We decided to make our own exhaust X-pipe to replace where the CATs go for when racing.

To me the X pipe should be close to where the bends are that trap gases and not further back where pipes are straight.

We also made slip flanges for the CATS so they can be easy swapped with X-Pipe when need be

We'll post more scanner data of test results to see what differences there are.

Three inch slip fittings for Collectors or Headers and 2 1/2 fittings to stock mid exhaust pipe.

Gut feeling is the exhaust pulses are better, exhaust sounds smoother, more low end torque and requires a PCM tune as O2s report leaner with the X pipe installed but further analyzing the scanner data should give the results

Here is the results of the 1st testrun with the X-pipe
No tuning changes were made so that we could see what the cause and effects of the exhaust change was

Engine leaned up about 5% more so assume better exhaust flow and you notice at idle (cell 19) was way more leaner so the X-pipe allowed this engine makeup to move out the exhaust better in cylinders.

Rearwheel gains look to be about 10/10 HP/Tq and shows the claims some vendors make of 30 HP is bogus.

Gutted CATs Left graph - Xpipe Right side

Here is a compare of launching to 120 MPH with the G (gutted CATS)
Then with the proper placement of our X-pipe

And then a superimposed scanner data of both as to HP and Tq.

You notice with the front placed X-pipe HP and Tq come on much faster and have aboust 3 MPH faster then G-Cats at the same distance

Here is 2nd round of testing.
Tests spread across 2 days but weather the same for both days at about 50 degrees.
Two tests with X-pipe done today to then compare the tests of yesterday which was one run with gutted CATs and one run with X-pipe.
Keep in mind the vette has 3.73 rearend gear with 335/30/18 tires with limited traction but all testcases are equal with that issue.

Also when values go negative that is due to wheel spin and standing start and having to make 4 manual trannie shifts.

Same road used in all tests.

Again X means with X-Pipe and C is when gutted CATs were used.

Clearly correct placement of X pipe ( using 3 inch to collector but then 2 1/2 inch pipe to exhaust pipes) as you see the first 7 seconds from dead launch X pipe is winner as to HP, Tq, and MPH

For this test we retuned the PCM for a perfect fuel trims around zero and WOT AFR at 12.8:1.
Also changed radiator fans coming on at 200 degrees to 185 deg

With the tune horsepower increased from 475 to 513 flywheel.

This makes it easy to see the difference as what happens for the 1st 7 seconds from launching.

As seen when the Xpipe is used the HP and Torque quckly increase and when Xpipe is used it gets to red line sooner so shifting to 2nd gear happens about 2 seconds faster

It looks great, JR. Quick question I have is did you have to alter your oxygen sensor locations at all?

No changes to any of the 4 O2s,

I designed this so rear O2s and their bungs stay in stock locations.

I wanted as little changes to the mid pipe as possible so only the front of pipes were cut shorter and rest of mid pipes stay in stock placement.

How would you think your design fairs against the Stainless Works setup I already have? I don't have cats, so the headers go into a section about 1 foot long, then another about 2 foot long that the rear O2's plug into, then into the X-pipe. So clearly your x-pipe would bring the x-pipe at least 3 foot closer to the headers, if not closer. I was just wondering if I could somehow just swap some of the parts around to make it happen, but I see with my current pipes I'm going to have to do some serious rearranging to get the O2 sensors where they need to be. I don't see how I can post a picture of my setup on this venue, but I can sent it to you privately.

Thanks,

Read this first

Exhaust flow

Good example Rick K reminded me of

Some years ago Rusty Wallace in Nascar out of the blue began winning several racers. The exhaust sounded different.
Others did not know why until he crashed and car ended up lying on one side and everyone then saw the exhaust design.

The closer the X is to the heads the easier it is for it to draw the exhaust out of the heads leaving better makeup of air/fuel burn.

Further back the X is means there is more exhaust volume backed up to where the X is.

The center of X is open which allows the exhaust flow of one head to pull the flow of the exhaust in the other sides echaust pipe.

Better image of X-pipe to collector of Headers and rear O2s in stock location

Rearwheel dyno numbers

Here is how Richard from Kimble Motorsports build a combined X-pipe and high flow CATs system for Bob R's C5

Next step in our designing a custom exhaust system is to baseline what the sound level (DB) our existing system outputs so that as we design after the X-pipe that we have a good flowing 3 inch system but is below a -94 DB sound level.

