Can having no cat converters cause rough idle problems?

Telewanger

New Member
Apr 4, 2009
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I just ordered some cats yesterday, so that I can pass NC inspection. These are high flow, 300 holes per sq. inch.

2.5" Thunderbolt Metallic Catalytic Converter 415250

Questions: 1994 Mustang GT, 5.0

1) A mechanic told me that my car will idle much better with cats on it. Is there any truth to this?
2) How much horse power will I lose by installing the cats?
3) Will I gain low end torque?

I have read some articles and different posts for the past week. So far, more people say to gut them out rather than to keep the metallic brick in it. A lot of the hot rod and racing websites say to use metal bands on it instead of welding it in, due to rust, vibrations, and ease of changing it later.

I guess about 40 percent of the people say to keep the metallic brick in it and weld the cats in place. Most performance websites say that backpressure is a myth. Backpressure is a resistance to flow, but you need a Delta pressure drop when measuring pressures from the cylinder to the exhaust tract, and that anything after the headers is just an obstruction to overcome, such as cats and the muffler. I don't know???

This crap is really confusing! I will need to put the cats on it to pass inspection but it will only be a visual inspection, so they won't test it.

My new cats will be arriving tomorrow, and I still haven't decided if I will just install them or remove the guts out of them. Help me make a final decision!

Thanks!
 
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One of the many problems with modding your car is, everyone's got an opinion of the "right" way to do it. Some of the knowledge you hear is old, or not based on any facts, or only applies to a certain type of engine, etc. I can only give you my experiences.

1. If your GT does not have any idle issues, cats will not noticeably affect your idle if at all. I bought my red GT with an OEM h-pipe, and I swapped on a Bassani X-pipe with hollowed-out cats. No difference in the idle that I could tell. However, I did have a rough idle both before & after the swap. Later, I was able to improve my idle by changing the fuel filter. So IMO, the cats will not affect your idle.

2. I've heard 3-5 hp loss with cats, but no evidence to back that up. Would you feel that loss of hp and torque? Maybe. Some cats flow better than others. Even if you do lose a bit of hp, you're likely better than the stock OEM pipe.

3. Doubtful. What the cats may do is change the torque curve a bit. Basically, your torque peak may appear at a slightly different RPM than it did before. Now, this is my opinion and I can't back it up with any facts, so take it as you will.

Exhaust bands make it easy to move and remove parts, but makes it easier to get exhaust leaks. My Bassani cat-back is held together by bands. I had to move the cat-back around this weekend to replace my lower control arms, and after I was done my exhaust was quieter. I also noticed the smudge of exhaust around some of the bands, so I had tightened those clamps more. So I've had some minor exhaust leaks for a while now, and this weekend I got rid of some (hopefully all) of them.

Welding, if done properly, eliminates those kind of leaks. But if you need to remove your exhaust for any reason, you'll have to cut and re-weld it back together. Personally, I'll deal with the leaky bands to have the benefit of shifting them around if necessary. They also make muffler swaps easy IMO.

All this is really confusing, you're dead on with that observation. All I can recommend is listen to all the ideas you'll hear, and make the best decision you can.
 
I am fine with my idle, but the guy at the inspection station didn't seem to like it. It is a little slow, but I can idle it up if I want. He says since I have no back pressure, it is causing the car to run rough.

After tomorrow, I will have cats on the car. Whether they are gutted out or not, I don't know yet. I will make a decision when they get here. I have been working on the car all winter replacing a lot of worn parts.

Here is a video of the idle and no cats at all on the car. This is the way the car was when I bought it. It had no cats. If you have already seen this video, sorry to ask you watch it again!

Thanks for your feedback!

YouTube - Mike Lynch - 1994 Mustang GT
 
Ah, I remember that video. Your lopey idle is due to the engine mods and the computer adjusting for them as best it can. Not due to "no back pressure". The guy has no idea what engine mods do to our cars and how it affects the idle. You could "fix" that idle with a tune in the computer, but if it runs well to you then you're set. I gotta admit, it sounds real mean with that lope.
 
I say throw some SLP or magnflow tips on it and some differnt wheels so its a lil more street..

It sounds good and you may not like the sound with cats that arent straight through or hollowed out

Nice freakin car.:nice: