Install a HF Cat for more Torque?
#11
03-11-2011, 11:10 PM
Champion
I can do better than that.. here is some exhaust theory from another thread, it isn't exactly what was asked, but its a good way to get the theory out there:
if an exhaust system doesn't restrict the flow of spent gasses, that means the piston- on it's exhaust stroke, doesn't meet near as much resistance when it heads north and it pushes that wad of spent air out and into the manifold/headers..
if the manifold/headers allow that wad of air (pulse) to pass unimpeded, and make room for the next pulse, that is better.. problems arise when the exhaust gets backed up and equalizes pressure too fast- which is why you want your exhaust to get HOT.. it creates a vacuum by creating an area of low pressure due to the pipes warming the air.. hopefully that draws the pulses out to the tip on a perpetual 'conveyor belt'.. if it does, you have a good scavenge going on.. if you don't- you have back pressure.. Back pressure allows the engine to rev faster, but by limiting the flow, the engine is revving but it's not pulling in (or expelling) as much air as it is capable of pumping.. THAT, robs power..
in such a condition, you think your getting more power out of the engine because the tach is climbing... not so.. think of it like this, which is just presented as a prop: an engine revving at 100% of it's capability (say, 6k rpm), but is drawing only 70% of the air it's capable of because the valves close before the vacuum in the cylinder is equalized on the intake stroke, your not producing but around 50% of the power the engine could produce - but revving is easier/faster.. your engine is lying to you...
if you imagine your cylinder is a hypodermic needle, and you violently pull the plunger downward, but cover the tip with your finger before it reaches it's full extension- you've created two things: the plunger will start to resist extending any further, and the vacuum in the tube will make the plunger want to return to the position where that air is equalized.. that 'help' given to the mechanical revolution of the crank allows the engine to rev a lot faster- but the force wasted trying to pull it down translates to loss of engine power, and to slighter compression (and weaker explosion) due to smaller volume of air in the cylinder.. not to mention that smaller volume of air has been laden with a charge of fuel that is too rich and leads to incomplete explosion.. leads to faster revs but loss of power..
the accompanying mods- cam, most notably, allows the valve to stay open long enough or create a large enough opening for enough air to get into and out of that cylinder..
what goes in has to get out just as efficiently, or you lose power trying to push air out of the cylinder on the exhaust stroke.. most exhaust valves are considerably smaller than intake- to create velocity, AND, a really hot charge.
the exhaust header/manifold has to create a passage large enough NOT to restrict the expelling air, and do it in time to NOT get caught up in traffic/bottleneck of the other cylinders on the same bank doing the same thing. This is why headers are superior to manifold.. each cylinder gets it's own corridor for it's exhaust stroke.. the larger (read: long tube vs. short tube) it's corridor, the more time the expelling air from the previous exhaust stroke has to clear the pulse before it bottlenecks or meets traffic from the other cylinders.. it still needs encouragement to get out of there though..
ceramic coating, or header wrap is intended to insulate the exhaust process- but not keep the rest of the engine bay cooler, but instead, to keep the exhaust manifold/header internally hotter.. the air coming out of that cylinder on the exhaust stroke is already hot, then it meets up with a corridor that is already hotter than it- which means it is drawn to it by means of a vacuum.. which means the engine is forcing the air out, and the exhaust system is pulling it out at the same time.. which clears the pulse a WHOLE LOT faster..
and then it meets it's first real obstacle- either the Y pipe (if you're running stock) or the catalytic converter.. which slows down the flow.. to their credit, cats gotta get really hot to do their job- which creates vacuum, which increases velocity, which almost negates the obstacle they create..
