No Exhaust Backpressure = Better performance?

[Matthew Jeschke]

I've been super confused by these YouTubers like Banks and MotoIQ saying NO backpressure is the universally optimal for an engine. This leaves me beyond confused. I've had questions on both these videos and nobody answers them. Their explanation is overly simplistic and doesn't seem to take into account anything of exhaust tuning or exhaust scavenging.

I had the cat back exhaust fall off my truck a few times now (I suck at welding). Every time it does my gas mileage utterly and miserably sucks and the power noticeably drops. I'd always assumed there was an optimal RPM (pressure) the exhaust is tuned for. Otherwise, unless your running WOT the exhaust IS tuned to be somewhat helpful. Am I crazy?

They just talk about smooth bends, large pipes, and less obstructions (cats, mufflers etc) as if that's everything to exhaust tuning. If it was that simple all the manufacturers would put on a super large exhaust pipe to improve MPG & EPA ratings?

I bought a Banks CAI because of the fancy tests they showed in videos and one claiming it would boost my MPG. It didn't have any noticeable effect in MPG (possibly it reduced my MPG).

Am I crazy or are these guys over simplifying stuff to sell parts?

 

[Pressureangle]

Yes of course they oversimplify, exhaust theory is more complicated than a theater pipe organ.

It's easier to visualize what happens in the exhaust if you think of the exhaust pulse as a slug of water- inertia is the property we're working with. This is why pipe size can't be too big, the gas slows too much to drag a vacuum behind it. It's also why when your exhaust falls off it may not work, the slug may not be long enough or the wavelength may be exactly wrong for the cam.

When Banks, and these other experts, make the simple claim 'zero backpressure is best' what they really mean is 'backpressure during exhaust valve open time is bad'. How much backpressure exists when the exhaust valve is closed has no effect. In a turbo system, there is always backpressure so the opportunity for reducing it meaningfully is more likely than in a NA system.

 

[Joseph Garcia]

I've been super confused by these YouTubers like Banks and MotoIQ saying NO backpressure is the universally optimal for an engine. This leaves me beyond confused. I've had questions on both these videos and nobody answers them. Their explanation is overly simplistic and doesn't seem to take into account anything of exhaust tuning or exhaust scavenging.

I had the cat back exhaust fall off my truck a few times now (I suck at welding). Every time it does my gas mileage utterly and miserably sucks and the power noticeably drops. I'd always assumed there was an optimal RPM (pressure) the exhaust is tuned for. Otherwise, unless your running WOT the exhaust IS tuned to be somewhat helpful. Am I crazy?

They just talk about smooth bends, large pipes, and less obstructions (cats, mufflers etc) as if that's everything to exhaust tuning. If it was that simple all the manufacturers would put on a super large exhaust pipe to improve MPG & EPA ratings?

I bought a Banks CAI because of the fancy tests they showed in videos and one claiming it would boost my MPG. It didn't have any noticeable effect in MPG (possibly it reduced my MPG).

Am I crazy or are these guys over simplifying stuff to sell parts?

You're not crazy. There is an optimal balance of sequential exhaust gases bursting out of the combustion chambers into the collective exhaust system.

 

[Marky Dissod]

The Great David Vizard said:
"Once the available flow exceeds about 2.2 cfm per hp, the gains possible by increasing muffler capacity drop to less than 1 percent."
… 2.2 cfm per horsepower …
A 2.75” Single is good for a 310hp engine with zero loss …
A 3.00” Single is good for a 370hp engine with zero loss …
(Why would the numbers for a 3.25" single be absent?)
A 3.50” Single is good for a 503hp engine with zero loss …
A 4.00” Single is good for a 657hp engine with zero loss …

However, each side of a Dual system only needs to flow 1.1cfm per horse, so …
A 2.25” Dual is good for a 457hp engine with zero loss …
A 2.50” Dual is good for a 513hp engine with zero loss …
(Why would the numbers for a 2.75" dual be absent?)
A 3.00” Dual is good for a 812hp engine with zero loss …

Oh, by the way, the Great David Vizard also said:
"The first misconception that needs to be cleared up is that a header relieves backpressure,
but a certain amount of backpressure is needed for optimum performance.
Just the opposite is true.
A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system.
When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder.
This is what the term 'scavenging' means."

 

[Marky Dissod]

The above numbers, however, also assume zero kinks crimps or folds.
If the tube / pipe is less than ideally 'mandrel-curved', it might help to go one size higher.

 

[Matthew Jeschke]

@Marky Dissod awesome detail. I've been kicking around idea of a Borla or Gibson exhaust. There's SO much miss information on aftermarket parts though. It's really annoying. Banks has VERY good quality stuff but their claims are biased by their need to sell product. I called Gibson with some more technical questions and they quickly fell on their face unable to answer my questions so I never pulled the trigger. I'm almost better off fabricating my own exhaust. However, per your comment, @Joseph Garcia, I'm unable to calculate the harmonic at which I get optimal exhaust performance / scavenging.

