Will stright pipes guzzle gas?

I got a 91 chevy LWB silvy. It has the factory cat on it with that cheap ass single inlet duel outlet muffler. The gas mileage is bad right now and the cat gets very hot. The heat from the cat has disclored the tranmission cross member. I figure it is bad and needs to be cut off. We dont have those emmisons test where I live so cutting it off is no problem. Duel stright pipes would be bad ass, but I heard that it can damage the valves, guzzle gas, and give you less HP. But I have also heard that it wont damage the valves, it will save gas and give you more HP. Whats true/false?

oh yeah it has a 350 5.7L V8

ummm it will prob guzzle gas but it will let more exhaust out of the engine, as for the valves i have no clue

I got a 91 chevy LWB silvy. It has the factory cat on it with that cheap ass single inlet duel outlet muffler. The gas mileage is bad right now and the cat gets very hot. The heat from the cat has disclored the tranmission cross member. I figure it is bad and needs to be cut off. We dont have those emmisons test where I live so cutting it off is no problem. Duel stright pipes would be bad ass, but I heard that it can damage the valves, guzzle gas, and give you less HP. But I have also heard that it wont damage the valves, it will save gas and give you more HP. Whats true/false?

If your O2 sensors are old, they probably need to be replaced. If you're dumping too many HCs into the cat, it will get hot even if it's not bad. Please don't cut the cat off, you're polluting the air everyone else has to breathe. There are high flow cats that barely restrict the exhaust, and they're supprisingly cheap (about $130). Having open exhaust improves mileage and horsepower, just like a high-flow intake. I've heard that it will damage the valves, but never heard any reason why. I don't see how they would be damaged.

Again, about the cat, please leave it on if it's not bad, or replace it if it is. Unless you use the truck in an environment that the cat could be hazardous, such as offroading through tall grass (the heat has been known to catch grass on fire, luckily, I don't know first hand).

I have dual staights on my 97 with the factory dual cats. driven about 30,000 miles that way without any problem. My buddy has a 2001 with the 5.3 and the same exhaust and he hasn't had any problems either.

Running straight pipes w/o cats wont hurt your gas mileage at all, the thing that will destroy your mpg is the increased weight of your foot with a really loud exhuast. I have never heard anything about damage valves but i guess it does seem logical to occur over time with decreased/no backpressure. And about the backpressure, some say you need and others say you dont. But basically with straight pipes you will lose low end power due to low/no backpressure but you will gain power at higher rpm's/mph cuz the exhuast can be pushed out of the engine much faster. I personally dont like straight pipes, it sounds like shit. Your best bet would be to forget about the converter and just buy a flowmaster 40 or something similar. It will sound good and maintain some back pressure.

I agree with most people on this board to keep the cats on unless they increase fire hazard with really tall grasses while off-roading etc. Some high flow cats aren't really too expensive, and yours may not even be bad to start off with, some sensors may just be bad and are letting a little bit of fuel into the cat so it burns there and get's hot. You can have straight pipes without valve damage, you just have to retain some back pressure. You can do this as long as your pipes aren't too large in diameter. Exhaust pressure can help fuel stay in the engine at certain RPMs so it doesn't burn while exiting the headers (hence valve damage) or in the cat. If you are going with a true dual system (instead of a Y-pipe and dual tips) the pipes should be a little smaller to retain pressure since there are two actual exits for the exhaust, instead of going into one. There are many possibilities with exhaust, you just have to know the limits. By the way, upgraded exhausts pretty much always increase HP and MPG. I have a 305 with a Y-pipe dual tip type. My stock cat flows very well and my 40 series FlowMaster muffler increases HP and my fuel's longevity without being too loud. :smile:

I can see how the backpressure would help a bit, but exhaust backpressure shouldn't be more than a few PSI, which is nothing compared to the hundreds of PSI in the cylinder. Plus, with straight pipes, the heat moves away from the valves faster.

You definatly want backpressure on a regular engine like that. The only time you would benifit from little or no back pressure is if you have a turbo (which is why turbo diesel trucks benifit from 4-5" pipes and no mufflers).

The only way you will gain ANY horsepower or torque is if your cat or muffler is really plugged up with crap. You'll get the most power/torque if you go with a high flow cat and a good quality muffler.

Like most have said, definatly leave the cat. It is illegal for you to take it off no matter what state you live in.

