Rick's Free Auto Repair Advice

Reduce exhaust back pressure?

Should I reduce backpressure?

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. 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.

Does an 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 performance website and all the links keep pointing back to the same originating post. So let’s clear this up right away. Backpressure in a performance car is ALWAYS a bad thing. Exhaust backpressure always reduces power in those performance or racing vehicles because they:

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

• Don’t care about emissions

• They’re not driving factory stock engines. They’ve altered the cams and totally changed valve timing to reduce valve overlap.

Why factory stock engines need some exhaust backpressure

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.

All stock engines have some valve overlap. That’s the period of time when the cylinder is near the top of the exhaust stroke and the exhaust valve is closing while the intake valve is opening. The inertia of the exhaust ejection can pull fresh air/fuel into the exhaust. Here’s how that happens.

Exhaust flow comes in pulses

The exhaust event for each cylinder is a steady outflow of gas. But once in the exhaust manifold, it’s a different story. There, exhaust flow is NOT steady—it comes in individual pressure pulses from each cylinder. Each pressure pulse is followed by a low pressure area, which essentially a vacuum. These pressure from the exhaust stroke pushes exhaust out and low pressure pulses in the exhaust manifold suck exhaust out out of the cylinders. The size and shape of the exhaust manifold and pipes all play a part in how the system works.

As I said earlier, during valve overlap, both the exhaust and intake valves are open in a cylinder. Exhaust manifold low pressure can literally pull a portion of the new air/fuel charge from the intake valve and “scavenge” it right out the exhaust valve, reducing a portion of the air/fuel charge needed for the next cycle.

If the engine doesn’t have variable valve timing or an aftermarket camshaft, this valve overlap period is fixed, meaning it doesn’t change from low RPM to higher RPM. I’ll talk about those engines first.

Exhaust backpressure 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 backpressure 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 exhaust valve closing to at or after TDC.

exhaust back pressure


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

Recall that 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 back 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 backpressure 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 backpressure issue changes at open throttle

At open throttle, pumping loss is low and exhaust backpressure is 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 aides 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 back 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 backpressure from the muffler give 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



Posted on by Rick Muscoplat


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