Log Head Arithmetic

 The Ford 144/170/200 inline 6 is a simple, durable engine that is easy to work on.  It is a "cam in block" design, meaning the camshaft is inside the engine block, and the valves are actuated by pushrods.  From 1960 until 1983, it was the workhorse used in the majority of Ford's small and intermediate cars.  

The one shortcoming of the engine is the cylinder head - the intake manifold and cylinder head were cast as one piece for simplicity and to save on production costs.  Known as the Log Head, because the intake manifold looks like a small log with branches feeding the cylinders, it is a restrictive design and provides an uneven flow of the fuel/air mixture.  Early engines, made from 1960 to 1967, had the small log head.  Beginning in 1968, Ford changed the design of the cylinder head and increased the intake volume, which provided slightly better performance.  These engines were produced up until 1983, in cars like the Maverick/Comet, Granada/Monarch, Fairmont/Futura, and Mustang, and are referred to as large log heads.        

(The small log head from my 1965 Ford Mustang - the log is behind the valvetrain with pushrods)

The best way to increase performance on a pre-1968 six cylinder is to swap out the small log head for a large one.  From there, you can add a dual outlet exhaust header, Duraspark or DUI/HEI distributor, and a bigger carburetor.  You can also buy a 2 barrel carburetor adaptor or modify the intake manifold to accept a larger carburetor.  

(Large log head on the same engine - this head was cast in 1973)

Swapping out the cylinder head is straightforward, but not something to take on if you're a novice mechanic.  The Ford Six Performance Handbook provides detailed instructions on the steps involved, expert tips, and things to watch out for.  In addition to the advice in the book, here are some additional things to I learned:  

• Cylinder heads are all interchangeable for all years and displacements, but you will want to factor in combustion chamber sizes and compression ratios so you don't end up with too much or too little compression.    
• The cylinder head itself weighs about 65 pounds.  You will need help lifting the old one and installing the new one.  
• Ford changed the sensor for the temperature gauge in late 1965, so you will need a new temperature gauge if your original cylinder head dates before August of that year.  
• The carburetor spacer for the small log does not fit the large log - the bolt holes are about 1/2 inch further apart.  Additionally, the bore for the intake is about a 1/4 inch larger in diameter. This difference, in addition to the wrong carburetor gasket, will cause vacuum issues, leading to stumbling and hesitation.  
• If you go with a different carburetor, you may have to modify the linkage for the accelerator pedal.  The only carburetors I know that don't require this modification are the Carter YFA, Holley 1904/1940 and the Daytona or Universal carburetor. This modification is not difficult but you will need to purchase the V8 accelerator pedal and linkage.
• If you use a Duraspark or DUI/HEI distributor, you will need a larger gap for your spark plugs - from 32-35 to .45 or more.    

Done right, these modifications can increase power by 30% while costing less than $1,200.  The result is usable, every day performance that approaches the base V8 models of that era, making the car more lively and enjoyable while still maintaining a vintage feel.