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WOT bog

T

tenderfoot

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I can't find a good thread to ask in so I'll just start this one. I've messed with this for a while now. Ive made it better but not go away completely. Edelbrock thunder series 650 on a sb350 with a very mild cam. HEI, timing is now set at about 17*. Was set at 11* for a while. Set the accelerator pump at leanest setting helped very little. Moved vacuum advance to full manifold at the carb and that helped a lot. Still the best I've had it can barely feel power surging in and out at heavy throttle. Seems metering rods are the only thing left I haven't messed with.
Then I had a thought earlier today banging my head against the core support. Cats! Could they be clogging up on me? Just enough to impede high flow? Well of course rob me down low too?
 
Link not working. Giving me warnings about hackers or something.:dunno:
 
It’s just the new settings on your browser.
Here’s the two tests you can do on your converter:

5. Backpressure – a good cat will impede exhaust gas flow minimally. If you see pressure in the exhaust pipe, before the cat, greater than 5psi, it's a sign of blockage. You can test exhaust pressure by removing the O2 sensor and inserting a tester like this.

22982995_wae_waebpt01_pri_larg.jpg


You can also make your own tool from an old O2 sensor, a pressure gauge and some careful drilling.

Your right foot is also a good tester. If your car seems to lack the gitup that it had in the past, but is otherwise running just fine, chances are you have an exhaust restriction. My truck seems to have plenty of power (it's a 7.4L). Moreover, I lack the funds to buy a tester and the initiative to make one, so I won't be doing this test.

6. Thermometer – a properly running catalytic converterwill have an operating temperature of roughly 300-500* at the inlet welds. The media will heat the exhaust gases another ~150* by the rear welds. Use an infrared thermometer to take the temperature of the cat at both the front and rear welds (not on the heat shield).

Temp at the inlet is 329*, so that is within range.

IMG_1058_zps44802a6e.jpg


Temp at the outlet is quite a lot cooler. Worse, the outlet temp varies widely from a low of 240* to a high of nearly 450*. The outlet temp is well out of range and highly erratic. This is a bad cat.
 
It's a '78 so I believe they have to be there. But can I cut em out, drill em out, with a hole saw or something and replace them? Anybody ever done something like that? Be damn messy right?
 
Also I know Edelbrocks are famous for bogging at higher rpms. But I've had almost this same exact set up that ran like a timex before. So I've a feeling I may still be scratching my head after I do this.
 
tenderfoot said:
It's a '78 so I believe they have to be there. But can I cut em out, drill em out, with a hole saw or something and replace them? Anybody ever done something like that? Be damn messy right?
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I used to do it on my friends’ Trans Ams and Mustangs all the time back in the late 80s and early 90s.
 
I used an air tool with a cutting disc. on Camaros and Trans Ams I would cut the heat shields off then cut the cat open and split the media in 2 or 3 pieces to get it out.
Then I would mig weld the cat back shut and weld the heat shield back on.

When I was done you couldn’t tell they’d been touched.

Mustangs were easier. Just drop the H-pipe and use a piece of rebar and a hammer from each end to bust it out.

People always said they wouldn’t run after this was done unless you put in a chip that was programmed for it. Total BS. They all ran better and got better fuel economy with the factory tune.

None ever had an engine light even come on.
 
tenderfoot said:
Also I know Edelbrocks are famous for bogging at higher rpms. But I've had almost this same exact set up that ran like a timex before. So I've a feeling I may still be scratching my head after I do this.
Click to expand...

If you don’t live in a county with emissions testing you can cut that off. I think it’s 82 or 85 when you started having to have a cat in Tx.
 
There are several businesses around here that are deleting semi trucks. No testing here on anything.
 
Same here, the only way an ECM would know if something changed is if it has an O2 down stream.
 
Have you checked to see if your timing is advancing fully? What do your plugs look like?
 
When I switch my vacuum from timed to full there is a major difference in idle speed. Other than that I don't know how to check if timing is advancing fully. Plugs are slightly brown but maybe just a little sooty. I'm going to work on gutting those cats today.
 
What about fuel delivery ?..perhaps your not getting enough volume and pressure to keep up with WOT on a big engine & carb,if your using the stock pump...or a gas filter that is clogging up or restricted..
 
There are advance weights in the distributor that may need to be inspected for working right or replaced to heavier springs that would change the advance rate.
 
diesel4me said:
What about fuel delivery ?..perhaps your not getting enough volume and pressure to keep up with WOT on a big engine & carb,if your using the stock pump...or a gas filter that is clogging up or restricted..
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I don't think I have a fuel delivery problem. I do have two filters in it. I put a second see through one in before the mechanical pump a while back thinking the same thing. It's stayed clean. Also have a mechanical gauge on the line just before the carb. Reads fine at idle but that doesn't help me while driving. Actually yesterday I pulled the gauge and replaced with a sender for an electric. Just have to get some wire and figure out where to run power from and mount it, somewhere. I need a tach too.
 
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