Here is some good reading, I just copied and pasted it here for everyone to read. Very informative specially for everyone who isn't going to force induce their car but wants to get as much power from it being naturally aspirated. Remember bolt on parts are good to get HP but to make everything work together, its BEST to get your car tunes to get OPTIMAL HP.
"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 ***** 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 un-burnable 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, hope this helps, peace. "
Reversion: at the beginning of the intake stroke during cam overlap, exhaust gas in the header is under high pressure (negative delta P) and is pushed back into the cylinder, diluting the new air/fuel charge.
Scavenging: at the beginning of the intake stroke during cam overlap, the momentum of the exiting exhaust gasses creates a brief vacuum (positive delta P) in the header, pulling out the remaining
exhaust gases from the combustion chamber, and allowing the new air/fuel charge to be full-strength.
Scavenging is also the reason for differently shaped headers (4-2-1, 4-1) and collectors. We use the momentum of exiting exhaust from one cylinder to scavenge exhaust from another that is next in the firing order! The different shapes allow for this to happen at different airflow velocities thus at different RPM bands.
Scavenging takes advantage of the momentum of the exiting gasses. In essence, the fast moving exhaust pulse pulls a vacuum behind it. Momentum is mass times velocity. So not only do we need to keep the velocity high to prevent reversion - but it greatly improves the scavenging effect.
Thus we have a balancing act (as others have pointed out). We want to minimize friction to lower the backpressure as much as possible - larger pipes have less friction because they have less surface area per unit volume. But we want to increase the delta P as much as possible to prevent reversion and increase scavenging effects - smaller pipes increase delta P because they increase velocity.
There are lots of tricks to try to widen the useful RPM band (stepped headers) or to increase the overall efficiency (ceramic coated exhausts), but it's still subject to this basic tradeoff:
Friction vs. Velocity
AKA: Backpressure vs. Delta Pressure
You want low friction and high velocity.
You want low backpressure and high positive delta pressure. "
Needing Backpressure - Myth or Reality?
The goal of any exhaust system is to efficiently remove burnt gases from the combustion chamber, prevent reversion at overlap, and by enhancing exhaust gas velocity leaving the chamber, create a vacuum to help draw or scavenge in more intake charge volume at cam overlap.
The key is maintaining exhaust gas velocity or energy as the gases leave the exhaust port when the
exhaust valve opens.
So as the exhaust gas leaves the exhaust port in a 4 stroke engine , it creates a series of pressure waves travelling at the speed of sound that move towards the exhaust tip (or forwards) and then some reflects back. Like the water waves coming onto the beach, forward and back, forward and back. The main overall direction is forwards but there is some reflection back to the exhaust port (reversion).
Simple enough...everyone knows this. So what's new and groovy?
The problem is at cam overlap (when both the exhaust valve and intake valve are both partially open and when the pressure in the chamber is greater than in the intake port).
If a high pressure wave is reflecting back and arrives at the exhaust port at the wrong time (i.e. when burnt gases still need to leave), it blocks the flow out. You see these instances when a high pressure wave is reflected back at the wrong time as dips in the torque curve AT REGULAR INTERVALS (usually in the midrange rpms).
If a low pressure wave is reflecting back at the correct time at the exhaust port it actually helps pull burnt gases out of the chamber and also helps pull in more intake air/fuel at overlap. You see these favourable low pressure reflected waves occurring on your torque curve as small torque increases AT REGULAR INTERVALS.
Now here's the first bone of contention and a source of debate between exhaust makers.
1. Is a reflected high pressure wave always bad?
Most of the experienced people I speak to and read on the various boards say YES! You never want backpressure and you want it as low as possible for as long as possible. The low backpressure assists in maintaining that high exhaust gas velocity. They then design anti-reversion chambers and/or place steps (increases in diameter at various proprietary points along the length of the header) to prevent the reflected waves from travelling back to the head.
There are also some pretty smart people who believe slightly differently ...They believe that if you have a high pressure reflected wave arriving a few milliseconds before exhaust valve closure, you prevent the loss of intake air:fuel out the exhaust valve at cam overlap. The exhaust backpressure at this crankshaft degree in the exhaust stroke prevents leaking out or bleeding out of you intake charge into the header and ensures all of it goes into the chamber for combustion.
However, these people do NOT use the exhaust diameter as a way to create this backpressure. That would be too crude or less precise, since the backpressure would exist at all times and they only want this backpressure over the few crankshaft degrees when the exhaust valve is just about to close ,when the intake valve is opening further, and the piston has reached TDC and starts downward for the intake stroke. Using an exhaust just to have backpressure then is like cutting butter with a chain saw.
