I had a bit of a 'mysterious' power loss issue with my LT1-power 97 Trans Am, and figured I would post up my findings for anyone that might be helped in the future. The story is I bought this car a few months ago already modified (would have preferred bone stock, but got a good deal) - 355 CI, 11.5:1 CR, Lloyd Elliot LE1 heads & IM, Comp Cams 306, long tubes, 58mm TB, etc etc. When I test drove it, it pulled strong and felt good.

The only thing I didn't love was the guy had essentially dumped the exhaust just behind the Y-pipe out the side, so it was very loud and smelly. Basically straight piped with no muffler or cat. His explanation was that the cats had grenaded / blown apart at one point, and all of the material from the cat guts clogged up the muffler. This killed the power, and he didn't get it back until he put the cut-out in place, and he liked the sound so he left it.

Fast forward to last week, I took it to an exhaust shop to put a proper exhaust back on it, and had it booked to go to the tuner the next day. When I go to the exhaust shop and explain the whole story the owner says it's very very unlikely a muffler could get clogged - it's a Flowmaster and he said he's never seen a straight-through muffler design like that get clogged, and in rare cases that more restrictive mufflers do get clogged, they heat up to the point that they usually blow a hole in the side or pop off. He convinces me to just replace the cut-out with a high flow cat and leave the Flowmaster alone, since there's likely nothing wrong with it.

However when it gets to the tuner, he's doing power pulls and the results are really disappointing. Despite everything checking out with AFR, timing, sensors, etc and the car just idling and sounding decent down low, the power curve quickly hits a wall and maxes out around 200 WHP - definitely very low for any LT1, let alone what's supposedly done to this engine.

I take the car home to check compression and do some troubleshooting, but I keep thinking back to the original owner saying he thought the muffler was clogged. So once I get it home I take it for a "baseline" rip in 3rd gear from 40km/h to 80 km/h, and time it around 4 seconds. Car pulls OK, but feels more like I'm driving the wife's CUV. Pull it in the garage and put it up on the lift and start measuring temperatures at different points in the exhaust with a heat gun, and find some pretty ridiculous temps - it's about 250-300 F at the bottom of the headers, and pipes leading up to the Y, but the new catalytic converter is glowing red and measuring over 1000 F! The muffler inlet area is up around 600 F, and the muffler outlets are back down to 250-300 F. Something doesn't seem right - why would the exhaust temps be 300+ F different from end of the muffler to the other?

I hack the muffler off and below is what I find - the inlet of the muffler is completely clogged, with what I'm assuming is old cat guts. And I mean clogged - you press on it with your fingers and it's packed in there tight! I was expecting to have to cut the muffler in half, and maybe find some small blockage that was restricting it somewhat, but nothing this conclusive and plugged tight. I'm surprised it could even make 200 whp like this, as I can't even see how any exhaust would get past this blockage. I drove the car and it was a night and day difference - that 3rd gear pull I couldn't even measure a time because the car was going sideways at even half throttle (summer tires and it's winter here in Canada now, no grip!). I've yet to get it back to the dyno, but I'm expecting nearly double the WHP gain from this.

Just goes to show you that a muffler can get very clogged up without glowing red or blowing up - maybe the exhaust shop I went to the guy wasn't the most experienced, but he seemed very adamant that wouldn't happen, and if there was stuff inside that you could shake it and hear it. In this case, the muffler was so tightly packed there was no loose bits to rattle or shake.

Muffler Inlet:


Over-Axle Pipe at Muffler Inlet:


Red-Hot Cat (1000+ F):

 

Sorry for the long read, but just wanted to put in all the details of the story for anyone that might have a similar situation.

The only outstanding item that I'm not sure about is the status of that cat - it was brand new, but I'm wondering if I screwed it up with it getting up over 1000 F? Even after cutting the muffler off it was getting quite hot. Not as hot as before, but the small section of pipe right behind the cat was glowing red, and the temps were still 800 F or so. I didn't drive the car far after cutting the muffler off, so I'm wondering if maybe it just has to burn up some 'stored' emissions that got trapped due to the muffler clogging?

