302HO with gt40p heads

synthartist69 · Mar 20, 2011

Hey guys,

I have a 65 mustang that needs a short block overhaul. I found a 302HO with gt40p heads on it already rebuilt out of a 97 explorer. I am considering buying it and changing the cam out to a comp. 270HR. I will be running the HIPO manifolds, weiand alum, intake, electronic ignition, a holley 650, and a 4 speed with 3.73 gears. What kind of HP should I expect to get out of this motor with that cam? I have never been really happy with my 289 with this set up but the heads are stock and the cam was a comp. 268H. Any input would be greatly appreciated!

mercurycapri · Mar 23, 2011

Im sure you are close to 260 hp. Ive heard that gt40p heads go flat in hp after 400 lift so duration is your friend. I have a simulair combo but my camshaft has allot of duration. 226 duration and .474 lift and it works great. It diffeniatly pulls real hard.

robbz28 · Mar 23, 2011

If you port match the heads to the intake, and get that holley and your timing dialed in just right you may be able to reach 300 horses...I think that 650 might be a little much carb, but it should do fine. The heads will definately be the bottleneck in your setup. Regardless 275 horsepower in these little mustangs is quite peppy.

tweet66 · Mar 23, 2011

I have a question...do they make headers to put "P" heads in a 65? My understanding is the spark plug was relocated on the P heads and normal headers won't fit without interference with the plugs???? If not, I'm putting my P heads on my wife's 302 for her 66

mustangmutt · Mar 23, 2011

The GTp heads are like GT 40's with smaller combustion chambers and the spark plugs are relocated for a more efficient burn. If you smooth out the bowls and exhaust ports and gasket match the intakes they should flow decent. If you dont feel comfortable doing this higher a pro. Depending on the Headers you have you might be okay. If you have plug to header clearance problems it will probably be on the drivers side. I don't know if you already have headers on the old 289. If so, just test fit them and see how they fit. Try Y's might clear better. I have GTp's on my '90 and was able to use the unknown brand headers I had by using 90 degree plug wires and bonking the #8 tube a little. I don't know of anyone who makes headers special for this set up in a classic Mustang. It does seem like a worthy challenge to see what works.

tweet66 · Mar 23, 2011

I had a similar problem when I built my first engine for my 66. I was running Windsor Jr iron heads. The ports where raised slightly and the plug angle was different than stock. I had to modify the #5 cylinder tube on the tri-y header to clear the plug. If the P's are similar the same could be done for them. IF that's the case I'm glad I kept those headers....

mustangmutt · Mar 23, 2011

I am curious to find out what you use. I have a 65 coup that I want to put a Fuellie 5.0 GT engine and I might use GTp's. I already have the intake off of a 2000 Mountaineer with no EGR passage, and since the GTp's dont have the AIR injection (smog pump) passages it would be a good head to use on an old car.

horseballz · Mar 23, 2011

These guys make tri-y headers specific for those heads:
New Page 1
(kinda pricey, but nice stuff)
I have "P" heads on my 68 and was sold a pair by an out of business supplier, from an unknown manufacturer that kinda fit with a little massaging for the bolts along with a little bit of trimming of the 45 degree boot for the #2 Ford Racing plug wire. I've heard rumors that the MAC long tubes (not quite as pricey as the FPA) will work and some of the Hedman units also.
HTH,
Gene

Hack · Mar 23, 2011

robbz28 said:
If you port match the heads to the intake, and get that holley and your timing dialed in just right you may be able to reach 300 horses...I think that 650 might be a little much carb, but it should do fine. The heads will definately be the bottleneck in your setup. Regardless 275 horsepower in these little mustangs is quite peppy.

He's running old iron exhaust manifolds and the GT-40Ps will be the bottleneck? Really? Doesn't sound right to me. I agree with 275 HP though. That's about what the '93 Cobra tested at. The Cobra had GT-40 heads which are very similar to the GT-40Ps, but it had short tube headers, which are superior to cast iron manifolds. 300 should be pretty easy to get if the exhaust flows freely.

To the original poster - make sure the valve springs are up to handling whatever cam you're looking at. If it has stock Explorer springs I don't think they will handle a performance cam. Also, if you end up replacing springs you should check on the valve job. Don't under-estimate the value of a good performance valve job. It's well worth the money.

mercurycapri · Mar 24, 2011

I do know that summitracing.com sells shorty spark plugs that will help with the clearance. Local auto parts store may be able to order them. You have to order the spark plugs for a ford explorer P heads.

Shorty Spark Plug - V8 - Spark Plugs - Ignitions & Electrical - EXPLORER - FORD - 1998 - SummitRacing.com

robbz28 · Mar 24, 2011

Hack said:
He's running old iron exhaust manifolds and the GT-40Ps will be the bottleneck? Really? Doesn't sound right to me.

When I was writing that at first I was in my hot rod frame of mind...but then I realized this guy didnt ask what he could do to make lots of power, but simply what power he would be making with the combo. So I was about to go into a detailed explanation about heads and horsepower and flow blah blah blah, and just cut it short...but yes, if you wanted to make some serious power, ditch those heads and get a higher flowing exhaust and so on and so forth.

302HO with gt40p heads

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