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Exhaust - MPG & HP Explained

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Old 08-07-2009, 01:27 PM   #1
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Exhaust - MPG & HP Explained

So we get a TON of threads regarding what muffler is better, what catback is better, is a catback better then a muffler, so on and so forth.

To help reduce these redundant threads, let's break it down, so any question you can in regards to performance and fuel efficiency will be answered right here. I'm going to try to keep this as short and simple as I can. There is a LOT that goes into exhaust tuning, more then most people care about.

MPG & Fuel Efficiency
- Will a catback or muffler change you fuel mileage?
Simple and straight forward - Not with todays vehicles. With the amount of emphasis put on fuel efficiency intake and exhaust systems will be designed for peak efficiency under normal driving conditions (low load). Changing the exhaust flow will effect performance under high loads, but have little impact on low loads.

If you see an increase or decrease in mileage it is most likely because you have changed your driving habits, even if they are ever so slightly.

Performance
- Will a muffler or catback give me power? Sort of. Depending on your current setup, you'll either make more power, or just change the arch of the power band.
- Which will give more gains? Depneds.
- How is power increased/decreased by changing the muffler or catback?
Short answer, by changing where the volumetric efficiency is the highest. By changing the flow characteristics of the exhaust system you will alter the power band (changing where the volumetric efficiency is at its best). VE is always the highest at WOT. Under partial throttle, VE is going to remain low, meaning fuel efficiency is going to remain unchanged (hence the no change in MPG). In simplest terms, a free flowing exhaust will provide more power higher in the RPM range, where a more volumetric restrictive exhaust will provide more power down low. unfortunately, you usually won't ever have both. Your torque curve is generally a hump, it goes up, peaks, then comes back down. The torque curve represents the power the motor is actually making (HP is a number that is calculated based on torque & RPM which is why its a climbing number that peaks at the highest RPM). I can dive deep into scavenging, exhaust pulses, header design, and so on if there are enough questions regarding optimal performance.


- What size exhaust is best for me?
The better question is.. where do you want to make the most power? Smaller exhaust piping will increase the exhaust velocity. Velocity = torque. So smaller piping is going to give you that power right way, but as you raise the RPMS, and more exhaust starts getting stuffed into the exhaust piping you can see the build up and back pressure that is gonig to happen, this is going to kill your top end. Larger exhaust piping will give you the volume that you need in the high rpm range to keep the velocity up. But your low end isn't going to produce as much exhaust. With less exhaust, the larger piping the exhaust gases are going to slow down creating a traffic jam in the lower band.

I would like to believe that the Toyota engineers knew a thing or to about engine tuning when they designed our trucks exhaust system. And with that in mind, OEM piping is the best all-around size for the vast majority of trucks. And I would recommend keeping the OEM size unless you want to change where your power band peaks.


- Which muffler will give me the best performance?
Again you need to ask yourself where you want your power band to peak. Below are a few pics of some popular muffler designs. Muffler backpressure is going to have the biggest unfluence oh your power band shape. A more free flowing muffler will provide more gains higher up, but could result in a loss of the lower end torque.

There are tons of 'Butt Dynos' claiming power gains from one kind of muffler over another, always take these claims with a grain of salt. The mind can make good things better then they really are, meaning a very sweet sounding muffler will trick you into thinking its faster because you enjoy it more. Only way to know for sure is to look at a real dyno, or flow tests of different mufflers.








- Do I want a muffler or a full catback?
Our exhaust is mandrel bent from the factory, meaning unless you plan on changing the pipe size of your exhaust, or doing a dual setup, the OEM will perform just as well, so find a good muffler. Now if you planning on running boost, or wanting to make more high end power, then larger piping is what you want.


So now that you have a good idea on the performance aspect of a muffler.. the next question, and arguably the most important one... the SOUND. Your gonna hear it, your neighbor will hear it, and that cutie that lives down the street from you is gonna hear it.. so make sure it sounds good.

