Exhaust system backpressure

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BRENDAN McCLAIN

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Ok my cats were clogged so I had them gutted and reinstalled. Sounds good with the flowmasters. One thing though, in the morning you can hear the hollowness of the cats.



But....I think I lost all back pressure. Complete turd down low, screams up high.



Is there a way to measure or track the pressure? Could I have a tune written to take advantage of the free flowing exhaust to cure the problem?
 
I don't know of any device that you could buy to check the back pressure of your exhaust. I know that they exist, but not they are not practical to buy for home use.



You really should have spend the money to get new Cats rather than waste it on gutting he old clogged ones. Your option is to put new cats on, or perhaps replace the gutted cats with a straight pipe. You are losing pressure because the exhaust is dumping into that large empty chamber.



...Rich
 
Dumping exhaust into an empty cat can, cause undesired turbulance. Ask me how I know..LOL.

If you state requires cats. Magnaflow makes cats in various weld in configurations. That is cheaper than buying the assembly. I have seen prices from $80 to $110. Shop around on the net.

If you have gutted the main cats you will eventually set off a CEL. In Texas, when the light ups go bad. They can legally be removed. The main cats must stay though.
 
You can find backpressure testers all over the internet, they are relatively inexpensive. If you purchase one don't get the kind that screws into the oxygen sensor hole. The one I have I drill a 1/8" hole in front of the cat and insert the gauge into the exhasut there, it has about 10' of hose attached to the gauge so I can run it up into the car and go for a test drive with it...think it cost me $40 some years ago.



You are not going to get a tune written to cure your poor exhaust setup. Get the exhaust setup right and you'll be amazed by what it can do. Not to dis on you but you could also do better than the Flowmaster as far as a muffler goes, they sound allright but aren't the best for making power. Just remember that when building an exhaust bigger isn't alwasy better...99% of the people who redo their exhasut make this mistake but it sounds really good so they think that it makes more power...noise does not equal power.



If you didn't have a backpressure tester what makes you thinlk the cats were plugged??
 
EddieS'04,6/4/2011 08:01 MT



. In Texas, when the light ups go bad. They can legally be removed. The main cats must stay though.
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I don't understand this. What everyone calls the pre cats or light ups as you referred to them as, actually have a specific job and it's not to aid in the functionality of what you call the main cat. While all the cats are 3 way cats the main cat is used more to control NOX than anything else. If the the pre cats are removed the main cat is not enough by itself to control HC and CO.
 
So would replacing with straight pipe be better or high flow cats? I live in Louisiana and they could care less about emissions in this part of the world. Insurance and lights are about all you need. Seriously!
 
I'm gonna say put cats on it, we all need to do our part environmentally speaking...call me a tree hugger if you want IDC. The other reason for putting cats on is the check engien light, while most people say "i don't care if the light is on" it actually helps to save you money. Lets say you don't have cats on it so now the light is on constantly, what happens when a problem starts to occur that you don't notice but would turn the light on but now you don't know it because the light is already on? Lets say that you are having a crankshaft position sensor problem and the pcm wants to report it, it does so by turning the light on, but you never notice anything amiss, now you are out in the middle of nowhere doing whatever and hop in the truck and it don't start...dam if there was only a way that we could have had an idea that there was a problem beforehand so we wouldn't have to pay this $100 tow bill....



So what made you think your cats were plugged in the first place??
 
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main cat is not enough by itself to control HC and CO.
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Scott, The main cats will control NOX, HC and CO. I cant give the documentation for the trac. My first car With OBD2. Previous cars. OBD 1 without the pre-cats. Got the same readings without them. On the dyno and sniffer. Previous obd 1 fords of mine, had the 3ways with air injection. For example if I order today a dual y-pipe from any other parts store than ford, in texas. They sell you a Non pre-cat, with main cats only. Not CARB approoved, 49 state only.



I compared my older obd1 test after I done away with pre-cats. The readings were the same. One test on my '94 stang. All my readings were lower. I had the bassani cat-x, with mains only.



Once the place I go. Had some sort of link up problem with the state. My trac has the rt-ypipe. No pre-cats. They did the dyno sniff. The numbers came in below tolerance. I think it is how well your motor is tuned.



One more example. A freind had an older lighting with the 351W motor. After going installing long tube headers. He stagered 2 mufflers and dumped at the side. No cats at all. He had it sniffed to see if he needed cats. His numbers came in below tolerance. Go figure..LOL
 
Eddie,

The reason some states emissions testing don't pick up missing cats is that they only operate the engine at idle. The EPA has multiple steps to their test procedures which include various engine speeds under load at certain engine tempuratures. Some vehicles/engines don't emit as much polutants at idle or until they have gone through specific warm up cycles



...Rich
 
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RL.

