The Mustang Chronicles: Carburetion

 For decades, carburetors were the primary devices for mixing fuel and air at a ratio to maximize combustion in gasoline powered engines.  These devices are simple in theory - they used the flow of air to regulate the metering of fuel, creating an ignitable vapor to generate power.  Over time, they became more complex to provide increased power and meet ever increasing emissions regulations.  With those changes, they gained a reputation of being clunky, inefficient, and unreliable.  This reputation is undeserved in most cases.  Carburetors have been used on everything from lawnmowers to large aircraft and are still used for many applications today. 

(Carter YFA one barrel carburetor)

The 1965 to 1967 six cylinder Mustangs originally came with the Autolite 1100 carburetor, as did most Fords with the Thriftpower six.  The Autolite 1100 is a one barrel carburetor developed by Ford as a replacement for the Holley 1904 and 1940.  It included an automatic, hot air choke and a spark control valve to manage ignition timing.  A common upgrade for these engines is to swap out the one barrel carburetors with two barrel units, and replace the Load-O-Matic distributor with a Duraspark or DUI/HEI.  This conversion can be done using an adapter for the Thriftpower's Log Head intake manifold, or by modifying the intake to accept a two barrel carburetor.  

Switching to a two barrel carburetor provides more horsepower by increasing the volume of fuel/air, which is measured in cubic feet per minute (CFM).  Increased CFM volume equates to more power through larger displacement, either due to the size of the engine, or by increasing the engine's speed, measured in revolutions per minute (RPM).  

But there is more to the math than just increasing CFM volume.  You also have to consider volumetric efficiency (VE), which is defined as the ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure, to the mass density of the same volume of air in the intake manifold.  Or in plain English, you can't shove more gas into an engine than it can expel as exhaust.  That's why bolting a Holley 750 Double Pumper on the Thriftpower six is the automotive equivalent of the Milk Gallon Challenge.  

I'm more worried about drivability than power.  I want better street performance from a torquey engine with good throttle response.  The engine is basically stock, with the original camshaft, pistons, crankshaft, and exhaust, so I'm not going to get much benefit from a two barrel carburetor conversion.  

A better choice is to replace the Autolite 1100 with another one barrel carburetor and upgrade to the Duraspark or DUI/HEI distributor.  Ironically, many people swap out the Autolite 1100 for a Holley 1904, 1940 or "universal" one barrel carburetor.  Or, you can go with a Carter YF or YFA, like I did.  

I went with the Carter YFA for reliability and compatibility with my stock setup.  The YFA was used on a variety of Ford cars and trucks from the 1970s into the 1980s and bolts on with no modifications to the fuel line or throttle linkage.  It also provides the improvements in performance I'm looking for.  At some point in the future, I'll probably do a complete tear down on my engine, upgrade the camshaft, exhaust, and go to a two barrel carburetor.  But for now, I'm more interested in driving my Mustang than rebuilding it.          

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.    

Mustang Chronicles: Gathering Parts

My simple job to replace a thermostat on my 1965 Mustang turned into this: 

(Ford 200 I6 engine block with new gasket before cylinder head install)

I broke a bolt off in the original cylinder head housing and had to drill it out.  My amateurish attempts to rethread the hole with an old tap and die set proved fruitless, leaving me stuck somewhere between my propensity to optimistically dive headlong into a project, and the limits of my skills.  So it seemed like a good idea to double down on optimism and take the opportunity to do some engine upgrades.   

The biggest shortcoming of the early six-cylinder Mustangs is the design of the intake manifold, which is cast as part of the cylinder head.  Ford decided they could save real money that way, but it results in an engine that breathes like a hamster with emphysema.  In 1968 Ford enlarged and improved the intake manifold and added larger valves in 1977.  The intake manifold looks a bit like a log, so pre-1968 heads are known as "small logs" and the 1968 and newer ones are "large logs".  Production of the engine spanned from 1960 to 1983 and it was used in just about all of Fords smaller/economy vehicles. 

(Remanufactured "large log" cylinder head from a 1973 Maverick)

The Ford 144/170/200 inline 6 will never be in the same league as the Toyota 2JZ or BMW N54 for performance tuning.  In various guises it makes between 90-120 gross horsepower.  But it's a stout engine, and it's possible to increase power output by 30-50% fairly easily and cheaply.  

In my case, it was cheaper to buy a large log cylinder head than have my old one rebuilt.  From there, I decided to also upgrade the carburetor and distributor with the goal of making 20-30 more horsepower.  Here are the details for my build:

• Remanufactured 69-79 Ford 3.3L 200ci L6 w/ 1 3/4" Carb Throat C9/D3/D5/D7 Cylinder Head from Allied Motor Parts. This head came from a 1973 Ford Maverick, but has the larger intake valves.  All Allied does is cylinder heads and they ship nationwide.    

• New Carter YFA 1 barrel carburetor from Mike's Carburetor Parts.  Mike's specializes in carburetors and has a lot of great technical and troubleshooting articles.  They do quality work and are made in the USA.  There are lots of options for carburetor upgrades on the Ford inline sixes, including two barrels using an adapter or modified cylinder head.  This article on Classic Inlines provides a wealth of options and information.  I chose the Carter YFA for the following reasons:

• I wanted something that would work with a "stock" engine, provide good low end torque and drivability on the street, and be reliable and easy to maintain.    

• I am not ready to modify the intake for a two barrel, or do a performance build on my engine.  I also didn't think a two barrel adaptor would provide better performance than the Carter 1 barrel, and I'd lose some of the drivability. 

• The Carter YFA was the standard carburetor for 1969 and 1970 inline 6 engines and a good match for the larger log head.  It was also standard on Ford's 240 and 300 cubic inch truck engines until Ford switched to fuel injection in the 1980s.     

• New Performance HEI Distributor - Besides the large log head, the biggest performance upgrade is to replace the Load-o-Matic distributor with a Duraspark or DUI distributor.  This HEI distributor got good reviews and was 1/2 to 1/3 the cost of either the Duraspark or DUI.  NOTE: You will need to replace the Load-o-Matic distributor if you swap out the Autolite 1100 carburetor because the LoM does not have a mechanical advance.  This article on Load-o-Matics from Classic Inlines will tell you everything you ever wanted to know.  .     

• New ARP Cylinder Head Studs from Vintage Inlines - these are overkill, and are 10 times stronger than the stock head bolts.  But after breaking bolts on several other parts of the engine, I felt the cost was good insurance, especially if I decide to tear down the engine again someday. 

I also bought the Ford Inline Six Performance Handbook.  This book is required reading if you want to rebuild and modify Ford inline six engines.  There is also a wealth of great information and knowledgeable people on the FordSix community website, who also maintain the links to the Classic Inlines tech info pages.  Finally, if you plan to work on one of these engines, please patronize Vintage Inlines, who are the go-to source for all things Ford inline 6.

Over the next few weeks, I plan to write about installing the cylinder head, setting up the carburetor and distributor, and any other tasks needed to get my Mustang back on the road.