Please feel free to ask questions. There are a number of members here that are also knowledgeable about exhaust tuning that can chime in and help as well.
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Old 08-07-2009, 01:28 PM   #2
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^^^^He knows what he's talking about^^^^
Thanks great post.
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Old 08-07-2009, 05:00 PM   #4
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A performance muffler should flow better, thereby reducing pumping losses, and increasing efficiency. This is half the reason a "performance" exhaust increases performance. This should yield a corresponding increase in efficiency, especially under load.(excluding all other factors, which should also favor a mileage increase, if the muffler is actually better than stock). I cannot fathom why the OP would suggest otherwise.....Practically speaking, most people never see an increase, but the cause is wanting to hear the exhaust....not that the potential is not there...
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Old 08-07-2009, 05:14 PM   #5
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thank you excellent post
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Old 08-07-2009, 06:50 PM   #6
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Quote:
Originally Posted by Isthatahemi View Post
A performance muffler should flow better, thereby reducing pumping losses, and increasing efficiency. This is half the reason a "performance" exhaust increases performance. This should yield a corresponding increase in efficiency, especially under load.(excluding all other factors, which should also favor a mileage increase, if the muffler is actually better than stock). I cannot fathom why the OP would suggest otherwise.....Practically speaking, most people never see an increase, but the cause is wanting to hear the exhaust....not that the potential is not there...
While you will definitely increase the efficiency when your under load, you usually don't measure your fuel efficiency while running under WOT.

I've read the claims that the lack of a muffler, or a free flowing performance muffler, will reduce backpressure on the pistons, and makes it more efficient, so on and so forth. A more free flowing muffler will change the scavenging performed by the exhaust system. I've already explained how a more free flowing exhaust is better for higher loads (more gas needs more volume) and hurts the low end (gases move too slow). If you remove back pressure, then you are in turn slowing down the gases at low volume (loads). Doing this will reduce the scavenging effect the exhaust will have (valve overlap on the intake stroke), and will require a performance intake to help counter.

Now of course you will always have the extremes. So to say that this will happen on 100% of applications is not accurate. With the technology that is used today, and the constant need for more fuel efficient vehicles, if a straight through 0 restriction muffler was really better, automotive engineers would use it. But if you look at almost all factory mufflers they are restrictive to serve this purpose. Create ideal scavenging at cruising loads.

Now if your cruising loads are higher, then I could see a performance muffler helping MPG, as you would be taking advantage of the more free flowing system. So when choosing your muffler you need to pick one that is optimal for your driving habits.
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Old 08-08-2009, 11:25 AM   #7
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Nice write up

Very nice write up. I wasn't looking to upgrade my exhast system and this even cements it more for me as my normal driving is low load.

Thanks for the info,
Joe
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Old 08-08-2009, 10:04 PM   #8
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Quote:
Originally Posted by XrunnIT View Post
While you will definitely increase the efficiency when your under load, you usually don't measure your fuel efficiency while running under WOT.
First, on the 4 cylinder taco's, they run 70 - 85% load on the highway at higher speeds. So this is really doesn't make sense....And on any engine, reduced pumping losses translate in higher efficiency, generally above 50% load, not just WOT. But greatest around 90% load

Quote:
Originally Posted by XrunnIT View Post
A more free flowing muffler will change the scavenging performed by the exhaust system. I've already explained how a more free flowing exhaust is better for higher loads (more gas needs more volume) and hurts the low end (gases move too slow). If you remove back pressure, then you are in turn slowing down the gases at low volume (loads). Doing this will reduce the scavenging effect the exhaust will have (valve overlap on the intake stroke)
Not true, scavenging is an effect of exhaust stream velocity. Exhaust velocity is only lost if the pipe is upsized. Reducing backpressure through the use of low restriction mufflers, and mandrel bent pipe help scavenging.......
(engineering basics on laminar and turbulant flow and velocity etc.)

Quote:
Originally Posted by XrunnIT View Post
if a straight through 0 restriction muffler was really better, automotive engineers would use it. But if you look at almost all factory mufflers they are restrictive to serve this purpose. Create ideal scavenging at cruising loads.
What a load!!!! The mufflers are somewhat restrictive on modern vehicles to keep them quiet. It has nothing to do with scavenging. How ridiculous to even say that! That implies a total misunderstanding of exhaust systems!!! Obviously a straight pipe is not an option on a new vehicle, and modern mufflers are not very restrictive. These myths that you are reciting are based on people who add larger diameter piping and mufflers, and suffer a loss of bottom end torque. It's the loss of velocity, not backpressure that causes this.

Scavenging effect is the opposite of backpressure, backpressure contains the intake charge, and exiting exhaust, while scavenging draws the hot expandind exhaust gases out. Scavenging is typically strongest if velocity is high, while backpressure is purely a product of downstream restriction.
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Old 08-08-2009, 10:21 PM   #9
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OEM MUfflers are in fact engineered with scavenging in mind. AND, Db levels.....Manufactures have to make an eficient engine, but also keep it quiet. They achieve this by making a large muffler, that doesnt interfere with the scavenging (or interfere as little as they can). Almost all after market "High performance mufflers are 2 times smaller than the OEM muffler.

