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Speedy Squirrel Speedy Squirrel is offline
Rocket Scientist
Join Date: Jun 2008
Location: Detroit
Posts: 896
The port area is related to momentum tuning. This isn't a Porsche invention. All engines are designed with this concept in mind.

Momentum is mass x velocity. The air has mass. Smaller cross-section ports produce higher velocity than larger ones, so for the same mass of air they have higher momentum.

When the intake stroke reaches bottom dead center and starts up on the compression stroke, the intake valve is still open, and air continues to flow INTO the cylinder. Why? Because the velocity of the inlet flow comes to a halt in the cylinder. This converts the momentum into pressure, which crams more air into the cylinder. The higher the momentum, the more air that gets crammed in.

The down side is that small cross section ports produce flow losses, so it takes more pressure to get air to flow. For the same boost you will get more power at the top end with larger ports.

Thus, port sizing is a compromise for low speed torque verses top end power. I'm sure when Porsche used EFI, full ignition mapping, with short, pulse conversion exhaust manifolds and twin turbos on the 911tt they found they had improved low speed torque so much that they could do with less momentum charging, so they opened the ports up some to make more power.

I would say an EFI car running with ignition timing control would be a good candidate to open up the ports a bit. I would personally not go over 40mm on the intake side for a single turbo 930.
Old 02-01-2010, 11:51 PM
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