cat converter back presure test???

[Dvldog 8793]

Howdy
This came up in another post. I have one experience with this test and it isn't a good one, maybe it is an isolated one hopefully. My mother was talked into replaceing the cat on her car because of a back-pressure test. The car ran fine prior and ran fine after. It was at a local shop that was doing regular service for her. I wasn't able to be there and told her to make sure and get the cat back from them. I took it to my neighbor that owns a salvage yard and he told me that this type of cat never fails.
Anyways, can anyone tell more about the Cat-back-pressure test? If it is legit it might also be used to find bottle necks in a intake system or exhaust system.
L8r
Conley

[Craig M]

Had a test done on the Catalitic converter on a gas engined vehicle a few years ago. Test should be similar for diesel cat. Was having major probolems and I told the muffler shop that I thought it might be the cat. They tested it for me and the cat was not the problem (can you believe that, an honest muffler shop). Test was to drill a small hole in the muffler before and after the cat. Install gauges and read pressure before and after cat. After test welded the small holes shut. Whole test took very little time.

[a5150nut]

Pin hole and pressure gage test work fine. But, you need to know what the backpressure is supposed to be when you start. My then stock 94 k2500 was 3 3/4 lbs before changes. Put in 3in tubing from cat to rear and an Allied muffler. Then re-checked and it was less than 3/4 lb pressure.

[Craig M]

This test was done to check the cat. If there was a large increase in pressure from the front to the back gauge, then the cat was "bad". If there was very little difference in pressure then the cat was "fine". A before and after test of pressure after removal of the cat tells you what you have gained by the removal of the cat and installation of a better muffler.

[patrick m.]

on gasoline engines, the GM service manuals all said 3psi was the limit (of course during a rev)
performance exhaust people look for no more than 1psi at full flow.

somewear around 3psi fuel milage, and performance on a factory eng begin to decline.

[tom.mcinerney]

This is good info. "3" is a benchmark, I guess. But we may need more for realistic judgement.
I think (don't know) most gassers use a metallic mesh construction. The 6.5Ls (at least the early ones) use a channelled ceramic, with tubular pores.
There are at least three different design/types of cats now in production for use on diesel vehicle engines. They modify different gasseous pollutants in different sequence, for different goals. One type is designed to be periodically 'regenerated' by blowing air thru it while it is being heated.
I spent a little while searching for a replacement for our OEM that would be effective for emmissions test and low restriction. A mesh style might work , but would be bulkier and near $2000 retail. Even if i had lots of money and couldn't think of anything else to buy, i wouldn't get an expensive cat until 2007, when 90% of the remaining sulfur in the road diesel will be removed {as has been case in europe for a few years}. I'll edit this post with some decent links when get chance....