T4 Engine tin: thermal design Options

[914werke]

So seeing a product recently got me thinking about the result of its application & my own observations of how well (or not) the T4 handles air flow, Engine block cooling & resulting oil temperatures. (IMG:style_emoticons/default/smile.gif)

Its somewhat well known that ...in the 914.. the T4 suffers from less than optimal cooling from the front mounted impeller/fan particularly to the #3 Cylinder
& that it is vitally important to seal the tins multiple openings to contain as much laminar air flow front to back over the finned cylinders.
In a gross approach, the goal was to seal the engine compartment "intake" air on top from the eng. heated "cooling" air below the tins.
When a motor is new and all its components serviced & refreshed, the design does, or did, a pretty good job.
After years of use (neglect), elements (oil dirt mice ext) conspire to restrict that air flow.
As well as eng. heat causing hardening of rubber pieces that are intended to help contain that cooling air below.
Now almost 50 yrs on I see fewer & fewer "STOCK" engine configurations which you can guess usually means more heat to deal with.
Jake addressed the cooling air via his(?) DTM solution, but IMO its not very practical for stock or near stock motors (& expensive)

The go-to response to eliminate that heat is usually an additional or a remote oil cooler which comes with its own set of challenges. (IMG:style_emoticons/default/confused24.gif)
How about KISS (IMG:style_emoticons/default/biggrin.gif) Has anyone used any of the Fiberglass based stick-on reflective heat shielding products on the engine-sides of the tin?
Idea being rather than allow heat absorption to all the those steel pieces reflect it back to carried away by the cooling air?

BTW Im not a Porsche engineer...but I did stay in a Holiday Inn last night (IMG:style_emoticons/default/happy11.gif)

[Superhawk996]

Bottom line: reflective heat shield on the interior of the tin isn’t going to amount to any noticeable cooling improvement.

Why not?

There are three modes of heat transfer taking place in cooling an engine.

Convection (this is the dominant one via the fan)
Conduction
Radiation

Let’s talk about radiation 1st. Radiation emitted from an object (the engine) is not affected by what surrounds it regardless of whether it is surrounded by air, steel tin (914) or a fiberglass shroud (911). Radiation emitted from the object is determined by the temperature of the object to the fourth power. I.e temperature of the object is the dominant factor.

Radiant energy travels though a vacuum. Radiant energy travels though gasses (air) with minimal loss of energy. Radiant heat energy isn’t effectively cooled until it is absorbed into another object, at which point that object is cooled by either convection or conduction.

Even if you were able to prevent the radiant heat energy that is emitted from the engine from being absorbed into the tin completely, you would still have heat conduction into the tin since the tin is attached to the heads and the case.

So you might ask . . . why not insulate the tin from the engine to reduce the thermal conduction?

Let’s consider 911 cooling shrouds that are fiberglass. We could also consider the new carbon fiber T4 “tin” that Type 4 Store is now offering for the 914. In either example, the conduction from the engine to the “tin” is greatly reduced because fiberglass and carbon fiber are not as thermally conductive as steel. Would you agree that neither of these (better insulated) shroud configurations or materials make a huge difference to the overall cooling?

Finally, don’t forget that the inside surface area of the tin (or 911 shroud) is also being cooled by forced air convection. By placing what will effectively be insulation layer inside of the tin, you’re losing the contribution of convective cooling of the tin that has absorbed that heat radiation the engine produced. Some heat is being conducted out of the engine into the tin by conduction, and then the inside surface area of the tin is then convectively cooled by the fan (on the inside) and ambient engine bay air on the outside of the tin.

Do you want to lose this convection cooling effect that the tin provides even if it is very minimal?

Sorry for the long answer.