intake ports are somewhat oval while others are somewhat rectangular (oval ports are usually not a perfect oval and rectangular ports are not perfect rectangles; these are simply terms to describe the type of port shape). Bigger numbers for the intake runner volume may sound cool but, as with other aspects of an engine, bigger isn’t always better. As runner volume increases, so does the amount of air/fuel that can pass through.
Sounds good, but as runner volume increases, this larger area can also slow the air/fuel velocity, which reduces throttle response. Smaller runners promote more velocity and crisper throttle response. The velocity at which the air/ fuel mixture can pass through is critical. The mixture must travel fast enough to keep the mixture in suspension. Otherwise, fuel droplets can fall out and not burn as part of the rich mixture. In other words, you want the fuel/air mixture to travel through as a fog or mist instead of fat raindrops. In a fuel-injection setup, the flow is mostly a “dry flow” of air until it reaches the back of the intake valve, when fuel is introduced.
Add to this equation the engine displacement. As displacement increases, the engine needs more runner volume. But there’s a limit of practicality. If the runners are too large for the engine, you may make more power, but the power is only at higher RPM. Smaller runner volume is generally better for the street; larger volume is generally better for topend speed at the track. You also have other variables, depending on the specific engine: bore/stroke, cam profile, etc.
There’s no such thing as one size fitting all when talking about intake runner volume. As with other aspects of the engine, runner volume needs to be matched to the specific application. That’s where you rely on the recommendations of cylinder head manufacturers.
The exhaust port shape and size is less critical. Basically, as long as the exhaust gas can move out of the head with no restriction at the entrance of the exhaust manifold or primary header tubes, you’re good to go. Keep in mind that the exhaust primary tubes should not be notably larger than the cylinder head’s exhaust ports, though, because this can have a detrimental affect on exhaust scavenging.
Many buyers tend to focus strictly on the port/runner volume numbers (again assuming that more is better). In reality, port length and cross-sectional area tells a more accurate story. A short runner length with a large cross-sectional area can actually flow better than a long runner. Again, this depends on the specific head design and other variables in the engine combination.
Porting and Runner Reshaping
One of the biggest mistakes a builder can make is to go hog-wild by regrinding the inside of the intake runners. Although smoothing the runners and eliminating all rough casting surfaces, lumps, and bumps inside the runners may look really cool, it isn’t necessarily going to make more power. In fact, this approach can kill power.
I can’t make blanket statements here because different engine designs and different head designs are affected, well… differently. In some cases, the head manufacturer intentionally leaves some surfaces rough in order to promote breakup of the air/fuel mixture (to keep it atomized). In other cases, a head can benefit from a smooth, uninterrupted finish. The head design may have included a small hump with a drop-off inside the runner in order to promote swirl. Before you go nuts with a die grinder, first understand what runner shape and surface finish is most beneficial.
Proceed with Caution: Modifying the ports and runners is a critical procedure if you expect to get the most out of your heads. This is a job best left to a skilled professional. Each runner (and port) volume also needs to be equal at each location in the heads. If you don’t have access to a flow bench to accurately measure and monitor runner volume and velocity, don’t touch them. Chances are you’ll do more harm than good.
With that said, there are a couple of mods that are always good to make, including port matching and volume equalization. Always match the intake manifold outlet port to the intake port on the head. This holds true for the intake manifold gasket as well. Make sure that the gasket has enough of a footprint to seal, without overhanging into the intake path, which creates a restriction. If the cylinder head manufacturer recommends a specific intake gasket, it’s best to follow suit.
CNC-Ported Cylinder Heads
No longer are CNC-ported heads reserved only for high-end race programs. In the past several years, and with enhancements in manufacturing technology, CNC-ported cylinder heads have been made a viable option for popular engine applications. CNC porting is more exact than hand porting cylinder heads, with an unlimited ability to exactly duplicate an original shape many times over.
Here are a few of the common advantages of CNC porting:
- Port volume, cross-sectional area, combustion chamber volume, and shape are consistent.
- Valve seats and portscan beblended seamlessly.
- Surface finishcan bemanipulated by machining.
- Cylinder headperformance and flowcharacteristics can be duplicated from engine to engine.
- There is no casting core shift; all features are precisely machined.
