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Since I am dealing with a problem with my tumble generator (it seems) I have been looking for a good definition of it's purpose. Here's the best so far:
The tumble generators are basically block-off plates placed just upstream of the fuel injectors, which create a vortex in intake air, promoting cleaner burn and leaner air-fuel mixture during cold-start and low-speed operation. The valves, which look and operate like throttle plates missing one side, open gradually beginning at 3000 rpm to allow intake air to flow freely.
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Sport Compact Car Web
 

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I believe they are there in an attempt to better mix the Air/Fuel by making the incoming air turbulent. SCC had a good def. What more do you want?
 

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changes circulation in combustion chamber

They modify the circulation of the fuel air mix inside the combustion chamber at low rpm.

Tumble is the motion you see when a wave breaks on shore, a horizontal turbulent motion, swirl is the motion you see when you watch water drain in a sink.

The open side of the tumble generator valves that stay open all the time promote high mixture velocities at low (< 3000) rpm. Since this high velocity flow is on only one side of the intake port, when the valves open it creates a "tumble" in the mixture in the combustion chamber. This makes it easier to get good emissions and very lean burn at low rpm and low load.

At high rpm (>3000) rpm the valve opens and you get full use of the intake port for better power at high rpm.

Larry
 

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Penguinking took out his TGV's and said there was NO gain.
 
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