Y pipes though- especially badly designed, cram all that exhaust traffic into a single pipe- and backs up the flow much more than even cats do..
an X or a H pipe though, they don't do that.. they simply bridge the separate banks together into one- and when one banks pulse(s) slow because the air is only being pushed and not pulled, the other one chips in and gives it a tug with the trailing end of vacuum on it's last pulse.. in effect pulling the stalling air out behind it just before the next pulse is crammed into it from behind..
designing/tuning an exhaust system is tricky business, but it's just as important as intake.. diameter of pipes to allow the expulsion of air need to be big enough to accommodate the volume coming out, but small enough to warm enough to create the flow.. engines are all about flow- more air in AND more air out, more power produced.. the pipes have to be small enough to get hot enough to help pull the air out when the force of the exhaust stroke is spent..
exhaust done properly- doesn't hurt the engine at all.. I gotta disagree with true duals hurting low end.. it doesn't.. it generally dis-allows the tach from climbing as fast as a restrictive intake/exhaust, but that does not translate to loss of power.. not exactly anyway..
exhaust pulses have a leading edge of hot air and a trailing edge of cooler air- which is complicated because the leading edge of hot air is pushing a wad of cooler air in front of it to clear it out..
your cylinders fire in rhythm and force the pulse to the tip.. but each bank has a depressed area where nothing is happening, which in effect stops the flow momentarily.. that leaves you with a dense wall of cooler air the next leading edge has to force through- which doesn't allow the exhaust gasses to flow as efficiently as they could.
this, in addition to blockages, deflectors, or harsh bends in the pipes creates backpressure- which robs you of power because the engine is fighting to clear the obstacles each time a cylinder clears.. Backpressure is BAD.. BAD.. BAD.. Backpressure is NEVER good..
having a connection between the banks- be it a y, an x, or an H creates a momentary vacuum, or area of low pressure which the high pressure (leading edge) pulses are drawn towards.. this is the basics of scavenging... Scavenging is GOOD GOOD GOOD... many folks toss the term backpressure around when they really mean scavenging..
your engine is a big ol' air pump that uses gas as a catalyst to explode the a/f mix.. more air in, more air out, more power.. for the best efficiency, you could run without any exhaust or headers/manifold.. your engine won't last long though.. obvious reasons.. but it would easily clear the spent air..
if your exhaust pipe diameter is too restrictive, it slows the exhaust down too much and creates backpressure.. we've already determined that is bad.. If it is too open, you won't warm the pipes enough to create that magic vacuum (scavenging), and you'll have air pushing out of the heads and then just sitting there- with a dense trailing edge that the next pulse has to push out of the way.. which i hope explains why both statements (open exhaust is absolute best; too open is bad) are both true statements.
your cylinders fire in rhythm and force the pulse to the tip.. but each bank has a depressed area where nothing is happening, which in effect stops the flow momentarily.. that leaves you with a dense wall of cooler air the next leading edge has to force through- which doesn't allow the exhaust gasses to flow as efficiently as they could.
this, in addition to blockages, deflectors, or harsh bends in the pipes creates backpressure- which robs you of power because the engine is fighting to clear the obstacles each time a cylinder clears.. Backpressure is BAD.. BAD.. BAD.. Backpressure is NEVER good..
having a connection between the banks- be it a y, an x, or an H creates a momentary vacuum, or area of low pressure which the high pressure (leading edge) pulses are drawn towards.. this is the basics of scavenging... Scavenging is GOOD GOOD GOOD... many folks toss the term backpressure around when they really mean scavenging..
your engine is a big ol' air pump that uses gas as a catalyst to explode the a/f mix.. more air in, more air out, more power.. for the best efficiency, you could run without any exhaust or headers/manifold.. your engine won't last long though.. obvious reasons.. but it would easily clear the spent air..
if your exhaust pipe diameter is too restrictive, it slows the exhaust down too much and creates backpressure.. we've already determined that is bad.. If it is too open, you won't warm the pipes enough to create that magic vacuum (scavenging), and you'll have air pushing out of the heads and then just sitting there- with a dense trailing edge that the next pulse has to push out of the way.. which i hope explains why both statements (open exhaust is absolute best; too open is bad) are both true statements.
if an exhaust system doesn't restrict the flow of spent gasses, that means the piston- on it's exhaust stroke, doesn't meet near as much resistance when it heads north and it pushes that wad of spent air out and into the manifold/headers..
