Exhaust back pressure

Exhaust back pressure versus exhaust pressure

Every day I seen amateur tuners brag that they removed the factory muffler and installed larger pipes to lower exhaust back pressure and help the engine breathe and produce more power. If that’s all they’ve done to the vehicle, they haven’t increased power at all. That’s the myth of exhaust back pressure. If it were that easy to get more power, car makers would be all over it. Every car would come equipped with headers and flow-through mufflers and they’d rely on electronic noise cancellation to deal with the noise issue.

The difference between exhaust backpressure and exhaust pressure

This is the first thing you need to understand when you get into the discussion about exhaust flow. There’s a HUGE difference in the terms exhaust backpressure and exhaust pressure. If you don’t understand this most important point when you read “experts” who point out that all exhaust backpressure is bad.

The term exhaust backpressure denotes a backward flow of exhaust that prevents proper engine breathing. Exhaust backpressure is always bad.

The term exhaust pressure is simply the resistance to flow you always get when pressurized air or gas flows through a pipe or duct. If you were talking about HVAC, it’s called duct pressure, not duct backpressure because there’s really nothing pushing air back towards the furnace. The same applies to auto exhaust.

Does a stock engine need exhaust backpressure?

If you do an Internet search for that question, you’ll see post after post with this title: Backpressure: The Myth and Why It’s Wrong. It’s all based on a single post from a few performance websites and all the links keep pointing back to the same originating post. So let’s clear this up right away. Backpressure is always a bad thing for both street and performance vehicles. Some degree of exhaust pressure, on the other hand, is necessary for factory stock engines. But less exhaust pressure can result in performance gains for racing vehicles. Here’s why lower exhaust pressure is better for racing vehicles:

• Racers don’t care about engine performance at part throttle and they don’t race them at part throttle.

• Racers don’t care about emissions

• They’re not driving factory stock engines. They’ve altered the intake, valve ports, valves, cams and totally changed valve timing to reduce valve overlap. They didn’t just slap on a performance exhaust and instantly get more power

Why factory stock engines need some exhaust pressure

On most street vehicles, idle backpressure runs around 1-psi. to 1.5-psi. At 2500RPM, the reading should not be higher than 3PSI.  GM recommends no more than 1.25-psi at idle, and no more than 3-psi at 2,000 rpm.

Factory stock engines produce peak power and optimal fuel efficiency between clearly defined points in their operating ranges. So the exhaust systems are designed to operate at maximum efficiency at precisely at those points in the power band.  At points below, between, and above maximum power and optimal fuel efficiency, the operation of the exhaust system represents several compromises between effective exhaust gas extraction, fuel efficiency, power delivery, effective exhaust gas pressure, and noise suppression.

What does that mean in practical terms?

It means that there simply is no way a “performance exhaust” can provide any meaningful improvement upon any of the above characteristics of a standard exhaust system without creating a penalty in another characteristic. Because changing any aspect of an exhaust system’s design necessarily affects one or more other aspects.

Keep in mind that I’m talking about STOCK FACTORY engines here that are designed to drive your kids to school. All stock engines are a compromise. They have to provide a reasonable level of pickup from a stop, yet work efficiently and provide good gas mileage while meeting emissions standards. That’s not easy to accomplish.

Exhaust pressure in an engine with fixed valve timing

Keep in mind that the core operating principle of an internal combustion engine is that it relies on the expansion of the burning air/fuel mixture to propel the piston down and turn the crankshaft. As the piston reaches the bottom of its power stroke, the combustion gasses are still expanding.

In an ideal world you’d want to extract the maximum amount of work from these expanding gasses before starting the exhaust stroke to expel them. If you open the exhaust valve before the piston reaches bottom dead center (BDC), the expanding gasses escape into the exhaust manifold while they’re still expanding and you lose that potential power. So in a best-case scenario, you’d want to delay exhaust valve opening until after the piston reaches BDC.

On the other hand, you also want exhaust gas pressure to drop to the lowest possible value before the piston begins its upstroke. That reduces the amount of work needed to raise the piston (so it doesn’t push against a still expanding gas). In that case, you’d want to open the exhaust valve before BDC to allow as much exhaust to escape before the upstroke begins. These are two conflicting requirements, the first requiring the exhaust valve to be open after BDC, the second requiring exhaust valve opening to be before BDC.

Exhaust valve timing is a compromise in a factory stock engine

Based on the two conflicting goals, engineers compromise. They design valve timing as a trade-off between the energy lost by allowing the still expanding gas to escape before it is fully expanded, and the extra work required to raise the piston while the gas is still expanding.

