lowend torque on a/t

ok, i think this is how it works and a shop guy told me the same. i was told that i need more backpressure than you would need with a manual, since i need lowend torque to pickup from a stopped position. i know that will hurt my highend hp a little, but are there any standard or average specs on amounts gained or lost? right now im almost running a straightpipe. i have a highflow cat and a straight-thru muffler. im going to ge putting in a pair of silencers. any guesses on what kind of change it will make other than sound?

if you put on 2 silencers then that would help alot with backpressure

Backpressure doesnt help power at any rpm, its a mith. Whats happening with your exhaust is that you have it TOO open. There is just too much space for the exhaust air to fill and it doesnt move out of the cylinder very fast, causing some already burnt air to get sucked back into the cylinder during valve overlap. So what you want for good lowend hp (why you were told you need it i dont know) is to have a smaller pipe. There is no exhaust that gives power at all rpms, but for lowend power, you have to use a thinner pipe. Hope that helps yah.

OMG! k c32bperformance let me ask you a question
if the exhaust is too big then the cylinder would have no problem letting all teh exhaust in with extra room, right, this is according to your belief. so then by what you say, all the exhaust will fit into the piping so its all good. i would like to note so far this is what you said. so why wouldnt all the exhuast leave the cylinder with bigger piping? why would it get sucked back in ?
why would smaller piping help ? oh and its not thinner piping, its smaller piping.

if theres no back pressure so theres no pressure or sucking in the exhaust, then it wouldnt matter what size your piping is, bigger or smaller theres no pressure or sucking in the piping. if anything you would want the biggest piping you can get so all the exhuast will get out, if you have smaller piping then maybe all the exhaust wont be able to get out since the piping is too small. i would like to note, this conclusion is based on what you said.

you guys who say theres no backpressure then i ask you to describe what you believe and then you guys describe to me exactly what backpressure is, then you guys say, yeah its just a myth.

k if you couldnt get the above statement heres kinda a simpler question

since you say with bigger piping, the exhaust will get sucked back into the cylinder during valve overlap, if this happens with bigger piping what stops it from happening with smaller piping ? since you say theres no pressure with bigger or smaller piping, its all the same then it wouldnt matter what size your piping is right ?

Originally posted by civickiller
OMG! k c32bperformance let me ask you a question
if the exhaust is too big then the cylinder would have no problem letting all teh exhaust in with extra room, right, this is according to your belief. so then by what you say, all the exhaust will fit into the piping so its all good. i would like to note so far this is what you said. so why wouldnt all the exhuast leave the cylinder with bigger piping? why would it get sucked back in ?
why would smaller piping help ? oh and its not thinner piping, its smaller piping.

if theres no back pressure so theres no pressure or sucking in the exhaust, then it wouldnt matter what size your piping is, bigger or smaller theres no pressure or sucking in the piping. if anything you would want the biggest piping you can get so all the exhuast will get out, if you have smaller piping then maybe all the exhaust wont be able to get out since the piping is too small. i would like to note, this conclusion is based on what you said.

you guys who say theres no backpressure then i ask you to describe what you believe and then you guys describe to me exactly what backpressure is, then you guys say, yeah its just a myth.

From an old, old post of mine on the subject...

"The Myth of Backpressure

…is probably the most widely misunderstood concept in engine tuning. IMO, the reason this concept is so hard to get around lies in the engineering terms surrounding gas flow. Here's the most important ones you need to be aware of to understand the things I'm about to say:

BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.
DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points.

One other concept needs to be covered too, and that's the idea of air pressure vs. velocity. When a moving air column picks up speed, one of the weird things that happens is it’s pressure drops. So remember through all this that the higher the air velocity for a given volume of gas, the lower it's internal pressure becomes. And remember throughout all of this that I’m no mechanical engineer, simply an enthusiast who done all the reading he can. I don’t claim that this information is the absolute truth, just that it makes sense in my eyes.

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).

The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the bitch of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to achieve optimal gas velocity and pressure at both these RPM points, given the need to flow such varying volumes.

These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; because is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhaust gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!).

