Difference Between GT40 and GT40P Heads

93GreenLX · May 4, 2005

Do they both bolt right up? Any special adaptations or spacers, etc needed for either? One better than another?

Car RamRod · May 4, 2005

not sure. ive got P heads on my car. Im trying to find out more also.

85_SS_302_Coupe · May 4, 2005

The Ps have a revised angle for the spark plugs that point the plug towards the dead center of the combustion bowl, improving the combustion in the bowl. This is why typical 5.0 headers won't fit on P heads, because the plugs stick straight out of the heads and hit the header primaries. I'm pretty sure that's the main difference.

DouglasOhio · May 4, 2005

IM 98% sure they are a direct bolt on. Also, i believe a difference is the emissions bump on the exhaust port right before the header exit, is smaller on the P heads. I think the P's flow a little better in the mid valve lift stages. If this is wrong someone feel free to correct me, i have a good site on the head differences if i can find it ill post it...

RedCoupe92 · May 4, 2005

85 SS is right, they are a direct bolt up, the exhaust bump is the only difference, they both flow the same, and I am pretty sure they accept the same headers as the regular gt-40 heads. i had a set of the first gt-40 cast iron heads, wish i would have kept them, they are alot better than the stock heads. either way, you will make about the same power, i believe the "p" heads just replaced the old heads.

85_SS_302_Coupe · May 4, 2005

RedCoupe92 said:
85 SS is right, they are a direct bolt up, the exhaust bump is the only difference, they both flow the same, and I am pretty sure they accept the same headers as the regular gt-40 heads. i had a set of the first gt-40 cast iron heads, wish i would have kept them, they are alot better than the stock heads. either way, you will make about the same power, i believe the "p" heads just replaced the old heads.

They're direct bolt on but there are issues with typical shorty headers not being able to clear the spark plugs. I'm pretty sure MAC sells a pair of shorties specifically for these heads, if not i know FRPP does. Most full length headers will clear the plugs though because they come straight out from the head further than shorties do.

darthcual · May 4, 2005

Yes I believe mac is the only header that will work w/ the p heads. Also they make a higher compression do to smaller valves than the sed gt40.

payupsucka · May 4, 2005

85_SS_302_Coupe said:
They're direct bolt on but there are issues with typical shorty headers not being able to clear the spark plugs. I'm pretty sure MAC sells a pair of shorties specifically for these heads, if not i know FRPP does. Most full length headers will clear the plugs though because they come straight out from the head further than shorties do.

89MustangGX · May 4, 2005

darthcual said:
Yes I believe mac is the only header that will work w/ the p heads. Also they make a higher compression do to smaller valves than the sed gt40.

New FMS headers also work with them now. They are a revised design.

twolo4u2c2 · May 4, 2005

Well my P heads dont have the passages in them or the bump. I wonder what they came off of. Anybody?

04sleeper · May 4, 2005

P heads have the same size intake valves but smaller exhaust valves than GT-40's.

DouglasOhio · May 4, 2005

twolo4u, r you sure you have P heads and not reg. gt40's?
Edit: You can check at the ends of the heads, the gt40p's have 4 indented bars and the gt40's only have 3. Also. at the "backs" of the heads next to the spark plugs the gt40's say gt and the gt40p's say gtp. Hope this helps

DouglasOhio · May 4, 2005

hey...
just found the site i mentioned before
www.stangpro.com
then you must click the articles tab up top
then scroll down the gt40 writeup/article. There is more info on the GT40p head there than i have found ANYWHERE else

RedCoupe92 · May 5, 2005

the size of the valves has nothing to do with compression ratio, it's the size of the combustion chamber that has something to do with the compression ratio, and also if you mill or deck the heads, thus bringing the mounting surface of the heads closer to the block, this will also raise compression.

Car RamRod · May 5, 2005

but anyway, for the most part, will long tubes fit on P heads? Anyone here actually using long tubes on p's? thanks

twolo4u2c2 · May 5, 2005

ya they say gtp and 4 bars. I got these from tri-state cylinder head last year.

V8SHME8 · May 5, 2005

Car RamRod said:
but anyway, for the most part, will long tubes fit on P heads? Anyone here actually using long tubes on p's? thanks

Not all long tubes will fit. Flowtechs will need to be modified alot. Some require dimpling. Mac long tubes fit. If you want shorties, then you need the Motorsport ones or Mac P specific shorties. You can buy the ones from Central Coast Mustang which are also made by mac, but are cheaper than what you can usually find for mac prices.

Difference Between GT40 and GT40P Heads

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