Back Pressure
06-22-2011, 07:31 PM
#21
Le Mans Master
...... Next time I go to the track I'm gonna take a bag of potatoes and cram 'em into the tailpipes 'til that 'ol backpressure sends me flyin' down the strip ................................. 
06-22-2011, 08:00 PM
#22
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Having build a few exhaust systems when I was working for a shop and doing some research at the time...
Needing back-pressure is a myth. The key is exhaust gas velocity. Too small a pipe and you create back-pressure and you are choking the engine. Too large and the gas cools and slows down as you are trying to get the gas out. As stated above the header does a lot of the scavenging of exhaust gases and can actually help pull gasses out of the combustion chamber as when the exhaust valve is open. On the flip side a poorly designed header can create a reversion and hold exhaust gas in the combustion chamber.
The hot exhaust gas is moving in pulses through the pipe. If it's properly designed the vacuum created behind one pulse will help pull another behind it. Even the best flowing catalytic converter will add an obstruction to the exhaust and slow things down. It's also a matter of what works best all around, a 3" pipe might work great for wide open throttle between 5k and 7k rpm and be great on a track car, but if you have a street car, you may be better with 2.5" to keep drive-ability down low and in the mid-range.
The early comment about Cats being needed because of oxygen sensors is because of how the ECU handles adjusting fuel trims to meet emissions requirements. If the computer doesn't think the catalytic converter is functioning properly it will throw a Check Engine Light. The engine doesn't need the Cat, the computer does (and the environment prefers it, haha).
Needing back-pressure is a myth. The key is exhaust gas velocity. Too small a pipe and you create back-pressure and you are choking the engine. Too large and the gas cools and slows down as you are trying to get the gas out. As stated above the header does a lot of the scavenging of exhaust gases and can actually help pull gasses out of the combustion chamber as when the exhaust valve is open. On the flip side a poorly designed header can create a reversion and hold exhaust gas in the combustion chamber.
The hot exhaust gas is moving in pulses through the pipe. If it's properly designed the vacuum created behind one pulse will help pull another behind it. Even the best flowing catalytic converter will add an obstruction to the exhaust and slow things down. It's also a matter of what works best all around, a 3" pipe might work great for wide open throttle between 5k and 7k rpm and be great on a track car, but if you have a street car, you may be better with 2.5" to keep drive-ability down low and in the mid-range.
The early comment about Cats being needed because of oxygen sensors is because of how the ECU handles adjusting fuel trims to meet emissions requirements. If the computer doesn't think the catalytic converter is functioning properly it will throw a Check Engine Light. The engine doesn't need the Cat, the computer does (and the environment prefers it, haha).
O2s are designed with a certain amount of heat in mind in the pipe. The gases need to get to a certain level, like 600F, for the voltage reading to stabilize from the sensor. If that heat isnt there, say from too big a pipe that scavenges the gas out quickly or lack of obstruction (same difference), the O2 readings force the ECM into open loop, and that kills the mileage for part throttle lugging around.
If the chip is retuned with these things in mind, the mileage will go back up and in my case even outperform its original levels.
Last edited by vader86; 06-22-2011 at 08:05 PM.
06-22-2011, 10:35 PM
#23
Le Mans Master
In regards to mileage, my 383 with 400+rwhp/430+rwtq still got 28mpg (there) and 31mpg (back) with 3.73s and a zf6 last weekend on a 200 mile journey each way. Home tuned.
Last edited by USAsOnlyWay; 06-22-2011 at 10:47 PM.
06-22-2011, 11:22 PM
#24
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not sayin back pressure in every instance, but its where stock engines get low end torque, among other things, you will gain power by opening up a restrictive exhuast, but when you get to the point where the intake wont keep up with it, you start losing your bottom end.