This last concept is why low velocity gas flow and backpressure hurt power production. Understand that during the exhaust stroke of a 4 stroke engine, it's not only important to get as much of the spent air/fuel mixture out of the chamber (to make room for the unburnt mixture in the intake system), it's also important that these exhaust gasses never turn around and start flowing back into the cylinder. Why would this happen? Because of valve overlap, that's why. At the end of the exhaust stroke, not only does the piston start moving back down the bore to ingest the fresh mixture, but the intake valve also opens to expose the fresh air charge to this event. In modern automotive 4 stroke engines valve overlap occurs at all RPM, so for a short period of time the exhaust system is open to these low pressure influences which can suck things back towards the cylinder. if the exhaust gas velocity is low and pressure is high in the system, this will make everything turn around and go the opposite direction it's supposed to. If these gasses reach the cylinder they will dilute the incoming mixture with unburnable gasses and take up valuable space within the combustion chamber, thus lowering power output (and potentially pushing the intake charge temp beyond the fuel’s knock resistance). So having good velocity and therefore low pressure in the system is absolutely imperative to good power production at any RPM, you just have to remember that these concepts are also dependent on total flow volume. The overall volume of flow is important because it is entirely possible to have both high velocity and high pressure in the system, if there is simply not enough exhaust piping to handle the needed airflow.

It’s all about finding a compromise to work at both high and low RPM on most cars, but that’s a bit beyond the scope of this post. All I am trying to show here is how the term backpressure is in reference to a bad exhaust system, not one that creates good low RPM torque. You can just as easily have backpressure at low RPM too, which would also hurt low RPM cylinder scavenging and increase the potential for gas reversion. And understand that these tuning concepts will also affect cam timing, though that is again probably beyond the scope of this post. At any rate, I hope this made some sense, peace."



So if you actually understand the physics behind efficient airflow, such that total kinetic energy increases with the SQUARE of velocity rather than linearly, you'll get how reversion is much less likely when the air is moving at very high speeds. Hence velocity = efficient flow, and backpressure = resistance to that flow.

To answer the original question, adding backpressure to the exhaust system, or in some other way choking off flow, will NOT help low end power output. Try a positive displacement supercharger such as the Jackson Racing kits if you're really that interested in low-end power on a small displacement motor.

:huh: ok civickiller... :bloated: if you couldnt understand the above statement, then read mine again. ;) But you shouldnt have any trouble since your so friggin smart. :rolleyes: Why do ppl have to talk shit? :confused: Cant they just question something civilized? :eek: Texan understands it better than i do, but the truth stands. :sun:

see the word backpressure itself may not be technically correct but what i said is true
why does exhaust flow sometimes get pulled back in, because the piston is sucking down so it pull the exhaust back in since its all a big vaccum.
so at low rpm why does that happen with big piping but not with small piping.
again i say if its only velocity from the engine then it wouldnt matter what size piping you have, so why when you have bigger piping the exhaust sometimes goes back into the engine than with a smaller pipe, maybe theres something in there pulling the gas out, but with bigger piping since the exhaust cant completely fill the piping, there is no vaccum pulling it out.
i know how engines work, but you didnt say anything about a vaccum

but yeah you did show us that the term backpressure is not technically correct, but i already knew that, but thats what peopel call it so thats what i call it.
since your engine is a big vaccum, as the exhaust leaves the engine and get into the exhaust piping. since its air tight as in the exhaust fills the whoel pipe, as it moves down your exhaust, what about all the space that your exhaust just left behind, it has to get filled by something, so again you got a vaccum inbetween your exhaust flow and the engine. then once the valve opens, then vaccum helps in pulling the exhaust into the exhaust piping and down the exhaust.
you can try and explain it all you want and try and call it a myth, but if you know how a vaccum works then you would understand it. like i said, i never noticed you say anything about a vaccum and thats an important part

its all good, just dont get angry next time you think somethins wrong. Yah win some, yah loose some. and your right, i didnt say anything about a vacuum, i forgot to. My boob:rolleyes:

Originally posted by civickiller
again i say if its only velocity from the engine then it wouldnt matter what size piping you have, so why when you have bigger piping the exhaust sometimes goes back into the engine than with a smaller pipe, maybe theres something in there pulling the gas out, but with bigger piping since the exhaust cant completely fill the piping, there is no vaccum pulling it out.
What's important is the momentum of the exhaust gasses. This is what creates the delta pressure to scavenge the cylinder.