if the manifold/headers allow that wad of air (pulse) to pass unimpeded, and make room for the next pulse, that is better.. problems arise when the exhaust gets backed up and equalizes pressure too fast- which is why you want your exhaust to get HOT.. it creates a vacuum by creating an area of low pressure due to the pipes warming the air.. hopefully that draws the pulses out to the tip on a perpetual 'conveyor belt'.. if it does, you have a good scavenge going on.. if you don't- you have back pressure.. Back pressure allows the engine to rev faster, but by limiting the flow, the engine is revving but it's not pulling in (or expelling) as much air as it is capable of pumping.. THAT, robs power..
in such a condition, you think your getting more power out of the engine because the tach is climbing... not so.. think of it like this, which is just presented as a prop: an engine revving at 100% of it's capability (say, 6k rpm), but is drawing only 70% of the air it's capable of because the valves close before the vacuum in the cylinder is equalized on the intake stroke, your not producing but around 50% of the power the engine could produce - but revving is easier/faster.. your engine is lying to you...
if you imagine your cylinder is a hypodermic needle, and you violently pull the plunger downward, but cover the tip with your finger before it reaches it's full extension- you've created two things: the plunger will start to resist extending any further, and the vacuum in the tube will make the plunger want to return to the position where that air is equalized.. that 'help' given to the mechanical revolution of the crank allows the engine to rev a lot faster- but the force wasted trying to pull it down translates to loss of engine power, and to slighter compression (and weaker explosion) due to smaller volume of air in the cylinder.. not to mention that smaller volume of air has been laden with a charge of fuel that is too rich and leads to incomplete explosion.. leads to faster revs but loss of power..
the accompanying mods- cam, most notably, allows the valve to stay open long enough or create a large enough opening for enough air to get into and out of that cylinder..
what goes in has to get out just as efficiently, or you lose power trying to push air out of the cylinder on the exhaust stroke.. most exhaust valves are considerably smaller than intake- to create velocity, AND, a really hot charge.
the exhaust header/manifold has to create a passage large enough NOT to restrict the expelling air, and do it in time to NOT get caught up in traffic/bottleneck of the other cylinders on the same bank doing the same thing. This is why headers are superior to manifold.. each cylinder gets it's own corridor for it's exhaust stroke.. the larger (read: long tube vs. short tube) it's corridor, the more time the expelling air from the previous exhaust stroke has to clear the pulse before it bottlenecks or meets traffic from the other cylinders.. it still needs encouragement to get out of there though..
ceramic coating, or header wrap is intended to insulate the exhaust process- but not keep the rest of the engine bay cooler, but instead, to keep the exhaust manifold/header internally hotter.. the air coming out of that cylinder on the exhaust stroke is already hot, then it meets up with a corridor that is already hotter than it- which means it is drawn to it by means of a vacuum.. which means the engine is forcing the air out, and the exhaust system is pulling it out at the same time.. which clears the pulse a WHOLE LOT faster..
and then it meets it's first real obstacle- either the Y pipe (if you're running stock) or the catalytic converter.. which slows down the flow.. to their credit, cats gotta get really hot to do their job- which creates vacuum, which increases velocity, which almost negates the obstacle they create..
Y pipes though- especially badly designed, cram all that exhaust traffic into a single pipe- and backs up the flow much more than even cats do..
an X or a H pipe though, they don't do that.. they simply bridge the separate banks together into one- and when one banks pulse(s) slow because the air is only being pushed and not pulled, the other one chips in and gives it a tug with the trailing end of vacuum on it's last pulse.. in effect pulling the stalling air out behind it just before the next pulse is crammed into it from behind..
designing/tuning an exhaust system is tricky business, but it's just as important as intake.. diameter of pipes to allow the expulsion of air need to be big enough to accommodate the volume coming out, but small enough to warm enough to create the flow.. engines are all about flow- more air in AND more air out, more power produced.. the pipes have to be small enough to get hot enough to help pull the air out when the force of the exhaust stroke is spent..
exhaust done properly- doesn't hurt the engine at all.. I gotta disagree with true duals hurting low end.. it doesn't.. it generally dis-allows the tach from climbing as fast as a restrictive intake/exhaust, but that does not translate to loss of power.. not exactly anyway..