One thing Im not seeing on here, and one thing that makes an after market muffler have little to no affect on MPG, is the engines pumping losses, and restrictions. At part throttle setting, the pistons are working against the largest restriction you can find in the engine....The throttle butterfly. Cruising at 25% throttle, it isnt gonna matter what kind of exhaust you have, as the throttle butterfly is creating the most resistance in the system.
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Old 08-08-2009, 10:22 PM   #10
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Quote:
Originally Posted by chris4x4 View Post
OEM MUfflers are in fact engineered with scavenging in mind. AND, Db levels.....Manufactures have to make an eficient engine, but also keep it quiet.

One thing Im not seeing on here, and one thing that makes an after market muffler have little to no affect on MPG, is the engines pumping losses, and restrictions. At part throttle setting, the pistons are working against the largest restriction you can find in the engine....The throttle butterfly. Cruising at 25% throttle, it isnt gonna matter what kind of exhaust you have, as the throttle butterfly is creating the most resistance in the system.
Ok I am officially confused, is this restrictiveness good or bad?
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Old 08-08-2009, 10:27 PM   #11
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Quote:
Originally Posted by dgerm View Post
Ok I am officially confused, is this restrictiveness good or bad?
Restriction is an "Old timers" term. Scavenging is the thing you need to consider with an exhaust. If your supercharged, or turbo charged, its less of an issue.
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Old 08-09-2009, 10:09 AM   #12
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Quote:
Originally Posted by Isthatahemi View Post
First, on the 4 cylinder taco's, they run 70 - 85% load on the highway at higher speeds. So this is really doesn't make sense....And on any engine, reduced pumping losses translate in higher efficiency, generally above 50% load, not just WOT. But greatest around 90% load
the more you increase the load, the more you will take advantage of more efficient parts.

so as I previously stated...

Quote:
Now if your cruising loads are higher, then I could see a performance muffler helping MPG
So if indeed the 4 cylinders are running at 70-85% load while cruising, then you will see a mileage increase.

Quote:
Originally Posted by Isthatahemi View Post
Not true, scavenging is an effect of exhaust stream velocity. Exhaust velocity is only lost if the pipe is upsized. Reducing backpressure through the use of low restriction mufflers, and mandrel bent pipe help scavenging.......
(engineering basics on laminar and turbulant flow and velocity etc.)
exhaust scavenging is the process of using the exhaust pulse from combustion to create a low pressure 'tail' to extract all the gases from the motor. This vacuum happens during the valve overlap on the intake stroke creating a vacuum for a fuller intake stroke.

Yes, reducing backpressure in the exhaust will help scavenging, BUT will do so at different loads.

Quote:
Originally Posted by Isthatahemi View Post
What a load!!!! The mufflers are somewhat restrictive on modern vehicles to keep them quiet. It has nothing to do with scavenging. How ridiculous to even say that! That implies a total misunderstanding of exhaust systems!!! Obviously a straight pipe is not an option on a new vehicle, and modern mufflers are not very restrictive. These myths that you are reciting are based on people who add larger diameter piping and mufflers, and suffer a loss of bottom end torque. It's the loss of velocity, not backpressure that causes this.
Restriction does not equal quiet. There are a TON of ways to eliminate and reduce exhaust decibels without relying on a restrictive muffler.

Take an OEM sized exhaust from the manifold back, and make it straight pipes. I GUARANTEE you will see a loss in low end torque. And you noticed, pipe sizing wasn't changed, backpressure was removed.

Backpressure and exhaust velocity are directly related. If you have too much or too little backpressure you WILL change the exhaust velocity, which will effect the scavenging (for the good or bad).

Quote:
Originally Posted by Isthatahemi View Post
scavenging draws the hot expandind exhaust gases out. Scavenging is typically strongest if velocity is high, while backpressure is purely a product of downstream restriction.
Not entirely true. Scavenging is at its best when it is timed properly. If that pulse is timed right, then it will result in higher velocity. Scavenging is not the result of velocity, but velocity is a result of the scavenging.
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Old 08-09-2009, 10:36 AM   #13
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Quote:
Originally Posted by XrunnIT View Post
the more you increase the load, the more you will take advantage of more efficient parts.

so as I previously stated...

