What to do about gt-40 explorer heads not having thermactor holes

I continued to be curious about the thermactor-to-head function and found a few interesting and plausible sources. It doesn't have much to do with the cats at all - or engine temperature, though the exhaust flowing through the heads would help warm things up (even without the AIR tubes attached). It's basically trying to complete the burn in the exhaust manifold, which would incidentally contribute to the protection of the front cats. Which also wouldn't necessarily be a good thing if you were running tube headers I'd imagine unless you like those blue / burned.


Here's what they said:

The P's aren't drilled from the factory. The EGR passage is probably a remnant of the original design and casting molds, but since the later model Explorers didn't use the thermactor passage, Ford simply cut one of the last production processes which was drilling of the crossover passage as a means of reducing cost. The Explorers took the injected the air directly into the exhaust manifold instead of through this thermactor passage. (jozsefsz comment - I guess this would be an interesting work-around, to weld a couple of bungs onto the exhaust manifold instead of drilling the heads).

Ford term for an early emissions control system, installed on cars from around 1965; by 1967, California specification vehicles were installed with it as standard. The name is a contraction of 'Thermal Reactor', and this type of system is also known as AIR (air injection reaction).

The purpose of the Thermactor system is to reduce the emission of unburned fuel and the products of incomplete combustion. It does this by injecting air into the exhaust manifolds; the heat of the exhaust gases is sufficient to ignite the combustion of anything burnable still remaining in the exhaust. The combustible gases, mixed with oxygen from the injected air, burn in the manifold.

On a later-model vehicle fitted with a catalytic converter, the Thermactor system also helps supply the oxygen needed by the catalysts to do their work.

The air comes from an engine-driven air pump, pushing air through rubber hoses to the distribution system for each manifold. In my 1967 Ford Thunderbird, this involves metal pipes, while in later engines this is integral to the intake manifold, to holes drilled through the cylinder head all the way through to the exhaust manifolds.

The system is complicated by two factors. The first is that if the throttle is closed suddenly, the Thermactor system can make the engine backfire; to prevent this, a device called the backfire diverter valve, operated by high manifold vacuum, diverts the air flow to the atmosphere. The second issue is that high exhaust system backpressure could force exhaust gases back through the Thermactor system. This is protected against by a one-way check valve in the system.

By itself, the Thermactor system certainly does improve emissions, but nowhere near any remotely modern standard; it does however provide a good adjunct to a catalytic converter. Ford used the Thermactor system at least through the 1970s and 1980s in this form.

Disadvantages of the Thermactor include power loss from driving the pump, from increased backpressure in the exhaust system, and greatly increased heat in the exhaust manifolds from the combustion taking place there. Also, it means yet more stuff under the hood and getting in the way of the engine. Although it's technically illegal, it's common for owners of presmog vehicles to remove the Thermactor system and replace the components with those from an unequipped car.

Just stumbled across this and a lot of my questions are answered. In review, my 96 Mystic Cobra had NO cats yet passed emissions in AZ (Tucson and Phoenix metro). Being an OBD2 system they just plugged into the port. They would only check for cats on pre OBD2 cars. However they also sniffled the pipe. Commyfornia is back to 67 and Nevada goes back to 1972. Total BS imo. I'm out of Las Vegas and back to AZ but will strip the emissions out of my 88 LX 5.0.