Classic Porting 4V heads By George Pence
Head porting is one of those jobs I prefer to farm out (i.e. pay somebody else to do it). I’m just not the most skillful guy with a grinder, and I realize it. However I can share with you the work I have seen performed to Cleveland heads many times by guys who are skillful with a grinder, and I can share the things that have been explained to me over the years by some very experienced practitioners of the Cleveland arts.
Modification of the 4V Cleveland ports is not as predictable as modification of the typical in-line valve cylinder head ports. The best names in professional race engine building found modification of the 4V intake port challenging. Modifications that provided improved readings on a static air flow bench and/or improved readings on the dyno did not always result in faster acceleration times. Finding the modifications that worked required great investments in time and money (trial and error); this is why those modifications are closely guarded secrets even today, and this is why the typical home porter should avoid getting carried away with modifying the cylinder head, especially on the intake side.
However, even modest street engine projects will realize a gain of about 20 bhp by porting the raw 4V cylinder head castings. The goal of porting the 4V head for the street is not to modify the passages, the goal is simply to smooth the intake and exhaust passages thus aiding the flow of intake and exhaust gases. Thus smoothed the 0.600″ valve lift air flow performance of the intake ports and exhaust ports improves by about 50 cfm in each port; air flow performance at 0.600″ valve lift shall be approximately 325 cfm at the intake port and 220 cfm at theexhaust port. But the numbers are not to become a goal, smoothing the ports is what is important.
A porting job for the street consists of opening up the constricted valve pockets and centering them on the valve guides, reducing the OD of the valve guides at least 0.100″, blending the walls and roofs of the valve pockets with a 1/8″ radius, and conservative valve unshrowding in the combustion chambers with a 1/4″ radius. The minimum cross section just above the intake valve seat is about the diameter it should be as-cast, there is very little “extra” material there, so caution must be taken to avoid removing too much material and making the cross-section too large. The final touch is 3 angle valve seat grinding and blending the cuts above and below the seats into the valve pockets and combustion chambers. The intake valve seats should be 0.060″ wide; the exhaust valve seats should be 0.080″ wide. Narrower seats would flow better but they would impact the durability of the valve job. Valve seat durability will also improve if valve seat run-out is held within a tolerance of 0.001″ or better.
Don’t be concerned about hurting the motor’s low rpm power, the only material removed from the intake port should be restricted to the valve pocket; if the port is worked at all it should only be smoothed, not enlarged. The exhaust port is where modification is often performed, although major exhaust port modification is not warranted for a street motor.
The 351C 4V exhaust port was not designed to the standards of the intake port. Ford’s racing department’s respect for the exhaust port was exemplified by their advice to mill it off the head entirely and install raised port plates! The exhaust port plates are not to be considered for a street motor however, for four reasons: (1) the benefit to an exhaust system employing mufflers would not be worth the effort (2) there are no off-the-shelf headers available for motors equipped with exhaust port plates (3) the plates constantly develop exhaust gas leaks (4) this modification eventually leads to cylinder head cracking.
The exhaust port performs better in operation (dynamically) than its static performance on the flow bench indicates; it performs just fine for naturally aspirated street motors. Don’t obsess over flow numbers. Adhering to the goal of smoothing the passage to aide gas flow, porting a street motor’s exhaust port entails rounding the port’s flat roof and removing the bump in the roof. The sides of the exhaust port are faired gradually (i.e. straightened) from the port exit to the ported valve pocket. The roof and sides of the exhaust port are where the most material is removed. Most head shops will radius (round) the exhaust port’s short side turn as well, but only a conservative amount; the casting is thin in this area and removal of too much material will break through into the water jacket.