Once all the bolt-on parts have been added to your turbo car it is time to go internal in the search for more power. One technique to increase power is to improve the flow characteristics of the cylinder heads by porting and polishing. The head is the gateway to to the combustion chamber. The shapes of the ports in the sides of the cylinder head and the length and diameter of the runners back into the back of the inlet/exhaust valves has a huge bearing on the overall efficiency of the engine. Some handy gains can be made simply by cleaning up the factory castings and machining marks in the cylinder head, which ultimately decreases turbulence of the air flow. Inlet port matching is also useful, as the factory rarely lines up the inlet manifold with the cylinder head perfectly. Each mismatch creates a turbulence which lowers air flow capability. This type of work needs to be done by an expert, with the aid of a flow bench, as it is just as easy to lose power as gain power from this type of work. Gains of between 10-15% have been experienced on late model Turbo Japanese cars.
TERMS:
BACK CUT
This describes an additional angle cut in the valve to improve flow. It is the top cut- the one closest to the valve stem. Back cutting the valves is said to have a similar affect as installing a camshaft with more duration. Back cutting increases the chances of failing a sniff test during a smog inspection.
BOTTOM CUT
The bottom cut is just as it sounds, the cut furthest into the valve-throat area. This cut opens the throat diameter to match valve diameter. A bottom cut is necessary whenever larger valves are installed or if the stock valves have been machined to take advantage of their full diameter.
BOWL PORT/BOWL BLEND/POCKET PORT
This is the single-most important step. The valve pocket is blended to remove any casting marks or edges left behind from machining the seats and throats. This procedure really helps power and fuel mileage.
CUT SEATS/GRIND SEATS
Most head shops use a special cutting tool to cut the valve seats, rather than grind the seats, as done in years past. Modern cutters machine several angles at a time, and cut a multitude of profiles. When valve seat work is performed, it's important to measure the valve's height relative to the deck using a dial indicator. Setting all valves to the same height ensures consistency of combustion-chamber volume and ensures that the valves wouldn't whack a piston.
FLOW BENCH
The flow bench is an instrument that is used to measure airflow through a port, carb or manifold. BPE uses a SuperFlow 600 flow bench. It's used for before and after comparisons. When examining the flow claims from various head shops, it's important to know how the measurements were taken. You'll need to know what valve lifts were used and the test pressures. Most shops test at either 25 or 28 inches of water. To compare tests with unlike pressures use these conversion factors: 25x1.06=28 or 28x.945=25
GASKET MATCH/PORT MATCH
Gasket matching opens up the port to match the intake manifold gasket. Note how the gasket in the photo is larger than the opening of the port. To match the port to the gasket, metal is removed from the flange and into the port up to two inches. Also note that the port is restricted deep into the port to allow room for the pushrods. Because of this restriction, hogging out the port at the flange is not necessary. Port matching is not required for low-rpm torque engines. A port match is generally performed when an aftermarket intake manifold is installed.
GUIDE BOSS
The guide boss is the portion of the valve guide that is cast into the head and protrudes into the port. Never remove the guide boss in an effort to improve flow. Removing the guide boss reduces valve support and leads to rapid wear of the seats, guides and valves. For high-rpm use, the guide boss can be streamlined.
LAP VALVES AND SEATS
Lapping is done by placing an abrasive lapping compound on the valve seats and then lightly rotating the valves in the seats to "sand" a fresh surface. Lapping should only be done to freshen a very recent valve job (it doesn't replace cutting the seats) or used as an inspection process for checking machine work.
MILLING
This is a machining process that removes metal from the head-gasket-surface area of the head for the purpose of increasing the compression ratio. Surfacing does the same thing, but less material is removed and the intention is to ensure that the head is flat and gouge free. If more than .030-inche is removed, one must consider how the ports have moved closer to the block. This relocation of the ports can affect the fit of an intake manifold on a V6 or V8. Excessive milling can also cause pushrod problems or alter the cam timing on an overhead cam engine.
MULTI-ANGLE VALVE JOB
Multiple angles are cut into the valve seat area to open up those areas to streamline flow. Often, five angles are cut. Opening the seats also allows the valves to be recessed into the head to take advantage of their full diameter. This is essentially like installing slightly larger valves.
POLISH
Polishing a port or combustion chamber does not alter the shape or size, but instead smoothes the surface to increase flow and reduce the build up of combustion deposits.
PORTING
Porting reshapes or enlarges by cutting, grinding or machining.
SHORT TURN
This refers to the shorter, tighter side of a turn in a port. Also known as the Short Side Radius
TOP CUT
The valve seat angle nearest the combustion chamber.
UNLEADED SEATS
Only the exhaust seats need to be equipped with valve seats capable of withstanding unleaded gasoline. Most OEM heads come with induction-harden seats. This process treats only the surface metal. Induction-hardened seats are good for only a couple of valve jobs before all the hardened material is removed. Aluminum heads must use seat inserts. Hard seats are what head shops install into both the intake and exhaust to repair damaged seats, to convert old leaded heads to unleaded. To install a hard-seat insert, the seat area is bored out and the insert pressed in. The outside diameter of the insert is generally .005 to .008 inch larger than the bore, so the seat will not fall out once installed. The seat is then cut.
UNSHROUD THE VALVES/FLY-CUT CHAMBERS
A fly-cutter is used to machine the combustion chamber around the valves to remove any material that might impede flow.
STAGES OF PORTING
Most cylinder-head shops offer port work in various degrees or stages. The work done will vary from shop-to-shop and depending on the head and end use, but the following is a good guideline of what to expect. The prices and work listed are for a V8 and assume the heads do not require repair work. A valve job is included in the price.
RELATED ARTICLE:
STAGE 1
Price Range $250-350 3-angle valve job Radius, blend and polish bowl areas Remainder of ports inspected and semipolished
STAGE 2
Price Range $350-600 Stage I plus port-match intake at flange Combustion chamber fully polished
STAGE 3
Price Range $600-1000 For race and high-rpm use Stages I and II plus ports enlarged and reshaped slightly Valves unshrouded in chambers Ports fully polished Flow tested
STAGE 4
$1500+ For race applications only
COPYRIGHT 1997 McMullen Argus Publishing, Inc
