Air cooled how?

gwl123 · 03-27-2007, 05:05 PM

I understand the 911 is oiled cooled but how and where is the air cooling the engine?

Jack Olsen · 03-27-2007, 05:36 PM

The engine's cylinders and heads have heat-dissipating fins. The main engine blower (a fan) pushes air over and through those fins. On top of that, there is an oil cooler mounted near the engine case (and sometimes also one in the front right fender). The main fan also pushes air through the engine-mounted oil cooler.

Technically, the engine is both air- and oil-cooled.

rlane · 03-27-2007, 05:38 PM

I have never seen estimates of how much heat is dissipated by the oil circulation/coolers versus direct heat transfer to the large amounts of air forced around the cylinders (w/cooling fins) by that large fan on the rear. I bet some of the real experts have some pretty good data on this

rlane · 03-27-2007, 05:39 PM

see ..Jack beat me too it and he IS rather more experienced.....

patkeefe · 03-27-2007, 06:06 PM

The oil is cooled by air also. Without delving into approach regarding crossflow heat exchangers, you can quantify with this:

Heat rejeced = 1.08 x CFM x Temperature differential

Airflow over the engine from the fan x 1.08 x (air temp difference) air temp into the fan - air temp leaving the fan. Tough to measure precisely this last quantity without burning your hand holding a thermometer under the engine.

Indirectly, you could do some more rigorous calcs with the engine as a control volume...heating value of fuel x fuel consumption rate x displacement x time -heat out the exhaust pipe will leave the actual quantity of heat being transferred by the cooling system.

Oh, and don't forget the oil...500 x GPM x delta T (oil temp in and out of the cooler).

gwl123 · 03-27-2007, 06:15 PM

I understand upgrading to an 11 blade fan would help to cool the engine and adding an extra oil cooler up front; how can I vent more air into the engine bay and expel the hot air. Thought about removing the engine lid grill.

khamul02 · khamul02's Garage

I think there was just a thread on how the 11 blade fan didn't improve things that much. The oil cooler up front is key. It's nice to have a fan up there as well during heavy traffic.

Jack Olsen · 03-27-2007, 09:04 PM

Quote:

Originally posted by patkeefe
The oil is cooled by air also.

I know it's not the point you're making, Pat, but viewed that same way, air cools the water in the 'water cooled' models too. We need to have some sense of what's unique about our cars.

Quote:

Originally posted by khamul02
I think there was just a thread on how the 11 blade fan didn't improve things that much. The oil cooler up front is key.

Except that for about a decade, a front oil cooler wasn't included or considered necessary. With a cylinder head temp gauge and the willingness to disconnect the fan, you could see how much work that blower does. Cool oil does not mean the entire engine is being kept cool enough. If you lose your fan belt, you can cook the motor before you see alarming engine oil temps show up on the gauge.

Quote:

Originally posted by gwl123
I understand upgrading to an 11 blade fan would help to cool the engine and adding an extra oil cooler up front; how can I vent more air into the engine bay and expel the hot air. Thought about removing the engine lid grill.

There is no bottleneck with the grille. Porsche engineers even saw fit to put a heat-releasing AC condenser there. As long as your engine fins are clean and unobstructed, an 11-blade fan will do a pretty great job of cooling the cylinders and heads. The water-cooled-world's belief that you need to get cool air into the engine compartment with scoops or tubes doesn't hold true for a 911 engine at any speed other than idle. When the motor is revving and the fan is working, huge amounts of cool air get sucked into the engine bay.

And the factory front-mounted oil coolers do a great job, too. Depending on your motor, a Carrera's cooler might be needed for track use.