Astur

I recently did a B&B Cross Pipes and GUNDO Hack modification that brings me consistent smiles and specially on cold starts. The problem is that on startup that wonderful exhaust note quickly goes away and I'm left with the "normal" exhaust sound.

I'm very curious as to why the exhaust sound at ~1,200 RPM during cold start is not the same as when revving the engine while standing still to the same RPM's after it has warmed up.

Would it have something to do with fuel/air mixture ratios on start-up? Whatever it is, I wish that I could keep that sound permanently throughout the day and throughout the RPM range.

vern1

Helmholtz resonator

Philster

On cold starts, the fuel mixture is very rich, the catalytic converters are cold and the overall ignition process is somewhat different. The intention during cold start up is to get the emissions system warmed up and to run a fuel mixture that won't let the engine accidentally run lean and to get combustion optimized and the pollutants down.

Once the ignition becomes proper and combustion is efficient, the sound is lost.... but that sound isn't necessarily healthy if you're going to screw up fuel/air mixtures and create poor combustion efficiency to get it.

Sounds like you want less muffler overall, and Gundo is just too conservative for you.

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Blue Chip

You should hear it with muffler bypass pipes. Glorious!

Astur

On my 997.2 I think that it would be a bit too over the top for me. However, I did have a muffler bypass (CUP) pipe on my 964 C4 which exited via the only resonator. Since that system only had one exhaust pipe, it wasn't "overly" loud but it did make itself known and elicited smiles all around.

MaddMike

With a X pipe on a DD, that may be a bit much.

Astur

Actually my saying that my car is DD is a bit of a misnomer since I'm retired and sometimes I feel like going out, sometimes not, sometimes I feel like going for a long drive, sometimes not...

Regardless of the use, I'm very happy with having done the center muffler replacement with the B&B pipes and the GUNDO hack. So much so that I may even go for the Super GUNDO hack that I've seen written up in this site. We'll see!

extanker

drill some holes in the muffler....that should give you the special effect you want

Petza914

By chance, does your car have PSE?

If so, upon initial start the vacuum valves are open (sport mode), but once the electric vacuum switch get's voltage, they close to the regular position. You can either push the dash button to move them back to Sport or you can disconnect the electic vacuum switch at its connector (not the hoses, the electrical connector).

vern1

Did u search what I recommended?

///zhp04

The different exhaust note at cold start-up is due to the additional air being fed behind the exhaust valves for the first 60 - 80 seconds of cold running. The air is fed to the engine by an electric air pump.

Secondary air injection is activated by the Motronic control module. The air supplied by the secondary air injection opens the check valve through excess pressure, the air is then blown through the lines to behind the exhaust valves of the respective cylinder heads.
The injection of secondary air leads to a reduction of the CO and HC pollutants that are increasingly created by operating with Lambda < 1 when running cold. Additionally, the catalytic converters achieve their activation temperature of approx. 350 °C more quickly due to the waste heat created during the afterburning.
Activation conditions are achieved during the first cold start process if the coolant temperature is between -10 °C and +42 °C. When running close to idle speed, secondary air injection is active for approx. 60 seconds, for a partial load up to 80 seconds.

Astur

No, no PSE. Just a pretty plain 2009 C2 with only major options being PDK and Sports Chrono.

Vern1: Yes, I did read up on your suggestion of the Helmholtz resonator. However and since I don't have droning problems, I don't quite understand it's applicability to my wanting a deeper sound throughout the RPM range.

GSIRM3

Yep. This is the answer.

Philster

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Astur wants to know how to get more of what the cold start produces, which is a slightly more agressive exhaust tone. Since he can't finagle his electronics to run a cold start procedure all day long, the answer is ____________________________________________.

I would say a more aggressive exhaust system. Astur, lose the mufflers; run resonators instead.

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BLU997

I know this is a bit of an old thread, but I too, was intrigued how the 997.2 "morning growl" was created.

Read on, our cars are much smarter than we know.

According to the 2009 997.2 DFI technical sales pitch, the EMS does some trickery to quickly heat the cats, and I quote...

"Then to quickly light off the catalysts, the DFI heats the exhaust rapidly through multipule injection. To heat the temperature of the exhaust gas to an even higher level, ignition timing is very late, further minimizing start-up emissions."

So that deliciously gruff cold start period serves to heat the cats asap, a combination of extra fuel bursts, and very retarded ignition timing.

Great noise, better for the environment, but "rich and retarded" is not a sensible tune for life, or for cruising around in our wonderful machines.