So if indeed the 4 cylinders are running at 70-85% load while cruising, then you will see a mileage increase.



exhaust scavenging is the process of using the exhaust pulse from combustion to create a low pressure 'tail' to extract all the gases from the motor. This vacuum happens during the valve overlap on the intake stroke creating a vacuum for a fuller intake stroke.

Yes, reducing backpressure in the exhaust will help scavenging, BUT will do so at different loads.



Restriction does not equal quiet. There are a TON of ways to eliminate and reduce exhaust decibels without relying on a restrictive muffler.

Take an OEM sized exhaust from the manifold back, and make it straight pipes. I GUARANTEE you will see a loss in low end torque. And you noticed, pipe sizing wasn't changed, backpressure was removed.

Backpressure and exhaust velocity are directly related. If you have too much or too little backpressure you WILL change the exhaust velocity, which will effect the scavenging (for the good or bad).



Not entirely true. Scavenging is at its best when it is timed properly. If that pulse is timed right, then it will result in higher velocity. Scavenging is not the result of velocity, but velocity is a result of the scavenging.

Just want to toss this out there. As long as this debate remains civil, I personally find it VERY educational so please continue!


Oh and thanks for everyone participating!
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Old 08-09-2009, 02:12 PM   #14
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So then....we agree that low restriction exhaust will help mileage to some degree.....The full benefit will show itself almost under full load (90% is closer to...) And inversly squared reduction in flow, from that point.....Meaning that at 25 %, there really is no benefit.....but thats at idle anyhow.
Reducing backpressure will help scavenging, if only there is little or no loss in velocity (pipe size the same etc...) However it does not depend on load, but on flow and velocity at any given point which is dependant on V.E. of the engine, which on Toyota's is quite high from 1500 rpm on. So a properly sized system should help almost off idle.....Wheras it would make no difference on say a honda at that rpm...
As far as a straight pipe losing torque, I have to disagree, but this is a moot point anyhow. I would point out that race cars are straight piped, not just for horsepower, but for torque as well.
Pulse timing and backpressure (as they relate to each other) are headpipe and manifold issues primarily, Larger exhaust systems with no account for this fact lose torque. Scavenging suffers as a direct result of headpipes (headers or manifolds, being too large to maintain the exhaust pulse movement, (kind of like trying to make a wind tunnel, with too small a fan, or too big a tunnel). Smaller pipes have more back pressure, but too small, and the high backpressure impedes velocity, and scavenging. New OEM exhaust systems don't really have this problem. This primarily is the reason we don't see huge gains in exhaust mods anymore.
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Old 08-09-2009, 10:46 PM   #15
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Quote:
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Our exhaust is mandrel bent from the factory
u sure? Mine definitely wasnt, and so aren't most of my friends'. In fact, I've never seen factory mandrel bent exhaust on 2005+ Tacomas. Unless X-runners are different?
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Old 08-10-2009, 05:51 AM   #16
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Originally Posted by blackhawke88 View Post
u sure? Mine definitely wasnt, and so aren't most of my friends'. In fact, I've never seen factory mandrel bent exhaust on 2005+ Tacomas. Unless X-runners are different?
He may be talking about the TRD exhaust. I think the X runners come with those from the factory.
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Old 08-10-2009, 07:59 AM   #17
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XrunnIT's QUOTE:
"exhaust scavenging is the process of using the exhaust pulse from combustion to create a low pressure 'tail' to extract all the gases from the motor. This vacuum happens during the valve overlap on the intake stroke creating a vacuum for a fuller intake stroke."

-Is this what headers are designed to do? increase the scavenging effect?
- My first vehicle was a 1983 ford truck with a 302 2barrel carb. I ran full duals exhaust no cats through 2 Cherry Bombs, did I prematurely kill this engine by doing this?
Yes, this was what headers were designed to do. And if you dig into header designs, everything from header primary lenght & diameter, to the collector length all play a huge roll in scavenging.

As far as your 83 truck, I would say no. You would have altered the power band and peak VE, but you wouldn't have done any damage to the motor.

Quote:
Originally Posted by Isthatahemi View Post
So then....we agree that low restriction exhaust will help mileage to some degree.....The full benefit will show itself almost under full load (90% is closer to...) And inversly squared reduction in flow, from that point.....Meaning that at 25 %, there really is no benefit.....but thats at idle anyhow.