Here we capture engine RPM and speed in MPH at the same time recording cabin level sounds ( with windows open) as to exhaust sound in frequency and DB level.
Average exhaust sound was around 3300-4000 Hz at around -60 DB

Click below and download 2 exhaust sound recordings.

As we design from the X-pipe to muffler tips we then can compare before and after sound end results and assure we are not above the maximum DB levels tracks limit too.

More images of X-pipe testing

First design has the exhaust from the long tube header collectors 46 total inches to the muffler tips.
As image shows the mufflers take up 24 inches of that length.
Weight of stock exhaust with no cats was 65 pounds, total designed system is only 23 pounds.

In testing the sound as to DbC level is too high,
average inside of car with windows up is about 110 DB and as much as 128 DbC near the rear wheels.
Measuring as to ear quality levels as DbA inside measured 92 and outside 102 DbA.
We want final design under 95 DbA and less then 85 DbA inside.

Important to keep in mind is what levels of sound human ears can take

Types of Hearing Loss

There are many different causes of hearing loss. The following are the general categories into which hearing loss falls:

Otosclerosis - is a disease that causes bony growth on the ossicles and causes the stapes to become immobile, thus not allowing sound to be transferred into the cochlea. This is the result of a disease that affects the movement of the stapes, located in the middle ear.

Meniere's disease - is a problem involving fluid pressure within the cochlea. It causes the sufferer to experience intermittent episodes of hearing loss, dizziness, and tinnitus. These episodes can occur anytime and for varying amounts of time.

They are often associated with stress.
Drug induced - some medications can result in damage to the auditory system with prolonged use.
They are called ototoxic. Here are a few drugs that are known to cause hearing loss: aminoglycoside antibiotics (such as streptomycin, neomycin, kanamycin); salicylates in large quantities (aspirin), loop diuretics (lasix, ethacrynic acid); and drugs used in chemotherapy regimens (cisplatin, carboplatin, nitrogen mustard).

Tumors - one of the common tumors in the ear is called a vestibular schwannoma. These tumors develop around the 8th cranial nerve, which is also known as the auditory nerve.
Trauma - trauma to the ear can include fractures of the temporal bone, puncture of the eardrum by foreign objects, sudden changes in air pressure, and very loud noises.

Presbycusis - this hearing loss is caused by natural aging of the human body and begins after age 20, but often it is not noticed until the ages of 55 to 65. Presbycusis affects the high frequencies in the speech range, making understanding and hearing speech difficult.

Noise-induced hearing loss (NIHL) - this is hearing loss due to exposure to either a sudden, loud noise or exposure to loud noises for a period of time. A dangerous sound is anything that is 85 dB (sound pressure level - SPL) or higher.
Dangerous Decibels focuses on noise-induced hearing loss.

Noise-Induced Hearing Loss (NIHL)

Of the roughly 40 million Americans suffering from hearing loss, 10 million can be attributed to noise-induced hearing loss (NIHL). NIHL can be caused by a one-time exposure to loud sound as well as by repeated exposure to sounds at various loudness levels over an extended period of time.

Damage happens to the microscopic hair cells found inside the cochlea. These cells respond to mechanical sound vibrations by sending an electrical signal to the auditory nerve.
Different groups of hair cells are responsible for different frequencies (rate of vibrations). The healthy human ear can hear frequencies ranging from 20Hz to 20,000 Hz. Over time, the hair cell's hair-like stereocilia may get damaged or broken. If enough of them are damaged, hearing loss results. The high frequency area of the cochlea is often damaged by loud sound.

Sound pressure is measured in decibels (dB). Like a temperature scale, the decibel scale goes below zero. The average person can hear sounds down to about 0 dB, the level of rustling leaves. Some people with very good hearing can hear sounds down to -15 dB.
If a sound reaches 85 dB or stronger, it can cause permanent damage to your hearing. The amount of time you listen to a sound affects how much damage it will cause. The quieter the sound, the longer you can listen to it safely.
If the sound is very quiet, it will not cause damage even if you listen to it for a very long time; however, exposure to some common sounds can cause permanent damage.
With extended exposure, noises that reach a decibel level of 85 can cause permanent damage to the hair cells in the inner ear, leading to hearing loss. Many common sounds may be louder than you think…

A typical conversation occurs at 60 dB - not loud enough to cause damage.
A bulldozer that is idling (note that this is idling, not actively bulldozing) is loud enough at 85 dB that it can cause permanent damage after only 1 work day (8 hours).

