Volume 1 Issue 3 - Diesel Articles

LLY Underhood Airflow

For the same kind of thing happens with the LLY, only with heat, not sound. As you drive happily down the road, the air coming into the airbox is generally close to ambient temperatures, due to the air pressure on the front of the vehicle. However, the stock LLY airbox is fully open to under-hood air. When the engine temperature starts to rise from towing, that warmer air from the engine does influence the air that is pulled in by the airbox. The vicious cycle begins when the cooling fan kicks in. The hot air coming off the cooling stack gets blown over by the fan to the area of the airbox. This hot air is then sucked into the air intake and ingested by the turbocharger and, ultimately, the engine. That in itself is not a good thing – remember the hotrodder principle: cold air is best for power – but it is only the beginning of the thermal feedback loop.

We have to take into account two other components in the Duramax intake system: the turbocharger and the intercooler. First, the turbocharger needs to compress the incoming air to the desired pressure in the manifold, around 20 PSI at peak fueling in this case. At this point, two phenomena cause the temperature of the compressed air to rise sharply over its inlet temperature:

1. The action of compressing air
2. The heat generated by the turbo compressor itself.

No compressor is ever 100% efficient. Extra heat energy is added due to the inefficiency of the turbo compressor – normal operation for any turbocharger or supercharger. Enter a second component, the intercooler. It improves performance dramatically by cooling the heated, compressed air that comes from the turbocharger. This system works reasonably well for its intended purpose, creating a dense, relatively cool, air charge for the engine. But what happens to air passing over the intercooler to cool the air inside coming from the turbo? It is heated: not only by passing over the intercooler but also by passing through the radiator.

These two graphics illustrate the airflow of the LLY engine. The first (above), occurs when the cooling fan is “off”. Notice that the area of the air induction receives relatively cool air. The second (below), occurs when the cooling fan is “on”. It is clear that the operation of the cooling fan sends heated air into the air intake area – the beginning of a disabling thermal feedback loop.

This creates warmer underhood air temperatures. Some of this warmer air finds its way into the air intake. This becomes a particular problem when the cooling fan engages: then there is tremendous temperature increase in the air that is pulled into the intake. Of course, that heated air gets fed

right back into the turbocharger, only to be heated much more as it takes yet more energy to compress it to the desired pressure. The superheated air ends up being cooled by the intercooler, this time causing the ambient air that flows over the intercooler to get much hotter. That now extra-heated air ends up flowing over the radiator and back in the intake again and our thermal feedback loop is in full, red tide bloom.