Can having no cat converters cause rough idle problems?

[Rush]

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

http://performance-curve.com/25thunderboltmetalliccatalyticconverter415250.aspx

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!

 

[Rice_slayer]

The mechanic is full of BS, my o/r pipe didn't cause ANY issues, just made the car louder. As for backpressure, that might be true as when I had my smog pump hooked to the o/r pipe, it would make crackling noises when I let off the gas, but it more of a cool rumble noise than anything...

 

[MustangMoser]

umm, install them then take them back off for what you're running right now. you'll lose some sound quality to your exhaust note, and possibly some hp/tq. you most likely will only notice a differance in how it sounds though. the cats will do nothing to your idle quality. I've never noticed a change in any of the mustangs I've put off road pipes on.

 

[94-427]

Idle quality will be diferant only if the car was tuned with cats then removed

 

[CC'S95GT]

By my seat of the pants meter, with the o/r h on it seems to have lost some low end torque but gained some higher rpm acceleration.
I also picked up a MPG or two with the o/r h on.

Cats are not supposed to cause a decrease in power. and because of my decrease in pwr I'm beginging to think my cats are getting clogged.

 

[Rush]

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!

Mike Lynch - 1994 Mustang GT

 

[Rice_slayer]

That thing is cammed, of course it's not going to idle smooth lmao

 

[Rush]

The stupid guy acted like he wanted to fail the car because of the cam idle. I told him that it is fine. He starts telling me that cats will cause the computer to make it idle 100% better due to the back pressure.
.

 

[ivan12]

That's BS. Will having cats make it quieter? Yes. Will installing cats make your rough idle go away? No. ^^^ it's your cam not your cats

 

[Paul]

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?

1. Nope.
2. With good high-flow cats? None, or very negligible.
3. Potentially. I've seen a back-to-back dyno with a mildly modified 5.0 where the car with cats made more torque.

Paul.

 

[94 DropTop]

You car is cammed. I take my car somewhere where they know what they are doing.

 

[mygreengt]

we dont have emissions testing in iowa

 

[Rush]

Now I need a smog pump on the car to pass inspections. This is really starting to suck. I have owned the car for well over a year and still can't drive the damn thing! When will it end? Am I going to have to turn this thing back to stock, or what?

 

[MustangMoser]

find a new mechanic, yours is an idiot.

What state do u live in?