When listening to music on earphones at a standard volume level 5, the sound generated reaches a level of 100 dB, loud enough to cause permanent damage after just 15 minutes per day!
A clap of thunder from a nearby storm (120 dB) or a gunshot (140-190 dB, depending on weapon), can both cause immediate damage.

In fact, noise is probably the most common occupational hazard facing people today. It is estimated that as many as 30 million Americans are exposed to potentially harmful sounds at work.

Even outside of work, many people participate in recreational activities that can produce harmful noise (musical concerts, use of power tools, etc.). Sixty million Americans own firearms, and many people do not use appropriate hearing protection devices.


Decibel Exposure Time Guidelines

Accepted standards for recommended permissible exposure time for continuous time weighted average noise, according to NIOSH and CDC, 2002. For every 3 dBs over 85dB, the permissible exposure time before possible damage can occur is cut in half.

Continuous dB Permissible Exposure Time

85 db 8 hours
88 dB 4 hours
91 db 2 hours
94 db 1 hour
97 db 30 minutes
100 db 15 minutes
103 db 7.5 minutes
106 dB 3.75 min (< 4min)
109 dB 1.875 min (< 2min)
112 dB .9375 min (~1 min)
115 dB .46875 min (~30 sec)

NIHL and Veterans

NIHL is of particular concern to veterans. Because NIHL is not immediately apparent (having a gradual onset), many veterans leaving the service are unaware of the full extent of hearing damage.
Although governments are now realizing the link between military service and NIHL, it took a long time and many lawsuits before any compensation was given to the affected veterans.
In 1999 the USA alone distributed $291.6 million in compensation for NIHL to some 56,792 veterans. The annual cost of compensation to veterans in France is estimated to be $60 million.
In Belgium two thirds of all payments made to veterans with disabilities correspond to NIHL. Many veterans damage their hearing during service.
They can, however, prevent more damage from occurring and they can save their remaining hearing after their military duties are completed.

Vehicle Exhaust Noise Level Certification

California law requires that all vehicles must be equipped with an adequate muffler to prevent excessive noise from the exhaust system.
It also prohibits the operation of a passenger vehicle (other than a motorcycle), or a truck with a Gross Vehicle Weight Rating (GVWR) of less than 6,000 pounds, that has an exhaust noise level greater than 95 (DbA) decibels, when tested under specified conditions.

If you have been issued a citation for operating a vehicle in violation of Vehicle Code Section 27150 or 27151, you must take your vehicle to a state Referee Center.

Note: The Referee is only authorized to inspect and certify passenger cars and trucks under 6,000 GVWR that have been cited for a violation of Section 27150 or 27151 of the Vehicle Code.
The maximum decibel level applies only to those vehicles. Citations issued to other vehicles (motorcycles, trucks exceeding 5999 pounds GVWR) are not part of this program. Check with the agency that issued the citation, and/or the court, to find out how to show proof of correction for these other vehicles.

The following steps outline the procedures you'll need to follow to get a certificate of compliance, which shows your vehicle's exhaust noise level is within the applicable standards.

STEP 1

If you feel your vehicle's exhaust system has been modified or has deteriorated so that the noise level exceeds current standards, you should have the exhaust system repaired before proceeding to step 2.

OR

If you have reason to believe that your vehicle's exhaust noise level meets the current standards, proceed directly to step 2.

Note that citations have a court appearance date.]
[Timely action can help you avoid additional fines and penalties.]

STEP 2

Have your vehicle tested at a state Referee Center. To make an appointment, call the Referee Scheduling Center at (800) 622-7733. Have the citation and vehicle registration with you when you call. The Center will tell you any fees that will need to be paid for the test.

