'git yer term's straight!!!!

back pressure= bad bad bad... every time.. no exceptions..

scavenging = good good good.. every time.. no exceptions.. it's often misunderstood and folks use 'back pressure' to express 'scavenging'..

scavenging is the conveyer of pulses from the exhaust port to and out the exhaust tip.. it is created by heat convection.. the engine pushes a wad of heated air (pulse) out of the cylinder at decent pressure, but that pressure dies pretty quick.. without it being pulled out of the way- it becomes a cooler wad of air that sits as an obstacle for the next pulse coming out of there..

how do you clear it?

... heat...

pipes get hot.. cats get hotter than exhaust pulses themselves.. they yank the pulse on through by means of convection.. remove a cat? loose the design of scavenging.. you gotta implement something new to replace it.. otherwise, in your pipes you'll have cool pulses of air getting even cooler and creating barriers for the one behind it, and you "lose bottom end".. it will only unobstruct when the engine starts creating enough energy to blow the pulses out and through the cooling ones.. that happens 2kRPM+.. your <2kRPM will suffer..

you can reduce the pipe diameter and create heat IF you remove or relocate a cat.. that sometimes works.. but you better break out your calculator because that crap is more science than what most folks are prepared for.. smaller pipes creates more velocity, but go too small, you'll have obstruction with the pipes themselves.. big pipes cool too quickly.. you'll create backpressure with the cooled pulses..

anyway.... take that and run with it...

 

There those are the right terms, i knew the exhaust came out in pluses but didnt know that cooler air was a barrier.

 

what about if you ran long tube headers with no cats, ive seemed to gain more bottom end torque out of it and after the LTs its 2 1/2" in exhaust true duals to flowmaster 40s. i still get the code but i drive it daily with no problems at all and even if i clear the code it all still reads the same on my a/f ratio

 

You gained more bottom end with no cats and LT's vs LT's with cats? That doesn't make any sense. Below 2000 rpms you definitely lose power if you don't have cats. The exhaust theory posted by drew is spot on. Your engine can't blow those cold pulses out of the way until it gets up to speed. The cats keep the air hotter and therefore flowing better at low rpms. At higher rpms you have enough umph to push through whatever, and then you start seeing the benefits of the LT's. If you look up some LT posts you will see plenty of people vouching for this. Hammer I know is one with LT's and he started with no cats then put them back on as he lost low end.

 

Yeah Drews Exhaust Theroy is simple and wasy to understand

 

ya i understand and would explain scavenging the same way, theres a video somewhere actually showing cylinder by cylinder scavenging that i learned from in high school automotive. im not arguing with this, after redoing my exhaust my 60ft went down and punching it from a dig i spin the tires more often so that was just my experience. i already sold my cats but maybe one day ill throw some back on

 

that shouldn't surprise you...

think about how long tubes work compared to manifolds or shorties; the primary tubes off each cylinder are longer.. they aren't competing to clear pulses- by the time the engine fires that cylinder again, the pulse in front of it is nearing the collector, which is plenty of room (and then some).. it hits the collector tube and collides with the other pulses either leading edge or trailing edge -and stays hot-..

it then goes to the muffler.. and this is where you're fortunate in your muffler selection: that 'chambered' flowmaster doesn't breathe as easily as perforated designs- and creates an obstacle.. that obstacle creates heat.. a chambered muffler gets hotter than a baffled or perforated one..

do you still have a resonator in the mix? That thing can help too with heat..

re bottom end: it's not really bottom end you're gaining or losing.. it's rev speed.. a grand example:

take an engine powered leaf blower- take the little diameter reducing end off the end of it.. point it parallel to the ground and squeeze the throttle- it revs, but it doesn't rev but so fast, and doesn't achieve the RPM like it does when you:

now, point the thing at the ground, with about a one to two inch gap between the ground and the nozzle.. squeeze the throttle.. the RPM's will get higher and quicker.. why? back pressure.. that's good, no? No.. back pressure is bad, yes? yes.. it is always bad..

what happens in that instance is the same thing that happens to your engine when it encounters back pressure.. (we're talking the air pump of the leaf blower, not the engine).. the cylinder expels it's exhaust pulse, and it either gets it all out, or it doesn't.. if the engine doesn't expel it, it has a portion of the exhaust pulse still in it when the intake stroke begins..

if it still has a portion of exhaust pulse in the cylinder when the intake stroke starts, there is less room in the cylinder for fresh air and fuel- well, the fuel is there, but the full volume of fresh air isn't.. that is manifested by a 'quicker rev', which gives the notion of 'more bottom end'.. it DOES rev faster and creates RPM's higher, but it's not burning a nice collection of fresh air and fuel.. since you don't have o2 in there like you should, the burn isn't as powerful.. it's all because the exhaust pulse didn't clear the port... as in 'back pressure'..

so- you get a quicker rev, but you don't have near as powerful a burn on your power stroke because a portion of the air in the cylinder is o2 depleted..

if you DO have fresh o2 in the cylinder- as in the exhaust pulse is clearing completely out and fresh air is brought in, the engine doesn't rev as fast, BUT what it does rev is a more powerful burn- hence, power to the terra..

