Vent or Venturi ?

[wit61]

Please settle a dispute between two guys who barely passed physics :

I recently yanked out the A/C plumbing in my E leaving two 2" diameter holes (one on either side of the engine bay) where the hoses and grommets once were.

Guy #1 maintains that those holes should be closed as the upper engine bay is a carefully designed chamber which is at peak efficiency only if sealed as the engineers intended. Guy #1 also says that these holes will be exit pathways for air, not entrances due to pressure, etc.

Guy #2 says "Poppycock!". Engine tin tolerances, throttle linkage holes, etc create many areas where the bay is not sealed, and in fact these are welcome points for air exchange based on temp, pressure, etc. These holes from the wheel-well will help, so leave them open.

Is either guy correct ?

[Steve@Rennsport]

Guy #1 wins (a prize),....

The engine compartment needs to be sealed so that no "false" air can enter except from the grille.

This maximizes cooling as the fan will draw air from the path of least resistance. You would be amazed at the differences in oil & cylinder heads temps when the engine sheet metal is not properly sealed or has open holes in it.

[roblav]

Hmmm.... I'll say something completely different.

1) Since it is well documented that these cars create lift at the rear end at higher speeds, then there is a low pressure area above the decklid.

2) The fan rotation increases the lift because it removes air from above the deck lid and forces it through the engine toward the ground under the car.

3) If it were my choice, in a racing vehicle, I would want to bleed air from under the car to above the car. You want the low pressure area under the car, not above it, so bleeding air from a high pressure side to a low pressure side is good - in this case.

4) But of course, we are all not racing these cars...

5) My opinion is that these cars were designed wrong - but of course all that was around the time of the development of the 908/910/917 cars... before 1977 with the creation of the Lotus tunnel car.... But... the Brabham suction car was designed around that time too.... The fans on our cars should be drawing air from below the engine toward the low pressure area above it. Anyone ever try running the airflow that way on our cars?

[wit61]

I gather from the responses that since I am more concerned about engine temps than high speed downforce, I should take Steve's advice and seal them up. It also wins me a beer since I am guy #1.

[Cornpanzer]

Ususally the big concern is whether the upper engine area is sealed from the hot exhaust temps. This is why the engine seals are so important in these cars. However, a couple holes leading to the wheelwell area would have negligable effect on engine temps.

[911pcars]

You need clean, relatively cool air for the following purposes:

1. Air for the engine intake
2. Air for the cooling system

If you bring air in from the bottom, the engine intake air is then sucking in heated air used to cool the engine.

If you bring air in from the bottom, you're also introducing a lot of road debris and dust.

The engines were not designed to cool with this air flow direction.

Want to try rotating the engine 180º and use updraft carbs? Watch out for reduced ground clearance and oil leaks.

Sherwood

[roblav]

Nice comment Sherwood - lol.

Updraft carbs eh? How about fuel injection? Will the MFI work upside down?

[Scott Clarke]

Porsche has tried "updraft" cooling. On the 917, if memory serves. Didn't work. This is discussed in "Excelence Was Expected."