BobbyT

Okay, so the shop manual says, "Checking resonance flap for operation: When the ignition is switched on, the resonance flap is closed by vacuum action. When the ignition is switched off, the resonance flap is opened (rest position). The opening and closing is audible in the engine compartment." And so it was before I did the 30,000 mile service. However, I don't hear it now when I cycle the key on and off. Can someone point me to the location of this component, and maybe shed some more light on this topic?

dhicks

What the hell is a resonance flap....surely Porsche did not put some mechanically operated flap to improve sound.....that was operated by turning the ignition key.....I am fascinated

BobbyT

Well, maybe the 95 doesn't have this flap...I've been looking at the manual, and I see what I think it is on a diagram of the Varioram intake, where it is refered to as a "tuning flap". Maybe what I was hearing before was the fresh/engine air flap next to the blower on the left side of the engine.

tessicini

My understanding is that the later 993's (guessing 96 thru 98) have varioram. Varioram is a technique for tuning the length of the intake path that the air travels on its way to the intake valves. This path wants to be long at low rpm and short at high rpm. I think at 5800 rpm the "resonance flap" is actuated to change the length of the path incrementallly. I think the flap is actuated at start up (key on) just to make sure it is working properly.

Bill Verburg

All 964 and 993(including the vrams) have the resonance flap . It woks as the original poster described and yes you should be able to hear it. It's purpoose is to improve air flow and cylinder filling at both low and high rpm

TomF

Bill, as usual, is spot on. The 95 and earlier cars had the two-stage resonant induction. It varied the length of the intake runners by use of this flap to generate more power. Varioram cars, generally 96-98, had a three-stage system. This included the flap and intake runners that actually changed length. Take a look at the schematics in the PET system. It is an ingenious system and it is partially how Porsche was able to fill in the torque curve on the later cars. AFIK, Panorama had an excellent article back at the Varioram's debut descibing the engineering and function. I think maybe in late '95.

BobbyT

Bill--thanks for the clarification. My 993 is not Varioram. Can you help me locate the resonance flap?

BobbyT

I feel like a dunderhead, but I can't find this "resonance flap" or anything that even looks close in the PET...if someone else can find it, I'd sure appreciate a page/item #.

Bill Verburg

Here is a description of the operation on a vram, the 964 and thru '95 993 don't have item #4 , the sliding sections. The resonance flap works the same in all of theses engines


A) Vibrating tube induction

If the gap (5) between sliding section (4) and central
intake distributor (6) is closed, the intake system is a
single-chamber vibrating tube induction system. Intake
air flows from the central intake distributor via vibrating
tubes designed for maximum torque to the cylinders.

B) Resonance induction

If the sliding sections (4) open, switching the system
from vibrating tube to resonance induction, the air flow
branches downstream from the air mass sensor (1).
One branch is connected to the central intake distributor
(6) of the single-chambersystem and one to the r~sonance
system (7) below the intake distributor.

From the central intake distributor, part of the intake air
flows through the intake pipe extensions,which act as
resonance tubes, directly to the intake funnels of the
short vibrating tubes and to the cylinders. The main intake
air stream flows via the second throttle flap and
the first large resonancetube of the resonance system
(7) to the resonancechambers.The resonance induction
system optimizes air flow to the short vibrating tubes.

C) Resonanceinduction with open tuning flap

The tuning flap (3) is installed in the second large resonance
tube. Initially,the tuning flap is closed. At a defined
enginespeed (5920 rpm), the tuning flap is opened,
increasing the flow area between the resonance chambers.
Air now flows to the resonance chambers via all
the connecting pipes.

Switchover conditionsfor intake pipe extensions
and tuning flap

1. When the ignition is switched on, the DME control
unit activates the solenoid valves. If the engine is
started, the intake pipe extensions and the tuning flap
are operated.When the engine has run up to idle
speed, the signal to the solenoid valves is deactivated
and the intake pipe extensions and tuning flap
are set to their basic positions.
2. If the engine speed reaches 5160 rpm and the
throttle valve is at least 50° open, the DME control
unit transmits an electrical signal to the solenoid
valves of the intake pipe extensions and the extensions
are operated, switching the intake system over
from vibrating tube to resonance induction.
3. If the engine speed is higherthan 5920 rpm and the
throttle valve is more than 50° open, the DME control
unit opens the tuning flap, increasing the resonance
tube cross section.

BobbyT

Hi Bill,
Thanks for a great, detailed explanation. I'll need to study this for a while to get a grip on how it works. The engine runs well since I did the 30,000 mile service, but it is missing that extra surge at WOT as it approaches redline, so I think I may have knocked a connection loose...I'm still looking for the actual physical location of this flap. It looks like it has to be very close to the throttle body.

JasonAndreas

In PET look at illustration 107-00, part #18. The resonance flap is located on the bottom intake crossover pipe (back of the motor). In the image below the red arrow is pointing to the resonance flap control arm. The flap is actually part of the crossover pipe.

chris walrod

My bad, when I saw 'resonance flap' on the title, I thought this was a thread about Mother-in-law's.

BobbyT

Thanks, everyone. I've found the resonance flap, and it does work. I'm just too hard of hearing to detect the sound of it actuating from the driver's seat when I turn the key. With a helper turning the key I can hear and feel the actuator click. All is well.