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1966 Mustang Hardtop 289 4 Speed
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Not the best pics for identification but I can see the "P" on one of your heads and the plug angle appears to match the P heads. I don't know enough about the engines to say one is better than the other though. I would have taken either engine if the price was right, I just happened to find a GT40P locally so I jumped on it.
 

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Discussion Starter · #22 ·
Not the best pics for identification but I can see the "P" on one of your heads and the plug angle appears to match the P heads. I don't know enough about the engines to say one is better than the other though. I would have taken either engine if the price was right, I just happened to find a GT40P locally so I jumped on it.
Good enough. Much appreciation to you Caper50 and the others who have put up info here. Dave R.
 

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Thanks Caper50. I learn something almost daily on these forums. So do you have an opinion on which heads are on this engine? Are the pics clear enough? And which head is the preferred one? Dave R.
Which is preferred really depends on the use. If you are upgrading a modern roller block 5.0, the GT40 heads are great, especially if you plan to add a supercharger because the 40s have larger chambers. If you are upgrading a classic 289 or 302, the P heads can be better because the smaller chambers keeps you from loosing compression. A down side of the Ps is the modified plug angle. It improves combustion, but most headers won’t fit with it. HiPo manifolds will work with a little trimming. If you are swapping a whole motor, then GT40 or GT40P doesn’t really matter except for the headers issue
 

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if you can’t find a buyer for the wh9le project I might be interested in the engine
 

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I believe after 97 and later we’re all GT40 P heads. Meaning the last year for the explorer to get the GT40 heads was in 96.
 

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GT40 Heads:

Looking at FRONT of each head (if standing in FRONT of motor), on the end of each head, you will see (3) vertical "bars". You do not have to remove any valve covers, or any other parts to see these markings.

Looking at the TOP of the heads, at the front corner (top, outside and near bottom corner of valve cover - towards water pump) you will see the letters "GT" stamped into the head casting. You can also see the same cast letters near the head's first bolt hole (again, closest to water pump). You do not need to remove the valve covers or any other parts to see the casting letters.

NOTE: The standard GT40 heads were used on all 1993-95 5.0 Mustang SVT Cobras and up to the mid-year 1997.5 Ford 5.0 Explorers.

~~

GT40P Heads:

Looking at FRONT of each head (if standing in FRONT of motor), on the end of each head, you will see (4) vertical "bars". You do not have to remove any valve covers, or any other parts to see these markings.

Looking at the TOP of each head, at the front corner (topside and near bottom corner of valve cover - towards water pump) you will see the letters "GTP" OR “P” stamped into the head casting. You can also see the same cast letters near the head's first bolt hole (again, closest to water pump). You do not need to remove the valve covers or any other parts to see the casting letters

GT40P's will not have the emissions crossover ports on the back of the head for the steel emissions tubing for smog pump/exhaust.

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As to which is preferred, GT40’s are the most common and usually the preferred head due to not needing “GT40P” specific headers. Trying to find GT40P specific headers TODAY is a PIA because most Companies that did make them have since closed shop OR have stopped production being there’s no mass demand for them anymore. You could find them used, but it’s like trying to find a needle in a haystack.

The problem w/ GT40P heads & header issues, lies mainly on the passenger side head due to the severe spark plug angles, with the worse being plugs # 3 & 4. You will need to get yourself a set of ignition wires that have a 90* boot, instead of the common 45* boot. You will also need to get High Temp ignition wire boots (non-flammable & heat resistant up to 1200*), because if you do not, you will be burning up ignition wires.

Even with using the specific GT40P headers (which used to be made by Ford Racing or MAC), changing out the plugs is still a PIA because of the spark plug angles.

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There’s this excellent archived web page that has the complete details on GT40/GT40P heads:


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Good luck with the sale - definitely an excellent asking price for all that is bundled.
 

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Discussion Starter · #31 ·
GT40 Heads:

Looking at FRONT of each head (if standing in FRONT of motor), on the end of each head, you will see (3) vertical "bars". You do not have to remove any valve covers, or any other parts to see these markings.

Looking at the TOP of the heads, at the front corner (top, outside and near bottom corner of valve cover - towards water pump) you will see the letters "GT" stamped into the head casting. You can also see the same cast letters near the head's first bolt hole (again, closest to water pump). You do not need to remove the valve covers or any other parts to see the casting letters.

NOTE: The standard GT40 heads were used on all 1993-95 5.0 Mustang SVT Cobras and up to the mid-year 1997.5 Ford 5.0 Explorers.

~~

GT40P Heads:

Looking at FRONT of each head (if standing in FRONT of motor), on the end of each head, you will see (4) vertical "bars". You do not have to remove any valve covers, or any other parts to see these markings.

Looking at the TOP of each head, at the front corner (topside and near bottom corner of valve cover - towards water pump) you will see the letters "GTP" OR “P” stamped into the head casting. You can also see the same cast letters near the head's first bolt hole (again, closest to water pump). You do not need to remove the valve covers or any other parts to see the casting letters

GT40P's will not have the emissions crossover ports on the back of the head for the steel emissions tubing for smog pump/exhaust.

—-

As to which is preferred, GT40’s are the most common and usually the preferred head due to not needing “GT40P” specific headers. Trying to find GT40P specific headers TODAY is a PIA because most Companies that did make them have since closed shop OR have stopped production being there’s no mass demand for them anymore. You could find them used, but it’s like trying to find a needle in a haystack.

The problem w/ GT40P heads & header issues, lies mainly on the passenger side head due to the severe spark plug angles, with the worse being plugs # 3 & 4. You will need to get yourself a set of ignition wires that have a 90* boot, instead of the common 45* boot. You will also need to get High Temp ignition wire boots (non-flammable & heat resistant up to 1200*), because if you do not, you will be burning up ignition wires.

Even with using the specific GT40P headers (which used to be made by Ford Racing or MAC), changing out the plugs is still a PIA because of the spark plug angles.

———

There’s this excellent archived web page that has the complete details on GT40/GT40P heads:


——

Good luck with the sale - definitely an excellent asking price for all that is bundled.
OK Cobra-Jet. Thanks for the additional info. Sounds like the P heads are a bit of a pain in the rear to fit headers onto. My knowledge base continues to grow by viewing the forum. Dave R.
 

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If that is a stock Explorer engine, looking at intake manifold...it would have the GT40 heads.

The upper intake is solid by the throttle body.

The upper intake with a notch (upside down vee) by throttle body has GT40P heads.

Still have to verify GT or P cast into heads or the bars.
 

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Discussion Starter · #33 ·
Ok Nos681. Thanks for that pointer on identifying this motor. I wish I had access to the car, motor and parts but as I stated above I'm in SW Ohio and that stuff is in central Alabama. I'll show my bro-in-law what he needs to look for on his next trip to 'bama. Dave R.
 

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Since I go to Anniston periodically, I am interested in looking at the parts.
 
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