47-55.1 Radiator question

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Unless that radiator is plugged, it should be more than capable of cooling the engine. Between aluminum construction and sheer size you should be more than covered at low load. Pusher fans are fine but I would try to build simple box or tube shaped ducts between radiator and fan to prevent air dispersing rather than install a shroud that restricts airflow when moving. And yes, the shroud in the pictures is very restrictive.

An air duct under the radiator can be very effective. The goal is to create a low pressure pocket behind the dam which draws air down through the engine comparment. It can be so effective that you can see the hood get sucked down on the highway. If the truck is overheating at idle the air dam is not the answer.

I'm not sure why folks say retarding timing will cause overheating. This is generally not true. When spark occurs it initiates a reaction which takes time to complete. We use spark advance to try and match peak combustion pressure to crank angle at 6-20 degrees after TDC. We start combustion before the crank is at TDC and the reaction continues even after the piston reaches BDC.

Retarding timing will effectively make peak pressure occur at a greater amount of crank degrees ATDC. To a point, the crank angle is greater, mechanical advantage created by the lever arm of the crank is greater, and cylinder pressure is lower but more effective. This means less wasted energy is used to heat the pistons, head, and block. But it also means the combustion reaction continues into the exhaust and can heat headers / manifold.

If spark is advanced we see a different result. Pressure begins to increase as the piston is forced to TDC by the crank. Pressure created by combustion attempts to force the piston down the bore while momentum and force created on other pistons is working to move the piston up. If there's too much spark advance we start to see big pressure spikes and heat as the "up piston" and "down piston" forces in the engine collide. First, engine oil endures extreme pressure as the rod bearing is forced toward the crank. This very quickly heats the engine oil which can result in oil thinning and even contact between bearing and crank. If the engine continues to operate with excess advance, energy that cannot be used to move the piston down is shed in the form of heat. The piston tops, cylinder head, and block are be quickly heated. Heat will transfer out of these parts but it happens more slowly than it is being added. The only way for pistons to cool are through contact with oil, by splash or from oil cooling jets, and by contact with the cylinder walls and minimally through windage. The primary cooling method for the heads and block is through heat transfer, first to coolant then to radiator then to air. It's very slow and can easily be overwhelmed. If you have old, heavy cast heads and / or a block with poor circulation paths it only delays heat transfer. And if any steam pockets form, well, you've just reduced heat transfer to near zero in those areas.

The absolute best approach is to reduce the amount of heat that's transferred to the heads, block, and piston. Especially on an engine that doesn't need to achieve maximum power or efficiency. I can tell you from experience that 2-3 degrees too much advance at cruise is enough to quickly take out bearings on a forced induction engine without any audible or performance signs there's a problem.

I would not be afraid to reduceg peak advance by 2 - 3 degrees and then monitor coolant temp. I would seriously consider installing an oil temperature gauge to monitor the effects of timing changes and changes in weather or fuel. And I would think about using a "wide band oxygen sensor" to check the air:fuel ratio of the engine as it operates through idle, acceleration, and cruise. Either I would buy one to use while driving or I would rent time on a chassis dyno. These tools and methods are frequently used in todays world of EFI tuning. When these tools are used correctly engines achieve higher levels of power without sacrificing reliability.

I would also consider adding an engine oil cooler to help ensure engine oil is not overheated. The engine oil cooler could be placed in a different location so as not to affect air intended for radiator cooling. An oil cooler placed under the radiator, like an air dam, might be very effective.

Your mileage may vary.