@Pressureangle do you know of an exhaust modeling software? I'd like to understand how I might optimize my exhaust, but don't have the background to calculate that stuff myself. I keep running into people like those two videos that sell the story short.

I've noticed a few things logging data, charting mileage, and tuning. The 3rd gen small block doesn't much like under 2000 RPM. It tends to knock there under load. Really hard to tune out, been fiddling with TCC lockup. However, under 2000 RPM is best fuel economy... Maybe due to fact that 4th gear in lockup is around 75 MPH at 2000 rpm. I get around 15 to 16 MPG then (air resistance likely is a big factor). However, if I back down to 1500 RPM, more around 55 to 65 MPH 4th lockup I've seen over 20 MPG on one tank before... This is also reflected in the fuel tables for the tune. That's where the least fuel seems to be used. Over time the 1500ish RPM MPG seems to be around 17 MPG.... Still a marked improvement.

I don't have a EGT sensor. Perhaps getting heat out of the cylinder under load at 1500 RPM is a factor too?

These knuckleheads sell us short on bigger is better with their simplified exhaust talk to sell huge aftermarket pipes. There's so much more!
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.
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I've a few other improvements I'd like to do but probably will never get around to due to cost... 205cc AFR heads (bit bigger than stock but claim higher vacuum at low RPM). And I'm stuck on gapless rings and dimpled Speed of Density pistons (damn things are $2000 though).

If I had a shop / bigger work area, I'd get into carbon fiber fab (I've done a little in past). Then mold new doors, esp rear hatch. Damn things is a boat anchor. These trucks are STUPID heavy.

 

[Marky Dissod]

Unless your horsepower peak exceeds 6000RpM, you're unlikely to need to worry about pressure wave / resonance tuning,
because our 4stroke engines don't get to take enough advantage of the 5th stroke (intake, squeeze, bang, exhaust, overlap?)
Stick with focusing on pipe diameter & sexy mandrel curves.
Everyone's claims are biased on selling product, so the question becomes, what are they trying to sell?
Sexy acoustics are not always the same as power.

Both those videos do a very good job of simplifying basics while hinting that there's more to it.

 

[Doubeleive]

I've been super confused by these YouTubers like Banks and MotoIQ saying NO backpressure is the universally optimal for an engine. This leaves me beyond confused. I've had questions on both these videos and nobody answers them. Their explanation is overly simplistic and doesn't seem to take into account anything of exhaust tuning or exhaust scavenging.

I had the cat back exhaust fall off my truck a few times now (I suck at welding). Every time it does my gas mileage utterly and miserably sucks and the power noticeably drops. I'd always assumed there was an optimal RPM (pressure) the exhaust is tuned for. Otherwise, unless your running WOT the exhaust IS tuned to be somewhat helpful. Am I crazy?

They just talk about smooth bends, large pipes, and less obstructions (cats, mufflers etc) as if that's everything to exhaust tuning. If it was that simple all the manufacturers would put on a super large exhaust pipe to improve MPG & EPA ratings?

I bought a Banks CAI because of the fancy tests they showed in videos and one claiming it would boost my MPG. It didn't have any noticeable effect in MPG (possibly it reduced my MPG).

Am I crazy or are these guys over simplifying stuff to sell parts?

 

[Matthew Jeschke]

@Doubeleive watched the video. He eluded to something which speaks of what I observed, the air track / pump really is from intake to tip of exhaust. The smaller pipe reduced fuel consumption and had slightly better power under 3000 RPM (smaller pipes richened their mixture). They shrugged that observation off. Part of my goal fiddling with my truck is to improve part throttle performance and efficiency. I wonder if there's a way to predict / model this stuff without buying parts and testing them. Wish I'd taken internal combustion engine course in college. Always wanted to take that.

 

[Matthew Jeschke]

The Great David Vizard said:
"Once the available flow exceeds about 2.2 cfm per hp, the gains possible by increasing muffler capacity drop to less than 1 percent."
… 2.2 cfm per horsepower …
A 2.75” Single is good for a 310hp engine with zero loss …
A 3.00” Single is good for a 370hp engine with zero loss …
(Why would the numbers for a 3.25" single be absent?)
A 3.50” Single is good for a 503hp engine with zero loss …
A 4.00” Single is good for a 657hp engine with zero loss …

However, each side of a Dual system only needs to flow 1.1cfm per horse, so …
A 2.25” Dual is good for a 457hp engine with zero loss …
A 2.50” Dual is good for a 513hp engine with zero loss …
(Why would the numbers for a 2.75" dual be absent?)
A 3.00” Dual is good for a 812hp engine with zero loss …

Oh, by the way, the Great David Vizard also said:
"The first misconception that needs to be cleared up is that a header relieves backpressure,
but a certain amount of backpressure is needed for optimum performance.
Just the opposite is true.
A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system.
When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder.
This is what the term 'scavenging' means."

Dang this David Vizard guy is cool. I'm going to have to watch his stuff!

