exhaust backpressure and cat question
 

hi, all. recently my 87 190e 2.3 8v broke it's driveshaft and flex-coupling at highway speeds. Consequently, the rear housing of the transmission was cracked, the O2 sensor was demolished and the cat was destroyed too. My esteemed tech made the necessary repairs using his parts car for what he could, and a few brand new parts aswell... it cost me a mere 1000 bucks, a steal. Heres the problem, i soon noticed that the car was very sluggish all over the rpm range, and it was using way too much fuel. It also developed a howling exhaust sound from the cat. Took the car back told him the symptoms. He seemed most concerned with the fuel consumption. 250$ later, i had a brand new O2 sensor (the one he replaced the broken one with was used). The car now gets decent mileage, but the power is still lacking (i got dusted by an old couple in a dodge shadow today). When I asked the tech what the howling was, he said that the cat he used on my car was hollow and had "no filter in it" and it was off a 2.6 motor. i'm weary of this.... my car's got a little more bottom end with the new O2 sensor, but anything over 3500 rpm leaves it gasping. i'm fairly sure that the engine relies on exhaust backpressure to run properly, but i'd like any advice i can get.

thanks alot.

I doubt that the loss of back pressure would amount to at most a couple horsepower. I'd think the loss of power would be because of restrictions in the cat or muffler and not from lack of back pressure. Is it possible that there is trash in the tailpipe or muffler? Also, I've never heard a properly functioning cat "howl".

If the exhaust is not restricted ... Changing the O2 sensor improved performance. I wonder if there is still some problem related to the reading from the O2 sensor.

I'd be interested in knowing what you eventually find.

poorstylishman,

I vote for an obstruction or some other problem in the exhaust. Taking away restrictions in the exhaust is a first step in getting a few cheap horsepower added to the engine output. I have never heard of it causing a problem.

Adding a configuration that changes lengths and diameters arbitrarily can change the tuning of the system, resulting in interfering pressure pulses at the entry to the cat or muffler, which can be a net restriction. Same with new bends and paths. And howling is usually the sign of a small hole, or series of small holes that whistle under certain pressure and flow velocity conditions.

Cobbling a system from another car with a different engine offers the potential to cause some problems. Sounds like you need a new exhaust designed for your car. Try www.timevalve.com. Good luck. Jim

thanks guys, i'm getting varying opinions on this... searching for related topics on this site leaves me thinking that the engine does, in fact need the back pressure from the cat... but logic tells me that a freer flowing exhaust should increase my power... i'm really stuck here. i don't want to spend 400 bucks on an aftermarket cat, if it's something else. Can any techs tell me for sure if this backpressure issue is my problem?

There is a good article in the May, 2002 issue of Grassroots Motorsports the deals with exhaust tuning and back pressure. I know there are several technical articles on this subject on the web, though I don't have them bookmarked on this computer. Note that the difference between no exhaust and a high performance tuned exhaust can affect HP by only about 8% max. I have driven my 190E 2.3-16 with no exhaust and with its custom stainless system, with and without a resonator and I cannot feel the difference. The differences are so subtle that it only shows up with a dynamometer.

You might consider disconnecting your exhaust, driving the car and see if you feel any difference in performance. You could certainly rule out whether or not the problem is due to a restriction.

Cheers,

Robby

I'm at a different computer. Here is a web link explaining exhaust flow in an automobile.

http://customimport.simplisticmedia.com/Exhaust/exhaust.html

thanks for the link. what i'm worried about is the whole concept that the engine relys on vaccuum to get enough air. so with no restriction at the cat, the amount of vaccuum at the intake is compromised... hence no juice... especially at the high end. What i'm unclear about now is whether having a hollow cat chamber (is that the term?) is creating a problem. The cat is empty, so it's not as if there is a simple pipe in place of the cat.

You mentioned that you suspect that there might be some crap further down the line. Where do you think that would be, and how would it have gotten there?!

An O2 sensor has no effect on general performance while a "cat" does not have a filter. Its possible that the mechanic is a little void in MB stuff. Most likely the used "cat" is obstruted. Also, what could make this car distruct like it did?

the drive shaft flex-coupling failed at highway speeds... consequently broke alot of different things. of all the things that were replaced, all i can imagine that would result in such a dramatic loss in power would be some sort of exhaust issue. Fuel economy has also suffered.

btw, some of these threads are keeping me on the fence about the backpressure

http://www.peachparts.com/shopforum/showthread.php?threadid=20716&highlight=backpressure+power

http://www.peachparts.com/shopforum/showthread.php?threadid=15614&highlight=backpressure+power

well, after a few horrid weeks of getting 200 miles to a tank of premium... and having no power over 3k rpm... i bit the preverbial bullet and bought an aftermarket cat... bolted it on. And whaddyaknow? It works... power is restored... it now runs double strong... i actually gave a jetta 1.8t a run for the money... (until he stopped smoking the tires, then he handed me my ass) but it was close up till about 80k! The hollow cat was the problem... the backpressure creates a vacuum at the intake... that's the story morning glory. regardless though, thanks for all your input, but my gut was right on this one.

Put a back pressure guage in the o2 hole and watch it freewheeling at 4k, if its over 6-8 psi drop the exhaust right behind the cat and retest.

I know I have seen monolith beds shatter and restrict mufflers in the past and if the cat took a hit on the bed casing you might have a chunk in the muffler.



Joe

One simple test thats sure to raise a few eyebrows in your neighborhood, drop the exhaust behind the cat a go for a drive. (make it a short one)

to joep: think I've ot that problem
 

Joep:

What you described (hit on cat casing) and chunk in muffler) is what I thinlk I've got.

I have a '77 450 SEL (Euro) ith fitted cats. When I got car, I replaced left one with universal. That was about a year ago. Ran fine for many months.

Then, in recent couple of months, I have developed problem. Car will rev fine and then develop a restriction-like thing on left side. I looked under the new cat, and saw a dent or crease. Can't remember left side hitting anything, but there it is. There's also something jiggleing around inside that cat. Muffler sounds a bit rythmic but not overly noisy in rear. It's fairly new. When I stop car, and thenr estart, it goes away for a bit. Maybe a couple of minutes at most. Worse under load, to point that I lose almost all power and it's an embarrassment on road.

Everything else has been ruled out.

Could a chunk have been broken off and be lodging in rear muffler or resonator? Is there any way for me to try and remove or break-up the obstruction?

thanks!!