Free flow muffler = fuel mixture errors?

[brucepick]

OK, I'll try to keep the background writing brief.

[pic added later for clarification]


I asked my mechanic friend if he'd install a smaller muffler in a different location, to enable me to do a better job covering the underside for aero improvement. The OEM muffler sits behind/above the rear bumper and I'd like to relocate a smaller muffler over on the side instead.

He says that the only muffler one could find that would fit would be a "performance" fart can. And that the reduced backpressure would fool the fuel management system into messing up the mixture which would defeat my purpose.

My '89 Volvo uses a Bosch Air Mass Meter between the airbox and throttle body, and an oxy sensor located in the cat converter.

He's basing his judgment partially on his own experience in his '85 Volvo that's turbo'd and has significant performance mods on top of that. When he opened up the exhaust system he had to remap his fuel management system. Both of us have 2.3 liter engines, his is turbo and has a mechanical air flow meter vs. my amm. My exhaust is 2" piping + cat + resonator + muffler. His is 3" piping plus cat + 3" exhaust pipe. No muffler.

What do you guys say? Should I go ahead and ask him to do it anyway? Or hunt for a restrictive fart can or add restriction to typical one?

[Coyote X]

Opening up the exhaust can possibly mess up the fuel tables. But the thing is how much it changes it depends on a lot of other stuff. If the cat and exhaust system all the way to the back of the car is stock then you swap out mufflers it might only change your fuel table a tiny almost unnoticeable amount. If the rest of the exhaust has been changed and you are now replacing the stock muffler with a different one it can have a larger impact. There is really not a lot of ways to predict how an exhaust will change things other than trial and error.

For economy type driving the flow really doesn't matter so it would be possible to put some reducers in the pipe before the muffler to choke it down to 1-1/4 then run a few inches at that size then go back to 2" to a bullet type muffler. That would help keep the low speed exhaust working well but would hurt the top end.

If you want to make it sound strange get a sweet thunder one chamber muffler It will take away all the higher pitched sounds and leave only the lower sounds. Nobody will have any idea what type of engine it is when they hear it run.

[RH77]

When I thought my exhaust was falling apart (just heat shields for now), I went over to an Integra performance forum and started the discussion.

Basically you have to tune the exhaust to make the most power at the desired RPM range. A free-flow muffler doesn't change the dynamics of the system further upstream. Exhaust diameter, header design, muffler choice...

I might suggest consulting a Volvo performance board for their take on things. Of course high-RPM operation is generally the goal in that application -- so look for something gains on the low-end.

RH77

[brucepick]

So far the exhaust is all stock, except for a growing crack in the pipe just before the rear muffler. 2" pipe.

I'm not really up for "tuning" the exhaust system. Not up for trying this and that and really doing it all without any instrumentation or test equipment. I just have a speedo and tach in this car, after all.

Should I mostly forget about finding a muffler that I can put on there that's physically smaller, that won't change the tuning of the system? I think you know, performance isn't the goal - stock performance is fine for my purposes.

[RH77]

Originally Posted by brucepick

Should I mostly forget about finding a muffler that I can put on there that's physically smaller, that won't change the tuning of the system? I think you know, performance isn't the goal - stock performance is fine for my purposes.

What is the goal of moving the muffler - aero?

My concern is the loss of low-end power with a free-flow muffler. I had one on a base Civic and lost a lot of low-end grunt. In a Winter "off-road incident", it got torn off, so I put the stock one back on. It was night-and-day. My Mom used the car one day and even noticed.

Since we generally use low-RPMs most of the time, whatever would benefit this end of operation. Perhaps a fart-can with a restrictor?

RH77

[brucepick]

Thanks RH77.

My interest is to relocate the muffler over to the right side so I can clean-sheet the underside. OEM muffler lives between the rear bumper and the gas tank so I'm a bit hesitant to cover it over. Heat buildup.

Since there's no room for the stock muffler where I want to put it, I'd need a smaller one. OK with me if it's a bit louder but I guess I'd need to find one that's not free-flow.

I guess you've stated it clearly - for whatever reason, low end power drops with a free flow muffler. If you or anyone can explain exactly how/why I'd be all ears (eyes??).

FE drivers "live" in the low rpm zone so a drop in low end power wouldn't be good. Especilly in my car which is kinda underpowered. The engine in these cars in particular is a low end grunter, it's happiest between 2-4K though redline is around 6K.

[Coyote X]

for a 2.3L engine a guess for setting it up would be put some reducers that are smooth and not abrupt transitions to reduce the pipe down to 1" or 1 1/4" then back to 2". Put that about halfway between the cat and muffler. Then you can do whatever you want to the muffler and it will probably stay in tune and run just fine. The restriction will help keep it from killing your bottom end torque. If you get lucky it can improve your bottom end a tiny bit.

[RH77]

Originally Posted by brucepick

I guess you've stated it clearly - for whatever reason, low end power drops with a free flow muffler. If you or anyone can explain exactly how/why I'd be all ears (eyes??).

I may have to retract my statement. Here's a breakdown of exhaust dynamics.

I'm not sure what I experienced then. Mileage may vary? I wonder if others experienced the same?

RH77

[brucepick]

Thanks Coyote X and RH77.

RH77, I just finished reading through the article. Very informative.
I wouldn't be so quick to discount your observations. If it happened, it happened.

I think the disconnect between our needs and the article's 'Mufflers' section is in the different goal - we're looking for FE. He says a stock muffler doesn't perform well, right after describing how it cancels sound pretty well. And he says that free-flow mufflers won't really help much because the pipe already is restricting flow. I think that he's saying, if you really want performance, you need bigger pipes and a freer flowing muffler.

That's probably true when you need performance. Wide open throttle (wot) between say, 3.5K and redline. That's performance driving, not FE driving or humdrum commuting. In performance situations just adding a free flow fart can won't give significant added breathing room if you've opened up the intake and maybe done some cam or valve mods. At lower rpms were we FE people live, even if at 75-80% throttle, things are different, I'd guess.

[brucepick]

Anyways, it appears that a bit of restriction might not be a bad thing for us FE fiends. So I think I could use a smaller muffler that's not free-flow - OR - find one that's free flow and just add the magic correct amount of restriction to it.