No Exhaust Backpressure = Better performance?

[Matthew Jeschke]

Startup of Truck after I built the 6.0 and had the exhaust disconnected

start_up (Original).mp4

drive.google.com

 

[Matthew Jeschke]

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

I'd never given that much thought! Makes since! Kind of like the highly energetic molecules hitting a bock wall.

 

[Matthew Jeschke]

Studying the parts catalog and comparing to the chart...

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."

Looks like the heavy half GMT800. That is the 1/2 ton that came with 6.0L had dual exhaust going back into a single muffler. This is what I called Gibson about and they said they didn't have anything like that.

I have the section of this exhaust in my attic. I could only immediately in front of the muffler back. When the junkyard pulled the motor they cut it there and I couldn't get the rest of it. I pulled it off a 2004 Escalade... I think only one year had this exhaust. Not sure why they dropped it?

Anybody fiddled with it?

 

[Marky Dissod]

Don't know how big those pipes are, but I'd bet their bends are not terribly restrictive.
Also I'd be hesitant (but not summarily opposed) to modernize the exhaust, for fear of moving the cats closer to the engine
Forced to guess, GM's pipes are SLIGHTLY undersized, but the 2-into-1 muffler is where the most power can be released.

In other words, instead of replacing the entire exhaust system, just improve the muffler & tailpipe(s?).

Or, consider the 8.1L 2500 exhaust system?

 

[exp500]

In the 70's and 80's there was alot about the crossover pipe effect on scavenging, Proper size and placement was difficult especially on 4x4's.

 

[Matthew Jeschke]

@exp500 I was curious about that. My C5 Corvette has a cross over pipe while the new corvette looks like it has the chamber mentioned in David Vizard's video (he mentioned them using his concept).

As for our trucks, I kind of wondered if there was something inside the muffler acting a bit like a cross over. Even on the dual exhaust it comes back to a single muffler.

Another thing that I notice cannot be good, is the volume on the passenger side bank (down pipe to collector) is much larger to cross to the passenger side frame rail. They didn't do anything to balance out the volume and I'm not sure why (maybe the harmonics are still tuned ~ wavelength works?). I'd have thought they'd have enlarged the cross over a bit to balance the flow idk?

 

[Marky Dissod]

In the 70's and 80's there was alot about the crossover pipe effect on scavenging.
Proper size and placement was difficult especially on 4x4's.

@exp500 I was curious about that.
My C5 Corvette has a crossover pipe while the new corvette looks like it has the chamber mentioned in David Vizard's video (he mentioned them using his concept).

As for our trucks, I kind of wondered if there was something inside the muffler acting a bit like a crossover. Even on the dual exhaust it comes back to a single muffler.

LT1 Caprice enthusiasts took note that the Ford Crown Victoria started using an h-pipe crossover, and started experimenting with h- & x-pipe crossovers,
although there are obvious limitations as to where the crossover can be placed to leave enough room for driveshaft droop.
The Holden Caprice basically uses a 2-in,2-out oval body muffler AS A CROSSOVER (that can neither be called an h- or x-pipe).

Another thing that I notice cannot be good, is the volume on the passenger side bank (downpipe to collector) is much larger to cross to the passenger side frame rail.
They didn't do anything to balance out the volume and I'm not sure why (maybe the harmonics are still tuned ~ wavelength works?).
I'd have thought they'd have enlarged the cross over a bit to balance the flow idk?

If both pipes are of the same internal diameter / 'cross section', the longer pipe will sing a lower note when air flows through it compared to the shorter one,
but neither has an inherent flow RATE advantage (without harmonics tuning somewhere else).

 

[Pressureangle]

@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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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.

There are quite a few engine dynamics software packages available, but for any of them more competent than a lawn mower you'll pay a fortune 500 rate upfront, and licencing after that. So no, nothing serious available to the general public, like freeware. Best to simply study the results and recommendations of genuine experts, and salt that with anecdotal experience of the end users. This forum is very good for that.