In a factory stock engine with fixed cam/valve timing, the valve lifts from its seat relatively slowly, so it provides a significant flow restriction for some time after it begins to lift. To compensate, engineers design the cam to start exhaust valve opening BDC. A typical exhaust valve opening timing is in the region of 50-60° before BDC for a production engine.”  mechadyne-int.com/

Ideally, valve timing would change to match engine speed and load conditions. That’s where variable valve timing comes into play. But in an engine with fixed valve timing, engineers have to find the optimum tradeoff. When the engine is under partial load conditions, engineers want to open the exhaust valve at BDC, since exhaust gas pressure will be lower. But under full load conditions, you’d want the exhaust valve to open before BDC due to the amount of time it takes for the valve to open.

Exhaust pressure and valve overlap

Now let’s look at what’s going on as the piston reaches the top of the exhaust stroke or top dead center (TDC). At this point, both the exhaust and intake valves are open. This is called valve overlap. And the duration of the exhaust valve opening can have a considerable effect on the amount of exhaust gas remaining in the cylinder at the beginning of the intake stroke. Under full load conditions, you want as little exhaust gas left so you can draw in the largest air/fuel charge. So you’d want the exhaust valves closing at or after TDC.

Now pay attention! This is where exhaust pipe diameter and exhaust pressure comes into play

Exhaust is a pulsed pressure wave that pushes and sucks exhaust out of the cylinder and tailpipe. The timing of exhaust valve closing is tied closely to the design of the factory exhaust ports, manifold design, exhaust pipes, muffler, and exhaust system back pressure. In a factory stock engine with fixed cam timing, changing the size of the pipes or muffler affects exhaust exhaust velocity and pressure—and not always for the better. Also, remember that fixed valve timing is a compromise.

Now, engineers determine exhaust valve closing based on whether the timing tradeoff is best for drawing exhaust gas out of the cylinder or providing exhaust pressure resistance to keep some exhaust gas in the cylinder. The timing of the pressure waves is directly related to engine speed, so engineers try to optimize exhaust valve closing for one speed range. Obviously, that can be detrimental at a different speed range.

For example, the engineer may want to retain some of the exhaust gas at partial load to reduce the cylinder’s ability to take in the new air/fuel charge. Why would they want this? Simple:

All normally aspirated engines suffer from pumping loss

What you have to understand here is that the throttle plate is the largest restriction in the entire intake system. At low RPMs and partial throttle, the engine experiences significant pumping losses because the pistons are  sucking against the mostly closed throttle plate during their intake stroke.

By purposely retaining some exhaust gas in the cylinder through valve timing and exhaust back pressure, engineers actually reduce efficiency. That forces the driver to push down on the pedal and open the throttle plate more.

Opening the throttle lowers pumping loss!  Now this part is important:

You gain MORE efficiency by lowering pumping loss than the energy you give up by keeping more exhaust gas in the cylinder! In partial throttle situations, backpressure is a net positive.

But the exhaust pressure issue changes at open throttle

At open throttle, pumping loss is low and exhaust pressure harms volumetric efficiency. You want high exhaust flow and velocity to help empty the cylinder at higher RPMs.

Does a larger exhaust pipe help flow?

Not necessarily. You obviously want an exhaust pipe that aids flow. But too large a pipe decreases gas velocity and it’s velocity that scavenges the exhaust from the cylinder.

What if you just install a high-flow muffler?

If you don’t increase pipe size but instead try to lower exhaust pressure by installing a high flow muffler, your performance will still suffer at lower RPMs and partial load due to pumping loss. But what happens at full load and higher RPMs? Full throttle and lower exhaust pressure from the muffler gives you more power.

So the question is: are you willing to sacrifice performance at part throttle for full throttle performance? If you’re a pedal to the metal person, go for it. If you’re a daily driver and you’re driving in stop and go traffic, reducing backpressure may cost you.

Then there’s the whole Variable Valve Timing issue

With the introduction of variable valve timing, engineers can have their cake and eat it too. They can change exhaust valve closing and valve overlap to meet varying engine conditions. Great, so it’s no longer such a compromise. But now it’s even more important that you don’t screw with it. The engineers designed in a certain amount of backpressure and the variable valve timing software is based on it. If your engine has variable valve timing and you replace the pipes and muffler, you throw all of that precise engineering out the window. All the assumptions made by the engineers are gone—you’ve substituted your own. Nice going.

©, 2015 Rick Muscoplat

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