So if reverse flow is so awesome, why don't all the manufacturers use it? The answer, of course, is cost. Reverse-flow cooling, while inherently better than standard-flow systems, requires some additional engineering for it to work properly. In any given engine, under most operating conditions, the coolant is boiling somewhere in the engine-usually in the coolant passages in the cylinder head. This is because the walls of the combustion chamber are much hotter than the boiling point of coolant. The layer of coolant directly in contact with the combustion chambers, particularly near the spark plugs and exhaust valve seats, is being vaporized. This is called nucleate boiling; it is normal and does not pose a problem to the operation of the cooling system as long as the vapor is not allowed to accumulate. Usually the coolant flow rate is strong enough to sweep the vapor away, constantly dousing the area with a steady stream of liquid coolant, and any vapor bubbles that did form usually condense back into the coolant stream. But if a steam pocket does form, the vaporized coolant can impede the flow of the liquid to that particular area of the water jacket and a hot spot will develop on the inside of the chamber-setting up a situation that can cause detonation at best, and engine failure at worst. This is because vaporized coolant can't transfer heat as efficiently as liquid coolant; it essentially acts as an insulator, allowing the temperature in the area to spike.
In a standard-flow system, the pattern of coolant flow is always carrying vapor bubbles up and out of the engine. In a reverse-flow system, however, the coolant flow rate is not strong enough to "blow" the vapor from the cylinder heads down through the engine block and out to the radiator. The vapor, instead, will remain trapped in the cylinder head, eventually filling it with steam until the engine melts down.
 Here are the components of...  Here are the components of the vapor-venting system. Steam enters the vent tube through the banjo fittings (A) on the rear of each cylinder head. It is routed through the vent tube (B) into the throttle body (C) before entering the overflow reservoir. |  The water pump is geardriven...  The water pump is geardriven off a toothed wheel (arrow) machined into the cam gear. The driveshaft protrudes through the timing cover and engages a splined collar that drives the water pump's impeller. |  Here is a cutaway of the Opti-Spark's...  Here is a cutaway of the Opti-Spark's optical trigger and the electronic signal it generates. |
The solution to this problem is very simple. The cylinder heads are vented at a location where coolant vapors would normally collect. On the Gen II engines, these vents are on the back of the cylinder heads. Steam and a small amount of liquid coolant escape through a banjo fitting into a metal tube that eventually runs to the coolant overflow reservoir. The opening of the banjo fitting acts as a restrictor-it is small enough to hinder liquid coolant from flowing through it, but steam will easily enter. As the steam passes through the banjo fitting, it experiences a pressure drop that allows it to condense more easily back to a liquid state.
This vent tube is an essential part of the cooling system. If you're pulling an engine from the junkyard, make sure that the vent tube is in fact there and that it's not kinked or damaged.
We talked to several engine builders to get their take on reverse-flow cooling. Their reactions were mixed but affirmed what we suspected: Reverse-flow cooling works well-better, probably, than a standard-flow system. But most cars just don't need it. For the type of driving the average person does, a standard-flow system does the job adequately, and without the added parts needed to make a reverse-flow system work. For a performance application, though, Chad Golen of Golen Engine Service and Mark McKeown of McKeown Motorsports Engineering are firm believers in reverse flow and say that the LT1 wouldn't be able to generate the power levels it's capable of without the reverse-flow design. McKeown actually modified one of the Ford Clevor engines he built for a recent Jegs Engine Masters Competition to run a reverse-flow system. "It just makes sense to cool the hottest part of the engine first," McKeown tells us. Karl Ellwein of Ellwein Engines-a builder who deals solely with LT1s-is a little less enthusiastic. He says that reverse flow may not contribute that much to the engine's performance; it doesn't hurt it, either, but it really limits the amount of aftermarket parts available for this engine.