You sound like a 'rabbit hole' guy, as I am (was). Don't forget the distinction between hobbies, tools, and careers.

 

[Mudsport96]

@exp500

Another thing that I notice cannot be good, is the volume on the passenger side bank (down pipe to collector) is much larger to cross to the passenger side frame rail. They didn't do anything to balance out the volume and I'm not sure why (maybe the harmonics are still tuned ~ wavelength works?). I'd have thought they'd have enlarged the cross over a bit to balance the flow idk?

While pipe balancing may seem to be something you would think is important... it is really not. SOME bends and crimps will hurt power, but if the pipe is sized correctly a bend will not hurt flow.


Dented pipe test

 

[Mudsport96]

Want to be baffled by engineering masterpieces? Ford v10 exhaust in the super duty trucks....

off of the manifolds 2.5 inches, reduced to 2 inches, before entering a 3 inch pipe going back to the rest of the exhaust.

 

[Marky Dissod]

My GUESS: improve low-RpM power at the expense of / to discourage or prevent high-RpM use?

 

[Matthew Jeschke]

You sound like a 'rabbit hole' guy, as I am (was). Don't forget the distinction between hobbies, tools, and careers.

110% got my degrees in engineering. Stuff I work with during the day sounds cool but the really fun is when I tinker with my truck. Has been a bit of a mad pursuit to improve upon factory design / configuration. I think I was able to accomplish that with the transmission build but engine despite being WAY more powerful has been very tricky to produce a better tune (intake system, exhaust, calibration, etc) to beat the factory. I'd be certain the manufacturer / engineers used some really awesome modeling and validated their exhaust design with software like you mentioned. I'm going to talk to some of my mechanical engineering friends and see if they have any wisdom to share.

Don't know how big those pipes are, but I'd bet their bends are not terribly restrictive.
Also I'd be hesitant (but not summarily opposed) to modernize the exhaust, for fear of moving the cats closer to the engine
Forced to guess, GM's pipes are SLIGHTLY undersized, but the 2-into-1 muffler is where the most power can be released.

In other words, instead of replacing the entire exhaust system, just improve the muffler & tailpipe(s?).

Or, consider the 8.1L 2500 exhaust system?

Reflecting on the David Vizard stuff you shared and it became apparent to me there's likely some design differences between the 6.0L exhaust and 5.3L / 4.8L exhausts. Such would be advantageous, although I see some of the catalog items are the same exhaust parts, a few are not. Given I swapped out my 5.3L for a cammed 6.0L it maybe prudent to find another one of the 6.0L exhausts to swap in.... I'd be really curious to see what the inside of the muffler looks like too but most of these companies aren't sharing that info. David Vizard said Walker (licensed?) and sold his exhaust design for a while. Wasn't too thrilled with sales team at Gibson, although maybe was just who answered the phone when I called.

While pipe balancing may seem to be something you would think is important... it is really not. SOME bends and crimps will hurt power, but if the pipe is sized correctly a bend will not hurt flow.


Dented pipe test

That's good to know. Kind of makes since as if there was a large difference I'd see one side being leaner than the other when I log data. They tend to run pretty similar.

Want to be baffled by engineering masterpieces? Ford v10 exhaust in the super duty trucks.... View attachment 466711off of the manifolds 2.5 inches, reduced to 2 inches, before entering a 3 inch pipe going back to the rest of the exhaust.

Looks like somebody tried it! I was curious, looks hard to make even for the factory. What year super duty is that off of? I looked up a 2005 F-250 and didn't see that cross pipe. Dang that's wild!

My GUESS: improve low-RpM power at the expense of / to discourage or prevent high-RpM use?

Maybe?

 

[Mudsport96]

Looks like somebody tried it! I was curious, looks hard to make even for the factory. What year super duty is that off of? I looked up a 2005 F-250 and didn't see that cross pipe. Dang that's wild!