#12
03-12-2011, 03:31 AM
Captain
^^^^^I've always thought that backpressure was more of an all inclusive term with regards to exhaust performance (unless there is too much). It may be a misnomer, but it gets applied everywhere in a lot of different contexts when folks are discussing the exhaust system (kinda like someone asking for a kleenex instead of saying 'hand me that nose tissue'). It may not always be the right word to fit the bill, but it is one that everyone accepts, and is generally understood to mean different things (depending on the words around it).
That being said, I am referring to the same beneficial forces you are speaking of, I regained some of the better thermodynamic properties of the system (scavenges better)... it's just shorter to say "backpressure" than it is to type (or rather; copy/paste) a dictionary and a half like you did
That being said, I am referring to the same beneficial forces you are speaking of, I regained some of the better thermodynamic properties of the system (scavenges better)... it's just shorter to say "backpressure" than it is to type (or rather; copy/paste) a dictionary and a half like you did
Last edited by Mad_Scientist; 03-12-2011 at 04:04 AM.
#13
03-12-2011, 07:42 AM
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It really isn't the same though. Back pressure is pretty much always bad. It means something is restricting the flow, last thing you want. scavenging/velocity are the keys. Heat plays a role in both, the farther back in the system you go, the cooler the gases and therefore the smaller/less volume they are, so your pipes need to be smaller to keep up the proper gas velocity to maintain scavenging at the cylinders.
Back pressure causes cylinder reversion whereby exhaust gases are sucked back into the combustion chamber on the next intake stroke. Without writing a novel, that's almost always bad, there are exceptions but not in these applications. Most big inch musclecars had 2.5" exhaust pipes and 2.25" tailpipes. Those cars sounded great and were engineered for best street performance. Sound has as much to do with compression, combustion chamber design, muffler configuration and even the design and sound deadening in your floor pan as it does pipe size.
Back pressure causes cylinder reversion whereby exhaust gases are sucked back into the combustion chamber on the next intake stroke. Without writing a novel, that's almost always bad, there are exceptions but not in these applications. Most big inch musclecars had 2.5" exhaust pipes and 2.25" tailpipes. Those cars sounded great and were engineered for best street performance. Sound has as much to do with compression, combustion chamber design, muffler configuration and even the design and sound deadening in your floor pan as it does pipe size.
Last edited by Randy_W; 03-12-2011 at 07:46 AM.
#14
03-12-2011, 11:39 AM
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I guess I need to get on a dyno, before I throw in this high flow cat. That way I can see were my torque curve is at. Then run it after the install on the dyno and see the torque curve. See if we cant put this question to bed. Either way, it will be good to have a cat on hand if we change our emission policy.
#15
03-12-2011, 11:46 AM
Captain
It really isn't the same though. Back pressure is pretty much always bad. It means something is restricting the flow, last thing you want. scavenging/velocity are the keys. Heat plays a role in both, the farther back in the system you go, the cooler the gases and therefore the smaller/less volume they are, so your pipes need to be smaller to keep up the proper gas velocity to maintain scavenging at the cylinders.
Back pressure causes cylinder reversion whereby exhaust gases are sucked back into the combustion chamber on the next intake stroke. Without writing a novel, that's almost always bad, there are exceptions but not in these applications. Most big inch musclecars had 2.5" exhaust pipes and 2.25" tailpipes. Those cars sounded great and were engineered for best street performance. Sound has as much to do with compression, combustion chamber design, muffler configuration and even the design and sound deadening in your floor pan as it does pipe size.
Back pressure causes cylinder reversion whereby exhaust gases are sucked back into the combustion chamber on the next intake stroke. Without writing a novel, that's almost always bad, there are exceptions but not in these applications. Most big inch musclecars had 2.5" exhaust pipes and 2.25" tailpipes. Those cars sounded great and were engineered for best street performance. Sound has as much to do with compression, combustion chamber design, muffler configuration and even the design and sound deadening in your floor pan as it does pipe size.
Last edited by Mad_Scientist; 03-12-2011 at 12:19 PM.
#16
03-12-2011, 11:50 AM
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