Finishing Bare Cylinder Heads
If you buy a set of bare/unfinished heads, size the valveguides and finishcut the valve seats. Bronze valveguides are honed (using a dedicated valveguide hone) to achieve approximately .0016- inch oil clearance with 11/32-inch valvestems (check for recommended stem oil clearance with your specific valves). Intake and exhaust seats are cut to angles of 15, 45, and 60 degrees, using a contour cutter.
The goal is to establish exactly the same depth (valve to seat) for each valve, to eliminate creating different closedvalve chamber volumes. Each valve seat is first rough-cut to accept each valve. The distance from the head deck to the face of the valve is then measured, and each valve position in the head is marked for plus/minus depth difference. By using a single valve location as the reference, each seat is then finish-cut to sink the valves to match the reference valve depth. Again, the goal is to achieve equal chamber volumes.
The seats and bowls are then blended to eliminate any sharp edges or overhangs that disrupt airflow. For this example, teflon valve seals were installed to the .560-inch guides. These seals allow enough room for up to .580- inch lift before coil bind or hitting retainers. Whenever you’re dealing with an aluminum cylinder head, it’s necessary to install a hardened seat at the base of the valvespring, to prevent the spring from digging into the softer aluminum. You also need a locating design to prevent the spring from walking around on the head.
If your heads don’t have machined reliefs at the spring base areas, you can use spring cups (featuring an OD lip to capture the spring’s outer diameter) or spring locators (featuring ID lips to register the spring ID). For example, if your valvesprings have an OD of 1.437 inch and an inner-spring ID of .640 inch, you can use a set of spring locators that fit over .560-inch guides with a .010-inch clearance (the inner locating lip of the spring locators provide an ideal fit to the inner spring ID). Specs for these inside locating shoulder types are .690-inch ID of inner spring, .060-inch locator thickness, 1.550-inch OD, .570-inch ID.
Inspection and Reconditioning
If you’re starting with an OEM engine core, never plan to reuse the stock cylinder head bolts. Most of today’s OEM head bolts are the torqueto-yield type, which can easily lose their elasticity if the heads have ever been serviced. Don’t take a chance. Toss them and replace them with new OEM bolts, high-performance aftermarket head bolts, or high-performance aftermarket head studs.
Because the GM LS engine is extremely popular today for performance builds, include a few LS-specific tips where appropriate. For example, when removing LS heads, be aware of the four inboard 8-mm “pinch” bolts. On a head that is dirty and covered with oil sludge, these bolt heads may be difficult to see. If you’re not aware of them, you can easily damage the head if you attempt to pry it off with force.
Before you disassemble the head, use a mallet to apply a single hit to the tip of each valve. This helps to break the valve locks free from their embedded positions. With the heads removed and placed on a workbench, use a valvespring compressor tool to compress each spring. With the spring compressed about 1/4 inch, remove the pair of valvestem locks using a pick or a pencil magnet. Carefully relax the spring and remove the retainer and spring. Don’t be concerned with keeping the springs in order if you plan to replace them. Even if you plan to reuse the springs, be sure to check them for rate and height anyway; specific location at this point doesn’t matter.
Before removing the valves, perform a vacuum test on each intake port and each exhaust port in order to inspect for existing valve seat sealing. With a valve closed, against its seat, a vacuum pull should hold steady. If vacuum leaks, the valve is not sealing and must be corrected by recutting or replacing the seat and/or the valve. Note the vacuum reading for each seat location as a reference.
Even if you plan to perform a complete seat and valve service, the initial data provides a reference point. If you plan to reuse the original valves, organize them for location reference. A simple method is to drill a series of holes in a wooden yardstick to accept the valves. Place a dividing mark at the center of the stick and label one side for the left bank and the other side for the right bank. Label the holes according to valve location on the heads.
Remove the valveguide seals and discard them. Use a pair of pliers or specialty seal pullers and a twisting motion during removal. If you have aluminum cylinder heads, a steel spring seat is located on each valvespring boss. The steel seats prevent the springs from gouging into the soft aluminum surface. Remove the spring seats. If you plan to use new springs of the same diameter, you may wish to reuse the original seats.
If you plan to reuse the original valves, use a micrometer to measure the stem diameter. Take the measurements immediately below the keeper groove, at the center of the stem and above the fillet. If any deviation exists, this indicates stem wear, so discard the valve. Also inspect each valve for runout by resting the stem on V blocks and checking with a dial indicator as the valve is slowly rotated. Any runout means that the valve is bent and must be discarded.