an engine is simply a great big air pump.. it uses expanding air to create energy by igniting it.. the cooler (denser) the intake air, the hotter the burn, the more the volume of that air expands, and the more powerful the engine.. the more air that passes through the engine from intake to exhaust, the more ponies you're producing.. internal combustion engines make use of fuel as a catalyst to more positively ignite the air, or to make the ignition possible in the first place.. the trick to producing ponies in ANY engine is getting the air through the engine..

put it in this example:

your engine is a great big hypodermic syringe.. you pull the plunger UP to suck air into it, and when you reach the top, you press the plunger DOWN to expel it.. let's say it is a 200cc syringe.. each time you pull and then push you are moving 200cc's of air through it.. it's efficient that the only restrictions of the syringe is the size of the inlet/outlet..

now lets say you start to pull the plunger up- but lets say you put your finger over the inlet where the volume of the syringe crosses the 150cc mark.. ... ... pulling the syringe plunger any higher just got difficult, no?

let's say you reach the top of the plunger travel, at the 200cc mark, but you only have 150cc of air in it.. it's a vacuum, and it's difficult to pull it, but you can force it there with enough power..

after you reach the top and let go, the plunger takes a dive back down to the 150cc mark all on it's own, but then you have to push it to get the reaming 150cc of air volume out... let's say you cover the outlet with your finger at the 25cc mark.. pushing that volume out becomes impossible because something is obstructing it, and the plunger bounces back toward your pressing finger.. you ignore it and start pulling on it again.. that 25cc never got out of the way, so it bounces back to the 25cc mark without effort..

you pull it again until it reaches the 150cc mark and you cover it again.. pulling it any further up becomes difficult, but you can manage to force it up.. when you reach the top you reverse course.. you expel 150cc's of air, of which 25cc's are 'reused' air, so you only moved 125cc's of fresh air..

this goes on and on..

an engine is the same at this fundamental level.. if it is 'assisted' via back pressure to where it can't expel the air, ( or gulp enough new air through the intake) it will be easier to move faster, but only by 'reusing' some of the previously used air.. an engine differs in that it needs 'fresh' o2 to burn more powerfully..

for this reason and this reason alone, and engine that is revving slower, but drawing air efficiently is moving MORE fresh air through it than an engine revving faster under obstructed conditions.. it produces MORE power because of such..

this is inaccurate ratios, but trying to quantify it: an engine revving 1000 RPM with 100% volumetric efficiency produces the same power as an engine revving 1500RPM at 75% volumetric efficiency.. the volume of air through the engine is the same, but one is working a heckuva lot harder to achieve it..

guess what? an engine operating at 100% volumetric efficiency won't rev as quickly or as high as an engine with obstructions.. it produces the same power, but doesn't spin the shaft as fast.. we're all about spinning that shaft, no? that engine has TORQUE, but doesn't have POWER.. if you were to grab the shaft with your hand on an engine 100% efficiency, you wouldn't have a prayer to stop the spin.. it's producing too much torque (spinning power).. if you were to grab a shaft spinning stupid fast, but not very efficient, you could stop the thing from spinning, but only after friction burning your hand, you stopped it's HP/work load (how fast it does a certain amount of work), but you exposed it's lack of torque..

there is a balance you want to achieve.. that balance is the quickness accelerating the speed of the shaft spinning (expressed with HP on engines), with how hard it is to stop that shaft from spinning (expressed on an engine as torque).. engines are designed to produce enough work load to get the thing to a decent speed, while at the same time having enough power of the spin not to easily stop it..

this is done by playing with the volumetric efficiency of the engine.. and it's all about how much air and how restricted it is.. and where that restriction is- on intake or exhaust.. ever heard an intake manifold that is good for bottom end? it's because it makes that syringe plunger harder to pull up.. ever hear of one for top end? it doesn't restrict the intake..

exhaust is the same for both engines.. you want that air out of there, because it needs fresh air to make a powerful (read: efficient) burn.. if it is obstructed, it will rev faster, but not produce as much power.. it will rev faster, and at some point the revs the engine is making will overtake the volumetric ratio, and will produce more power (but not as efficiently).. it will get hotter (manifested in loss of energy to heat transference).. but it WILL rev quicker and give quicker power to the terra..

the trick is finding what will work for YOUR application.. torque? power? faster revs? more efficient? damn efficiency, just quicker? ... this is why we do what we do trying to fiddle with these things.. any number of combinations can help or hurt your goals.. key word: combinations.. in the case of the LT's without cats and a flowmaster? you maintained a level of restriction similar to what you had to begin with, and didn't lose low end.. want that bad bear to produce more power (but less torque)? lose the flowmaster.. it will not rev as fast, but it will be more efficient in the end, and produce more power on the top..

 

I was just schooled in Exhaust lol

 

Definitely a good read.

 

that explains a lot, very good read and explanation. this thread needs to be changed to airflow! i used to have magnaflows and even with cats but no resonators i could feel it lost the low end a bit. my setup is fine by me because with a truck im not shooting for over 120mph efficiency, i just want to get there as fast as possible. so next is the gears, wanting 4.10s now