STEP 3

The Referee will conduct an exhaust noise level test in accordance with the Society of Automotive Engineers (SAE) standard J1169, May 1998, to determine if your vehicle's exhaust noise level exceeds the noise standard (95db).

If your vehicle meets the standard, the Referee will give you a "Certificate of Compliance." Take the certificate to the court (See Step 5).

If your vehicle does not meet the standard, the Referee will give you a report that lists the recorded decibel readings. Your vehicle must be repaired to meet the standard of not more than 95 decibels (See Step 4).

STEP 4

If your vehicle does not meet the standard, additional repairs must be made before you can make a second appointment with the Referee Scheduling Center.

STEP 5

Once your vehicle receives a "Certificate of Compliance" from a Referee Center, present the certificate to the court as proof that your vehicle is in compliance with California law.

NOTE: Inspection fees and any necessary repairs are your responsibility. You may also be required to pay a fine and other legal costs when reporting to the court. Also, note that most citations have a deadline. Timely repairs can help you avoid additional fines and penalties.

If you have any questions, please contact the local law enforcement agency that issued the citation or the Bureau of Automotive Repair at (800) 952-5210

Second round of design and testing of the Team ZR-1 model NFW consists now of custom designed mufflers which reduced DBA at rear of car from 102 DBA to 94 and by driver area in a range of 82-90 DBA

We're in the process of doing testruns with OBD scanner to analyze the data as a compare to design 1 tests

Not a smart thing to do as this owner put in cutouts that is forcing all the time hot exhaust over time delaminates the fiberglass spring, causing failure of spring and also adds more heat to the transaxle that already gets too hot

Third round of NFW exhaust design has been completed.

We wanted a balance so that for those that wanted less DBA inside the car but not lose the sound or increase backpressure at the exhaust tips they could now choose to their liking by either using design stage two or :

We designed 3 inch tailpipes from our custom designed mufflers to ending out the back end.

Testing showed the DBA inside the car now running between 80 and 88 DBA with only a 2 DBA drop at the end of tailpipes.

There is a 2 DBA difference inside the car depending on if the windows are open or closed.

This allows those that buy our NFW system to choose the race ( higher DBA) and later could purchase our tailpipes of buy the total system and install or take off the tailpipes when need be

Attached is some audio recordings from part throttle to WOT, with recordings captured inside and with the mic mounted by license plates.

You will need to unzip the files to listen to the recordings.

Scope sound levels at part throttle

Here is results of before the NFW exhaust was installed and then with it on.
This is with the full length design with the tailpipes.

Weather was 85 degrees adding 1,500 feet to the 4,000 feet elevation tests were done at
Notice the lower torque comes up sooner and faster now with the end to end system being 3 inch

Design testing of end of tailpipes 2,5 and 8 inch longer then in design 2.
Each additional length to bring end of tailpipes further away from underside reduced sound level in drivers are from mid 80s DBA to 78 DBA at idle but no loss to sound level at the end of tailpipes.

Final testrun of the NFW Team ZR-1 custom exhaust design with the double wall end tips which are stainless steel
Inlet is 3 inch with 4 inch outlet.

Winter test of our NFW exhaust design
Weather 36 degrees and cloudy. Windows up and radio on and off to see if internal exhaust sound was at a proper dBA level.



Part throttle to WOT


WOT



Recorded performance data during the WOT run in part from zero to 110 MPH

Team ZR-1 Corvette Racers NFW custom exhaust test at 60 degrees and windows closed.

Part throttle



WOT run

Here is idle with windows open and then closed
Measuring sound at the tailpipe tips is 94 dbA
where in drivers area sound level is only at 80 dbA with windows open and 77 dbA with windows closed



Here is stop/go city driving that includes both windows open, closed, drivers or passenger window open or closed, radio on and verbal talk
Weather 55 degrees and cloudy



Launch from a dead stop :

This is a cold start idle. Weather is only 42 degrees so normal to see water vapor until everything gets up to operating temp

70 degrees, windows closed, 4,000 feet elevation, up to 160 MPH

Again in either case you want them to be located as close to collectors as possible

Compare after 10 years of my custom NFW exhaust design on July 5th 2019
Bullet mufflers used still sounds good, has not degraded over these years

Weather 80 F Degrees, A/C off and windows closed

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.