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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Speedy Squirrel.


I like what our are saying and love the video.

Now I am just spit balling here but here is what I am thinking.

With a carb it only pulls fuel when the intake pulls air. Little of the fuel is going to sit in the intake tract.

The vid is of an electronic injection of unknown timing and duty cycle.

CIS sprays fuel full time. I do not know that it puddles but some of it probably coats the intake port like when we use Windex on our windshield.

Thus, no mater how well the injector atomized the fuel it may not matter as the spray settles.

I like your point about the beginning and ending lift making for the highest volocity. This is true.

However, on a 930 when the valve is only about 30% open the port determines the velocity.

Also, when the valve opens and closes it is not when the piston is moving very fast do to the poor angle of the rod and crank shaft. The bulk of the air filling the cylinder is going to be when the sweep of the crank is at its highest, the valve is mostly fully open, and the port is determining the air speed.

In the vid after the initial rush you can see to fuel sheathing down the ports.

I could be all wrong but this is what I am thinking.

I just do not see the runner being long enough to make for much of an increase in cylinder filling no mater what the port size is.

A 3.3's 32mm intake ports have about a 40% higher velocity than those on a 993TT.

They are probably that much more restrictive.

As to the WUR's. Many makers CIS WUR's have as you noted a provision to change the AFR under load. When vaccum in the intake goes away, control pressure is lowered and the AFR's get richer to support higher output. I do not know the goals, but say 14.7 is the goal at idle and cruse, 13/1 might be the goal with acceleration.

The 3.3 Turbo dose not have this function. It keeps its idle and cruse AFR untill boost reaches about 5 psi, then control pressure is lowered, even more so than on N/A CIS cars.

The 3.0T however did, it enriched with loss of vac, not with achieving a boost hurdle.

A 930 has a lot of volume between the metering plate and the throttle body.

When the TB is opened, the metering plate is isolated by the turbo compressor, the metering assembly has sizable volume, the intercooler, and all the many feet of plumbing that connects them. Thus, there is a lean surge or condition with acceleration off idle and for a brief time off curse. Instantly lowering control pressure with acceleration helps and I found it quite noticeable on my turbo.

I believe this is part of the reason many 930 tuners recommend setting idle CO to 3 to 3.5%. This is closer to a 13/1 AFR that better supports making power.

I could be off on this but this is where my studies and gut are taking me so far.

Last edited by 911st; 02-05-2010 at 08:06 AM..
Old 02-04-2010, 08:02 PM
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