Check Head for Damage
After complete disassembly of the heads to a bare state (remember to remove any plugs, such as threaded plugs or expansion cap plugs), the heads must be thoroughly cleaned in a jet wash or hot tank to remove all traces of contaminants. If media tumbling is performed, the heads must then be rewashed to remove any particles.
Before performing cylinder head assembly, inspect the head for cracks and signs of coolant leakage (which could be the result of a crack or casting porosity). Depending on the fault (crack or porosity), the head may be rescued by a skilled cylinder head shop using tig welding and/or vacuum resin impregnation. Depending on the extent of the damage and the estimated cost of repair, make a judgment call to decide if the head should be repaired or replaced.
Check Head for Straightness
Place the head on a clean workbench and carefully check the deck surfaces for warpage. This includes the block deck, intake deck, and exhaust deck. Use a precision machinist’s steel straightedge and a feeler gauge. Only use a precision-ground straightedge. Avoid using a scrap piece of metal that may not be straight and flat.
Position the straightedge lengthwise on the deck from front to rear at the center of the combustion deck and insert a feeler gauge between the straightedge and the deck. OEM specs usually call for an allowable maximum of .003 inch along a 6.00-inch distance, or .004 inch along the entire length. Although this tolerance range may be acceptable for a routine rebuild, ideally there is no gap whatsoever. If any deviation is found, the deck should be resurfaced.
Perform the same flatness check with the straightedge positioned from each corner to the opposing corner in a diagonal manner. The same tolerance specifications apply. If any warpage is found on the combustion deck, resurfacing is required. If more than .005 inch must be removed in order to achieve flatness, a thicker head gasket may be required in order to retain desired compression ratio and alignment from head to intake manifold.
Restoring surface finish and/or deck flatness is one reason to mill the deck. Another reason is to reduce combustion chamber volume when fine-tuning to achieve a target compression ratio.
Although (in general) a .005-inch clean-up cut shouldn’t affect intake manifold mating angle, a more severe material removal can result in a mismatch between the head and the intake manifold. Depending on head design, this can also bring the valves (especially with oversized valves) dangerously close to the edges of the cylinder bores or pistons.
Instead of milling the head decks parallel to the existing deck plane, the decks can be angle milled. This involves positioning the head at an angle relative to the cutter to remove more material from one side of the head (inboard or outboard) than the other side. For instance, .100 inch might be removed from the exhaust side of the deck, to meet zero at the intake side (removing material from the exhaust side while leaving the edge of the intake side alone). If the heads are angle milled, the top of the head bolthole locations of the head must then be spot-face milled to allow the underside of the bolt heads to retain full contact with the heads. Otherwise, the bolt heads meet the head bosses at an angle, eliminating full contact/support for the head bolts. The intake manifold deck surface of the heads must also be angle milled to retain full and even contact with the manifold. As part of your head inspection, perform the same flatness check at the cylinder head’s intake deck. Typical specification calls for a maximum allowable deviation of .0031 inch. The exhaust deck specification is generally an allowable maximum of .005 inch. Some builders of older engines perform angle milling (taking more material from the exhaust side) in order to reduce the valve angle and promote a more direct airflow. With today’s readily available high-performance heads and advanced designs, this is rarely needed.
Whether new or used, inspect all valvesprings. First measure each spring’s pressure at the installed height, then at the maximum-lift specification, and check for coil-bind height. Compare your findings with the manufacturers’ spring specification. When checking dual springs, perform this measurement check with the spring retainer in place because of the locating step in the retainer.
With dual springs, you probably have about .100-inch difference between inner and outer spring height, so the inner spring can enter coil bind before the outer spring.
Closely examine combustion chambers for burrs or other sharp spots. These can lead to hot spots during engine operation, resulting in detonation (preignition). If any sharp edges are found, lightly smooth them out.
If valve seats require refacing, the valve seat angles of 30, 45, and 60 degrees must be maintained. Each valve seat must be inspected for runout, using a valveseat runout gauge. This check allows you to verify valve seat concentricity, relative to the centerline of the valveguide. An OEM specification may provide a maximum-allowable valve-seat runout of as much as .002 inch; for a blueprinted precision build, a preferred maximum runout should be less than .001 inch.
