How Much Restriction Is Your Exhaust Creating at Maximum Power — and How Can Exhaust Cutouts Help?

Your engine may make great power at part throttle, sound good on the street, and feel strong during normal driving.

But what happens when you put the throttle to the floor and ask the engine to make maximum horsepower?

At that point, airflow increases dramatically.

More air enters the engine.

More fuel is burned.

And a much larger volume of exhaust has to get out.

That raises an important question:

How much restriction is your current exhaust creating when your engine is trying to make maximum power?

If the mufflers, resonators, tubing, bends, or other downstream components can't move enough exhaust efficiently, pressure can begin building in the exhaust system.

That is where an exhaust cutout can become a valuable performance tool.

Your Exhaust Has to Flow What Your Engine Produces

An engine is essentially an air pump.

Air enters through the intake system, mixes with fuel, burns inside the cylinder, and then has to leave through the exhaust.

The more horsepower the engine makes, the more airflow it generally has to process.

Think about the difference between:

A 300 HP naturally aspirated street engine

and

A 900 HP supercharged or turbocharged engine.

They may both have a 3-inch exhaust system, but the amount of exhaust gas trying to travel through that system can be dramatically different.

An exhaust system that flows perfectly well at 300 horsepower may eventually become a restriction as engine output increases.

What Is Exhaust Backpressure?

As exhaust gases move through the exhaust system, they encounter resistance.

That resistance can come from:

• Mufflers

• Resonators

• Catalytic converters

• Pipe diameter

• Tight bends

• Crush-bent tubing

• Exhaust transitions

• Restrictions inside mufflers

• Small tailpipes

• Turbocharger exhaust systems

• Other exhaust components

When the exhaust cannot move through the system freely enough, pressure builds upstream.

That is generally referred to as exhaust backpressure.

SAE International has published research showing that increased exhaust backpressure can negatively affect engine power and torque, while reducing excessive backpressure can create opportunities for improved engine performance and efficiency.

Technical reference:

https://saemobilus.sae.org/papers/study-improve-engine-efficiency-reducing-backpressure-2023-01-0946

Does an Engine Need Backpressure to Make Torque?

This is one of the biggest myths in performance exhaust.

You'll often hear:

“You need backpressure or you'll lose torque.”

What an engine really needs is a properly designed exhaust system.

Exhaust velocity, tubing diameter, scavenging, pressure-wave behavior, collector design, cam timing, RPM range, and engine displacement all matter.

Simply adding restriction is not what produces torque.

In fact, excessive backpressure can force the engine to work harder to push exhaust gases out of the cylinder.

The goal isn't necessarily zero restriction everywhere.

The goal is:

The right exhaust flow characteristics for the engine and power level.

Why Restriction Becomes More Important at Wide-Open Throttle

During normal cruising, your engine is only producing a fraction of its maximum horsepower.

The exhaust system may have no problem handling that airflow.

But put the throttle to the floor and everything changes.

Cylinder filling increases.

Fuel demand increases.

Combustion energy increases.

Exhaust volume increases.

RPM climbs.

Suddenly the exhaust system that seemed perfectly adequate while cruising may become one of the restrictions limiting maximum power.

That is why some exhaust changes show relatively little difference during normal driving but produce measurable differences on a chassis or engine dyno at wide-open throttle.

Where Can the Restriction Come From?

It isn't always the exhaust pipe itself.

For example, you might already have a 3-inch exhaust system.

That sounds like plenty of flow.

But your exhaust could still contain:

A restrictive muffler

Multiple resonators

Sharp bends

Reducers

A restrictive Y-pipe

Small tailpipes

Poor transitions

Each component contributes to the total pressure drop through the system.

Research summarized in engineering literature has found that mufflers and resonators can account for a significant portion of exhaust-system pressure drop.

That means simply having large-diameter tubing does not automatically mean the entire exhaust system is free flowing.

This Is Where an Exhaust Cutout Can Help

An exhaust cutout gives the exhaust gases another path.

