gpjli2

Fwiw Variocam activates in the 3200 rpm range. You should hear a noticeable change in engine note, something like the engine waking up w a low growl (certainly not a 'hiss') as you go through the low 3K's. Listen for it next time out. If what you are hearing is at 3900 You need to look elsewhere.

I doubt anyone here can diagnose this based on your description. If you are truly concerned about it you need to go for a ride w a qualified tech in the car before making the call.

Gewgaw

Thanks for the Variocam information! Yeah, I didn't think anyone could definitively tell me what was going on just from this verbal description -- it's hard enough when the mechanic is looking at the car in person sometimes! I just wanted to know if anyone else had this problem and if so what it turned out to be. Sounds like once I get a tech to figure this out and post the outcome, we'll all be learning something new.

PTParks

I believe it works on oil pressure (not a vacuum operated system).

door2416

Just thinking out loud, it started doing this right after an oil change. Did you seat the filter properly or maybe you knocked something loose up top when you filled the oil.

Gewgaw

Good theory, I'll go check the air box area. Filter is probably seated properly since I don't see a pool of oil underneath the car.

Bruce In Philly

Coupla items:

1 - My variocam appears to actuate at around 2800 RPM... this may be more to the way I drive and that I feel the annoying "dropout" as it transitions there.
2 - The shop foreman noted to me that around 2800, a plenem flips also... he thought the dropout I feel is due to both things happening at the same time
3 - Hissing: I never posted this but..... after I swapped out the center muffler for the Sharkwerks x pipe, I hear a distinct and clear "pshhht" when this transition occurs. No doubt it is there. I can only hear it if my windows are down. I suspect the hissing was always there but the center muffler muffled it.

Whatever........ I view all of this as part of the charm. I hate the dropout but interestingly, the X pipe seemed to lessen the impact. I gave up trying to figure it all out... I think it is a lazy actuator for the variocam but I get no codes thrown.

Peace
Bruce in Philly

Gewgaw

My mechanic finally got to looking at this, and his opinion is that this hissing sound at 3900 RPM and again at 4600-4800 RPM is the Variocam activating, and is completely normal.

Bruce In Philly

Is your tech a dealer tech? This "again" at 4600-4800 appears to completely different than anything I have heard or read about variocam including discussions with my dealer. I have what is believed to be a variocam issue and have no symptoms in that RPM range.

The variocam system is an all or nothing system that activates once as you accelerate.... as I understand the system. As you accelerate, for example, the system operates in a range of about 10K RPMs from around 2300 to 3300 RPMs..... so I remember... I may have some values incorrect so someone please correct me. The load, driver inputs etc, feed the ECU and it decides when to actuate the transition from low to high lift. This happens once.... there is no other tier of operation for it to happen "again". As I understand the system. There is also an intake plenum flap that actuates in a similar fashion around the same speed... so my dealer tech told me.

My lift transition, when I get the obvious thump/pfhhht happens mostly around 2800 but I can get it to transition at around 3100 RPMs by taking my foot off the gas on the highway,.... then the speed and rpms drift down to around 3000, then applying the gas... I can usually get it to thunk when I apply the gas there.

To diagnose a lazy actuator, JFP at RennTEch gave the this advice... which I have not done as my Durametric version had a bug that prevented live value reading. The bug is fixed in the software so I may give this a try this weekend. Read about it here: http://www.renntech.org/forums/topic...ocam-actuator/

There are many threads on this thump/suckout variocam issue and yours is the first having any symptoms in those upper RPM ranges.

Another: https://rennlist.com/forums/997-foru...am-issues.html

Good luck, keep posting.

I keep threatening to change the actuators when I did my next plug change, and then I totally forgot about it a few weekends ago when I did my 80K plug change. It is only about $200 for two actuators if I remember correctly.

Peace
Bruce in Philly

Gewgaw

Bruce, this was an independent Porsche service shop who told me it was the Variocam. He said there were two stages of lift in Variocam Plus, which correspond to the two points on the RPM range that produce the noise.

But what you said makes sense to me, as I also understand Variocam varies with throttle input and driving style, so why would the hissing sound ALWAYS come on at exactly the same RPMs?

I'm going to be taking it into the local Porsche dealer for a second opinion next week. Stay tuned....

Ericson38

From the rear, are both exhaust tips at the same height ? Looking for a sagging motor mount that could change a clearance to the chassis in the exhaust system, thinking that it is a resonance of something not in the engine, but connected to it, made out of metal.

Ever get an opportunity to have someone go through that RPM band with their foot in neutral while you listen from the back, possibly under the car (on a rack) ?

ChicagoSpeed996

Actually your two stage theory got me thinking. The 997 has a Helmholtz chamber that opens/actuates at two rpm ranges:

The air filter of the 3.8 litre engine differs
from that of the 3.6 litre engine
through an active on-demand resonance
reservoir integrated in the upper part of
the air filter. This reservoir is activated
via a vacuum-controlled flap as a function
of engine speed and using temperature
compensation. The flap operates in
the temperature range between 32 °
and 86 ° Fahrenheit, opening between
approx. 4,600 and 4,800 rpm and
closing between approx. 6,000 and
6,250 rpm. Opening and closing the
resonance reservoir produces an improvement
in the intake noise of the
3.8 litre engine, particularly in combination
with the specific intake system
of the 3.8 litre engine.
The sound opening of the air filters for
the new Carrera engines has been modified
to largely prevent the possibility of
hot air intake from the engine compartment
even under critical engine operating
conditions (e.g. high ambient air temperature).

Ericson38

Bruce-

Have you seen this description of the RPM related activations/deactivations ? Don't know if these points were changed for the DFI engine.

https://rennlist.com/forums/996-foru...-a-primer.html

Bruce In Philly

Oooo you are good. To be clear, this flap is for sound effects only. It is not part of the Variocam system.

Peace
Bruce in Philly

Gewgaw

Yes, the resonance chambers are for the PSE, correct? This car does indeed have PSE, but that hissing sound occurs whether the sport exhaust is activated or not, or even unplugged entirely. Good theory though!

ChicagoSpeed996

The chambers described by the excerpt I quoted are actually in the airbox and I think all 997s (PSE or not) have it. It actuates in sport or normal mode. It is just to augment odd sounds or to improve the intake sounds. The interesting thing is that it happens at two rpms. I thought a hiss at two RPMs might be from a leak in this area of the intake/air box possibly.

If the mechanic says its the Vario, and normal, sounds like you are all good.