390 GT identification
Logistics Pro
Joined: Oct 2005
Posts: 3,785
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From: Mddl A MexCans
I no guru here so ???? But as I understand it the GT will have a rather odd exhaust manifold bolt pattern. Unlike the FE basic one atop the other it has stagger to the bolt pattern. (More bolts per exhaust hole)
Post Fiend
Joined: Mar 2004
Posts: 6,056
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From: Union, Washington
Standard heads have 8 holes one each above and below the exhaust ports. The GT heads have the vertical holes and extra holes on the sides of ports. The standard manifolds are a straight log type (rectangle) where the GT ones kinda wiggle around and look like they flow better (they do). Thunderbirds never had a GT motor from the factory.
Posting Guru
Joined: Oct 2005
Posts: 1,235
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chances are pretty good unlrss it was a cusom set of headers that they weren't GT heads... i've seen only one set of headers that fit trucks with the 14 bolt pattern, and they were custom ordered(not built though) from stan headers.. JMO
Senior User
Joined: Feb 2006
Posts: 355
Likes: 1
'66-69 GTs had the 14 bolt pattern. Headers for the 16 bolt CJ will physically bolt on in most holes - - but the ports wont line up. Not good for power, but most guys don't know to check.
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Hotshot
Joined: May 2003
Posts: 11,141
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From: south louisiana
Only thing that set the GT 390 apart was the staggered exhaust pattern and camshaft, other wise it's really the same engine the bigger cars got. The 390 in my 68 Merc Monterey's got the same heads, (minus the staggered pattern), same comp ratio pistons, same block, crank , rods etc. Only this one got a 2 bbl intake and carb at the factory. The camshaft & Holley carb made the difference in the power rating on the GT.
Old School Hot Rodder
Joined: Feb 2006
Posts: 6,472
Likes: 11
From: Exmore, VA
The real reason for the extra bolt holes was chassis clearance on the smaller cars. The over and under bolt pattern would not have cleared the shock towers in the center. If anyone has ever changed plugs in either a 390GT or CJ you know what I mean about tight clearance. Ford also designed the CJ exhaust manifolds to flow better than the old log style, the only better cast iron manifolds for FE engines were the 406-427 style. Some 390 heads had the bosses cast for the extra holes, but not drilled and tapped.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
Originally Posted by 85lebaront2
The real reason for the extra bolt holes was chassis clearance on the smaller cars. Some 390 heads had the bosses cast for the extra holes, but not drilled and tapped.
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Originally Posted by 85lebaront2
The real reason for the extra bolt holes was chassis clearance on the smaller cars. The over and under bolt pattern would not have cleared the shock towers in the center. If anyone has ever changed plugs in either a 390GT or CJ you know what I mean about tight clearance. Ford also designed the CJ exhaust manifolds to flow better than the old log style, the only better cast iron manifolds for FE engines were the 406-427 style. Some 390 heads had the bosses cast for the extra holes, but not drilled and tapped.
Old School Hot Rodder
Joined: Feb 2006
Posts: 6,472
Likes: 11
From: Exmore, VA
Trust me, I learned all the tricks. I used to own a tune-up and carburetor shop. I still have a 24" 3/8" drive extension I bought for big block Mustangs and intermediates. The worst were the 4 speed models with air pumps.