There’s a lot of confusion surrounding the various factory cylinderhead designations on FE engines. With many aftermarket intakes being marketed as “high rise,” it only made things worse as every parking lot hot rodder and online merchant is convinced that he has a real high riser on his hands.
The various “riser” designations came from Ford’s factory race efforts in the early to mid 1960s. In the beginning, the FE had a single, basic intake port layout, which saw the normal incremental changes as development continued.
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As factory involvement in racing took hold in the early 1960s, it developed a specialized racing cylinder head that was used first in limited production lightweight Galaxies and later in the Fairlane Thunderbolt. This race-oriented head had numerous unique characteristics, including a different valve-cover mounting-rail angle and rocker stand heights. But the intake port was the key feature. It was dramatically taller than those on the traditional FE head. This mandated a matching tall intake, which in turn required a lump in the hood–the instantly recognizable teardrop bubble–and the term “high riser” was coined.
High-riser heads have considerably larger valves than traditional passenger car heads, along with a wider distance between the valve centerlines. Also, a much larger combustion chamber—at around 86 to 88 cc—required a domed piston to achieve high compression. The rocker system for the higher riser at first glance looks like any other FE, but it actually uses a shorter pedestal height in order to accommodate the casting’s raised intake port.
High-riser heads were always rare, only came in limited-production specialty vehicles for a couple of years, and have become extremely valuable. You will not find them in a salvage yard or at a garage sale. They were always race parts and most of them led a hard life with extensive modification and repair being common. It is safe to assume that all remaining usable castings are in the hands of either collectors or racers.
So, if the new race head was a “high riser,” everything else needed a name too, and thus they became “low risers.” While this best describes the earlier 390, 406, or 427 performance heads, the low riser designation has become a default term used to describe every non-performance factory casting as well. As a result, there is a dizzying array of head castings that fall into the low riser category. Fortunately for the guy building on a budget, most of these castings will physically bolt on and work on all popular FE engines. The 427-based heads that would cause any valve interference issues are readily identified collector items; a casual builder is unlikely to stumble upon a set.
Most factory low-riser heads are functionally interchangeable, sharing a common port entry, rocker pedestal position, and chambers that are reasonably similar in volume. Some castings are more desirable than others, but the differences and advantages are generally modest. Exhaust port position can cause some issues with header compatibility. The volume header manufacturers did not recognize that, although the bolt pattern remained the same, there are two different port heights relative to the deck.
The medium-riser combination actually followed after the others, and was in part an effort to get a raceoriented 427 into a production vehicle without the hood bubble. The medium-riser heads share the standard head’s valve-cover rail and reasonably similar rocker-stand heights. Medium-riser intake ports have a raised floor and a more squared-off port cross section, compared to other FE heads. This was said to result from experience and research showing that most airflow occurred at the top and sides of a port, rather than along the bottom, and that the reduced port cross section for a given amount of airflow made the head more efficient. While seemingly intuitive in today’s context, the move to a physically smaller port cross section was quietly revolutionary considering that every other performance engine, including those from Ford, would still be going down the “bigger is better” path for several years.
The GT 390 heads are often visually mistaken for 428 CJ heads, but they are best used for a numbers matching project. Designed originally to ease exhaust-side fit in the 1967 Mustang and Fairlane platform, they have a unique exhaust pattern (14 bolts) that properly fits only the stock manifolds. Even headers, ostensibly intended for the application, don’t fit the exhaust pattern well. With their normal intake port and standard-size 2.03-inch intake and 1.56-inch exhaust valves, and none of the performance enhancing features that make a Cobra Jet head desirable.
Cobra Jet heads are a key component in what would be the “last hurrah” for factory-installed FE high performance engines. It’s intuitive to want to categorize these as a “low riser” or a “medium riser,” but in reality they are a blend of the two, along with an important feature only found on these—the 16-bolt exhaust flange. A Cobra Jet head has an intake gasket face that matches the low-riser gasket dimensions, but the port itself transitions to the medium riser’s general cross section and floor a short distance into the port. The Cobra Jet heads use the same basic valve cover and rocker parts as the traditional passenger-car 390, but have larger valves at 2.09 and 1.65 inches in diameter. The exhaust side of the head was altered to accommodate eight additional horizontally oriented fasteners, easing installation in shock-tower-equipped Mustangs and Torinos. This exhaust face, coupled with the C8OE-N part number, make a Cobra Jet head readily identifiable.
Written by Barry Robotnik and Republished with Permission of CarTech Inc