Thread: Convert to reverse flow cooling

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I like your style Max –

Possible? Well, years ago Smokey Yunick developed and built reverse-flow system for some of his Trans-Am race cars – but then again Smokey had pretty much unlimited resources. Several independent engineer / engine builders have toyed with reverse cooling but as you already know the LT1 was the first “mass-produced” engine with reverse-flow cooling.

There are differing opinions on the effectiveness of reverse-flow cooling. It would appear to be better than standard-flow systems but it certainly requires additional engineering to make it work properly. In a “standard flow” engine the coolant is typically boiling somewhere in the engine – more than likely in the coolant passages within the cylinder head as this is the hottest point the “coolant” comes into contact with. The temperature of the interior engine walls surrounding the combustion chambers are far above the boiling point of any known coolant. The coolant directly in contact with the back-side of the combustion chambers, particularly near the spark plugs and exhaust valve seats, turns to vapor. This forms a turbulent natural convection where the vapor forms bubbles that collide causing a condition known as “nucleate boiling”. This condition is not harmful as long as the vapors are continually moving and not allowed to accumulate (like when an engine runs cry – very bad things happen). Under normal engine operating conditions, the coolant flow rate is sufficient to move the vapors along while continuing to flood the area with a steady stream of liquid coolant. As the liquid hits the vapors, any of the above mentioned “bubbles” that may have formed condense back into the coolant stream.

If a steam pocket forms, the vaporized coolant may prevent the flow of the coolant to that area of the water passage and a hot spot will develop on the inside of the chamber. This condition can cause detonation and potentially lead to engine failure. Vaporized coolant cannot transfer heat as efficiently as liquid coolant so in essence, it acts as an insulator, allowing the temperature to go into the stratosphere.

In a reverse-flow system, the coolant flow not strong enough to expel the vapor from the cylinder heads down through the engine block and out to the radiator. The vapor, instead, will remain trapped around the cylinder head, where it will become super heated and in all likelihood - melts your engine.

GM came up with a solution to this problem. They vented the cylinder heads where coolant vapors would normally collect. On the Gen II engines, these vents are on the rear side of the cylinder heads. Steam and trace amounts of liquid coolant escape through a banjo fitting into a tube that goes into the coolant overflow reservoir. The opening of the banjo is small enough to limit liquid coolant from flowing through it, but allows steam to easily pass. As the steam travels through the little hole in the banjo fitting, the pressure drop and it condense to a liquid state as it returns to the overflow reservoir.

So to “convert” from standard to reverse flow cooling, you’ll have to have the steam holes and a means to regulate / restrict the passage of steam vapor and fluids. You’d also need to capture and return to the cooling system.

Picture of how an LT-1 Reverse Flow (from GM Manual)

Regards,
Glenn