When the cutout is closed:

Exhaust continues through the normal exhaust system.

When the cutout is opened:

Exhaust can bypass some of the downstream restriction.

Depending on where the cutout is installed, opening it may allow exhaust to bypass:

• Mufflers

• Resonators

• Long sections of exhaust tubing

• Restrictive bends

• Tailpipes

The result is a shorter and potentially much less restrictive exhaust path.

Explore Granatelli exhaust cutout systems:

https://granatellimotorsports.com/collections/exhaust-cutouts

Does Opening a Cutout Automatically Add Horsepower?

No.

And that's important.

An exhaust cutout cannot remove a restriction that isn't there.

If your existing exhaust system already flows everything your engine needs, opening a cutout may produce very little additional horsepower.

But if your exhaust is becoming restrictive at high RPM and maximum engine load, opening a properly positioned cutout can potentially free up power.

The bigger the difference between:

Exhaust flow required by the engine

and

Exhaust flow capacity of the existing system

the greater the potential benefit.

How Do You Know If Your Exhaust Is Restrictive?

There are several ways.

1. Chassis Dyno Testing

One of the best methods is extremely simple.

Make a dyno pull with the cutouts closed.

Then open the cutouts and make another pull under comparable conditions.

Compare:

• Peak horsepower

• Peak torque

• Horsepower curve

• Torque curve

• Air/fuel ratio

• Boost

• RPM

If horsepower increases with the cutouts open, your normal exhaust configuration was likely creating some measurable restriction at that operating point.

Even more interesting is where in the RPM range the difference begins.

You may find both configurations make nearly identical power at lower RPM.

Then as airflow increases, the open-cutout run begins pulling away.

That can be a strong indication that exhaust restriction was increasing as the engine approached maximum output.

2. Measure Exhaust Backpressure

For a more technical approach, exhaust pressure can be measured.

A pressure sensor or gauge can be installed at an appropriate location in the exhaust system.

Then the engine can be tested under load.

The important thing isn't just looking at pressure during idle.

You want to know what happens when the engine is:

At high RPM

Under heavy load

Making maximum horsepower

Exhaust restriction that isn't noticeable at idle may become substantial at wide-open throttle.

3. Compare Performance With the Cutouts Open and Closed

If the vehicle has electronic cutouts, you have a very useful built-in testing tool.

Closed:

Full exhaust system

Open:

Reduced downstream exhaust restriction

Comparing the two configurations can help show whether the rest of the exhaust system is affecting performance.

This is one of the reasons a cutout can be more than a sound modification.

It can also become a useful testing tool for your engine combination.

High-Horsepower Engines Can Benefit the Most

As horsepower increases, exhaust-flow requirements increase.

Imagine modifying a car over several years.

Originally:

450 HP

Then:

550 HP

Then:

700 HP

Then:

850 HP

The same exhaust system may remain underneath the car through every stage.

At some point, the exhaust that worked perfectly with the original combination can begin restricting the new combination.

Opening a cutout can provide an alternate path without requiring the car to permanently run an extremely loud race-style exhaust.

Turbocharged Engines

Exhaust restriction is particularly interesting on turbocharged engines.

Before the turbocharger, exhaust energy drives the turbine.

But after the turbine, reducing unnecessary restriction can help improve the pressure differential across the turbine.

A restrictive exhaust downstream of the turbo can work against the system.

This is why large downpipes and free-flowing exhaust systems are common on high-horsepower turbo combinations.

A properly positioned exhaust cutout can provide another high-flow path when maximum performance is desired.

Supercharged Engines

A supercharger pushes more air into the cylinders.

More air allows more fuel to be burned.

That creates more exhaust.

As boost and horsepower increase, the exhaust system must handle that increased volume.

A system that works perfectly on a naturally aspirated engine may become restrictive after a supercharger is installed.

Opening a cutout can provide additional exhaust-flow capacity when the engine needs it most.

Nitrous Engines

Nitrous combinations can also dramatically increase exhaust volume.

