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Speedy Squirrel Speedy Squirrel is offline
Rocket Scientist
Join Date: Jun 2008
Location: Detroit
Posts: 896
The atomization of CIS is very poor by today's standards, but that was not the purpose of the 32mm port.

Carburetor 911's (2.7's) have 36mm ports, and they have even worse atomization. The CIS 3.0L has 39mm ports.

The "other CIS WUR's lower control pressure with loss of intake vacuum upon acceleration" statement is misleading. The warm-up regulator enriches the full load mixture on an N/A engine due to loss of vacuum by lowering control pressure, but the acceleration enrichment is from the metering plate overswing (see "Automotive Electric/Electronic Systems" by Robert Bosch GmbH, any edition).

Paul Frere states in "The Porsche 911 Story", the turbo port size is for improved load speed torque, which as I mentioned before, is a very common engine design tactic.

The evaporation of fuel is dominated by droplet size, fuel temperature and air temperature. Port velocity is effective primarily on gaseous fuel/air mixing. The intake valve produces flow velocities many times higher than the highest port velocity, and that is where the evaporation action is at; when the intake first opens. Here is a link to a video showing the actual effect: YouTube - In Cylinder Video ( How a 4 stroke engine works ).

I would not be afraid of ignition problems due to increasing the port size.

The S/C piston top is another long story ...
Old 02-04-2010, 08:00 PM
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