It's true that exhaust back pressure isn't strong in relation to other types of pressure, but it doesn't need to be. It's only to really keep the fresh fuel in the cylinders. Higher RPMs or a quick change in engine speed can still let a little fuel out the exhaust. Since no matter how exact engine timing may get, there will always be some slight imperfections because engine parts move so fast. A little pressure is good. :)

I live in Ky. Those damn farmers drive those out of tune tractors around spitting out oil and hydraulic fluid and filling the air with black and white smoke....So I don't want to hear no BS about the environment...When the hick farmers fix their massive leaks, put mufflers and cats on their old ass steal wheeled tractors, i will buy a new cat. Why should I spend $130 on a new cat when I can just cut it off

Because you drive your truck every day for extended amounts of time, and the "hick" farmers are only running their tractors on occasion when required for discing, plowing, etc.

Over the lifetime of your truck vs. a tractor, you're going to pollute a great deal more. Take some responsibility and do the right thing.

I live in Ky. Those damn farmers drive those out of tune tractors around spitting out oil and hydraulic fluid and filling the air with black and white smoke....So I don't want to hear no BS about the environment...When the hick farmers fix their massive leaks, put mufflers and cats on their old ass steal wheeled tractors, i will buy a new cat. Why should I spend $130 on a new cat when I can just cut it off

"Those damn farmers" grow your food for you. Without the tractors, you probably wouldn't be eating much.

It sounds as if you are upset about the farmers polluting. How do you think everone feels about you taking off a part that was designed to make everyone's air a bit cleaner? The old tractors may have visible smoke coming out of them, but the pollution that is worst for the environment is invisible.

You said:
"When the hick farmers fix their massive leaks, put mufflers and cats on their old ass steal wheeled tractors, i will buy a new cat."
...Maybe they're waiting for you to buy one.

As was already said, those tractors may see as many miles in a year as your truck does in a few days. Also, how many cars do you think there are for every tractor? Maybe several thousand? There may be even more than that. Show some damn respect for everyone else and replace the cat if it is bad, or leave it on if it's not. A farmer taking his produce a few miles to market is nothing compared to everywhere you drive. :banghead: Some people can't see past today, or think about anyone but themselves. Maybe you should try a little bit harder.

well hey, I'm one of those dam hick farmers and our tractors don't put out smoke when just driving along. They will when you hit the gas or put them under a huge load, but so will any other diesel vehicle out there.




So, to sum things up, I hope you burn in hell.

hey chevyman84, first off, it sounds like your cat is plugged/shot. i see a lot of people giving you flak about removing it, but that's what i'd do whether you were going with duals or not. the extra emmissions are minimal. around here its legal to chop em on a vehicle under '94. if you are feeling guilty about hurting the environment, there are other ways to help cut emmissions/save energy.

...the extra emmissions are minimal...

...if you are feeling guilty about hurting the environment, there are other ways to help cut emmissions/save energy.

Have you ever seen, or even heard, of a 4 (or 5) gas analyzer? The extra emmissions are NOT minimal! The hydrocarbon emmissions alone will be thousands of times higher with no cat, unless the engine is in PERFECT tune. Even if it is, HCs will be much higher than with no cat.

I don't think he gives a shit about anyone who isn't him, and apparently you don't either. There are other ways to reduce pollution, but why not do everything you can?

I don't have a cat on my Silverado, but if I did, I'm sure I would start several brush fires. That is why I havn't put one on it. It didn't come from the factory with a cat, and there's really no room for one anyway. I don't drive it very much either. I've got a cat on my daily driver, if it goes out, I will replace it.

well, hats off to you j-ri. you're quite an environmentalist. maybe arseholes like myself would install overpriced power robbing mods to our vehicles if we could just get it thru our thick heads to listen to every tree hugging/seal lover we encounter. well, we all now know that i'm a shithead that will not put a cat on his truck, but j-ri, don't get self-righteous unless you yourself are "doing everything you can" . by the sounds of things you have at least 2 vehicles, how about your girlfriend/wife, does she have one? well that's a lot of vehicles to upkeep/put GAS into. how about your house? are you living in a straw hut in the woods w/no electricity/generator? is it solar powered? windmill? how do you heat it? do you have any other recreational vehicles, 4 wheeler, dirtbike, seadoo, motorboat? how about a lawn mower? better go trade it in for some environmentally friendly scissors. maybe you should take that $ you got saved up for goodies for your truck (which likely will help nullify any gas milage you gained by using acetone), and donate it to a charitable organization which will in turn spend 80 cents on every dollar earned buying CATS for those too unfortunate to be able to afford their own.
i'll admit that i'm every bit as much but no more of an asshole than joe blow down the street with the inefficient furnace, or the huge energy wasting house, car that leaks oil/freon.
sorry for ranting. i'm not trying to pick a fight; i find you very knowledgable and helpfull so i hope not to offend you. just trying to make a point.
you'll have to excuse me now while i go save a seal and hug a tree. L8r

