MustangForums.com - Article in Modified Mustangs about Backpressure...

I actually disagree with this article. Let me explain why, and then I'll get down to the details about changing backpressure, etc.

What I don't like about this article is that it makes a distinction between HP and Torque. It basically says that "backpressure is good for TQ but is bad for HP". This is a mistake in thinking about HP and TQ that many people in motorsports make.

Torque is a FORCE, just "twisting" instead of "pushing" or "pulling".
Horsepower is a rate of work being done or in other words, energy-per-unit-time.

...and horsepower is calculated FROM torque. For American units the equation is HP = Torque * RPM / 5252. So when you test a car on the dyno, the dyno is measuring the torque (Force at the rear wheels), and then the HP is calculated from the torque number.

As you can see the two values of HP and TQ are linked together in a specific relationship. HP comes FROM TQ.

So let's say we have a stock mustang and we dyno-test it. Then we put a new exhaust on it and test it again. Suppose, like this article says, that we gained some TQ in the lower RPM ranges. For example:

Stock: 300 ft-lbat 3000 RPM.
After: 310 ft-lb at 3000 RPM.

Well, let's do the math and see what the HP is (note the formula above):

Stock: 171 HP @ 3000 RPM
After: 177 HP @3000 RPM

...you can see thatincreasing the torque at a given RPM point also raises the HP at that same RPMpoint.

The problem with the article isit says "torque" when it really means low-end power and it says "horsepower" when it means high-end power. It is ludicrous to say that a mod "looses torque but gains horsepower" becasue this is mathematically impossible, unless you are talking about different points in the powerband, in which case that should be explicitly stated.

Now then, down to business about the backpressure situation.

When you are talking about a fluid (liquid or gas) flowing through a pipe, there are several factors at play here. There are what engineers call "head losses" which is basically things like fluid friction or resistance to flow caused by rough pipes, bends, and that kind of thing. There is also the dynamic effects of the flow, which incorporates resonance and change in fluid velocity as the diameters of the piping changes, etc.

This article seems to be grouping both of those things together into one item and calls them "backpressure", when in reality they are two different things. Let me explain further:

Backpressure is resistance to flow, and it is caused by two seperate factors:
1) imperfections in piping, such as rough pipes, crushed bends, rough joints, little bits of weld sticking into the flow area of a pipe, or outright flow obstructions such as a catalytic converter or some kinds of mufflers. This kind of flow restriction is ALWAYS present regardless of operating conditiosn or RPMs, etc.
2) dynamic effects like resonance, which are quite complicated to understand...but any pipelike object has one or more resonant frequencies. This works just like a musical instrumentsuch asa pipe organ or a trombone: the piping in the instrument resonates at a given frequency depending on it's length and shape. Depending on what the resonant frequency is and what the input frequency (RPM) of the engine is, there may be backpressure generated at some certain RPMs.

Backpressure, wherever it comes from, is ALWAYS bad for performance becasue it restricts the flow of exhaust of of the cylinder. Backpressure increases the pumping losses (wasted power) in the engine becasue the engine has to work harder in order topush the exhaust out. It also can interfere with proper cylinder filling, which means you get less fresh air and gas coming into the engine, which also makes less power.

So we can clearly say that backpressure source #1 from the above is ALWAYS a bad thing. This is one of the benefits to aftermarket exhausts--they typically are smoother and offer less static resistance to flow...and that's good.

Now the #2 part is where things get a little confusing. ANY exhaust system is going to have resonance of some kind or another. But resonance can help things or it can hurt things...it depends on how well the resonant characteristics of the exhaust system is matched to the engine. If you change the resonant properties of your exhaust, then you may help things or you may hurt things....or perhaps even both (at different RPM points).

Resonance is extremely complicated and the math involved is frightening to say the least (and this is coming from an engineer no less). But generally speaking, an exhaust system has the LEAST resonant backpressure whenever the engine RPM matches the resonant frequency of the exhaust. Thus, an exhaust system offers the best power boost whenever it is designed such that it matches the intended powerband of the engine.

For example, headers with long, thin, tubes are commonly used on trucks and off-road vehicles where the engine rpms are relatively low, and added power at the lower end is most important. On the other hand, on a pro drag race car that runs high rpms most of the time the headers are shorter and have much larger pipes. That kind of header has a higher resonant frequency and thus makes more power at the higher RPMs.

...but any system also has less flow outside of it's sweet spot. So, those "short fat" headers flow great at high RPMs but their resonant characteristics don't match well at lower RPMs, and thus at those RPMs the headers actually increase backpressure due to the resonant mismatch.

Going back to that article..what happens when you swap out your stock cats for some high flow cats?

Well, first off the bat you are decreasing your static backpressure becasue the aftermarket cats are more "open" than the stock ones (this helps your power everywhere)...and second you are altering your resonace properties...which will probably help in some places and hurt in others. But that last part isn't really a backpressure issue at all. It's a very complicated dynamic flow problem.

You may have heard people say something like "a car needs backpressure to run right. I took the mufflers off my old T-bird and it ran like crap..that's why I know".

Well, backpressure is only PART of the problem here. When Billy Bob takes a hacksaw to his exhaust pipes and cuts off the mufflers from his T-bird there are two things changing. Just as above, first the backpressure drops due to the lack of the muffler restriction. And second, the resonant properties of the pipe changes becasue it's length and shape changes. If the "bad part" of the resonant changes are worse than the benefits from the decreased static backpressure, which could very well happen, then guess what...it LOOKS like "reducing backpressure made the car run bad"...but that's not really the case. It's just that the resonant properties of the exhaust are not something that most people think about...and for good reason, it is dreadfully difficult to understand and model.