Last month, we showed you a buildup on Tim Moore's budget 5.0L Ford. We pulled it from the junkyard and added a Lunati hydraulic roller cam, some head mods, and an intake and carburetor and made a very impressive 375 real street horsepower. An important part of keeping the investment in line was retaining the production GT40P heads. These are Ford's best-flowing production cylinder head for the pushrod 5.0L. Due to space considerations, we didn't have room in the first story to detail all the tricks that went into these heads. Plus, as is usually the case, there are several ways to achieve those goals. The trick is to upgrade the valvesprings so the entire valvetrain can handle the lift and loads imparted by the longer-duration, higher-lift cam. But there's more to this than just swapping springs.
Let's start by reviewing the evolution of these heads. The GT40 first appeared in '93 and can be found on Mustang Cobras and Lightning pickups. This design was slightly reworked and installed on the '97-and-later Explorer and Mountaineer SUVs as the GT40P. The GT40 valve sizes are 1.85/1.54 for the intake and exhaust, while the GT40P went to a smaller 1.46-inch exhaust valve. There were other changes to the GT40P, including a slightly smaller combustion chamber reduced from 65 cc to roughly 60, which benefits the compression ratio. One of the biggest changes to the P heads was relocating the spark-plug placement to improve combustion efficiency. Unfortunately, this causes header-to-spark-plug interference when using standard small-block Ford headers with the GT40P head. There are a few headers out there that are designed specifically for the GT40P head, so you might want to check on that.
The GT40P incorporates a rotator into the exhaust valve retainer, which, through a series of ball bearings, allows the valve to rotate slightly each time it opens. This helps prevent exhaust valve seat erosion. The valve rotators are thicker than standard valvespring locators and retainers, though, and require a spring with a shorter installed height on the exhaust valve. The installed height is the distance from the valvespring seat in the head to the underside of the retainer. On these heads, the installed height of the intake valvespring is around 1.780 inch, but the exhaust is less at roughly 1.600 (the installed heights can vary by 0.050 inch or more). Because of this discrepancy, the exhaust valve's valve-lock groove must be shorter to allow sufficient room above the retainer for the guided rockers. This reduces the actual distance between the bottom of the retainer and the top of the valve guide and seal, so this combination can't accommodate any kind of performance cam with more than 0.450 inch of lift.
The production Ford GT40P iron head offers decent flow as a stocker, but it needs a few modifications in order to accommodate more valve lift.
The easiest way to identify the GT40P head is these four vertical casting marks on the outside of the head. To verify, GT40P is also cast into the head (usually!) below the valve cover in the intake side of the head.
The keeper groove in the stock exhaust valve (left) is lower than with non-rotator-style valves because GT40P heads use shorter springs and a thicker retainer. We decided to use new valves from Manley (right) with keeper grooves in the same location as the intake valves, making assembly simpler and easier. Other valvespring kits compensate with different retainers or keepers to communize the installed heights.
A valvespring's installed height is the distance between the spring seat and the bottom side of the spring retainer. In order to use the Lunati dual springs, this required machining the lower guide boss (arrow) to reduce its 0.990-inch O.D. to clear the inner spring, which has a 0.730-inch I.D. Slover's Porting Service did this machine work at the same time that the heads were ported and machined for screw-in studs.