Free flow muffler = fuel mixture errors?

[RH77]

Can someone help me with this:

Vehicles that I can think-of, which have spring-loaded valves in the muffler:

Nissan Altima
Mitsu Lancer Evo
Scion xA

On the Scion, I found a post with a cut-away of the device and discussion

So, what is the purpose of this? I know on the Evo, it would open to let more exhaust through when at full-boost, high RPM (and consequently louder).

RH77

[diesel_john]

tuning for either sound or power, most likely power.

[Frank Lee]

Originally Posted by diesel_john

Exhaust systems on vehicles by their nature have a lot of vibration, so what the manufacturers do is they very carefully position the mounts at points called node lines. Based on the mass and stiffness of each component in the exhaust system and their resonant frequencies the designers use finite element modeling to predict where the best place is to put the mounts. then after testing the physical parts and tweeking the models for everything connected to the exhaust system they eventually agree on the size, weight, strength, type of rubber yah da yah da yah da. this is all happening at the same time the engine guys are saying they need this and they need that to change. Most of this is modeled now days, of course. Which translated, means: they can iterate on a solution faster than ever.

Long story short, it probably will rattle, it probably will cause a premature crack somewhere else, and it probably will change the torque curve of the engine. Will it matter once you have modds everywhere else on this car? No.

No kidding? The guy at the corner muffler shop doesn't look like a rocket scientist, and his systems are as good as factory. And, those rubber hangars all look pretty uniform, interchangeable, and universal to me.

[Coyote X]

putting a flapper valve has been shown at various times to help bottom end and economy. I have been planning to put a flapper valve on my metro once it is built into the new body. I also planned on using 1" exhaust pipe to get the engine tuned for a lower RPM

I can't find the link on here but there was a discussion about adding a flapper to the end of the tailpipe. Jegs sells one that is pretty cheap and could have a spring fit to it to adjust the amount of pressure required to open it. Putting it at the end of the exhaust is not as good as closer to the front but I am sure it will still help out.

[RH77]

Originally Posted by Coyote X

I can't find the link on here but there was a discussion about adding a flapper to the end of the tailpipe. Jegs sells one that is pretty cheap and could have a spring fit to it to adjust the amount of pressure required to open it. Putting it at the end of the exhaust is not as good as closer to the front but I am sure it will still help out.

I'd love to see the results of such an addition. I would definitely consider one if it worked properly and results were solid.

RH77

[Coyote X]

I tried it on my camaro with a huge cam in it that makes it have a horrible idle. I just covered the exhaust tips and left a 1" hole. The idle was way smoother and the car felt much smoother when I left on my way to work. I blew them off the car on the way to work though so I have no idea how much it actually helped mileage. I do know you could actually stand around the back of the car with it idling and normally it would choke you to death if you just walk by it.

If the weather would warm up I would really like to try that again and this time clamp them on much tighter. That car would be the one to show the biggest improvement I figure since it gets horrible mileage normally.

[presidential]

Originally Posted by RH77

Can someone help me with this:

Vehicles that I can think-of, which have spring-loaded valves in the muffler:

Nissan Altima
Mitsu Lancer Evo
Scion xA

On the Scion, I found a post with a cut-away of the device and discussion

So, what is the purpose of this? I know on the Evo, it would open to let more exhaust through when at full-boost, high RPM (and consequently louder).

RH77

There is also the Mitsubishi 3000 GT VR4 (and it's Stealth AWD turbo sibling), the Corvette Z06 (optional on the LS3 08-?? Corvettes as well) and certain years of the Infiniti M45 (Q45s may have this as well, I am unsure). As far as I know, most of those systems are operated by a solenoid, none are vacuum operated (where one could rig up a rising rate regulator based on engine vacuum).

If you wanted to get really clever, install an electric cutout (available from Summit amongst other places) and have it go to another exhaust system.

The purpose on a boosted application is to relieve the exhaust of any backpressure. Most high-performance turbocharged applications like as little restriction as possible for optimum power output, hence my reason for putting 3" exhaust on all of my past turbocharged applications. I could go on and on but I'll shut up now. Hope some of my drivel helped out.

[Daox]

Modifying the exhaust will alter your powerband. However, this is much more drastic on a turbocharged engine. On a naturally aspirated engine the effect is much less and the ECU should be able to compensate for it.

[trebuchet03]

Originally Posted by Frank Lee

No kidding? The guy at the corner muffler shop doesn't look like a rocket scientist, and his systems are as good as factory. And, those rubber hangars all look pretty uniform, interchangeable, and universal to me.

Yep - they actually do that analysis. And when they're done, they actually physically test The difference between them, and the corner muffler shop is that when they have a failure - it's not one guy coming in to spot weld a custom job. Factory recalls/service cost much more than proper analysis and initial testing. They don't always get it right - but they have some damn good FEM guys

The corner muffler guy also isn't as concerned about dropping 2dB by moving a hanger 8cm; auto mfrs. typically are. Optimizing your mechanical system could mean you can save a few dollars per car by using a different muffler

[Frank Lee]

The point was there is lots of latitude for what will work to make a functional exhaust system. Trying to dissuade/scare someone off by talking about nodes and s*** is, well, B.S.