It's my guess that this whole "backpressure helps low end torque" myth started way back in the early carburetor days when if someone reduced the exhaust restriction, frequently he lost some power. There wasn't some magical power thing happening because a bunch of exhaust gas got trapped in the cylinders (due to the restrictive exhaust). Carburetors meter out fuel to the cylinders in two ways. Primarily when the air is rushing downward through the carb towards the intake valve, and secondarily by when the intake manifold air gets momentarily "burped back" up the carb venturi when the intake valve opens (releasing the residual cylinder pressure, consisting of burnt gas, back into the intake manifold). When the burp happens, air that just milliseconds earlier pulled fuel out of the venturi, reverses direction and passes through the venturi again, gathering fuel again. Once the piston starts to go down, creating suction, the twice-fueled air passes the venturi again getting additional fuel. This happens all the time and is not a big deal as long as you have correctly jetted your carb. Now if you reduce the exhaust restriction, you reduce the burp magnitude. This reduces the volume/mass amount of air that gets fueled multiple times, leaning out the mixture. When you lean things out (say from 12.5A/F to 15 A/F) you're going to lose power. A lot of guys misread what was going on in the engine, and passed this misinformation on to succeeding generations. Had WBO2 sensors been commonly available back then, as they are today, the "tuners" would have been able to measure what was actually happening. But technology most often isn't around as soon as it could be used or appreciated. Back to the early tuner: If the carburetor jetting was increased after the exhaust upgrade (getting the A/F ratio back to the 12.5 target) the driver would've noticed increased power&torque at all RPMs due to the increased VE caused by better cylinder cleaning/scavenging.
That's it in a simple nutshell, from a former Alaska resident (where I bought my '69).
06-22-2011, 11:41 PM
#25
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not sayin back pressure in every instance, but its where stock engines get low end torque, among other things, you will gain power by opening up a restrictive exhuast, but when you get to the point where the intake wont keep up with it, you start losing your bottom end.
06-23-2011, 11:28 AM
#27
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Guys, I'm not convinced that exhaust velocity is the magic thing here (regarding post-header exhaust). To get a fluid to go quickly through a pipe you need pressure. This means there is going to be a pressure drop across the length of the pipe. The head-pressure initiating this flow is called, in this topic, backpressure. We all agree that is not desirable.
I think we can also all agree that the same amount of fuel/air mass that goes into an engine has to go out of it. As long as out equals in, why do I care about the exit speed? We can all picture in our mind the idea of exhaust flow where the gas going through a "small" pipe has to go faster than the same gas going through a "large" pipe, for the same mass per second rate. Look at any engineering textbook regarding fluid flow, and the formulas show that the pressure drop is proportional to the pipe/tube length (makes sense) , to the square of the flow rate, and inversely proportional to the diameter. I think we can all envision that a larger pipe would reduce the proportion of the exhaust gas that is slowed by "wall friction", agreeing with the formula.
We can all make our own choices for our exhaust system. Obviously I have my own preferences.
I think we can also all agree that the same amount of fuel/air mass that goes into an engine has to go out of it. As long as out equals in, why do I care about the exit speed? We can all picture in our mind the idea of exhaust flow where the gas going through a "small" pipe has to go faster than the same gas going through a "large" pipe, for the same mass per second rate. Look at any engineering textbook regarding fluid flow, and the formulas show that the pressure drop is proportional to the pipe/tube length (makes sense) , to the square of the flow rate, and inversely proportional to the diameter. I think we can all envision that a larger pipe would reduce the proportion of the exhaust gas that is slowed by "wall friction", agreeing with the formula.
We can all make our own choices for our exhaust system. Obviously I have my own preferences.
06-23-2011, 12:54 PM
#28
Instructor
Is it possible that the velocity component is most helpful to reduce reversion? As was previously stated, the exhaust is a pulsing flow, if the velocity is high, the exhaust mass has greater inertia with which to reduce reversion. If the velocity is too high, (small pipe) the mass flow is reduced, lessening the inertia, and allowing greater reversion. Just a guess from someone who doesn't know.
06-23-2011, 03:57 PM
#29
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St. Jude Donor '03-'04-'05-'06-'07-'08-'09-'10-'11-'12-'13-'14-'15-'16-'17-'18-'19-'20-'21-'22
'23-'24
FWIW, here's a real world dyno comparison.........
I have a substandard exhaust system, I've always known that. I wanted to get an idea how much I could gain so I had my car strapped to a dyno a few years ago. I ran back to back, first thru my exhaust (Hookers into a 3" RT Cat, then out thru 2 1/2" Flowmasters); then dropped the exhaust and ran again straight out of my collectors. I gained 20 RWHP and increased the torque by 5lbs by dropping the exhaust. It sure proved to me that my exhaust system sucked (it still does) but what surprised me was the increase in torque. I now believe backpressure is an urban myth as frequently mentioned above. At least as far as WOT is concerned.
