GT40 versus GT40P

[BrianB31]

What is the big difference between the GT40 and GT40P?

I'm looking at changing from the E7 heads I have now and go to something with more performance. The AL ones are way out of my budget but the Cast Irons are not.

Thanks!

[Racinger]

The GT40's have 1.85"/1.54" valves, GT40P's have 1.85"/1.46" valves, and the stock E7TE's have 1.78"/1.46" valves.

Based on that information, I would expect the GT40 Iron head to significantly outflow the P's, but that would prove an incorrect assumption. The P's not only outflow the original GT40 heads, but they do it everywhere. There is no advantage for the GT40 iron's until above .500 lift. Since the GT-40P heads aren’t designed for optimum flow above .500, and the overall flow of either head doesn’t support the kind of flow you’d expect to get from a wild cam combo, the high lift flow isn’t as important as it would be with a more aggressive combination. The P heads also come with 59cc combustion chambers, which will increase your stock 5.0HO engine compression to about 9.5:1 while the GT40's 65.5cc chamber hurts your compression dropping it to about 8.8:1. Compression ratios are not the most important thing in engine performance, but so long as you’re staying under 10:1 you should be able to pick up some free horsepower here.

Back when the P heads first hit the scene you could pick up a set for only about $400. Now they are becoming scarce, and you should expect to pay in the neighborhood of $500/set. A set of brand new, stock GT40P’s will flow 190cfm from .400 to over .500 lift intake, peaking at about 195cfm. That's a serious improvement over stock, and well matched to mild head/cam/intake combinations. E7TE’s flow up to about 155cfm intake side peak, at .500 lift. A home ported set will get you about 165-170cfm, while a pro ported set is in the neighborhood of 190-200cfm. To get more than that, you’re going to have to pay big bucks for the porter to replace the valves to a larger size, and really put some work in.

When it comes down to making choices for your heads, it’s important to decide where you’re going, and what you need. The GT-40 Iron heads do sport some enhancements beyond the P’s. For a forced induction combination, the Iron’s do have a lower compression ratio, and in the case of more aggressive camshafts the Irons usually came with GT-40 valvetrain, good for about .520 lift while the stock P head springs only go to .500. If you were to use an XE series camshaft with a great deal of lift, the GT-40 Iron head would probably work better than the P head for pure, peak flow due to the more linear flow characteristics the Irons have continuing past .500. The added advantage is the GT-40 heads work with common headers, and the P heads require specific, and more expensive GT-40P headers in almost all cases. The GT-40P head; however, has it’s advantages too. A more efficient combustion chamber design delivers a better burn, a little more compression for more power in a N/A configuration, more peak flow at the valve lifts most combinations involving these heads will experience, and a superior area under the curve. By under the curve, I mean the flow rate viewed as a graph. If you take the horizontal X axis, and label it "Lift", and the vertical Y axis, and label it "Flow" plotting the flow curve out between the heads would be interesting to look at. It all adds up to a definite advantage for the P head in most applications.

Specifications:

Head
Material
Comb Chamber
Intake Valve
Exhaust Valve
Peak Flow In.
Peak Flow Ex.
Stock Max Lift

GT-40P
Cast Iron
59.0cc
1.84"
1.46"
196cfm@.500
139cfm@.500
.500"

GT-40 Iron
Cast Iron
65.5cc
1.84"
1.54"
192cfm@.500*
128cfm@.500*
.520"*


*GT-40 Irons only feature flow rates up to .500 lift, although flow at .600 lift may be slightly higher.

[Ranger50]

Originally posted by BrianB31
What is the big difference between the GT40 and GT40P?

spark plug angle. p's require either a different header or some funky sparkplugs and/or wires.

Brian

[denniruz]

For the money, the World Products Windsor, Jr. heads outflow the GT-40(p) and are about $700 for the set and don't require special induction or exhaust manifolds (Which I believe the GT-40 variants do, but I may be wrong.)

Dennis

[Slimjim]

I just got mine back from power heads, bigger valves and CNC ported, let me get some pictures up.

Bigger Valves (stainless) 1.90 X 1.60




Porting



Springs



They also have better springs, bronze guides, milled .015
Not bad for $638.00