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measuring exhaust back pressure


careldo

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Hi guys,

my cat on my 92 eldo 4.9 i believe still has the original cat and was wondering if i should just replace it. I hate to replace it without just cause so i am trying to figure out how to measure back pressure. In the manual there is some mention of installing some tool in the exhaust and if the reading is above 1 PSI it would indicate an exhaust restrication. Has anyone on this board done this before? Where does one install a gauge?

careldo

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I had just replaced my Catcon just a few months back. I did not measure the Back Pressure. I knew to replace it when it sounded like I had a big can of rocks under my car. Ran great up to that point. Mine lasted 185,000 miles on my 93 STS... MD

Cop didn't see it, I didn't do it!

93 STS 225,000 Miles

99 STS 111,000 Miles

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I was just watching Two Guys Garage http://www.twoguysgarage.com/home/index.php and they used an EXHAUST BACKPRESSURE GUAGE, it screws into the O2 Sensor location. I don't know who produces it. Here is an experpt from the show:

Increasing exhaust flow through exhaust tubing and changing the muffler.

Sam and Dave (McBride) want to get a throatier sound out of an F-150. The truck runs fine, but they want a better sound, and wouldn't mind a little extra power either. Before they removed the existing system, they wanted to check it to make sure they had no excessive back pressure. The system checked out o.k. so they got to work installing the new AeroTurbine performance muffler. It's designed to improve airflow over standard mufflers, and it built to last longer than a typical muffler. Mandrel-bent pipe was the only piece left in the equation, so Dave stopped by Shafer Classics and Warren Shafer showed us how mandrel-bent pipes are made. Sam and Dave finished up the F-150 with the mandrel-bent pipe, which also promotes airflow, and the F-150 now has a satisfying growl when you step on the accelerator. Mike

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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So what are we using as the limits here? In other words, what reading constitutes a "bad or plugged" CAT and what is a good CAT? At what rpm? What is a normal pressure reading at idle?

Never underestimate the amount of a persons greed.

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loucar1, That may indicate a loose "brick", but it certainly won't reveal a plugged CAT. If a brick loosens, it can twist in it's housing, partially blocking the openings and reducing flow. But thats not really plugged is it? From a practical standpoint, your assesment is probably correct, and you don't need a gauge to tell you that!

But in fact, exhaust restrictions can happen INSIDE THE EXHAUST PIPE! Saw it myself on a '72 Chevelle. Back then, OEM exhaust pipes were made of two layers of steel for several reasons that I won't get into. They may still be made this way for all I know. Anyway, my brother's wife backed in to a snowbank after a nice "HOT" run to work. After that, the car wouldn't go above 35 mph, would spit and sputter when revved in neutral, and backfire through the carb. After changing the timing chain (backyard mechanics) I suggested that my brother drop the pipes at the manifold and take it around the block. The results were immediately apparent, the exhaust was plugged. The outer exhaust pipe looked undamaged, but the inner pipe had folded, restricting the flow to about 1/4 or less.

I suppose that my point to all this is that you can't sell a gauge and expect it to tell you that the cat is bad unless you rule out a few other things and have some parameters with which to make a judgement.

Never underestimate the amount of a persons greed.

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Yeah, a double-wall pipe. The Northstar has this on the front section I believe. I understand it keeps heat contained much better than a single wall pipe does. Almost like an integral heat shield.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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For what its worth the show that I was watching (Two Guys Garage) stated that modern muffler systems have little or no backpressure, unless bent or restricted. The current Two Guys Garage episode goes into this in detail check it out. See www.tvguide.com to see if you can still catch the current episode, Mike

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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And probably 1 psi or less at idle. That, Guru, pretty much sums it up. Don't even bother with that gauge, it wouldn't give you any hints at all as to what is really going on. Particularly if jadcock's statement is correct, and the double wall pipe has collapsed BEFORE the o2 sensor port! The readings then would be extremely low since the restriction would be before the port. The GOOD thing about all of this is with very much restriction, the car would struggle to make 40 mph...("significant performance loss")..it would be pretty obvious that there was some sort of serious problem. And more than likely a hissing noise from somewhere to tip you off. In other words, your CAT is probably fine.

Let's see, 14 inches of mercury, if one psi were 32 inches of Hg(?), that would be a little under .5 psi! Pretty tough target to hit!

Never underestimate the amount of a persons greed.

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