Will stright pipes guzzle gas?

[Chevyman1984]

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?

[Chevyman1984]

oh yeah it has a 350 5.7L V8

[Pat_OD]

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

[J-Ri]

Originally Posted by Chevyman1984

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).

[Ridesagain]

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.

[Silverado Brethern]

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.

[98SilvyKid]

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.

[J-Ri]

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.

[goinbig]

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.

[98SilvyKid]

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.

[Chevyman1984]

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

[SageCuda]

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.

[J-Ri]

Originally Posted by Chevyman1984

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. Some people can't see past today, or think about anyone but themselves. Maybe you should try a little bit harder.

[goinbig]

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.

[clampjockey]

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.