Variable length Velocity Stacks??

[Dpmulvan]

Has anyone messed around with variable length velocity stacks for a flat six?? Seems like it might be a worthy/ relatively inexpensive project to undertake if it hasn’t already been done. I see it’s being used on super bikes with success and reliability. What would be the main challenges to adapt for a 911 motor?

[Classic 911]

Looks interesting.


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[JoeMag]

The technique is interesting raising and lowering a complete stack over a smaller one vs sliding. Can get a bigger change. One that they show is a massive change - looks like adding a 3" stack on top of a 1" one.

Would've been good for them to show a dyno across rpm range with stack up and stack down. Then can figure out where to lift. Similar to what you do with an intake with resonance flap.

Mechanically, just about anything can be adapted. One challenge I see for 911 motor is filtration. Their's is inside airbox so no issue.

[stownsen914]

I've seen some vendors that make easy to swap stacks to facilitate this. I think Danst Engineering might be one? Packaging in a 911 can be a challenge, since you need room for the filters as JoeMag noted, and there isn't a ton of room above most 911 induction systems before you hit the roof of the engine bay. Easy enough to test on a dyno though.

[callard]

In the '80s I was using an engine dyno to extract maximum HP from my two liter carbed 911 race engine. I tried many variations of the bolt on "HP increased" parts available then. I played with engine and cam timing (906 cams), different height and shaped intake manifolds, factory vs. aftermarket headers, choke sizes and velocity stack heights (I made adjustable ones).
Every change in stack heights (longer and shorter) hurt the numbers. It seems that the Porsche engineers spent a lot of time doing their homework and getting it correct. Another thing that impressed me was that when a factory intake manifold run was slightly bent (the two outside runs), the inside diameter went from round to curved. None of the aftermarket manifolds did that and lost some flow.
My biggest gains were in compression and more modern exhaust systems (pipe sizing and controllable back pressure).

[MoreGAS]

Yes, Porsche, it is called Varioram.
More gains(power and efficiency) likely found as all OEM's have gone w modern variable cam timing.
That system in the video looks like it would be an on off switch, not truly variable.

Kevin
GAS Motorsport

[Dpmulvan]

In internal combustion engines, a variable-length intake manifold (VLIM),variable intake manifold (VIM), or variable intake system (VIS) is an automobile internal combustion engine manifold technology. As the name implies, VLIM/VIM/VIS can vary the length of the intake tract in order to optimise power and torque across the range of engine speed operation, as well as to help provide better fuel efficiency. This effect is often achieved by having two separate intake ports, each controlled by a valve, that open two different manifolds – one with a short path that operates at full engine load, and another with a significantly longer path that operates at lower load. The first patent issued for a variable length intake manifold was published in 1958, US Patent US2835235 by Daimler Benz AG.[1]

There are two main effects of variable intake geometry:

Swirl
Variable geometry can create a beneficial air swirl pattern, or turbulence in the combustion chamber. The swirling helps distribute the fuel and form a homogeneous air-fuel mixture. This aids the initiation of the combustion process, helps minimise engine knocking, and helps facilitate complete combustion. At low revolutions per minute (rpm), the speed of the airflow is increased by directing the air through a longer path with limited capacity (i.e., cross-sectional area) and this assists in improving low engine speed torque. At high rpm, the shorter and larger path opens when the load increases, so that a greater amount of air with least resistance can enter the chamber. This helps maximise 'top-end' power. In double overhead camshaft (DOHC) designs, the air paths may sometimes be connected to separate intake valves[citation needed] so the shorter path can be excluded by de-activating the intake valve itself.
Pressurisation
A tuned intake path can have a light pressurising effect similar to a low-pressure supercharger due to Helmholtz resonance. However, this effect occurs only over a narrow engine speed band. A variable intake can create two or more pressurized "hot spots", increasing engine output. When the intake air speed is higher, the dynamic pressure pushing the air (and/or mixture) inside the engine is increased. The dynamic pressure is proportional to the square of the inlet air speed, so by making the passage narrower or longer the speed/dynamic pressure is increased.