Mech33

I took my car to the track for the first time yesterday at Laguna Seca for a 92 dB day (had a blast, but that's another thread!). I was so paranoid about failing 92 dB sound after all the meatballing stories that I ended up preemptively trying to make my car quieter, and it worked.

Looking at the OEM PSE exhaust valve system, it's pretty simple as most on here know: with the valves CLOSED, exhaust is forced to take the longer path through the side mufflers and then back through the center muffler. With the valves OPEN, both this longer side path as well as a new shorter parallel path direct through to the center muffler is used. Here's a quick pic to illustrate:



The physical valve operation is also pretty basic: there's a vacuum line going to the bottom of a diaphragm, with atmospheric pressure applied to the top of the diaphragm, such that when vacuum is present on the vacuum line it sucks the diaphragm down. The diaphragm is tied to a rod which, as it moves up or down, spins a butterfly valve in the exhaust pipe between the entrance to the side muffler and the center muffler to either open or close the side muffler "bypass".

The valve is configured such that when vacuum is present, the valve is closed, which is the "quiet" configuration. When vacuum is removed (e.g., just applying atmospheric pressure to the vacuum line by disconnecting it), the valve opens, which is the "loud" configuration.

So when you push the PSE button to make the exhaust "loud", a vacuum solenoid in the car is just flipping to disconnect vacuum from the valves so that they'll flip to the "open" configuration.

One side effect of this valve configuration is that if vacuum is no longer present in the intake system (such as at full throttle), then all of these vacuum lines will typically lose vacuum, so the valves will naturally open at full throttle regardless of if the PSE button is pressed or not. So the PSE button just handles making the exhaust loud when the car is under partial vacuum (idle, part throttle).

So what happens if you want the exhaust to be "quiet" all the time, especially at full throttle?? Only someone driving at a place like Laguna Seca on a low-dB day would want this.

Well, ideally we'd just find a way to apply vacuum to the valve *all the time*. Maybe there is a special vacuum source in the car that is created either via a pump, or a check valve run off of the natural vacuum to hold it during brief periods of full throttle, but I couldn't find any reference to such a source after searching around for a bit.

The next best thing I could think of was to instead mechanically hold the valves in the closed position. The mechanism is reasonably easy to access (you can just lay on the ground and fiddle with the valve), so as a last minute solution the night before my track day, I just bent some coat hanger wire in a shape to pull the valve shaft down (closing the butterfly valve), and then wrapped it around the valve body to hold it in that position. Here are a few pics of my temporary hack:





The car definitely sounded quieter afterward, but I was nervous that it still wasn't quiet enough, so I continued to take it easy at the appropriate sections on track to keep from getting meatballed.

During the second session, I decided to do a sound check towards then end to see if I could go full throttle without violating sound. They sent me out for two sound check laps, and I made sure to drive as close to the sound check station as possible between turns 5 and 6, and be at full throttle up around 8,000 RPM where the car was really wailing.

The result? 92.3 dB!! It was within ~0.1 dB on both runs. They said I was within the margin for a 92 dB day and that I had another ~0.5 dB of margin for they'd actually flag me.

So, long story short, there is a very easy solution for 991 GT3 owners at Laguna Seca 92 dB days: simply hold the valves shut. I'm sure there is a small reduction in power in exchange for this, but I refuse to believe it is any worse than regular supertrapp-style solutions (in fact, I'm convinced the large side mufflers are less restrictive than a supertrapp plate stack). The car felt great, and fast! And still sounded mean at 9k...

My next plan is to try and build a more clever flip that is reusable and easy to attach and remove (the hanger wire takes some manual fiddling and and I have to un-bend it to remove it). I'll keep folks posted on that progress in case you want to replicate what I use in the end.

996FLT6

What was your best laptime at track? Mike

Mech33

1:44 so far in the GT3, but I'm a big wuss... 1st full day on track with a 911, so lots of room to improve as I learn how to drive the car.

MaxLTV

This is awesome news! Maybe this plus simple turn-away tips would work for even 90db days.

GregJGT3

You could also just use some .030" or heavier safety wire to do the same thing. Loop it wound the valve lever and then tie off any where convienent.

awtiger1

Impressive ingenuity there, MacGyver! I wish I had thought of that before my day at Laguna last Thursday. I was fine as long as I upshifted early and did 1/2 throttle up the hill - but at the end of the day I wanted to try for a fast timed lap and was immediately black flagged and kicked off the track (with a letter from Monterey County stating that I did 98.5 Db). Bummer to end an otherwise great day.

I do think LS has a lot of issues given that most days are now 90Db and they lost an anchor client (165 days). Will be a shame if this great track gets destroyed by a few NIMBYs (who moved there decades after the track was built / was already an icon).

rockitman

Great idea ? May work for Limrock...which is 88dB for most track days...

Mike in CA

Mech, I'm glad our little exchange of pics and conversation in that earlier thread about a possible solution paid off!

mwar99

Any worry that forcing the valves to stay closed will damage the valve system in any way? I have to think there is some strain put on the system when forcing the valves to stay closed.

orthojoe

Great post and info, Mech33! Thank you very much for this. I think I'll try this route along with some turnaway pipes to pass 90dB next month!

Now if someone can figure out a nifty device to switch it on/off with a button just for turn 5!

MaxLTV

Laguna DRS hahaha

devenh

Maybe Porsche could build it into the Sport Chrono App

Mech33

Thanks for the comments! Glad some of you guys might be able to try this out at Laguna or other tracks, too.

I thought about this and decided the risk was small. The actuator consists of a diaphragm attached to a rod, which is spring-loaded to hold it in the "open" position by default. Then when vacuum is applied to the bottom of the diaphragm, the pressure difference across the diaphragm overcomes the spring force and pulls the rod down.

So the difference is that in normal operating conditions, the force on the actuator comes from pressure on the diaphragm, and the force on the rod itself and the rod-diaphragm interface is actually pretty small (it's just along for the ride as the diaphragm moves). In the "forced close" condition, the force on the diaphragm to overcome the spring is transmitted from the rod itself, so the force on the rod-diaphragm interface is higher.

It's hard to tell without cutting one of the valves open, but I suspect the internals of the actuator are designed such that forces on the rod throughout the entire range of movement is not a problem.

Anybody have a stock exhaust valve actuator they don't mind donating in the name of science?

mooty

yes you can do that, but you will eventually forget if it's on or off. i had that on 997.

also shutting the valve all the time to do 1-2 trk day is ok, prolonged use is bad.

Mech33

Why is it bad? Trying to figure out why running exhaust through the big side mufflers is any worse than blocking the exhaust with supertrapps.