Factory 99-02 exhaust y pipe

 

[Marky Dissod]

Reflecting on the David Vizard stuff you shared; it became apparent to me there's likely some design differences between the 6.0L, vs 5.3L / 4.8L exhausts.
Such would be advantageous, although I see some of the catalog items are the same exhaust parts, a few are not.
Given I swapped out my 5.3L for a cammed 6.0L it maybe prudent to find another one of the 6.0L exhausts to swap in ...
I'd be really curious to see what the inside of the muffler looks like too, but most of these companies aren't sharing that info.
David Vizard said Walker (licensed?) and sold his exhaust design for a while.
Wasn't too thrilled with sales team at Gibson, although maybe was just who answered the phone when I called.

Things to consider about the 4.8L / 5.3 L vs 6.0L exhausts:
4.8L & 5.3L were rated @ under 300horse. LQ4 was rated @ 300horse or 330horse. LQ9 was rated @ 345 horse.
Just looking at those ratings, I'd bet that the 4.8L 5.3L & at least the early LQ4 shared most of their exhaust stuff, with the LQ9 maybe getting some better stuff.
Realistically speaking, if you want more from your exhaust, either look at Gen4 6.0L stuff, or Gen4 6.2L stuff in terms of pipe sizing.
Remember what Vizard said about mufflers - you want straight-thru perforated pipe(s) surrounded by sound insulation stuffing.
If you can't see thru from one end to the other, forget it and move on.

 

[Doubeleive]

Don't know how big those pipes are, but I'd bet their bends are not terribly restrictive.
Also I'd be hesitant (but not summarily opposed) to modernize the exhaust, for fear of moving the cats closer to the engine
Forced to guess, GM's pipes are SLIGHTLY undersized, but the 2-into-1 muffler is where the most power can be released.

In other words, instead of replacing the entire exhaust system, just improve the muffler & tailpipe(s?).

Or, consider the 8.1L 2500 exhaust system?

I asked the muffler shop when I last had a cat replaced if they could move it back an inch or 2 from the engine, I figure anything helps from getting them burnt, I think having the 3rd (non-monitored) cat hollowed out also helped get some of the heat out.

 

[Mudsport96]

Things to consider about the 4.8L / 5.3 L vs 6.0L exhausts:
4.8L & 5.3L were rated @ under 300horse. LQ4 was rated @ 300horse or 330horse. LQ9 was rated @ 345 horse.
Just looking at those ratings, I'd bet that the 4.8L 5.3L & at least the early LQ4 shared most of their exhaust stuff, with the LQ9 maybe getting some better stuff.
Realistically speaking, if you want more from your exhaust, either look at Gen4 6.0L stuff, or Gen4 6.2L stuff in terms of pipe sizing.
Remember what Vizard said about mufflers - you want straight-thru perforated pipe(s) surrounded by sound insulation stuffing.
If you can't see thru from one end to the other, forget it and move on.

I would agree with that except for one thing I know for sure. And trust me I love David Vizards information, it can be invaluable.



The 2002 8.1L Vortec (L18) engine is a cast iron 8.1-liter (496 cubic inch) V8 with 330-340 horsepower and 440-455 lb-ft of torque

2002 LQ4 is a 6.0L iron-block V8 engine from GM's LS-family, producing approximately 300-325 horsepower and 380-385 lb-ft of torque

i know for a fact because I worked on two identical 2002 gmc hd 2500s but one had a 6.0 and one had the 8.1, the exhaust is the same. The only difference is where the y pipe meets the block, the 6.0 y pipe has the ends cut and necked down and angled to meet the smaller outlets on the ls manifolds. Other than that the full exhaust system is identical. So obviously it flows enough that the 8.1 makes more power than the 6.0. Is it ideal? Probably not, or aftermarket systems wouldn't make even more power. But it can flow enough to support 70 lbft more than the lq4 made. So if a guy was inclined, and wanted make a little more power and keep it quiet enough for good highway manners. Find a 2500 burb exhaust and use that. It should flow plenty for a warmed over 5.3 or 6.0. And more than enough for just about any reasonable 4.8 that would go in a heavy suv.

