gt40 vs gt40p

[Unit 5302]

This appears to be an ancient topic, but since the recent comment was made, I feel a need to chime in on this debate once again.

Out of the box the GT-40P's flow better, burn fuel more efficiently, and have a smaller combustion chamber (higher compression), than the GT-40 Irons. The GT-40 Irons you'll usually find have a better valvetrain capable of taking cams with lifts somewhat higher than .500 whereas the GT-40P head stock valvetrain should not be used with cams having a lift over .500 unless you like valvespring bind (which you don't). There should be no debate still out there on which head is superior. The GT-40P is by far the better head, no ifs, ands or buts.

As to the aluminum aftermarket heads vs. a set of GT-40P heads, there is some debate. Bigger is not always better, and the GT-40P heads can make excellent power while not killing your performance below 4000rpm on a small block 302, unlike any aluminum head that's going to be really suited for a 331 or 347 stroker.

Building an engine is not like building a computer where the parts work independently.

If you put a set of big, high performance aftermarket aluminum heads onto a 302 without changing the intake and the cam, you're just going to get an unresponsive, torqueless gas guzzler because the port velocity is going to be terrible as the intake won't be able to flow to match. If you put a big intake on the engine keeping with the idea you're going to build the car bigger later, you're still going to have poor intake/head flow velocity because the cam isn't going to allow sufficient overlap, duration or lift to efficiently fill the cylinder. Okay, so what if you cam the little 302 to the moon. Gratz! You now have an engine that should have good port velocity... at 6500rpm, which you can't rev to with the stock rev limiter, and your flywheel, stock bottom end, fuel pump, transmission, fuel injectors, cooling system, and exhaust are incapable of safely supporting. If you manage to upgrade all of those components, you'll have one heck of a powerhouse 302, but for $10,000. Oh, and your fuel economy will suck, and you'll have to keep the engine above 4,000rpm to notice the extra performance. Your low end torque will be significantly worse so you had better be running at least 4.10 gears making 1st gear, and rush hour traffic a nightmare.

In summary, there is absolutely nothing wrong with GT-40P heads for the 302 owner that wants drivability, 320hp or under without forced induction, and the ability to stomp the mismatched 5.0 in the next lane.