Momentum is just mass * velocity. The exhaust gas velocity is affected mainly by two things: the velocity of the piston and the cross-sectional areas of the flow path (cylinder, valve, head, exhaust manifold...). The mass is just the total amount of air and fuel injected into the cylinder during the intake stroke.

At a given RPM, the only simple thing you can do to change the momentum of the exhaust gas is to change the size of the header tube. The smaller the tube, the higher the velocity (and thus higher momentum and thus higher delta pressure). Prove this to yourself by blowing out a candle from 12" away - you can't do it if you open your mouth wide; but if you purse your lips to a small opening you can get a higher velocity stream of air. (It's a matter of getting a fixed volume of gas through an opening in a fixed amount of time - the smaller the opening, the higher the velocity has to be through that opening.)

So why not always make header tubes tiny for maximum momentum? Friction. At high RPM the exhaust velocity will gets too high, and as the friction goes up close to the square of the velocity, it starts to take more power to push the exhaust gas through the little header than the engine gains due to high delta pressure.

As texan says, it's all about compromise (like all engine components).
:)

backpressure is just the exhaust creating a vaccum helping pull out the next exhaust. i know its not technically the right word to use.

i know the velocity pushing the gas out creates this vaccum. with bigger piping at low rpm, the exhaust doesnt fill up the whole piping so no vaccum is created or very little and thats why the exhaust sometimes gets sucked back into the cylinders.
the candle thing is not good, think of a hose. if you plug up half the hole in the opening then you get the water to shoot out but your water is restricted and doesnt flow as much as when you let the whole hose opening be open. thats kinda the thing but you dont have a vaccum in a hose or you blowing out a candle. allot of people just dont understand the vaccum thing so they dont get it and they think i am wrong.

i know its not all vaccum, its everything combined that makes it work. i dont like it when people say its a myth, because they cant understand the whole thing, they just know parts of it. and you guys dont need to explain it to me, i am taking college level pysics, chemistry, calculus II, and i am a good mechanic and i know engines really good.

if you read all my posts above, i am not saying anyone is wrong, i am just trying to add to there explanation to make a whole complete definition and not a partial noncomplete definition, that can get people confused and give wrong advice or maybe sound stupid when they explain what they think is a complete answer when its not

civckiller, i didnt say it was a myth cuz i dont understand it...

Oh and your wonderful, take those college classes...
You arent the only one in college. That was really arrogant.

he didn't say he was in college- he just said he was taking "college level" classes. When I was in college, there were many different levels of physics classes. As far as newtonian (classical) physics go, they had three or four different levels. I suspect that classes called "college level" are not taught at a college...

Whats the difference, he still said it, and someone would have to be really thick to not see what he was infering...

i am not trying to brag or anything. i just said you dont need to explain how an exhaust flows or the equations needed to determine all these things because i know them. i did not say i am the only one in college. i am just sayin you dont need to explain it to me.

you can just say what you need to say, you dont need to explain it, say well blah blah blah velocity and then have to explain how to get the velocity of something.

just trying to save you guys alittle time so you dont need to type as much.

Well, thats cool that you weren't bein a jerk. But what I was mainly getting at was that you seemed to be implying that I dont understand the stuff, and that that was the reason I said it was a myth.

NM then.

well thanks for all the help... damn, i didnt mean to start a war over this shit:D for whatever reason, the silencers did help the lowend, it picks up quicker. not to mention its so much quieter, i love it.

on a side note, i got pulled over. and that ass told me to pop my hood. so now i have a great excuse to buy new legal headers. i meantioned them before, but does anyone know about the chikara torkstep headers? i know theyre supposed to "broaden" the tork curve, but does it hurt hp at all?
i also put my stockbox back in. but i put in a k&n filter and cut out the back corner of the box to help with air flow. so if i get pulled, i wont get busted for it again.

hp is a direct relation of torque hp= (torque x rpm)/ 5250
so if you increase torque you increase hp, if you lose torque at a certain rpm then you will lose hp