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Are CAT's really that restrictive???

11K views 43 replies 12 participants last post by  CavemanB52 
#1 ·
I was at a car show yesterday and spoke to a tuner who told me that the main CAT's just behind the manifolds, where not that restrictive as compared to a straight tube if you were under 620 hp. That seemed completely contrary to every thing I have heard for years. However he said that he had objective data to prove it from 2 cars he tested. I was told that there was less than 3 hp gained with open pipes but a definite sound increase inside the cabin. Those comments made me wonder. Has anyone else ever done a comparison between the stock CAT's vs an open pipe assuming the rest of the exhaust remains the same?
 
#2 ·
Coming with no empirical data, I would probably say this is probably true. Maybe back in the day car manufacturers had more restrictive exhaust systems but nowadays with better technology they can push the limits more.

I know there is a lot of discussion on backpressure (is it good, bad, necessary, etc) and sometimes open headers just don't help as much. I've seen smaller NA motors lose power with bigger diameter exhausts installed.
 
#4 ·
I don't think being restrictive is the problem with cats, I have has two modded supercharged vehicles (2003 Silverado SS 700+hp and my 2009 Cts-V 640rwhp) and in both the cats disintegrated after a couple months even hi flow cats didnt last, maybe I just have bad luck with them or drive to fast lol
 
#8 ·
rpg6373, I love the electric cutouts. That is a modern twist on old school trick.

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What if the CATS were mounted further back like 3-4 feet so they would be in the middle of the car. Granted they would not heat up like if they were closer but would still be emission compliant and would quiet down the car. Just a thought.
 
#12 ·
The stock cats are slightly restrictive. Switchin to the high flow will open up top end hp. But u would only do that if u upgrade the headers

Cutouts are mainly for stock catbacks that are very quiet and power robbin u put them before the mufflers.
 
#14 ·
With a supercharger you need some restriction. If you run catless you will gain top end hp but lose on bottom and mid power, (power being hp and torque)

Put in high flow cats and let the power come to you.

We dynoed a supercharged car with and without out cats and the verdict was 5 hp gain at max rpm with a 15-20 power loss from 3500-5500rpm

was it worth it, AH NO!, cut the ac belt if you want more power, we have seen gains of 10hp by simply cuttin the ac belt...

Ppl will go to extremes to win races here, even if it costs them ac while drivin home after a victory race...
 
#15 ·
The muffler is the only restriction you need to keep from losing HP due to back pressure. The low end HP loss your speeking of is from straight pipes. The test numbers are on this forum. Done with the same car and motor. You are right for the most part but there is no power loss with no cats. There is just not much of a power gain unless over 600 hp or there abouts. So you can't talk generalities. Most who have done more than just added an exhaust ie pulleys, meth, porting will see additional all around improvement with power and torque due to headers. Still need back pressure but a stock muffler to going to provide that. Cats or no cats doesn't make a difference except maybe sound increase. Cats for what they can and can't do is like talking a about purple dragons and wizards.
 
#16 ·
Caveman and Naf bring up interesting points. I did a search on this forum and found countless comments and opinions about exhaust and CAT's but was more interested in objective evidence comparing an exhaust with Cats on a stock exhaust vs one without the cats. Granted everyone's other power components very and results will probably very accordingly.

But as a general rule of thumb for cars in the 580-640hp range I wonder what the difference would be in sound and power?
 
#17 ·
I don't know about the close-coupled cats, but putting the main cats (the rear set) back in and re-tuning only cost me a little more than 1 RWHP.

It's been awhile (ca. 2009), but IIRC, Wait4Me built and tested a completely catless and muffler-less 3" mandrel-bent system connected to the stock exhaust manifolds (separate downpipes to replace the close-coupled cats were an option). With no other mods, again IIRC, the net change was only 6-8 RWHP or so. He reported his findings here, so they're probably in the forum archives somewhere.
 
#18 ·
I've seen 10-25 hp increase for various header install before and afters but the one test I saw for the difference between cats or no cats was a 1 hp increase in the middle of the power band for a split second. So essentialy no change. Just some extra sound. I don't like dealing with the possibility of melting them. Besides they are useless after they go in the landfill they breakdown and release carbon into atmosphere anyway... so why even use them in the first place. Tree huggers do more damage to the environment than they anything else.
 
#21 ·
Same problem here. Decided to just through Kooks headers and Xpipe connected to factory muffler. If you do this ask for a modified connection pipe from Kooks to fit a coupe (If you have a coupe). Cause otherwise you will need to weld stainless steal and do your own modification to the exhaust. They cut and flanged mine to fit.
 
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