 

[Marky Dissod]

The smaller pipe reduced fuel consumption and had slightly better power under 3000 RpM (smaller pipes enriched the mixture).
They shrugged that observation off. Part of my goal fiddling with my truck is to improve part throttle performance and efficiency.

They shrugged that observation off because their goal was peak power, and sized their pipes accordingly.
Ever notice that most dyno charts don't even begin under 3000RpM?
If you chose pipes 'one size too small for peak power' - only ONE, not two - that'd bias your engine to make more torque/power UNDER the torque peak.
1.625" - 1.750" headers, for example ...

 

[Matthew Jeschke]

Just watched the video on David Vizard and gives me a bit more to go on! Now this guy's an engineer!

I could use his findings 2.3 CFM / HP, then look at engine flow based off of RPM and operating range to figure out a tube length, diameter, etc for lossless pipes at that operating range.

I'm assuming if I go much larger at that point I'd start to loose inertia and end up tuning more power at higher RPM at that point? I don't have any models / formulas to go off of to figure out the inertia (when it starts to peak / fall off).

 

[Doubeleive]

my Denali has 3.5" factory
I put 3.5" on my previous trucks, always sounded good anyway.
I generally have my foot in it a lot, so any low rpm power it takes is not really a problem for me

 

[Matthew Jeschke]

Interesting, if the factory put 3.5" it's probably optimized for part throttle. What year was your Denali? I forget...

 

[swathdiver]

Matt, I think the scavenging effect, how the exhaust is tuned is more important than backpressure. Well, you don't want any backpressure.

The 2011+ 6.2s in the SUVs and Pickups use 3.5" Y-Pipes and Exhaust Systems. These are 400 hp motors and if your engine was built anywhere close to that, that's what I'd go with.

 

[Doubeleive]

Interesting, if the factory put 3.5" it's probably optimized for part throttle. What year was your Denali? I forget...

2012

 

[Marky Dissod]

Just watched the video on David Vizard and gives me a bit more to go on! Now this guy's an engineer!

I could use his findings 2.3 CFM / HP, then look at engine flow based off of RpM and operating range to figure out a tube length,
diameter, etc for lossless pipes at that operating range.

I'm assuming if I go much larger at that point I'd start to loose inertia and end up tuning more power at higher RpM at that point?
I don't have any models / formulas to go off of to figure out the inertia (when it starts to peak / fall off).

2.2CFM not 2.3 ... and assume if you go ANY larger at any point, you'll lose inertia.

I had the cat back exhaust fall off my truck a few times now (I suck at welding).
Every time it does my gas mileage utterly and miserably sucks and the power noticeably drops.

This is because colder atmosphere is quite a restriction to hot exhaust gasses.

 

[ZKWBQD]

I've been super confused by these YouTubers like Banks and MotoIQ saying NO backpressure is the universally optimal for an engine. This leaves me beyond confused. I've had questions on both these videos and nobody answers them. Their explanation is overly simplistic and doesn't seem to take into account anything of exhaust tuning or exhaust scavenging.

I had the cat back exhaust fall off my truck a few times now (I suck at welding). Every time it does my gas mileage utterly and miserably sucks and the power noticeably drops. I'd always assumed there was an optimal RPM (pressure) the exhaust is tuned for. Otherwise, unless your running WOT the exhaust IS tuned to be somewhat helpful. Am I crazy?

They just talk about smooth bends, large pipes, and less obstructions (cats, mufflers etc) as if that's everything to exhaust tuning. If it was that simple all the manufacturers would put on a super large exhaust pipe to improve MPG & EPA ratings?

I bought a Banks CAI because of the fancy tests they showed in videos and one claiming it would boost my MPG. It didn't have any noticeable effect in MPG (possibly it reduced my MPG).

Am I crazy or are these guys over simplifying stuff to sell parts?

Correct, if you eliminate your entire exhaust system, your engine will give you a bit more horsepower. That's why race cars don't have mufflers. However, when you drive around with a noisy car, everyone hates you, especially your neighbors.

 

[Matthew Jeschke]

@ZKWBQD I eliminated my muffler on my 1994 Z28 can speak from experience on neighbor thing lol. They always could tell when I was coming and going.

The MotoIQ guy got back with me. I think his video only played half way through. I re-watched it and he goes over pipe length and exhaust velocity. He recommended a book but looks like it's for turbo vehicles.

Still trying to find some info on pipe length and diameter calculations... I think David Vizard may have some of that covered but is a bit of a different perspective CFM / HP for zero loss system.

 

[homesick]

Correct, if you eliminate your entire exhaust system, your engine will give you a bit more horsepower. That's why race cars don't have mufflers. However, when you drive around with a noisy car, everyone hates you, especially your neighbors.

I put headers on my Camaro, in '78. Driving that thing, with open headers, to the muffler shop was some of the best fun ever. Just sitting in the driveway, idling, had the whole neighborhood buzzing and vibrating.

It was great.

joe