The last tree I came in contact with was with my front bumper because it was in my way. Not sure about seal, but I don't think it would taste as good as deer. If I seem a bit "extreme" here, it's to counteract the "extreemness" of others. That way, we might just meet in the middle. I maintain my vehicles well, would do a bit better if I had more money. My Silverado is my vehicle for everything other than driving around town. Towing, hauing, offroading (I was out all day today, used 25 gallons of gas, spilled a few quarts of gear oil, lost two running boards, and knocked 4 trees completely down, hit a few others). When I said "do all you can", I meant the bigger things that don't sacrifice your comfort or enjoyment. Will I go without heat, hell no! I'll cut down a forest before I get cold. If I don't go mudding at least once a month, you better put up a big fence, cuz I might just drive down every yard on your street. Is it really so bad if one person cuts the cat off? No. But when 1,000,000 people think that only themself and the guy down the street have cut 'em off, then we'd have a problem. If you can't afford a cat, you may have to end up cutting it off. If you can afford it, even if it means buying a 12 pack rather than going to the bar, do that.

Likewise, I didn't mean to offend either of you. I would, however, like some fresh air for my kids to breathe. If I get going now, maybe I can catch a seal, choke it to death, grill it up, and find out if it tastes anything like swordfish (which is aweful, btw).

Quote"Is it really so bad if one person cuts the cat off? No. But when 1,000,000 people think that only themself and the guy down the street have cut 'em off, then we'd have a problem. If you can't afford a cat, you may have to end up cutting it off." Quote
is it really so bad if one person offroads with a gas guzzling , fuel inefficient, oil leaking, large tired swamp buggy? no. but when 1,000,000 + do, then we'd have a problem.
is it really so bad when one person buys electricity from a coal burning, environmentally unfriendly utility company? no. but when....
is it really so bad if a poor bum on the street has a barrel fire to keep warm even though it emits toxic fumes? no. but when a million bums have a million barrel fires...
is it really so bad to mix your colors with your white's? no really is it? i don't know how to do laundry.
i think the both of us could go on quite some time hashing this out, seeing who cares more about saving whatever. i like nature, i fish, hunt, go offroading. a lot of the same things you seem to do. so we don't see eye to eye on me not wanting a cat on my truck; that alone shouldn't write me off as someone with 0 morals.

If you figure that mixing colors with whites, let me know... all my clothes are pretty much grey now. I think it might be from throwing everything in together. :)

I didn't say you have no morals, I was trying to push the both of you to put a cat on. If that can't be done, it can't be done. Not a huge deal. I've got good friends that have cut their cats off. I haven't staged any protests against them or anything.

Several of you guys are getting a little testy over the cat issue. When I lived in the Peoples Republik of California I had a 1981 Chevy 1 ton dually with a gas 350 which did not have a cat on it from factory. When I got it it would not pass California emissions-- too high HC level. I asked the test station if it would be permissible to put a cat on it to reduce HC levels. His reply was that it would immediately fail because it was not original equipment and that it did not matter that it would meet all emissions requirements. What I am trying to say is that government does not really care about the environment. I moved very soon after that to Arizona where my truck was fine. It is not illegal for you to take off the cat on your own vehicle as some posters would like you to believe. It is illegal for anyone else to do it for you. I have removed cats from several of my vehicles and realized a gain of from one to three miles per gallon. If you live in an area that does not require annual emissions tests then its a personal decision ; not one that anyone here on the forum should castigate you for making. It does not make you a bad person; It just means that you don't buy into all the government propaganda about global warming etc. (Despite all of Al Gore's hot air the overall global average temp has dropped over the past seven years; and the period from Jan 2007 to now has seen a drop in av. global temp equal to the rise in temp since 1930's)

You are dead wrong on the cat issue, this is a federal law, and although it would be virtually impossible to get caught in a non emissions state the federal penalties and guidelines cross all states. Politics aside an engine tuned to run with a cat will suffer without one as will the air, this applies more to tuning used in obd11 cars/trucks, but they are by themselves much cleaner then say a 92. Since this thread is way to old to have continued, I am closing it, but feel free to start a new thread on the subject if you so choose.