OK, everybody here are my questions. I have seen many comments about back pressure with the 5.3L engine. What exactly are you all talking about here? I don't seem to be missing any low end power, which is what I think everybody is saying. What is the difference/effect of leaving the resonator on with regard to back pressure, if any?
When I started looking at muffler systems, I couldn't believe the wild a** claims on the various manufacturer's website's about horsepower gains with their cat-back systems, so I called Magnaflow about it. Their rep told me that 80% of the gain from a cat-back system comes from the mufflers themselves, so if you gained 10 hp from the cat-back, you would gain 8 hp by just changing the mufflers. True or false?
Any and all comments from all you fine folks will be deeply appreciated. Thanks!

Mark

 

Mad, when you change things on your car, the torque curve changes.

Torque response is how quickly your car jumps or doesn't jump when you get on the throttle at various speeds and RPMs.

Most cars are designed with good back pressure which gives you a lower torque curve and allows you to manipulate normal traffic situations with great throttle response.

A lot ofmodeifications to cars allow them to hit high speeds with a minimum of RPMs and great throttle response at higher speeds.

Theoretically you sacrafice lower speed and RPM throttle responses with high flow exhausts. VVT and VTEC systems are compensation for this.

The only way to know if you have back pressure problem is on a dyno.

 

Magnaflow is right on the money with this. Changes in the piping help, but aren't where the most resistance is. It is mainly in the mufflers themselves.

The muffler bounces the exhaust around quite a bit to create sound cancellation waves and basically keep the car quiet. The aftermarket typically doesn't try to keep it quiet. Their goal is to make a nice sound, but they are more concerned with gaining power.

 

No matter what anyone says backpressure is never good, never. The more straight through your exhuast is, the more power your going to gain, that is true 100% of the time

 

A certified BMW engineer said,

Some say that "an engine needs backpressure to work correctly." Is this true?

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

And there is basically everything about backpressure if anyone ever thinks its good, just show them that.

 

I loved the two Montes I've owned, but right now it's a Silverado 5.3L.

Lot's of research I've done on back pressure points to one thing: in a decent flowing exhaust, varying back pressure adjusts where the torque and power curves fall.(You can't have both low and high rpm top performance on a 5.3L the way it's designed. It takes lots of mods.) On Magnaflow dyno charts for the 5.3L, the chart is above 4,000 rpm and it looks great. My rig cruises at1600 rpm at 60mph. We tow a travel trailer occasionally.Itgets toaround 3,000 rpm on a downshift going uphill.I want my power curve tohelp me at those lower rpms where I drive 98% of the time. That's not what the Flowmaster on my pick-up is giving me. Low back pressure on a 5.3L costs low rpm power. Many posts and others I talk to confirm that. Back pressure has a purpose.

 

Exactly the point I was trying to make. I never mentioned increased torque, I mentioned ashift in the torque curve....big difference.

 

Exactly

 

aw man.... we have this debate about once every 2-3 months or so. no one ever convinces anyone of anything, and whoever brought up the question lands even more confused than they were before.