While the GT-40 Iron heads are significantly better than the stock units, they are not really worth the kind of money you'd spend to upgrade.
There are some drawbacks to the GT-40 iron heads. One major one is the combustion chamber size. At 65.5cc's it'll drop your compression down to about 8.8:1 from 9.0:1.
The GT-40 doesn't flow exceptionally well at low lifts either. It's much like the Windsor Jr's and Sr's in that regard. They are very linear in the way the flow increases. Quite frankly, after looking at the flow numbers, I'm completely unimpressed with all three of those heads in stock form.
Expect max flow out of the GT40's to be about 190cfm intake, and about 130cfm exhaust. Even with the larger than stock (and GT-40P) exhaust valves, the GT-40 iron seems to be deficient on exhaust flow.
Valve sizes on the
GT-40 iron are 1.84" 1.54"
GT-40P 1.84" 1.46"
E7TE 1.78" 1.46"
The GT-40P heads make much better flow numbers at lower lifts, in addition they feature 59cc combustion chambers that will effectively raise your compression ratio to about 9.5:1.
Coupled with the additional compression, and the additional lower lift flow (equal flow from about .400-.600) the GT-40P is significantly superior to the GT-40 Iron heads, and they are a great head with just a little exhaust work to get up to a better 80% of intake exhaust flow.
I've also heard of deals as good as $400 for a complete set of GT-40P heads. It may be worth calling around in your area to locate a 1996-2001 Ford Explorer with the 5.0. If you can get a solid deal on the Explorer V-8's 5.0, you could be setting yourself up with a solid deal for performance. The 5.0 in the Explorer also comes with the Explorer intake, (GT-40 Lower/Explorer (nearly the same as the Cobra) upper.)
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