 

[Matthew Jeschke]

@Mudsport96 dang that's good to know that the 8.1L is a very close fitment for Gen 3 SBC.

@Doubeleive what were the results? Have your cats lasted longer? I think if you have a heavy foot it'll burn them up pretty quick. I noticed fiddling with mine, that light driving doesn't heat them up too bad... but if you start getting into it more, WOW they heat up FAST. There's a good chance a bunch of the MPG is wasted on Cat Over Temp Protection to cool them. It'd be fun to turn that off on your tune and see what effect in temps your modification made. I believe the only reason factory puts them so close to engine is so they heat up faster on startup (or so Christian Koenigsegg eluded to in some of his stuff I watched).

I have another crazy idea to plate the exhaust and create a Thermoelectric Heat Pump cooler out of it. This has a cooling effect as the dissimilar metals create a current flow under heat. The byproduct is one side of the plating gets cooler, the other get hot (but no hotter than the exhaust gases). It also creates current flow. You can capture the electrons back into the charging circuit / electrical system... then measure the potential off of the heat pump to get your exhaust gas temperature. Would be pretty cool. You'd benefit from lower EGT, could potentially advance timing a bit more, and your cats would also be cooler, lasting longer and potentially less need for dumping gas during COT cycle.

 

[Doubeleive]

@Mudsport96 dang that's good to know that the 8.1L is a very close fitment for Gen 3 SBC.

@Doubeleive what were the results? Have your cats lasted longer? I think if you have a heavy foot it'll burn them up pretty quick. I noticed fiddling with mine, that light driving doesn't heat them up too bad... but if you start getting into it more, WOW they heat up FAST. There's a good chance a bunch of the MPG is wasted on Cat Over Temp Protection to cool them. It'd be fun to turn that off on your tune and see what effect in temps your modification made. I believe the only reason factory puts them so close to engine is so they heat up faster on startup (or so Christian Koenigsegg eluded to in some of his stuff I watched).

I have another crazy idea to plate the exhaust and create a Thermoelectric Heat Pump cooler out of it. This has a cooling effect as the dissimilar metals create a current flow under heat. The byproduct is one side of the plating gets cooler, the other get hot (but no hotter than the exhaust gases). It also creates current flow. You can capture the electrons back into the charging circuit / electrical system... then measure the potential off of the heat pump to get your exhaust gas temperature. Would be pretty cool. You'd benefit from lower EGT, could potentially advance timing a bit more, and your cats would also be cooler, lasting longer and potentially less need for dumping gas during COT cycle.

remains to be seen if anything has made a difference yet, but I have my fingers crossed I am probably right around 1 year since the last one was replaced

 

[Marky Dissod]

I would agree with that except for one thing I know for sure. And trust me I love David Vizards information, it can be invaluable.
The 2002 8.1L Vortec (L18) engine is a cast iron 8.1-liter (496 cubic inch) V8 with 330-340 horsepower and 440-455 lb-ft of torque
2002 LQ4 is a 6.0L iron-block V8 engine from GM's LS-family, producing approximately 300-325 horsepower and 380-385 lb-ft of torque

i know for a fact because I worked on two identical 2002 gmc hd 2500s but one had a 6.0L and one had the 8.1L, the exhaust is the same.
The only difference is where the y pipe meets the block, the 6.0L y-pipe has the ends cut and necked down and angled to meet the smaller outlets on the ls manifolds.
Other than that the full exhaust system is identical. So obviously it flows enough that the 8.1L makes more power than the 6.0L.
Is it ideal? Probably not, or aftermarket systems wouldn't make even more power. But it can flow enough to support 70 lbft more than the lq4 made.
So if a guy was inclined, and wanted make a little more power and keep it quiet enough for good highway manners. Find a 2500 burb exhaust and use that.
It should flow plenty for a warmed over 5.3 or 6.0. And more than enough for just about any reasonable 4.8 that would go in a heavy suv.

Or, consider the 8.1L 2500 exhaust system?

TL,DR much?

 

[Matthew Jeschke]

TL,DR much?

Yeah I hear ya.