When the nitrous system activates, the engine can suddenly produce substantially more horsepower than it does during normal driving.

That means the exhaust system suddenly has to move considerably more exhaust gas.

An exhaust cutout provides a way to increase available exhaust flow only when you want maximum performance.

Naturally Aspirated Engines

Naturally aspirated combinations can benefit too.

Large displacement engines, aggressive camshafts, high RPM, ported cylinder heads, larger throttle bodies and intake modifications can all increase airflow.

But with naturally aspirated combinations, exhaust sizing and scavenging can be especially important.

This is why testing is better than assuming.

Run the vehicle both ways.

Closed vs. open.

Let the dyno tell you what the engine wants.

Exhaust Cutouts Aren't Just About Peak Horsepower

Peak horsepower gets all the attention.

But don't only look at the biggest number on the dyno sheet.

Look at the entire curve.

An exhaust change might produce:

No difference at 3,000 RPM

A small improvement at 4,500 RPM

A larger improvement at 6,000 RPM

That tells you something valuable.

It suggests exhaust demand is beginning to exceed the capacity of the closed system as RPM and airflow increase.

For racing, that additional power at the top of the RPM range may be exactly where you need it.

Why Electronic Cutouts Make This Practical

You could build a completely open race exhaust.

But then you'd have to live with it everywhere.

An electronic exhaust cutout provides a different solution.

Driving Around Town

Cutouts closed.

Exhaust travels through the normal system.

Highway Driving

Cutouts closed.

You keep your mufflers and quieter exhaust note.

Car Show

Want more sound?

Open them.

Dyno

Want to see if the exhaust is restricting power?

Open them.

Drag Strip

Want maximum available exhaust flow?

Open them.

That is the advantage.

You don't have to permanently choose between a street exhaust and an open race exhaust.

Granatelli One-Touch Electronic Exhaust Cutouts

Granatelli Motor Sports electronic exhaust cutouts are designed to make switching between those two configurations simple.

Granatelli systems feature a Smart Chip-controlled one-touch operating system.

Rather than holding a switch while the butterfly valve opens or closes, you press the switch and the controller completes the operation.

The system automatically stops the actuator when the butterfly reaches its intended position.

Granatelli designed this system to help prevent unnecessary over-torquing of the butterfly shaft and internal drive gears.

Learn more:

https://granatellimotorsports.com/pages/exhaust-cutouts

Built for Exhaust Flow

Depending on the system, Granatelli electronic cutouts feature:

• CNC-machined 6061 aluminum bodies

• Stainless-steel butterfly plates

• Stainless-steel blade shafts

• High-torque DC gear-reduction motors

• High-temperature steel drive gears

• Plug-and-play wiring

• One-touch electronic control

• Single and dual configurations

• Slip-fit and weld-in configurations

• Round and oval designs

Granatelli also offers multiple sizes including:

2.25-inch

2.5-inch

3-inch

3.5-inch

4-inch

That allows the cutout to be matched more closely to the exhaust system and power level of the vehicle.

What About Lowered Cars?

One of the problems with adding a cutout to a performance vehicle is ground clearance.

A traditional 3-inch round tube is approximately 3 inches tall before considering additional hardware.

For lowered vehicles, Granatelli developed its 3-inch oval electronic exhaust cutout.

The oval cutout is approximately 2.25 inches tall, giving roughly 0.75 inch additional ground clearance while providing the free-flowing characteristics of a 3-inch system.

That can make a major difference on:

• Lowered Camaros

• Corvettes

• Mustangs

• Pro Touring builds

• Street/strip cars

• Custom exhaust systems

Where Should You Install the Cutout?

Location matters.

The farther upstream the cutout is installed, the more downstream exhaust components it may bypass.

But installation location also needs to consider:

• Catalytic converters

• Ground clearance

• Floor pan clearance

• Driveshaft

• Transmission

• Crossmembers

• Suspension travel

• Exhaust heat

• Wiring

• Exhaust direction

Granatelli notes that its exhaust cutouts installed after the catalytic converter do not require a computer reflash simply because of the cutout installation.

