Building The Perfect LT4 Cooling System! Pressure Vs Flow... What is Too Much Or Little?
Over the past two years... I've heard it all!! Every Possible combination for the LT4's Supercharger Cooling System and no matter what people do, the air is always there, robbing power!!
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"I Just Bled The Supercharger... Where Is The Air Coming From?!?"
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"I have the TriFecta of LT4 Cooling... THREE Heat Exchangers!! My MATs are still High!"
- "I've added a bigger pump, a fender tank and an Ice Tank in the trunk...where's the Air Coming from?"
So let's think about this...
The SC pump and the number of Heat Exchangers (HX) and/or auxiliary tanks contributes to the creation of bubbles in your Supercharger's cooling system. Here's how, but let's first tap into your knowledge of a 2-liter Bottle of Sprite. When it is sealed, the Sprite is perfectly clear... but opening the cap too soon causes thousands of bubbles to magically appear, creating more pressure and expanding!! The bottle is under pressure from the carbonation inside... and when you release that pressure, the pressure change versus thermal dynamics releases the oxygen in the mix and forces the oxygen molecule to escape! Your LT4 Supercharging Coolant system is not far off from this chemical reaction!
For the sake of this technical description, the term "Water" refers to the Supercharger's Coolant mix, however most motorsports do not allow the use of any glycol-based coolants.
The SC pump and the number of Heat Exchangers (HX) and/or auxiliary tanks contributes to the creation of bubbles in your Supercharger's cooling system. Here's how, but let's first tap into your knowledge of a 2-liter Bottle of Sprite. When it is sealed, the Sprite is perfectly clear... but opening the cap too soon causes thousands of bubbles to magically appear, creating more pressure and expanding!! The bottle is under pressure from the carbonation inside... and when you release that pressure, the pressure change versus thermal dynamics releases the oxygen in the mix and forces the oxygen molecule to escape! Your LT4 Supercharging Coolant system is not far off from this chemical reaction!
For the sake of this technical description, the term "Water" refers to the Supercharger's Coolant mix, however most motorsports do not allow the use of any glycol-based coolants.
Diving into the technical realm now!!
The affect on bubble creation depends on whether the bubbles are created by gaps in the system, pockets where air can collect or water vapor in the form of steam due to the chain reaction of moving molecules. If its from the air pockets, the solubility of gases (like oxygen and nitrogen from the air) in water increases as temperature decreases and increases as pressure increases... this is why I keep telling people having high pressure pumps is NOT the answer, but having the flow without pressure drops across the system is. In the LT4, it is common to see the temperature drop from 200°F to 120°F as the water makes a the pass through the system. As this cooling removes BTUs (the measurement of heat energy), the water's capacity to hold dissolved gases, or bubbles suspended in solution decreases. The issue begins as soon as the water leaves the hoses under pressure created by the pumping action, and it expands in the multiple tanks or heat exchangers... with this expansion, the pressure will drop, causing dissolved gases to escape out of water and form bubbles... this is known as outgassing.
The net effect on dissolved gas bubbles depends on which change is more significant, but the pressure drop is the more common OOPS for poorly designed, multiple capacity SCHX systems. This issue becomes even more prevalent after shutting down a hot engine without idling or bleeding following a track session or hard pull where the bubbles degas, finding their way to the highest location in the cooling system.
Mechanical Creation of Bubbles in a Sealed System:
Mechanical Creation of Bubbles in a Sealed System:
Cavitation happens when the impeller of the pump spins faster than the fluid can maintain contact with its vanes. When this happens, the molecules of the fluid are heated by the friction of the water being spun at force, no doubt this releases the air molecules suspended in the fluid. Imagine a boat propeller and how it makes a trail of bubbles on the lake, the faster/harder that prop spins the more bubbles are produced and this is called "Slip" or a "loss of efficiency". Air bubbles are now a byproduct of the pump pulling water from a low pressure that is flowing with lower viscosity and higher speed, because hot water flows faster as the molecules are less dense with lower friction. Bubbles from water vapor form when the inlet pressure drops to or below the water's saturated vapor pressure at that temperature, then being pressurized by the rotor function and pushed into the hose to soon expand and release that pressure upon entering the heat exchanger or auxiliary container.
The direct answer to the question, "I Just Bled The Supercharger... Where Is The Air Coming From?!?" is answered by... the temperature drop from an estimated 200°F to 120°F lowers the saturated vapor pressure of the water combined with the pressure drop going from the pump to the HX which reduces the external pressure on the water giving you the air bubbles you keep finding! If the pressure in the HX drops below the saturation pressure at the given temperature, even at 120°F... vapor bubbles will form and expand due to Boyle's Law. Think about the bottle of Sprite! Now, imagine if you add ICE to a tank, and when you do that... you actually push this phase shift to a greater temperature drop and can create MORE bubbles!! Just remember that colder fluid has a higher density, thus more drag and slower movement which actually creates more friction with the surfaces. This is a very common source of problematic bubbles in pumps and other hydraulic machinery like the pumps and system designs I developed for Commercial Jets that could reach altitudes greater than 40,000 feet where its -60ºF outside!!
The direct answer to the question, "I Just Bled The Supercharger... Where Is The Air Coming From?!?" is answered by... the temperature drop from an estimated 200°F to 120°F lowers the saturated vapor pressure of the water combined with the pressure drop going from the pump to the HX which reduces the external pressure on the water giving you the air bubbles you keep finding! If the pressure in the HX drops below the saturation pressure at the given temperature, even at 120°F... vapor bubbles will form and expand due to Boyle's Law. Think about the bottle of Sprite! Now, imagine if you add ICE to a tank, and when you do that... you actually push this phase shift to a greater temperature drop and can create MORE bubbles!! Just remember that colder fluid has a higher density, thus more drag and slower movement which actually creates more friction with the surfaces. This is a very common source of problematic bubbles in pumps and other hydraulic machinery like the pumps and system designs I developed for Commercial Jets that could reach altitudes greater than 40,000 feet where its -60ºF outside!!
Let's Wrap This Up...
I hope this description of why adding more pumps, high pressure drops and developing a system like I have with one main SCHX that slows the fluid flow and has an extremely low pressure expansion across the entire system works flawlessly for the LT4 platform. Any pressure drop is the primary driver for bubble formation in a flow scenario, either by releasing dissolved gases or by causing the water to flash to vapor due to cavitation as the pressure falls below the vapor pressure. The temperature change alters the fluid's properties, its viscosity, vapor pressure, gas solubility and influences the degree to which these actions occur. I have spent the past year perfecting how to slow the process of cooling so it will remove the most amount of heat without triggering a vapor point and more air in the system. I have also added aids to alleviate the pressure drops with the bleed-valve so ONLY COOLING is the greatest change and this will increase power for your LT4 without the need for the ECU to pull timing due to air in the system, pump cavitation or higher MATs!
-David A. Wilks
