Installing GT-40P Heads: Iron Head Horsepower

We guarantee plenty of burnout action when you add GT-40P heads to your 5.0 Mustang.

For real performance, stock E7TE heads leave a lot to be desired. However, stepping up in the performance head market will set you back more than $1,200 for a decent pair of aluminum castings--and that's straight out of the box, with no porting. A good port job can set you back another $500-$1,000. After a couple grand you have a killer set of heads, but there's got to be a less-expensive way of making horsepower. Sure, you could race-port the existing E7 castings, which many have done, but then you're putting quite a bit of change into stock heads. There is a better way. Central Coast Mustang sells GT-40P heads at a great price, making them a viable alternative in a market flooded with performance head choices.

Commonly referred to as Explorer heads, GT-40P iron castings are found on current 5.0-equipped Explorers. Similar to regular GT-40 heads, the biggest difference between the two is that the P-heads feature a revised spark plug angle which extends the plug into the combustion chamber, putting the flame front close to the absolute center of the chamber, resulting in a smoother combustion process. P-heads have a smaller combustion chamber at 59cc, which will bump the compression ratio up on an otherwise-stock 5.0 engine. Because of this, your tuneup will probably need to be modified. With the GT-40P heads you will need to back off the timing a couple degrees and bump up the fuel pressure. Obviously, tuneups will vary. You just want to keep it out of detonation.

While we've seen engine dyno tests performed with GT-40P heads replacing factory E7TE castings and resulting in a boost of more than 25 hp, we have reluctantly set our horsepower gain goal at 25 ponies, as well. The reason for this is that we're actually replacing mildly worked E7TE heads. For this reason, we're not sure what kind of boost the GT-40P heads will give. Dennis Hilliard of Central Coast says we can expect to gain three-tenths in the quarter-mile. This kind of improvement may not seem like much, but the owner of our test car, Jamie Holten, runs in FFW Trophy Stock where three-tenths is a lifetime. Holten's best time is a 12.63 at 105.9 mph, and if we gain the desired result he will be right in the thick of things in Trophy Stock competition.

Along with the heads, we used MAC headers for GT-40P heads. MAC is the only manufacturer of these headers, and the parts we installed fit like a glove with no problems whatsoever. With the P-heads requiring pedestal-mount rockers, we also called up Crane Cams for a set of 1.6 pedestal-mount roller rockers. If you're running a stock cam, 1.7 rockers will work perfectly. However, since Holten's car has an E303 cam, we didn't want the pistons and valves becoming too close, so we chose the 1.6 rockers. As with all head installations, check piston-to-valve clearance before final startup.

We did the installation at Holten's place of employment, Motorworks, in Longwood, Florida. It took him and yours truly eight hours from start to finish, using air tools and a lift to gain access to the header flanges. We turned to Rare Auto Design in Orlando for dyno tests to find out how much horsepower we gained. A trip to Orlando Speed World provided us with updated quarter-mile times.

Boy, were we surprised with the results. We gained 45 hp and 40 lb-ft of torque. At the dragstrip, we gained two-tenths and 2 mph. We know what you're thinking--only two-tenths from an extra 45 hp? Let us explain. Holten runs the factory fan, but he had removed it when he ran the 12.63. However, when we dyno'd the car before installing the GT-40P heads, the fan was on the car. Holten tells us removing the fan is worth two-tenths in itself, which tells us the fan soaks up roughly 20 hp. After we installed the heads and with the fan off, he ran a 12.46 at 107.9. We then dyno'd the car without the fan to get some consistency and were rewarded with a peak of 316 hp at 5,500 rpm, and 348 lb-ft of torque at 4,000 rpm. If we subtract the 20 hp that the fan is worth, we end up with a gain of 25 hp.

Now we know why Dennis Hilliard of Central Coast Mustang is so high on these heads. With no other changes we gained 25 hp using factory iron heads. Through Central Coast you can have the heads fully prepped for your combination. As a matter of fact, Central Coast has an '89 coupe with GT-40P heads that has run a best of 11.98 in the quarter-mile.

All in all, the addition of GT-40P heads to any 5.0 Mustang is a worthwhile venture, especially if you're on a budget (aren't we all) or shooting for the Trophy Stock title.