Harris county is one of the strictest in Texas. They do the sniff on the dyno at various rpms. From idle to 2500 at normal operating temps.
 
Eddie,



I know what the cats are and aren't capable of doing, afterall I do hold a State of CA advanced smog license and there is no doubt that the State of CA has the toughest emission requirements in the country. We have a state run emissions program here in AZ as well and I have to say that it is the biggest joke in the world, I would have to say that 50% of the cars that pass here would not pass in CA. I don't know of any factory service manual that accurately explains the operation of the various cats on the vehicle. Another thing to take into consideration on stock vehicles with pre and main cats is that the factory has to build these and make sure that they will pass emissions for 8 years or 80,000 miles, to this day I do not know of any aftermarket setup that can guarantee that kind of performance. So while a person probably can yank the precats and get by with just the main cat in the short term it is not going to be able to maintain those levels in the long term.



ANother thing to think about on a pre and main cat setup..have you ever wondered why there isn't an O2 sensor behind the main cat for monitoring it. Simply put, what people consider the main cat is more for NOX control than anything and while it does clean a bit of HC and CO in the process it's main function is still NOX control. The pre cats are there to reduce HC and CO, without proper HC and CO reduction the NOX reduction can't take place.
 
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Brendan,6/4/2011 08:54 MT



No exhaust flow, horrible fuel mileage. When the honeycomb was pulled out you could barely see light through the other side
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When cats plug the most obvious sympton is that the vehicle will just not accelerate when getting on it hard...the motor will try to rev but the vehicle just won't accelerate, alot of techs go chasing the maf for this issue as it can act the same way as a plugged cat, and sometimes is accompanied by what some describe as a jet engine sound coming out the exhaust. Typically a plugged cat is not noticeable at idle and the vehicle will still continue to cruise ok once it gets up to speed. Some of the GM midsize cars are notorious for plugging cats and the usual customer complaint is that the car won't move when getting on it hard but drives fine otherwise with no other symptons, on these vehicles I have seen as high as 30 psi of backpressure in the exhaust on a wot run but surprisingly they still somewhat accelerate and while cruising backpressure readings are normal
 
Eddie,

Sorry about the strict emmissions testing in places like Houston and Dallas Here in Waco, they don't do any emmissions testing. :grin: Smog and air polution are far less of a problem here.



...Rich
 
Brendan, here is an article about back pressure.



As mentioned above, if you get a guage, get the one you drill a 1/8th inch hole and slip the tip of the tester into the hole.



Super easy to use and they normally come with an aluminum plug you simply tap into the hole you drilled...
 
I guess I'm confused. I think we know that he has a lack of back pressure because his cats have been gutted. So why should he spend money for a meter to measure the amount of back pressure that he does not have??? His best move is to save the $40-$50 he would spend on the meter and just put that money towards a the purchase of a cat? :banghead:



...Rich



 
Scott,

Im not dissing your certs or knowledge. Im just explaining my personal experiances.

I have had an '88 t-bird with a non-stock 302 HO. Also a '94 stang with a non stock 302 base Cobra motor. The bird had walker cat-Hpipe no pre cats for 8yrs and over 90K miles.

The stang with the bassani cat-x, no pre cats for 12yrs over 80k miles.

As I told rich. Both of these motors always came in as clean or cleaner than OEM. In one of the modt strict counties in texas.

Am I lieying? Hell NO..

Nonthing explains my freinds gen 1 lighting non stock 351W, with no cats, Passing emissions. I was there I saw the print out.

So your certified and Im a lier, not.



ANother thing to think about on a pre and main cat setup..have you ever wondered why there isn't an O2 sensor behind the main cat for monitoring it
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Have you looked under your trac. Their is an O2 sensor right after the primary main.



I have my personal experiances. You have your certs. They cancell each others lotto card out...:argue:
 
Eddie,



Not doubting your experience with the cats one bit, I have also seen similiar results so no need to get defensive. I also apologize if you thought I was trying to call you out on something.



In regards to the O2 locations, the scenario I described above relates to setups that have a O2 sensor before and directly behind the "light off" cats. On a 3 sensor setup the catalyst sensor will be mounted behind the "main" cat.
 
Scott,



Were cool. I havent had much sleep the last few days. Up here at lake livingston. Helping mom and staying up too late and getting up too early. 4hrslast night. :haveabeer:



Mom and I have even had misunderstandings.
 
I must say.. it's nice to see a "normal" conversation with out two people getting hostile about the subject matter being discussed. Unlike people do on other forums.



I am still interested in what someone had said about Flowmasters not being the best for performance. WHat is the best? I would like more "exhaust " tone, for a bit of added excitement, but I would really like more power if possible.



Thanks

Brian B
 

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