Always follow applicable emissions and noise laws for your location.

Your Mufflers May Be Perfect for the Street and Restrictive at the Track

This is worth emphasizing.

A muffler isn't necessarily "bad" because it creates some restriction.

For a street car, you may intentionally choose a muffler that keeps the vehicle comfortable.

You might want:

Less drone

Lower cruising noise

A quieter cold start

A comfortable interior

Then when you go racing, your priorities change.

Now you want maximum exhaust flow.

That's exactly where an electronic cutout makes sense.

You don't have to replace the street exhaust.

You simply give the exhaust another way out when maximum performance matters.

Use the Cutout as a Diagnostic Tool

This may be one of the most overlooked benefits.

If you've continued modifying your engine and you're wondering whether the exhaust has become restrictive, the cutouts can help answer that question.

Make a controlled test.

Cutouts closed: 610 RWHP

Open them.

Cutouts open: 610 RWHP

Your current exhaust probably isn't costing much power at that output level.

But imagine this:

Closed: 610 RWHP

Open: 635 RWHP

Now you have something worth investigating.

The cutout didn't magically create 25 horsepower.

It exposed horsepower that the engine was already capable of producing but couldn't fully realize through the more restrictive exhaust path.

Your actual results will vary, which is why testing your specific combination is so valuable.

How Much Restriction Is Too Much?

There isn't one universal backpressure number that applies to every engine.

Different engines, turbo systems, camshafts, exhaust designs and operating speeds react differently.

Instead of chasing one universal number, look at what your particular engine is telling you.

Ask:

Does horsepower continue climbing normally?

Does opening the exhaust change the power curve?

Does boost change?

Does torque improve?

Does the engine pull harder at higher RPM?

What does measured exhaust pressure show?

Those answers are more useful than assuming every engine wants exactly the same exhaust configuration.

The Real Question

Instead of asking:

“Do exhaust cutouts add horsepower?”

Ask:

“Is my current exhaust restricting the horsepower my engine is already capable of making?”

That's the better question.

Because an exhaust cutout doesn't manufacture horsepower.

It gives exhaust gases another path.

If the normal exhaust is already flowing everything the engine needs, the horsepower difference may be small.

If the exhaust is restricting the engine at maximum airflow, opening the cutout can potentially make a significant difference.

And unlike permanently installing an extremely loud exhaust system, an electronic cutout lets you close everything back up when the race is over.

The Bottom Line

Your exhaust system has one major job:

Move the exhaust gas produced by the engine.

As horsepower increases, that job becomes more demanding.

An exhaust system that works perfectly at one power level may become restrictive at another.

Granatelli electronic exhaust cutouts give you the ability to bypass downstream restriction at the push of a button.

That can give you:

Normal street exhaust when you want it.

Aggressive sound when you want it.

More exhaust-flow capacity when your engine needs it.

And perhaps most importantly:

A simple way to find out whether your current exhaust is holding your engine back.

So the next time you're chasing more horsepower, don't only look at what's going into the engine.

Ask what may be stopping everything from getting back out.

Shop Granatelli Exhaust Cutouts

Explore Granatelli round, oval, electronic, manual, single and dual exhaust cutout systems:

https://granatellimotorsports.com/collections/exhaust-cutouts

Learn more about Granatelli electronic exhaust cutout technology:

https://granatellimotorsports.com/pages/exhaust-cutouts

Technical Reference

SAE International — Study to Improve Engine Efficiency by Reducing Backpressure:

https://saemobilus.sae.org/papers/study-improve-engine-efficiency-reducing-backpressure-2023-01-0946

 

Meta Description

Excerpt

Your exhaust may flow fine during normal driving but become restrictive when your engine reaches maximum power. Learn how exhaust backpressure can affect horsepower and torque, how to determine if your exhaust is holding you back, and how Granatelli electronic exhaust cutouts can provide a less restrictive path at the push of a button.