Thread: Catalytic Converters

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I did a search on catalytic converters and read through about 30 threads on exhaust systems but I did not find any thread on just the converters. I looked up the definition of a "Sound Decibel" and found it is given by the formula DB = 10 log (I/I0) where I0 is a standard sound level detectable by the human ear. I am used to using decibels in electronics as db = 20 log (E/E0) so the scale is slightly different but still related to the power of 10 in the ratio of the sound level to some standard. My question is that I am worried that one of these years the greater Richmond area will flunk the Federal guidelines on ozone levels. As I understand it if a certain level of ozone (O3) is exceeded four times in a given month the gasoline sold in this area will have to be reformulated more like it is in the North East and California. So far I believe the ozone levels in Richmond have been over the limit three times in July the last three years or so and if we ever get to four days of high ozone levels that may not only trigger a higher price for gas but make exhaust sniffing a part of the State Inspection procedure just as it is already in some counties in Northern Virgina and D.C. In reading about the mufflers it seems that most rodders want a certain sound, just as I did when I was 16, but what I want is a "whoosh" not a glass pack crackle. I would prefer not to annoy the local police or the members of the State Trooper Barracks within 1/2 mile of my home. So my question is what does a "Universal Cat" do to exhaust sound and to flow? Advanced Auto has Universal Cats which I can insert in my exhaust system now so that I won't have to add them later and that brings up more questions such as can I run with just cats and no muffler, how loud would that be? Also if I use both a cat and a turbo muffler in each pipe how much does that restrict flow? To be specific I guess I will use 2.25" tubing with tight-fit headers on a SBC-350 with a mild torque cam and a 600 cfm carb. My understanding is that the cat has to be in a hot area close to the headers to work best. So here are the questions:

1. Can I use just catalytic converters as mufflers and will it be fairly quiet?

2. If I use a cat AND a turbo muffler will that be essentially a stock restriction or will turbo mufflers increase flow even with cats in the same system?

In a few places in the muffler threads some folks asked for actual decibel levels, but there are so many different combinations of engines and mufflers that nobody seemed to come up with quantitative data and relied on what sound they liked. I would favor a fairly quiet muffler as long as the flow is good.

Don Shillady
Retired Scientist/teen rodder

Don,

Cats don't provide a lot of sound deadening - as evidenced by the rumble of my '93 Vette when did a cat-back system replacement with 2.5" pipes. I have set off car alarms pulling into a parking place. It's not cackly - just a nice hot-rod rumble. However, I don't get a second look from the police or at the safety check.

If you use cats with no muffler, you WILL attract attention.

Cat's aren't horribly restrictive, either. With your engine combination, I don't think you'll give up much if you put them in the system. However, mufflers are an absolute necessity.

Don that's one 150.00 i wouldn't spend until i had to. you was asking about a st insp. law yesterday so i assume you are getting your car st. inspected. To get a sticker in VA. you have to have a muffler on you car. the conv. wont take the place of the muffler.

Originally posted by techinspector1
I have a pair of 2 1/4" x 36" glasspacks with the louver openings facing forward on my F250 and it's just right.

I agree... I had the stock exhaust on my F150 replaced with a glasspack and love the sound. It's only loud when I really put my foot in it but for normal driving then it's not bad at all. When I had it put on the guy said it would change my driving habits and he was right . I like the sound and the crackle when the RPM's are dropping but maybe that's just me.

Thanks all. lt1s10 helps a lot knowing Va Inspection rules and HenryRifle says cats alone are too loud and maybe I will look into the whoosh-mufflers as Streets says, but now I know I have to have mufflers and maybe the high flow cats are optional for a '76 SBC in a car titled as a '29. Thanks for the info.

Don Shillady
Retired Scientist/teen rodder

I would personally go for a set of turbo mufflers and check your current cats for any type of resistance. Many times if you are getting backpressure of your cats they have melted due to rich mixture Aka afterburning. If they are backing up, go for the high-flows. They are about the same price, and theres no point in spending the same for OEM if you can get a good set of high-flows.