[pav]

I have a 1964 1/2 mustang with 289 and stock heads with headers looking to replace with gt40 heads, will they fit and if so would they be and inprovement over stock ? also would i have to make any changes for them to fit
thanks, gary

[sixtsixstang]

They will fit and will be an improvement. Just remember that they will require specific headers because of the positioning of the spark plugs. On a standard head, the plugs are at an angle. The GT40 plugs come straight out. From what I understand, the GT40P heads are a little better.

[The Greek]

I'd like to clearify something about the cast iron GT-40 series heads. The first heads that were introduced, "GT-40" has the sparkplugs located in the angled position like a regular production head. There is no problem with header fitment because of spark plug position. The later heads "GT-40P" have the plugs relocated to where standard headers will not fit. An easy way to ID these heads, (There's others), is the number of vertical lines that are cast in to the ends of the heads. The GT-40 head has 3 lines and the GT-40P has 4.

[sixtsixstang]

Thanks for that clarification. That is why I love this site. I learn something every day here.

[The Greek]

I don't think that anyone knows everything about these cars. I'm still learning, too.

[silver69]

Aother option for the 40P heads is a 289 HiPo cast manifold if you dont want hedders. NPD and Mustangs unlimited both sell the manifolds new. You can buy the matching H pipe and finish duals that way too. My oldest brother figured this out and we are running 2 GT40P engines this way in our 69 Convertibles with no issues at all. The way the hipo manifold is designed everything clears with ease.

[Pig Muffin]

Originally Posted by silver69

Aother option for the 40P heads is a 289 HiPo cast manifold if you dont want hedders. NPD and Mustangs unlimited both sell the manifolds new. You can buy the matching H pipe and finish duals that way too. My oldest brother figured this out and we are running 2 GT40P engines this way in our 69 Convertibles with no issues at all. The way the hipo manifold is designed everything clears with ease.

MAC also sells headers that are shaped for the GT40p heads.

[pav]

Thanks for all the great info, but still not sure what to do . I am looking at a set of windsor jr 's would this be a better option over gt-40? and with the Jr's or gt-40 will I notice power gains over stock and around how much ?
thanks

[zr2_wolverine]

I might be wrong but it looks like you will lose hp.You should do more research, yes you will get better flow with the gt40 heads, they have 1.84", 1.54" valve and bigger intake port but going to lose alot of compression if you only change the heads and not the pistons. ford made change in the middle 64 prodution, the 5 bolt belhousing paturn have 8.7:1 with 2 brl carb and 9.0:1 4 brl carb comp ratio and the 6 bolt have 9.3:1 and 10.0:1 all depends on which 289 you got your comp ratio right now is somewhere between 8.7:1 to 10.0:1 and your 289 head has 52-55 cc chamber, 1.78" and 1.45" valve and smaller intake port, the gt40 has 64-68 cc chamber. the bigger chamber of the gt40 head will drop your compression ratio big time. you should keep comp ratio around 9.5 to 10.5:1.

you can rebuilt you head with bigger valve and mild porting job, they are going to be better than the gt40. you can shave the gt40 head but i dont think you can them that much to keep your comp ratio. 69-73 351w head has 1.84", 1.54" valve and 60 cc chamber. most aftermarket head has 60cc chamber.

[Dave]

Nobody has asked what cam you're using. Lot depends on that also. Here is a mother lode of Ford head flow data to look thru.
Dave
Ford Head Flow Data and Specs