Celica Forums General Discussion Exterior The Myth of Back Pressure

Ok this is long ... I did not write this and I am not going to argue with anyone.

I just remember reading this and thought it was interesting.

Thanks, Darren

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"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:

BACK PRESSURE: 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 its 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 Im no mechanical engineer, simply an enthusiast who done all the reading he can. I dont 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good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).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 ***** 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 unburned 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 fuels 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.

Its all about finding a compromise to work at both high and low RPM on most bikes, but thats a bit beyond the scope of this post. All I am trying to show here is how the term back pressure 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, hope this helps, peace. "

Written by:

Martin Chase
Sr. Tuning Mechanic
Yamaha Inc.

ttt

I don't need your pity

i would have replied but there's nothing to say, so pity reply #2

Third time's the charm. #3

I hate you guys ... j/k

Its all good . Thanks for the info though Rice. Whether this is true or not I don't know, whether its called backpressure or whatever, I don't know. But some have felt loss of power from making their exhaust bigger.

this is just some random guy who is making a sweeping argument about some simple misunderstandings HE has about what the definition of backpressure is..

the simple fact is, the MORE air in your engine at spark the bigger the boom because O2 fuels fire as much as gasoline does. (the reason N02 is SUCH a combustable fuel is because nitrogen is highly unstable since it has only 1 electron, so it combusts very easily, and O2 is needed to fuel combustion, a spark is all the energy needed for NO2 to split into nitrogen AND oxygen it's like gas that COMES with it's own air, which combined are a perfect fuel).

So when your pressure drops, you have less air.. once air is burned it exhaust needs to be removed for more air to come it.. the key is HOLDING the air long enough for it to be used to fuel the flame, so IF you have air moving through too fast, then O2 is being pumped out before it can be used. and your pressure drops. this is why it's a delicate ballance. YOU NEED BACKPRESSURE to keep the air in the engine long enough to be used. The simple fact is, this guy is just mis-informed or misunderstood what he read.

That is some HIGH QUALITY INFORMATION Rice

All i can say is, VERY GOOD INFORMATION


good work Darren

First off, NO2 doesnt exist. I think you're talking about N2O(nitrous oxide) since the oxidation number of N in this formula is -3 and O is +6, this way they balance(2*-3 + 6 =0(neutral). N2O is about 36% oxygen by mass. In normal air, oxygen only makes up a percentage in the 20s(not exactly sure how much).

Why is oxygen important? Because without it, combustion cannot take place. Oxygen and fuel combine are mixed in the induction chamber and then ignited by the spark plug causing the explosion that pushes your piston down...and finally making your car go. The bigger the explosion, the harder/faster the piston is pushed down(increased power). So why cant we just add more fuel to make the explosion bigger? Because in a chemical reaction, you can only get out what you put in. If there isnt enough oxygen mixed with the fuel, then there will be left over fuel that wasnt burned because there was no oxygen left for it to combust with, this would be a scenario where your car is running rich, which is bad on your engine because you get carbon deposit on the internal from the left over gas.

This is why intakes and other forms of getting more oxygen into the engine add HP, because they allow for more fuel to be burned thanks to the increase in oxygen. Nitrous oxide does the same thing in a different way. If 10L of air are let into your engine, you may only get 2.5L of oxygen, but if 10L of Nitrous oxide are let into your egnine, you get 3.6L of oxygen. Once again this allows more fuel to be burned thus increasing power.

But first the N2O have to be separated. This occurs naturally in the engine when the temperature reaches somewhere around 600 F.(once again i'm not sure of the exact temp.) This way you get all that oxygen in there so more gas can be burned. Thanks to N2O's high(compared to air) oxygen concentration, it is more efficient than turbo's or superchargers which just pump in lots of air. This is why you can get so much more power out of Nitrous oxide than any other form of forced induction.

Now i know there was a point to my typing this, but i really cant remember it now. If i remember i'll post it, but till then, sorry for wasting for time, lol

ummmmm, I am pretty sure that NO2 is called Nitrogen Oxide and it is measured through emissions. I used to be a mechanic so I am pretty sure that I am correct here. Why would he be talking about Nitrous Oxide?

Assuming that the first post is absolutely correct, couldn't someone pattent a special midpipe for all RPM ranges? Basically a pipe with a valve in it that opens wider and wider as your RPM climbs and then closes as it lowers, keeping the perfect pressure. Wouldn't be too hard to tune, either.

Emissions is CARBON DIOXIDE (CO2)

"Nitrous Oxide" (laughing gas). N20... like C02 is CARBON Dioxide and is the emission of burning carbon based fossil fuels (like gasoline) using oxygen as a catylist... If you were a mechanic you should know that CARBON DIOXIDE is what is measured through emissions. What produces the spark is the rearanging of atoms in an excited state, giving off energy.

N20 or Nitrous Oxide, (proper term) is 1 Oxygen atom bonding with 2 Nitrogen Atom to form a stable state (instead of 2 oxygen bonding with one carbon atom which is C02 and what you are mistaking for N20, which is what I was talking about). Anyone with college level chemistry should have studdied this (not really something you learn as a mechanic though, I guess). Oxygen can bond with anything that has an odd number of electrons. So it bonds easily to carbon or nitrogen, however carbon is much more stable than nitrogen, nitrogen having only 1 electron will quickly cause less stable molecules to split in order to bond with it to become more stable, which is why it's so flamable.

I did say N02.. you are right. just a typo. N02 would be impossible. odd number of electrons.

[ 05-04-2003, 05:07 PM: Message edited by: FX-MAN ]

Just for clarification NO2 is

Nitrogen dioxide (NO2)

Nitrogen Oxides in vehicles are caused by excessive combustion chamber temperatures. Some of the common causes of high NOx emissions are problems with the vehicle's Exhaust Gas Recirculation System (EGR), improper ignition timing, lean air/fuel mixture and malfunctions in systems that control engine temperature, such as the thermostat and cooling fan, and vacuum leaks. Due to the complexity of the internal combustion engine, other components may cause high NOx as well.