One of these days I'll actually get a decent exhaust on my car........ especially if I ever hook up the N20 again
I have a substandard exhaust system, I've always known that. I wanted to get an idea how much I could gain so I had my car strapped to a dyno a few years ago. I ran back to back, first thru my exhaust (Hookers into a 3" RT Cat, then out thru 2 1/2" Flowmasters); then dropped the exhaust and ran again straight out of my collectors. I gained 20 RWHP and increased the torque by 5lbs by dropping the exhaust. It sure proved to me that my exhaust system sucked (it still does) but what surprised me was the increase in torque. I now believe backpressure is an urban myth as frequently mentioned above. At least as far as WOT is concerned.
One of these days I'll actually get a decent exhaust on my car........ especially if I ever hook up the N20 again
06-23-2011, 11:44 PM
#30
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Guys, I'm not convinced that exhaust velocity is the magic thing here (regarding post-header exhaust). To get a fluid to go quickly through a pipe you need pressure. This means there is going to be a pressure drop across the length of the pipe. The head-pressure initiating this flow is called, in this topic, backpressure. We all agree that is not desirable.
I think we can also all agree that the same amount of fuel/air mass that goes into an engine has to go out of it. As long as out equals in, why do I care about the exit speed? We can all picture in our mind the idea of exhaust flow where the gas going through a "small" pipe has to go faster than the same gas going through a "large" pipe, for the same mass per second rate. Look at any engineering textbook regarding fluid flow, and the formulas show that the pressure drop is proportional to the pipe/tube length (makes sense) , to the square of the flow rate, and inversely proportional to the diameter. I think we can all envision that a larger pipe would reduce the proportion of the exhaust gas that is slowed by "wall friction", agreeing with the formula.
We can all make our own choices for our exhaust system. Obviously I have my own preferences.
I think we can also all agree that the same amount of fuel/air mass that goes into an engine has to go out of it. As long as out equals in, why do I care about the exit speed? We can all picture in our mind the idea of exhaust flow where the gas going through a "small" pipe has to go faster than the same gas going through a "large" pipe, for the same mass per second rate. Look at any engineering textbook regarding fluid flow, and the formulas show that the pressure drop is proportional to the pipe/tube length (makes sense) , to the square of the flow rate, and inversely proportional to the diameter. I think we can all envision that a larger pipe would reduce the proportion of the exhaust gas that is slowed by "wall friction", agreeing with the formula.
We can all make our own choices for our exhaust system. Obviously I have my own preferences.
Chapter Two-- applying your knowlege of fluid dynamics -- in redneck terms.
(Keep it simple St--id. with lots of recognizeable pitures)
1. The venturi effect: does it apply in any way to a exaust system when a "tuned header" is developed for a four stroke engine?
Consider: in your two stroke dirt bike the expansion chamber of a properly tuned exaust will make a very noticable "power band".
2. What is the ratio of expansion on volume of a 14.7 fuel to air ratio when heated from 90 deg c to 900 deg c?
and how does that effect volume of gas in vs volume of gas out?
Consider: what is the effect of the rapid expaision of gasses in a gas turbine or rocket motor .... or..... your Likker stil = (Liquor Stil)
extra credit: how many times does water expand going from 90 deg c to 900 dec C..
(what happens when you build too big of a fire under your pot of beans... why did the beans boil out of the pot????)
3. For the water utility guys. Thinking of line loss, pipe size and head pressure....
Explain the "water hammer effect" and how would it relate to the intake filling of a cylinder on your next high performance engine build?
and...Can you explain volumetric efficiency?
Consider: (When your local volunteer fire department abruptly cuts off a flowing fire hydrant... why do all the houses in the local area spring leaks and/or break water lines?)
4. Could you apply the physics of siphoning gas out of you roommates z28 gas tank in college to a six figure job of increasing the output of Dale jr's motor to help in his quest to win a cup title??. Could you apply that to his exaust system?
Do you think there is a little more than just moving fluid through a pipe going on here.... thats why we call it exaust velocity.... the dynamics in the real application seldom fall under one simple formula.
Square that? or just go talk to your local speed shop and quit talking to your neighbor who still thinks his grand am is a mustang eater because he put on a K&N filter, installed platium plugs, and
put 104 additive in his tank.... and thinks SBC need a little back pressure to run right...Remember bench racers and mouthy beer are a leathal combination.
http://www.youtube.com/watch?v=O-74Z...eature=related
06-23-2011, 11:53 PM
#31
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Is it possible that the velocity component is most helpful to reduce reversion? As was previously stated, the exhaust is a pulsing flow, if the velocity is high, the exhaust mass has greater inertia with which to reduce reversion. If the velocity is too high, (small pipe) the mass flow is reduced, lessening the inertia, and allowing greater reversion. Just a guess from someone who doesn't know.
