What do you guys think - should we try to describe how the SVX cooling system works? And how different issues affect the system with a holistic perspective?
Please feel free to argue, correct, add and comment :-).... my sole purpose is to get an agreement on the basics. This is just free flow of thoughts.... not a comprehensive description nor written in the most logical sequence.
- The thermostat controls engine (and pump) inlet flow temperature, which is the result of all bypass flows and the radiator exit flow combined. This is adjusted by the thermostat - the control "signal" is the bypass temperature. The relative flow rates are defined by flow resistances (by pass circuits vs throttled radiator circuit).
- The thermostat starts to open at 78-80C and is fully open at 91C.
- The water pump flow figures in the FSM are given at 85C coolant temperature (thermostat app 50% open).
- Normal engine exit temperatures are app 87-88C (based on my lap top).
- The fans are triggered on at 92C and off at 88C (again, based on my observation) - engine exit temperature (sort of funny). Vehicle speed sensor 2 and AC operation also have an effect.
- Based on all above, the target engine inlet temperature is 85C. Would be nice to measure at the low part of the block casting - there are plugs which could be used.
- The system design is based on high flow rates and low delta T across the engine - only approximately 7 C. Main reason is probably tight critical tolerances within the engine, efficiency and emissions.
- The bypass flow temperature to the thermostat is very close to engine exit temperature 87-92C. Heater core will have an effect on this.
- To keep the thermostat at least 50% open the radiator exit temperature must be in the order of 80C or more (to keep the engine/pump inlet temp at 85C).
- If the radiator exit temperature is significantly less than that the thermostat will decrease the radiator flow and increase the bypass flow, which - judging purely by the diameters - has a higher flow resistance and will thus lower the absolute pressure at the water pump inlet.
- Highest heat load from the engine to the coolant is in the exhaust valve seat area.
- The control signal for the thermostat is a bulk flow reading at the engine exit.
- Steady state operation may differ quite a bit from fast transients because of natural "thermal inertia" within the system.
- Sudden increase in heat flux from the exhaust valve seat area may cause local boiling because of the total system thermal inertia - the increase in radiator flow (thermostat opening) may be too slow to catch up. This is especially relevant in turbo charged engines.
- The radiator design looks "flimsy" :-) - maybe to keep the exit temperatures up (and thermostat open)?
- Tony has shown that the air flow thru the radiator is quite low - this is due to aerodynamics and other issues. The radiator sealing to the vehicle body is there for a reason - as is the underside plastic protection - the undertray.
- If the radiator is uprated w/o other modifications the radiator exit temperature goes down and this leads to the thermostat staying in a more closed position, higher bypass flow rates and lower pump inlet pressure, higher pump pressure ratio, lower mass flow - all will increase the potential of cavitation at the impeller vanes at high revs. Keeping the fans on all the time (when not required) works the same way. Relocating the thermostat to the upper hose will help in this respect, but it will work against one of the core design points - controlling the engine inlet temperature.
- The more heat we can keep within the process the higher the BMEP -> higher output and efficiency (w/o detonation)
- From the cooling systems point of view the most critical thing fighting detonation is to avoid local hot spots which either cause local boiling or are the cause of steam generation causing catastrophic decrease of heat transfer. This applies to air pockets too - and steam from any kind of cavitation, either at the pump or elsewhere in the system.
- Increasing the margin to local boiling will help in that respect - some have reported potential to increase boost and/or timing a few notches.