Changing the exhaust note of your Aston Martin DB9 is easy. ‘Uncorking it’ as its been called is as simple as pulling Fuse 22 from the correct fusebox. This is probably the most popular ‘tweak’ owners do to there cars. In fact, the dealerships would do this for you as you picked up your new baby.
The forums are also full of questions, and myths. Does it really make a difference? How much louder does it get? Where is the fuse box? Which fuse is it? What does uncorking really do? Does it improve the gas mileage better? Does it make the car run richer at idle?
What does Uncorking really do?
The DB9 has a 2 stage exhaust. There is a ‘long way’ through the rear muffler box that is the quietest path. If all the exhaust is flowing through this path your car will be ‘purring’. There is also a ‘shortcut’ that bypasses the long way. The shortcut is turned on/off by a bypass valve. When the bypass is opened, the exhaust flows more freely out the tailpipe (skipping the restriction of the muffler), and has more of a ‘growl’ sound to it (listen to the video below).
With the cork still in, the bypass is controlled by the computer to be normally closed (between about 800 and 3000rpm), and then automatically open when the engine is at higher RPMs (allowing the engine to breath easier while its working hard).
If your car still hasn’t been uncorked, you actually can hear the difference right when you start the car when its cold. Have you ever noticed that right when you start it up, you get that terrific ‘GROwwllllllll’ that starts loud and then fades to a ‘Purrrrrr’ a few moments later? That’s the system at work. The bypass is open at startup, and then is closed automatically after the engine is started at low RPM.
The valve is actually operated by a vacuum diaphragm (see videos below). Vacuum is used to PULL the valve CLOSED. The cars electronics only control an electric exhaust valve vacuum pump that creates the vacuum to be applied to the diaphram. The electronics don’t actually operate the valve directly.
With the engine off, there is no electrical signal and thus no vacuum. The valve itself is designed to be NORMALLY OPEN, so when parked (and off) the bypass is actually sitting in the open position. Start the car, the vacuum pump starts, the electronics kick in and tell the valve to close the bypass. Hence, this is why you can hear the uncorked sound at startup.
This explanation should dispel the myth that uncorking your car is bad for the vacuum pump and diaphragm. Your car probably spends 90%+ of its time parked and off already. Pulling the cork merely changes this to 100% of the time. Since the valve was designed to be open at rest I don’t think we are doing any damage, no more than would happen to a ‘show car’ that almost never runs.
This should also dispel the myth that not allowing the vacuum to flow somehow make the car too rich at idle. Presumably the worry here was that the reduced exhaust backpressure at low RPM will somehow mess with the air flow rates and fuel injection. I am skeptical of this. The engine uses a air mass flow sensor on the air intake to compute the exact amount of air entering the engine, and then adds the exact amount of fuel for that amount of air. Regardless of how freeflow the engine is running, the amount of fuel is always precisely calculated dynamically.
Is it bad for the car? I doubt it. If the dealers were doing it while delivering the new car, I highly doubt it is harmful as they wouldn’t want to void a warranty.
Enough theory, lets get onto the good stuff…
What do I need to do to uncork it?
Remove Fuse 22 from the Trunk (boot) fuse box. It’s that simple, just pull a fuse out. OK, everybody knows that, but where is the fuse exactly in the car? [Note: This applies to model years 2004 thru at least 2012, but I am not sure on late model (2013 and beyond) cars. Let me know if you have info on this…]
You will be working on the Trunk (Boot) fuse box. You can refer to your owners manual for its location, but here is where it is:
Here is the official fuse listing for the box and layout of the box.
Here is what Fuse 22 looks like removed. It is an industry standard Yellow 20 Amp ATO fuse. If you’ve lost it or need to replace it, you can get a replacement at your local automotive parts store for a couple of bucks (no need to go to Aston for this).
Once you have removed Fuse 22, your car (and dealer) will be aware of it. The cars Powertrain Control Module (PCM) will log the event as P1488 but will NOT turn on the idiot light (Malfunction Indicator Lamp or MIL) on the dash. Next time you or the dealer connect and OBDII reader to the car they will see the P1488, but this is normal and not an issue.
Here is my video clip of accessing the fuse box and removing fuse 22. Wearing a mechanic cam (I needed my hands free) so I apologize for the camera motion and positioning.
How much louder does it get?
I think it improves the sound at lower RPMs. It adds a bit of growl while you are driving around normally, but never an amount that will annoy the neighbors.
I’ve tried to give you a taste of the change in this next video. I have setup a sound meter (thanks Rob for the loan) about 18″ from the exhaust tip and measure the sound level with the bypass open and closed. With the bypass closed, the idle sound level is about 69 decibels (dB). With it open, the sound is closer to 72 dB. A little bit louder, but not straight pipes! Have a listen to the change.
And this is a great video from guygittens where you can really hear the difference in the sound (even though its a V8 Vantage in this video, the change is similar to the DB9).
Wanna see the actual valve and how it works?
Here is a video of the actual valve under the car at idle, and I show how the diaphragm opens and closes, and you can hear the corresponding sound changes.
Here is another good video of the valve in action while Fuse 22 still in place (courtesy of YouTube user AMG32).
Is there any other way to uncork my car?
I’ve read about two other ways people have approached this issue.
You can disconnect the vacuum hose from the diaphragm. This accomplishes the same result (leaves the valve always in the bypass position since the vacuum never reaches it to pull it open). You should plug both the vacuum hose (to stop it from being a vacuum leak) and cap the opening to the diaphragm (allowing humidity and debris in to damage the unit). Sure, you don’t need to touch the fuse, but you need to take apart this other stuff and run the risk of some actual damage. I don’t see why to do this approach since the Fuse 22 is a much more elegant and clean solution.
Another way it to replace your rear exhaust with an aftermarket system from a performance exhaust manufacturer. The whole point of the performance exhaust is to improve exhaust flow. This is a cool way to do it and probably improves exhaust flow a lot more, but it’s costly and maybe not a DIY job.