06-23-2011, 11:56 PM
#32
Its all about having a "TUNED" combination( from the air cleaner or air inlet to the Exhaust header outlet or tailpipe outlet for your intended application.
There are excellent articles from Smokey Yunick & David Vizzrrd on this topic from 20 years ago.
Published papers.
I believe in a free flowing exhaust too.
Works best with a camshaft overlap of at least 60 degrees I think.
Ed Iskendarien's 5th cycle theory.
Without pointing fingers here tonight,
"The Kadance theory" has been proven not to be 100 % correct.
Finate Amplitude waves at work.
They are invisible to the human eye.
But present inside of a running 2 or 4 cycle internal combustion engine.
Have to read on your own to get a good understanding.
There are excellent articles from Smokey Yunick & David Vizzrrd on this topic from 20 years ago.
Published papers.
I believe in a free flowing exhaust too.
Works best with a camshaft overlap of at least 60 degrees I think.
Ed Iskendarien's 5th cycle theory.
Without pointing fingers here tonight,
"The Kadance theory" has been proven not to be 100 % correct.
Finate Amplitude waves at work.
They are invisible to the human eye.
But present inside of a running 2 or 4 cycle internal combustion engine.
Have to read on your own to get a good understanding.
06-24-2011, 12:13 AM
#34
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Very well put!... Now move to the next chapter and discuss the following:
Chapter Two-- applying your knowlege of fluid dynamics -- in redneck terms.
(Keep it simple St--id. with lots of recognizeable pitures)
1. The venturi effect: does it apply in any way to a exaust system when a "tuned header" is developed for a four stroke engine?
Consider: in your two stroke dirt bike the expansion chamber of a properly tuned exaust will make a very noticable "power band".
2. What is the ratio of expansion on volume of a 14.7 fuel to air ratio when heated from 90 deg c to 900 deg c?
and how does that effect volume of gas in vs volume of gas out?
Consider: what is the effect of the rapid expaision of gasses in a gas turbine or rocket motor .... or..... your Likker stil = (Liquor Stil)
extra credit: how many times does water expand going from 90 deg c to 900 dec C..
(what happens when you build too big of a fire under your pot of beans... why did the beans boil out of the pot????)
3. For the water utility guys. Thinking of line loss, pipe size and head pressure....
Explain the "water hammer effect" and how would it relate to the intake filling of a cylinder on your next high performance engine build?
and...Can you explain volumetric efficiency?
Consider: (When your local volunteer fire department abruptly cuts off a flowing fire hydrant... why do all the houses in the local area spring leaks and/or break water lines?)
4. Could you apply the physics of siphoning gas out of you roommates z28 gas tank in college to a six figure job of increasing the output of Dale jr's motor to help in his quest to win a cup title??. Could you apply that to his exaust system?
Do you think there is a little more than just moving fluid through a pipe going on here.... thats why we call it exaust velocity.... the dynamics in the real application seldom fall under one simple formula.
Square that? or just go talk to your local speed shop and quit talking to your neighbor who still thinks his grand am is a mustang eater because he put on a K&N filter, installed platium plugs, and
put 104 additive in his tank.... and thinks SBC need a little back pressure to run right...Remember bench racers and mouthy beer are a leathal combination.
http://www.youtube.com/watch?v=O-74Z...eature=related
06-24-2011, 12:23 AM
#35
Dave D. understands best out of anyone here on C4.
He signed out.
If you eliminate mufflers from a blower car or Turbo car,
More available boost can be had at WOT.
Why guys are always installing large 5 " inch single exhaust systems on Modern Chevy Duramax & Ford 6.0 diesel trucks & Dodge Cummins.
I know everyone around the USA has witnessed that in the last 10 years.
They run pretty fast too for a 6,000lb to 10,000 Vehicles.
Just citing a daily example of free flowing exhaust at work everywhere today on the roads in the USA.
He signed out.
If you eliminate mufflers from a blower car or Turbo car,
More available boost can be had at WOT.
Why guys are always installing large 5 " inch single exhaust systems on Modern Chevy Duramax & Ford 6.0 diesel trucks & Dodge Cummins.
I know everyone around the USA has witnessed that in the last 10 years.
They run pretty fast too for a 6,000lb to 10,000 Vehicles.
Just citing a daily example of free flowing exhaust at work everywhere today on the roads in the USA.