The people who agree with this will often tell you that combustion chamber and intake port pressures are higher than the pressure in the exhaust just before exhaust valve closure . So some intake flow into the chamber can get pushed out the closing exhaust valve by the higher combustion chamber pressures.
So all you guys that say backpressure is a good thing...I don't believe so...not at all crankshaft degrees which is what you get with a restrictive diameter exhaust. You don't want to have too big a diameter (actually it's cross-sectional area) that will slow or kill velocity or energy. But no backpressure most (99%) of the time is good.
2. How do we get low pressure waves and high pressure wave to arrive at the correct time?
The conventional way to get the exhaust gas harmonic to do this dance of low pressure to pull in more intake charge and high pressure to prevent bleeding off all at the right time is by changing the tube layout on the header: using lengths, diameters, collectors with various merge angles. But these are limited to one harmonic or exhaust gas speed.
So some Japanese engineers at Yamaha (figures, it's always some genius engineer at some bike manufacturer that comes up with these wild ideas) thought: "What if you have an exhaust throttle valve (located in the header collector or at the entrance to the secondary tubes in the first merge collector) that could control the pressure wave behaviour?".
The throttle valve angle would vary as the speed of the exhaust gases changed to control the reflected waves. In an 11,000 rpm bike, the valve opens progressively as the rpms climb as the tubes are "in step" with the engine harmonics and less reflected waves occur but at around 7000 rpm, the valve is closed down to 40-60% of wide open when the harmonic is "out of step" with the engine and at 8500 rpm the exhaust throttle valve is progressively opened. How much to change the throttle angle is based on crankshaft angle input or ignition signal input to an ECU with then controls the throttle valve angle knowing the harmonics of the engine.
you keep reading "you need velocity", but nobody ever quantifies how much, in what conditions, where in the system, or how much is too much. To me, this is no different than saying "bigger is better". A lot of velocity will hurt power because pressure losses increase exponentially with velocity. That's the whole idea behind restrictor plates in racing. The air velocity is very high through them, but they certainly aren't helping power output. Most people seem to attribute a power loss from a large exhaust to a reduction in velocity. However, they never really investigate the cause. In all of the cases I've seen, the larger exhaust made the engine run lean, and that was the true cause. Correcting the mixture strength back to LBT typically restores the lost power plus a little extra.
It's sort of both. When you increase the exhaust size after the collector/cat/whatever, you're decreasing backpressure, and allowing the engine to induct more fresh charge during overlap. When that happens and you don't compensate by adding more fuel, the engine runs lean. If you run significantly leaner than LBT, you'll lose more power than you'll gain from the VE increase. The leaner the mixture is from LBT, the slower the burn becomes, and the more timing you'll need for MBT.
There is definitely a point of diminishing returns. As long as you have a good diffuser for your collector and you tune the engine, I don't know if there is a point at which a larger exhaust will hurt you for power output. However, the exhaust will still be heavier, so overall performance can still drop. It will also be louder.
Basic physics would suggest that the momentum of the exhaust gasses in a smaller exhaust would help to induce more vacuum at the exhaust port, but based on the testing I've seen, it's more complicated than that.
Now that I think about it more...
The only reason that the velocity is higher with smaller tubing is because the smaller cross-section restricts flow more, which means more backpressure is developed. When more backpressure is developed, there is a larger pressure gradient to the atmosphere, so velocity has to increase accordingly. The advantage comes in the form of timing. When the velocity increases like that, a delay is added into the system. Air speed takes a little more time to develop, but due to its momentum and the inherent increase in the pressure differential, the air speed will drop back to "normal" at a later crank angle. If you can take advantage of this phenomenon with your valve events, then it can be useful (namely, in the case of exhaust primaries and intake runners). If you can't, then all you're doing is increasing backpressure and decreasing charge purity. Unless you're spending the time to try and optimize the system, you're most likely going to be stuck with just more backpressure.
MBT ("Mean Best Torque") is the timing advance needed to produce best torque, and
LBT ("Lean Best Torque") is the mixture strength needed to produce best torque.
To add to this...
If your collector is crap, like most of the generic stuff, the first ## inches of the exhaust tubing effectively becomes part of the collector. The collector should act sort of like a one-way valve, so momentum effects become more relevant.
Also, if you're running a cat, just put something big after it. The honeycomb will pretty much kill any sort of gains you could get from resonance or inertial effects.
Source: Backpressure- - Automotive Forums .com Car Chat