Inspect each valveguide’s inside diameter using a dedicated valveguide bore gauge. When measuring the inside of the guide, check diameter from the top to the bottom of the guide, inspecting for taper wear. If you find any taper, replace the guide. Based on the valvestem diameter of the valves to be used, the guide diameter must allow the proper amount of oil clearance. Refer to the cylinder head manufacturer’s spec for recommended valveguide clearance. Depending on the application, this clearance can run from as little as .0012 inch for intake locations and .0016 inch for exhaust locations to as much as .0015 for intake and .0025 for exhaust. Worn guides can be replaced or drilled out, reamed, and fitted with bronze guide liners that are then honed to size.
Flow Bench Testing
The only way to verify airflow of a cylinder head (aside from taking the head manufacturer’s word for it) is to run and monitor airflow at a constant pressure. The pressure drop is measured across the port/chamber area, determining the ratio of the pressure drop across the calibration. Readings are then monitored through the range of valve lift (from the moment of opening to maximum lift, and especially just before the valve closes. Pressure is readjusted to calibrate test pressure before each check (at 10 percent of valve movement, 20 percent, etc.). Controls on the flow bench allow the operator to switch from sucking air through intakes to blowing air through exhausts.
A flowbench measures this airflow in relation to an applied pressure differential, in inches of a water column. Wet flow testing uses a solvent similar to fuel in specific gravity. Wet flow testing allows you to not only measure airflow, but how fuel/air is distributed within the port as the valve operates.
A special pressure differential (PD) valve, such as the one made by RTS Tools, uses a special valve with a hollow stem where the stem passage connects with orifices in the PD valve’s face. This allows you to determine not only how much volume (in cubic inches) exists in the port, but where the fuel/air is being distributed and in relation to valve movement (based on valve lift). There is not space here to delve into flowbench testing, but this is how advanced race engine builders develop power-boosting cylinder head performance (by obtaining base data and being able to monitor port volume and flow as they modify their ports). Having this data removes the guesswork when choosing cylinder heads.
Cylinder Head Installation
You must first ensure the block decks and cylinder head decks are clean and dry. Then carefully wipe the decks with a fast-drying solvent and a lint-free rag. Examine the deck surfaces closely to verify that nothing is trapped between the decks such as oil, particles, lint, etc. Then you can begin installing your heads.
Position the head gasket (using the deck dowels to locate the gasket). Pay strict attention to head gasket orientation to make sure that alignment is correct and that critical coolant passages are not blocked. (Always refer to the head gasket manufacturer’s instructions for gasket location.) Your hands must be clean and dry while handling the head gaskets. Some MLS gaskets have a seal coating that may be sensitive to finger acids. Make sure that the head is fully seated on the dowels by gently taping the head with a rubber hammer.
If you’re using head bolts, always measure each bolt for length and compare to installed depth (bottom of female threaded holes to top of cylinder head bolt locations) to verify that bolts don’t bottom-out. If they do, you don’t achieve clamping load.
If you’re using head studs, clean them with a fast-drying solvent to remove any buildup of preservative grease that may be present on the threads and to remove any particles that may have accumulated during handling. Apply a bit of oil to the bottom threads (or use a locking compound if you prefer a more permanent stud mount). Install the studs only hand tight. Tightening them completely can result in slight splaying of the stud angle. The clamping load is achieved when the nuts are tightened.
Locking the studs in place is really only needed if you plan to service the engine frequently (as with a race engine). However, if you decide to lock the studs in place, do not use an anaerobic compound (Loctite, etc.) because it expands when cured. Depending on cylinder wall thickness, this can result in excess pressure against the cylinder walls, which can lead to cylinder wall cracking. Instead, use JB Weld or a similar compound. Again, unless you plan to routinely service the heads, simply lube the lower stud threads before installation.
Apply a film of moly lube (such as ARP Ultra Torque) to the upper stud threads and to the bottom of the nuts. Install the flat washers, install the nuts, and tighten. Begin by tightening all nuts to 15 ft-lbs. Continue to tighten in steps until you reach the final recommended torque value for your application. For example, if a head calls for a final value of 110 ft-lbs, tighten at 15, 30, 55, 85, and then 110 ft-lbs. Always follow the specific tightening pattern recommended by the head manufacturer or by the aftermarket stud or bolt manufacturer. This is generally a spiral pattern, starting at the center and working outward in a clockwise spiral pattern. Follow the same tightening pattern during each step.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks