I have done search upon search with all sorts of different answers. Then I asked Mike T. in the Chips and Programming forum and he just blew me off saying he didn't have time to teach. All I want is to truly know whether having no back pressure on a 4.6 really does hurt the engine and what exactly it hurts. And with the way this "expert in performance" has blown me off about it twice I am beginning to thank that having no back pressure won't hurt them at all and that it is a myth thought up to scare you into thinking that you better not touch anything on your trucks. So please if someone has repetitive proof that it hurts the samethings, i.e. samething happening to two different people, please give me some info. And yes I am talking about no cats and mufflers, but I just want to understand how for my own knowledge.

Sorry I am just a little pissed of how he avoided answering a question that might have probable have taken two whole sentences to answer.

 

id like to know too iv heard both sides and am confused... i have a 5.0l

 

Ok I will answer my own question. Removing the cats does not hurt the engine. Reason is because any of the answers that people have given make absolutely no since at all if you sit down and think about it. For one if there was problems such as the valves warping, then all of the performance places would sell modified valves, as another purchace to people buying headers and what not for their vehicle, to counter act it. And the way to keep your low end power while making a true duel set up is to install a x pipe.

 

Probably not much help, but I was told the same thing by a tech at a muffler shop when I had a dual exhaust put on a truck a couple years ago. He didn't say specifically that it would damage the engine, but would decrease your torque and that he didn't recommend putting a true dual exhaust on. I ended up just doing a cat-back dual exhaust with a Flowmaster FM40 muffler.

 

Best way I found to describe it....

"The truth - exhaust gas velocity

Here's the real deal about choosing an exhaust that is too big. I'm not a physicist, my understanding of fluid dynamics is very limited.... but here's my understanding.

Exhaust gases moving out of your exhaust can be used to pull other exhaust gases out of your cylinders. This is why headers are sized and the tubes matched the way they are. This effect is called scavenging. The faster the gases move, the more you benefit from scavenging.

The larger the tubing, the slower the gases move, the less scavenging occurs. (but, more ultimate flow with less restriction is possible)

The smaller the tubing, the faster the flow. (but obviously, the resistance can quickly overcome the benefit of scavenging if you start reducing your tubing size... a drinking straw simply can't flow enough for a motor)

Notice I didn't say anything about backpressure. Backpressure is just a measure of the restriction in the system. It can be caused by bends, narrow tubing, badly flowing mufflers, etc. Backpressure will be present in any system - but the less that is there the better. A large diameter system with a restrictive muffler (and therefore a high backpressure) will not gain any velocity. So now you will have to worst of both worlds - reduced scavenging and a restriction to free flow at higher rpms.

Some people claim to lose low end power when strapping on a big bore exhaust. This may be true, but it's because of a decrease in the scavenging effect, not because you don;t have enough backpressure."

Basically, if you remove the cats/mufflers on the 4.6 and the engine can no longer "fill" the exhaust system fast enough to keep X amount of exhaust velocity, then you loose power at certain points, usually in low end torque. At lower RPMs, the engine isn't filling the system fast enough to promote scavenging and thats where you loose power. You might gain some HP at high RPMs, but that's typically not where you want to be all the time on the street.

 

Let me add my 2c I have true duals with no cats and magnaflows. My truck is extremely loud and i have lost all power. My girlfriends cavalier has more punch than my truck. The only thing i have learned is that ford enginered those exhaust systems for a reason. If you can get the sound you want from a single in dual out then go for it and stay away from the headaches i have expereniced. I am going to get aftermarket cats and a h pipe to try and get some of the power i have lost. Its to the point where ill probally half to add major hp to make the bigger free flowing exhaust work.

Ben

 

I think the major problem with having a exhaust that is to free flowing (ie. true duals)is it reduces the amount of backpressure. This is evident in the loss of "get up and go". Less backpressure equals less torque. However less torque is not harmful to your truck, but the loss of backpressure is. Without the proper amount of backpressure you can't seat your valves correctly. This can be terrible for an engine, especially during the cold months. When the valves do not get seated properly and cold air is sucked into the pipe and up into the motor, now your talking about detination of your motor. Best thing to do is leave the cats and cut out the "Y" pipe and add a dual in/dual out muffler. My dad has this setup with dual glasspacks and its so loud in the truck you can't carry on a conversation. If that is not loud enough for you, then go buy a different truck.

 

Backpressure is bad.

Too much flow = gas loses velocity and cools (and gets more dense as it cools) in the pipes, and it saps energy to push this gas out.

Nice post, Gddyap.

 

Thanks GDDYUP for the good post espacially explaining about the backpressure. Reason why I say that is because a lot of people here like bonefish's reply is that these engines are designed to have a certain amount of back pressure which isn't true. Lets say that they were and either the muffler or cats get clogged or a hole is knocked into the cats changing the amount of backpressure, it would ruin your engine. So no they don't need backpressure to keep from hurting the engine, but it will, like GDDYUP said, would change the location of the power. My conclusion on this subject is that the people who say that it will hurt the engine is because everyone knows about the old engines and that removing the cats got better milage and power. Then the new ones came out, all wired up like someone in a hospital, and muffler shops said you couldn't do that because it will mess the computer up due to the downstream sensors checking to make sure the cats do their job. What I am getting at is that when people don't understand something they get afraid that the worst will happen. Ignorance breads fear and shaddy assuptions.

 

Less exhaust velocity equals less torque. Remember that backpressure is just a measure of the amount of restriction, it has nothing to do with torque. By removing all restrictions, you loose velocity, which in turn reduces the amount of scavenging, which is what kills your low end torque. With the loss of scavenging, you cant fill the pipe fast enough to create enough velocity to help evacuate the cylinder.

Also, "backpressure" really has nothing to do with seating the valves. Valve events are made and coordinated by the camshaft, lifter, and valve spring. The valve closes and stays closed via spring pressure which is not influenced by exhaust "backpressure". If you measured the amount of backpressure in your system, you'de find maybe something in the range of 10-20 psi or something in that range. Nowhere near enough pressure to affect valve performance.