For the 1GR motor, idle ranges between 12-13% for a n/a vehicle, and cruising (at 55mph-75mph) is about 19-22% load (documented logs supporting these facts). At 60% throttle (a brisk acceleration) the motor is at about 83% load (for a 1GR with bolt on and a tune).

So at 25% load there is really no benefit, aka normal cruising speeds.

Quote:
Originally Posted by Isthatahemi View Post
As far as a straight pipe losing torque, I have to disagree, but this is a moot point anyhow. I would point out that race cars are straight piped, not just for horsepower, but for torque as well.
Horsepower is a direct reflection of torque. The more torque you make,the more HP your gonna make. Horsepower is more linear because it climbs base on RPM. Torque is the raw power of the vehicle. Your peak HP numbers come from how much torque your making at redline.
Horsepower = (Torque x RPM) / 5,252
And you are right when you say race cars run straight pipes for performance. But remember, they are tuning those for high end power, and very high loads. Not a daily driving street vehicle.

If you still don't believe me go out and do two dynos, one with a full exhaust, and one with headers only (catless if you have the option).

Quote:
Originally Posted by Isthatahemi View Post
Pulse timing and backpressure (as they relate to each other) are headpipe and manifold issues primarily, Larger exhaust systems with no account for this fact lose torque. Scavenging suffers as a direct result of headpipes (headers or manifolds, being too large to maintain the exhaust pulse movement, (kind of like trying to make a wind tunnel, with too small a fan, or too big a tunnel).
That is correct.

Quote:
Originally Posted by Isthatahemi View Post
Smaller pipes have more back pressure, but too small, and the high backpressure impedes velocity, and scavenging. New OEM exhaust systems don't really have this problem. This primarily is the reason we don't see huge gains in exhaust mods anymore.
Exactly my point. With today's engineering that goes into vehicles, they try to get the perfect balance of power and economy. Favoring more to one side or the other based on the vehicles purpose.

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Originally Posted by chris4x4 View Post
Quote:
Originally Posted by blackhawke88 View Post
u sure? Mine definitely wasnt, and so aren't most of my friends'. In fact, I've never seen factory mandrel bent exhaust on 2005+ Tacomas. Unless X-runners are different?
He may be talking about the TRD exhaust. I think the X runners come with those from the factory.
The XR exhaust is mandrel bent from the factory.

I may be wrong saying all the '05 + are. I was under the impression that the exhaust was the same minus the muffler and exhaust tip.

Can some one take some pics of an OEM 05+ exhaust so we can confirm if its mandrel bent or not? THANKS!!

The XR doesn't come with the TRD exhaust, just a plain 'ole exhaust with a tip, and a different sounding muffler.
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Old 08-10-2009, 07:15 PM   #18
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so if putting doug thorley headers on a 4cyl will increase the power band, hows my mileage gonna look by that? cause essentially, im runin a jardine catback now and then with headers is that gonna screw it up performance wise?
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Old 08-10-2009, 07:26 PM   #19
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Quote:
Originally Posted by XrunnIT View Post
As far as your 83 truck, I would say no. You would have altered the power band and peak VE, but you wouldn't have done any damage to the motor.
I would disagree. A vehicle that age would most certainly run lean with better scavenging. Especially a carbed one. So my opinion is that you may have contributed to its premature death.

As for straight piped exhaust being worse for power. I disagree. This is a misconception based on lack of consistant pipe diameter, and lack of proper fueling. All else being equal (mixture, cam & valve timing etc), an unfettered exhaust flow will mean less parasistic losses, and more power (actual, or calculated).
So I guess we disagree
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Old 08-10-2009, 08:27 PM   #20
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Quote:
Originally Posted by Isthatahemi View Post
As for straight piped exhaust being worse for power. I disagree. This is a misconception based on lack of consistant pipe diameter, and lack of proper fueling. All else being equal (mixture, cam & valve timing etc), an unfettered exhaust flow will mean less parasistic losses, and more power (actual, or calculated).
So I guess we disagree
You say we disagree, but your arguing against something I didn't say.

I never said "straight pipes is worse for power". I've only stated that they move the power band higher in the RPM range

The difference is where the power is made. Your not gonna put straight pipes on a vehicle and increase the power from 1500rpm to redline. Its not gonna happen.

Two exhausts, exact same pipe diameter from the headers back:
Straight pipes is going to make more torque in the higher RPM range, which equals more peak HP.

An exhaust that has more restrictions is going to make more low end torque, build HP faster, but fall off at the end, resulting in lower peak HP.
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