Talk about LOUD !

Look at the 2023 Z06 Corvette

The Dba even 5 feet back is way over federal law allows and even worse 2 feet away from exhaust tips

Being I designed this custom exhaust back in 2008, 14 years ago, thought it was a good time to measure the exhaust sound in Dba
to see if anything had degraded in all that time

I used 3 test instruments

1. DB meter set to measure in Dba (shown on left side) was 1 1/2 feet from exhaust tip at a right angle
2. Output of that to a multi meter to record the sound in K Hertz (shown on right side)
3. OBD-II scanner

Weather was around 60 F degrees
Recording is from cold engine start to full warmed up
Since manual tranny, in neutral

Ran RPMs from idle which is at 850 RPMs due to CAM grind narrow LSA, to create more vacuum to 2,000 to 4,000 RPMs

My video could have been better as sun was to my back and did not know when I was at certain angles the Sun blocked out the multi
meter on the right side but, its recorded data is shown below

Idle cold/hot measured between 90-94 Dba, and at higher RPMs max recorded was 107 Dba
So in all these years nothing in the exhaust design or small mufflers as degraded


Crank your volume UP !

Exhaust systems are extremely complicated. What looks like just a bunch of metal pipes merged together is actually an engineering masterpiece, every fold, bend, and pipe diameter is carefully selected to achieve a specific effect.

This “effect” has a lot to do with balancing out the three most important characteristics of the exhaust system:

• Exhaust Scavenging,
• Backpressure, and
• Exhaust Velocity.

This is true for both OEM and aftermarket exhaust systems, with the exception that OEM exhaust systems are more geared towards compliance with emissions standards over outright performance.

Before you upgrade your exhaust system, it’ll help to familiarize yourself with concepts such as scavenging and backpressure.
We’ve touched on these concepts in one of our articles that explains how exhaust systems work and what components are involved.
In this article, we’ll briefly discuss the scavenging effect what it is, what it does, and why you should understand it.

What Is Exhaust Scavenging?

Put simply, scavenging refers to the process of replacing spent gases in the engine cylinder with a fresh charge of air and fuel.
Your engine is basically a large air pump that makes power by rotating pistons through a crankshaft, powered by multiple explosions of air and fuel sprayed in a fine mist.

Every modern four-stroke engine follows the same cycle:

• Intake: Air-fuel mixture enters the combustion chamber.
• Compression: Piston travels up to top dead center and compresses the air-fuel mixture.
• Combustion: The spark plug ignites the air fuel mixture, and the piston gets pushed to bottom dead center.
• Exhaust: Piston travels back up to top dead center, but this time it pushes the spent exhaust fumes out through the exhaust valve.
The whole point of the exhaust stroke is to expel the spent gases that remain in the engine cylinder (better known as the combustion chamber) after the combustion cycle is completed.

When the spent gases are evacuated, it frees up the much-needed space for the next combustion cycle.
But have you ever wondered how exactly the exhaust system draws the spent gases out of the engine cylinder?

Sure, the piston does some of the work by pushing the gases out when the exhaust valve is open, but there’s a lot more at play, to enter the scavenging effect.

How Does Exhaust Scavenging Technology Work?

There are two main forces involved in scavenging:

1. The piston pushes out exhaust fumes while travelling back up to top dead center, and
2. The pressure differential between the combustion chamber and the exhaust manifold causes the exhaust gases to get pulled out.

Allow us to explain, the pressure within the combustion chamber is 6 to 7 times higher than atmospheric pressure, and the pressure outside the combustion chamber (in the exhaust manifold) is equal to atmospheric pressure.

Naturally, the exhaust gases are going to want to move towards the low-pressure area, that is, out of the engine cylinder, and into the exhaust manifold.
This happens because gases tend to travel from high pressure to low pressure, that’s why wind exists.

And it is this pressure differential that causes exhaust gases to leave in the first place.

There’s a lot of misinformation out there about automotive exhaust systems and the concepts associated with them.
Exhaust back pressure is one such concept that’s widely misunderstood. This is mostly a consequence of misinterpreted test results and good ol’ hearsay.

Back pressure is actually quite simple, as the name suggests, it is the pressure that pushes back on your exhaust system against the direction of flow. Even though it isn’t desirable, back pressure plays a vital role in the functioning of your exhaust system, as it dictates a lot of the design characteristics. More on that later.

When exhaust scavenging, back pressure, velocity, piping diameter, valve timing, and overlap are all optimally balanced, what you end up with is an ideal exhaust system setup. However, this is subject to many variables. We’ve explained this in detail towards the end of our exhaust scavenging guide.
Remember that most of the rules change when it comes to cars fitted with an aftermarket turbocharger; or even OEM turbocars for that matter.
In this article, we’ll explain what back pressure is, and shed some light on the popular myth about back pressure being good for your engine.