I'm going to try to take it for another rip tonight and see if the temps drop. I don't want to put a new muffler on it and have that one get full of cat guts all over again. I know most will say just get rid of the cat, but in Ontario we have Drive Clean emissions testing, and it's getting harder and harder to cheat. I also prefer not having the stank of an open exhaust.

 

Thanks for posting. That is downright incredible!

I wouldn't worry too much about your cat. With the reduced mass flow through it, I would argue that it got no hotter than it gets at max airflow / max power.

 

My worry with the cat is just that it was still running literally red-hot, in the 800 F - 1000 F range. Is that sustainable? Most 'generic' internet sources seem to think that running a cat that hot means it's only a matter of time until the insides explode. I have no particular issue with it, if it can hold up and run at those temps.

 

Cats routinely run at 1000 degF. Did you measure that with an IR heat gun?

 

I don’t know if I’ve ever had a muffler get clogged, but I’ve had cats fail and clog the exhaust. Midas replaced a failed cat one time and when I drove it off the lot it was running just as poorly as when I took it in for replacement. I ended up putting the car in storage as a result. Years later, when I was installing headers & exhaust, I learned that Midas had simply gutted my cat and that I had paid a handsome fee for nothing more than their labor. Catalyst had blown back into the exhaust which is why it was still running poorly. I don’t believe I ever checked the muffler, but it’s certainly possible it was also full of catalyst.

I was pissed when I found out what they had done. If so much time hadn’t passed, I would have probably notified somebody, perhaps the Georgia attorney general,

 

Yes, with one of those point-and-click heat temp guns. Before removing the muffler / with tons of back pressure, it was glowing red and reading 560 C (1040 F). After the muffler removal the small pipe section just behind the cat was still glowing red, and the cat was around 500 C / 900 F. If that is 'normal' then it makes me feel better, I've just never noticed a cat / pipe literally glowing red.

 

looks like previous owner used Metallic substrate cat that blew apart.

 

Wow, never seen that before(muffler clogged like that).

 

Cats should not run red hot unless you were at WOT for an extended period of time and they should cool back down quickly. You most likely are running too rich or it is the restriction in your exhaust and getting red hot. You might want to think about getting a test pipe put in place of the cat. I had a cat get red hot once back in 96 and the insides burnt up with the cat sounding like a metal ball in a can when driving around.

 

hey! that car looks very familiar! my suspicion was right after a lot of trial and error figuring out what the issue was. miss that car a tone but glad it went to a good owner. a lot of people on this forum will say not to put a Flowmaster back in a go for a MagnaFlow muffler that's less restrictive. my hearing probably isn' the greatest but I loved that open cutout sound/smell lol.

good luck when you return to the dyno I'm curious to see results!

 

I had that happen but with straight through absorption mufflers the cat guts came out like tapeworm.

 

Happy ending to this story:

Ended up replacing that clogged muffler with a Borla, took it back to the tuner and saw a huge difference instantly - first pull was up over 325 whp from barely 200 with the clogged muffler. After all was said and done, made a max of 358 WHP at 6300 RPM (power curve was just leveling off at this point). This was on a Mustang dyno as well, which reads a little low (not that I'm super caught up in the magnitude of the numbers anyway).

Brett at BR Tuning did a really good job on this one, and was very helpful and patient in helping to get this sorted out. Would definitely recommend him to anyone in the Toronto area.

 

I'm having an issue that, through research has pointed to the cat being clogged. Symptoms are: squealing noise during rev from exhaust gasses escaping through manifold, very very low performance due to the pcm putting tranny into "limp" mode. Any ideas? Corrections? Thoughts and opinions?

 

If you have a digital thermometer (the laser point and shoot type) you can see how hot your cat / exhaust is to pinpoint where it's clogged. If the cat is clogged it should be getting really really hot, and then substantially cooler right after.

And if you feel quite sure and want to test that theory, either disconnect or cut the exhaust before the cat and see what happens.

 

I appreciate that advice. So could having my air injection system disconnected have any part in this issue?

 

Are you talking about the air pump? I wouldn't think it should cause any issues . . . as far as I know, the purpose of the air pump is to pump more oxygen to the cat to heat it up even more to help burn more emissions. I doubt that would cause issues but I'm not too familiar with it. Generally if you're running rich / tuned rich then that will blow up your cat eventually.

 

that sucks thanks for letting us know that i never knew