i just bought a set a gt40p heads without really researching them (stupid) just from what i heard they were an improvement over stock, but now im finding out that they may not fit and i may need special headers, has anyone used these heads and will they work in my 66? any help would be appreciated.
gt40p in a 66'
- Thread starter bigarchie
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12sec67
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not only will the p's require other headers but athey also have bigger cc chambers than your 66 289 heads which will lower your compression.i believe 289 heads are 53cc's - 55cc's and the new heads are 64 cc's.
lower compression=lower horspower and torque
lower compression=lower horspower and torque
They are good heads and a nice bump up in performance over stock. If you can live with the special headers they should be fine. Otherwise a set of regular GT-40's are really the same thing. Also, you can mill the heads down to 54cc's with no problem for about $100 to get the compression to where you want it.
The GT heads aren't as good as other aluminum performance heads, but are a great value and should net you a good 20-30 hp over the stock ones.
The GT heads aren't as good as other aluminum performance heads, but are a great value and should net you a good 20-30 hp over the stock ones.
Glen's 1965 5.0
Founding Member
I have the World heads that are basically the same head. I had them milled also to keep the compression up. I finally had to bite the bullet and buy the ceramic Powertrain headers that are made to fit these babies and all is now good in Mustang land. 
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