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Does a Straight Pipe Affect Gas Mileage?

Does Straight Pipe Affect Gas Mileage

You’ve likely heard the phrase, straight piping before.

All it means is replacing your vehicle’s stock exhaust system with a straight pipe.

The first thing you’ll notice about your car, truck or SUV after installing a straight pipe is the louder exhaust note. And the first thing you’ll feel is the responsive acceleration.

Among other things, a straight pipe doesn’t have sharp bends that restrict air flow like stock exhaust systems.

An advantage to straight piping when it comes to racing is a more predictable exhaust flow.

Installing a straight pipe into your vehicle will affect gas mileage. In what follows we’ll look at some things that installing a straight pipe will also affect.

How Does a Straight Pipe Affect Gas Mileage?

Among other things, a straight pipe will increase your MPGs.

How?

Unlike a stock exhaust system, a straight pipe eliminates all the parts of an exhaust system that while essential, restrict the flow of exhaust gases and cause back pressure, which we’ll also talk about.

Since a straight piped engine can breathe more freely, fuel economy is increased. As such the engine doesn’t have to produce more power to fight against the back pressure.

There are only two ways to increase horsepower in an internal combustion engine: 1) increase fuel or 2) increase the amount of air going into the engine.

Straight piping increases the amount of air going into the engine which conserves the amount of fuel needed for combustion.

One happy result is better fuel economy.

Let’s take a look at the parts of an exhaust system that get eliminated in a straight pipe as we consider what makes them essential to the exhaust system.

What’s Involved in a Straight Pipe?

Every vehicle with an internal combustion engine has an exhaust system.

This system begins at the engine with an exhaust manifold where exhaust gases are collected and ends at the exhaust tip where exhaust gases are expelled from the engine.

Key components of an exhaust system are the exhaust manifold where exhaust gases are collected.

From the exhaust manifold exhaust gases travel to the catalytic converter where a chemical process cleans the exhaust gases to make it less hazardous to you and the environment.

Some vehicles like luxury cars may be equipped with a resonator which the exhaust gases then pass through to dampen noise as they travel through the rest of the exhaust system.

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The exhaust gases travel through another pipe called the catback to the muffler where the sound of the exhaust gases is dampened and cleaned again.

And finally, the exhaust gases find their way out through the tail pipe.

But a straight pipe eliminates the catalytic converter, resonator and muffler leaving only one (or two) long pipes stretching from the exhaust manifold to the tail pipe.

The Importance of Diameter Size

The Importance of Diameter Size

This not only applies to straight piping, but all custom exhaust systems.

A lot of research, development and engineering has gone into your vehicle to design an exhaust system that maintains a balance between frictional loss and the velocity of exhaust gas flow.

The diameter of the pipes you install will affect engine performance.

A small diameter pipe can increase exhaust gas velocity, which is how quickly exhaust gases can evacuate the engine. Or the volume of exhaust gas that can evacuate the engine – depending on how you want to look at it.

Diameter size improves performance.

But if the pipe is too small it could result in frictional loss which puts resistance against the exhaust gas flow.

And if the diameter of the pipes is too big it can consume too much of the engine’s horsepower and torque, which is the exact opposite of why you’re installing a straight pipe.

An exhaust system manages:

  • exhaust velocity
  • back pressure,
  • and scavenging.

Since we’ve already looked at velocity, let’s now turn our attention to the other two.

Exhaust Back Pressure

Back pressure is often misunderstood.

It’s sometimes confused with exhaust scavenging (which we’ll get into next).

Back pressure is an opposing force acting on a gas that’s trying to move through a confined space – basically, it’s something that prevents exhaust gases from being expelled from the engine.

As an engine pulses exhaust gases resulting from combustion. The pulsing generates exhaust flow (or positive exhaust flow) as the gases make their way to the exhaust tips.

A restrictive exhaust system creates back pressure because parts like the catalytic converter provide resistance against positive exhaust flow resulting from the fact that the catalytic converter can only clean a finite volume of exhaust gas at a time.

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This, limits horsepower.

A straight pipe eliminates these enemies to positive exhaust flow. This is why the size of the straight pipe is vitally important.

The right size will help the engine breathe well while creating a high enough exhaust flow.

Exhaust Scavenging

Warning: generalization.

Simply put, exhaust scavenging is the process of replacing exhaust gases in the cylinder of an engine with a fresh air and fuel mixture.

An exhaust system is a masterpiece of carefully organized pipes. Every internal combustion engine goes through a cycle of intake, compression, combustion and exhaust.

The exhaust portion of the cycle is our focus here.

How does exhaust gas come out of the combustion chamber and into the exhaust manifold? And what does this have to do with straight piping?

When the exhaust valves sitting on top of the cylinders open, the cylinders do some of the work of pushing spent exhaust gases out.

The pressure differential between the cylinder when it pushes the exhaust gases out and the exhaust manifold also cause the exhaust gases to get pulled out of the combustion chamber and into the exhaust manifold – this is exhaust scavenging.

Benefits of a Straight Pipe

One of the benefits of straight piping is fuel economy. But this isn’t the main reason why people choose to install a straight pipe to their vehicle.

No, the main reason why you go through the trouble of straight piping your vehicle is to gain horsepower and torque.

Since there’s no back pressure created by components like the catalytic converter and muffler, the engine can produce more horsepower.

Exactly, how much horsepower?

Like ten.

I know it doesn’t sound impressive. But for racing applications, every pony counts – The immediate throttle response in a straight piped vehicle is exhilarating.

Another benefit to installing a straight pipe beside increased fuel economy and horsepower is the louder sound your vehicle makes.

I personally don’t see this as a benefit – straight piping can be excruciatingly loud unless you’re on a race track. But hey I include it as a benefit as many enthusiasts seem to really enjoy this feature.

Drawbacks of a Straight Pipe

Which conveniently leads me to the drawbacks of a straight pipe.

Chief among which is emissions.

Remember that I said that a straight pipe eliminates all the essential components of an exhaust system in the name of increased horsepower.

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A straight pipe is unforgiving in that it’s going to make your vehicle produce A LOT of emissions. In the United States most states have emissions laws that all vehicles with an internal combustion engine must pass in order to legally operate on public roads.

If the automobile you plan to straight pipe is your main mode of transportation you’ll have a challenging time passing emissions testing.

Not only that but unless you work on your own cars if your vehicle is in need of repair most mechanics won’t touch your ride if parts of the exhaust are missing.

Does Adding a Straight Pipe Require a Tune?

Does Adding a Straight Pipe Require a Tune

An engine is a big air pump.

Any time you adjust how much air is coming into the engine or how much air comes out by way of exhaust, you’ll need a tune for the engine to keep up.

Since a straight pipe eliminates restrictions to exhaust flow, it helps to increase the volume of exhaust flow.

As such, a straight pipe requires a tune.

It may even require fuel injection modification. Without a tune not only will the check engine light stay on so you won’t know when you have other problems with the engine.

But straight piping may cause your engine to run lean which can lead to problems of its own.

Remember how we said that most states require the passing of emissions testing in order for a vehicle to use the public roads legally?

Without the elements that straight piping a vehicle eliminates like the catalytic converter, resonator (if so equipped) and muffler. A vehicle won’t be able to pass inspection since all those parts were specifically designed to control emissions.

Not only that but some states and local laws have noise ordinance which means that a straight pipe is too loud for many cities and town.

Generally, the maximum decibel for a vehicle’s exhaust is 95 dB. Which is why a straight piped vehicle is best reserved for track use only.

Sources:

unitedautosonline.com; greengarageblog.org; autocornered.com; low-offset.com