What Is Exhaust Back Pressure?

Back pressure refers to a pressure buildup in the exhaust system that interferes with the outward flow of spent gases.
It is the pressure opposing the desired exhaust flow think of it as the exact opposite effect of scavenging.
The entire exhaust system is basically one large flow restriction that we can’t do without for multiple reasons:

• The exhaust pipes need to redirect the fumes away from passengers.
• The catalytic converter needs to clean up spent gases to meet emissions regulations.
• The muffler softens the exhaust note to meet noise regulations.
The list goes on. Each of these components adds some amount of restriction. And with restrictions comes back pressure, this is never desirable.
To understand why this isn’t desirable, you need to know how scavenging works because both concepts are extremely intertwined. Let’s recap real quick.

Exhaust Scavenging is the process by which spent gases are drawn out of the combustion chamber and expelled into the exhaust system.
This happens because there’s a pressure differential between the part inside the engine cylinder (combustion chamber) and the part outside the engine cylinder (exhaust manifold).
Gases always travel from areas of high pressure to low pressure.

The pressure within the combustion chamber is 6 to 7 times higher than what’s in the exhaust system. That’s why the spent gases gush out at the moment when the exhaust valve is opened.
Spent gases exit through multiple cylinders at high speed and pressure, in the form of pulses.
Each time these pulses merge at different points in the exhaust system, a trailing negative pressure wave is created, especially right after the exhaust valve shuts.

This negative pressure wave travels back to the next exhaust valve and helps to evacuate the gases even quicker (because of the same reason; gases like moving towards low pressure). If there’s valve overlap, the scavenging effect can also help draw in more air into the engine simultaneously.
Engineers strive to tune exhaust systems to time everything perfectly and allow the exhaust valves to open right at the point when the negative pressure wave arrives.

Back pressure hinders this process, it is counterproductive.

Is Back Pressure Good or Bad?

It’s clear that exhaust gases like to flow from high pressure to low pressure. The higher the pressure differential, the easier it is for the exhaust fumes to flow out of the cylinder. This means you want a maximum pressure differential to achieve high volumetric efficiency.
Adding back pressure is only going to lower the pressure differential, making it harder for exhaust gases to escape (poor scavenging).
This creates two problems:

1. Volumetric efficiency drops, that is, exhaust gases don’t get vented effectively, which means that an insufficient amount of air and fuel gets drawn into the cylinder (there’s only so much space to work with). This reduces power output.

2. Because some of the exhaust gases fail to escape, the next combustion cycle gets affected negatively.

With that in mind, back pressure is clearly a bad thing. And guess what causes back pressure restrictions.
The more restrictive your exhaust system is, the more bends it has, the more back pressure you’re going to get.
The reason why aftermarket exhausts are desirable is that they have fewer restrictions compared to your stock setup.
If you’ve tried researching this topic in the past, you’ve probably heard that some amount of back pressure is actually good for your engine this is a logical fallacy.

Do All Exhausts Need Back Pressure?

You don’t need back pressure. More accurately, back pressure is an inevitable consequence of trying to increase performance. No matter what you do, your exhaust system will naturally generate some amount of back pressure.
This is mainly because exhausts are transient state systems, not steady-state, that is, they’re dynamic. As the rpm and exhaust velocity change, what the engine needs in terms of exhaust design also changes.

But you can’t really change your tubing length on the fly; you’re pretty much stuck with what you have installed.
That’s why it’s always a trade-off if a certain tubing length gives you peak performance in a certain rpm range, it’ll give you worse performance in other parts of the rev range.
It’s up to you to decide what part of your rpm range you want the peak performance to be in. That way, you get better scavenging where you want it, and worse scavenging where you don’t care. Either way, you’re going to end up with back pressure somewhere unless you’re running unequal length exhaust runners

Negative Results From Less Back Pressure

All of this considered, there are some people who have experienced poor drivability as a result of removing back pressure, perhaps by removing restrictions such as the catalytic converter or the resonator.

While they’re not wrong, their assumption that “removing back pressure” was the culprit behind poor drivability is incorrect.
When you remove restrictions from your exhaust system without tuning the PCM,
or without compensating for the new exhaust flow characteristics, it creates problems.

So long as you compensate for the changes you’re making in your exhaust system, removing or reducing back pressure will always be a good thing.
Take a look at the exhaust headers on drag cars for instance, they point straight to the sky. No back pressure whatsoever.

This myth has more to do with exhaust velocity than back pressure.
Someone probably did a bunch of experiments with variable pipe diameters, got better results with narrower pipes, and assumed that it was the back pressure that’s helping performance. It must’ve been faster exhaust velocity and better scavenging that was doing the trick.