Another dreary rainy day here, so I'm going to have a rant about one of my favorite topics, "Pumping Losses".
I'm sure many of you have heard me mention this in various posts.
But, do we really know what they are?
In case some don't, I'm going to expound a bit on it.
Pumping Losses
or
"Our stealthy power robber".
So, what are these mysterious "Pumping Losses" of which I speak?
Well, they are just that. Power losses from pumping air around in the engine, and it's ancillaries, which rob power from our rear wheel, and generate excess heat inside the engine where we don't want any excess heat. It's a type of efficiency problem, and it affect all engines. Even if you want to keep your Bullet "bone stock", you are experiencing pumping losses, and you can improve your power output, fuel economy, and engine longevity by reducing them.
So, when people talk about engines, many times they refer to them as an "air pump".
And this is sort of a decent analogy in a way, but not totally accurate.
But, I'm going to use that analogy for right now, because it illustrates some points I want to get across.
An air pump is not an "engine", it is a "pump". And it pumps air around, and it uses power to do that. Now, that's okay with us if we want to pump air, like a compressor does. But, in our engine, air pumping is something that is just a "means to an end" of getting power out to the wheel. So, we want to move the air around where we need it to go, but we don't want to use alot of power up in the process, because we have something else we'd rather use that power for.
And the power we use up to move that air around is called "pumping losses".
Pumping losses happen in the damndest places. Alot of times you are getting pumping losses that you didn't even know you had.
Let's take the crankcase breathing system for example.
Heck, isn't that the thing that just serves to spew oil all over the back end of my bike?
Well, not really. It actually does have a purpose. And it's purpose is to reduce pumping losses in the crankcase.
You see, when that piston comes downward to draw air in thru the intake system, it's also pushing down on the air inside the crankcase, acting like an air compressor. And if we didn't have a breather on the crankcase it would compress that air down until it reached bottom, and started to go back up again, and relieve the pressure. Like an air spring.
Well,this isn't good. It's taking a whole boatload of power to be compressing that air on every down stroke of the piston. So we don't want that, and we have to do something about it. So, we put in a vent. And we run that vent to a duckbill or a PCV system, so that it lets that excess air get pushed out, but closes a one-way valve when it tries to come back in. And with a system like this, the crankcase is quickly pumped-down to low pressure in a short time after you start it up, and it stays at low pressure all thru the running, and uses up less power than it otherwise would have. It also provides a better environment for the rings to seal, and reduce ring blowby, and you get better power from it too. So, it's a double-bonus to have a good functioning breather system.
Now maybe that was an easy one, that many of you already knew about, but now were going to go to the easiest one that everybody knows about.
This is the exhaust and intake system, and nearly every one of us has experienced the power gains when we added the "free flowing" exhaust and intake filter to our bikes. This one is the one that we've really "felt in the seat of the pants" when we helped the breathing with those speed parts.
And guess what? What we really did was to reduce pumping losses. I'll bet alot of you never thought that pumping losses were so dramatic.
Let's look at these items in some depth.
Okay, for the intake, when we "free flow" the intake system with a more open filter, we've "reduced restriction". Most of us intuitively already know that. But, what is "restriction"? Restriction is drag or friction of the air that we are trying to move. It's not moving as fast as we want it to.
And what does that create?
It creates a partial vacuum in the inlet tract, which then is at a lower pressure potential than the atmosphere, and thus the air does not rush in so fast, because there is less pressure difference between the incoming air, and the vacuum we create with the descending piston that "pulls" the air in. See, it's all a matter of different pressures working to fill a void. Lower the pressure of what the engine "sees" as the external air, and less comes in. This happens because the inlet tract is restricted more than the external atmosphere, and so cylinder filling is reduced.
At the same time, the friction of the air is so great, and the resistance to being compressed to try to move down the inlet tract, causes the piston to have more trouble moving downward, and uses up power that was stored in the rotating flywheels. Sort of like trying to suck a thick milkshake thru a straw.
Remove that restriction, and there's less resistance to the air trying to move in from the outside, so that it can do it better, and less vacuum is created in the inlet tract, so that more of the atmospheric pressure can better fill the vacuum in the cylinder.
You see, the maximum inlet pressure you can get in a normally aspirated motor is 14.7 psi, which is 1 atmosphere of pressure. That's the air outside. The piston going down is attempting to create a "full vacuum", which it never can do, but it's trying. The more differential in pressure between the outside air, and the vacuum in the cylinder, the better the fill. Intervene in that process, and you reduce the intake flow and get losses in power production(less cylinder fill), and increases of power consumption trying to do the work. Double whammy.
Get it?
Well, there's a bit more to it than that, but that's a basic understanding of why removing restrictions in the inlet system will reduce pumping losses.
Now, for the exhaust system.
Well, reducing pumping losses in that works too, but in the opposite way.
We have high pressure in the cylinder after combustion and power stroke, and we still have 14.7 psi outside in the air. And we have a pipe in between. So, what we want to do is to get those gasses out of the cylinder, without having to use the piston to push them out. So, we use a technique of valve timing called "blow-down". Blow-down happens when you open the exhaust valve early enough in the power stroke, where the gas pressure is still pretty high, but you've gotten all the power you are going to get out of the power stroke. So, if you open it up when pressure is still over 70psi in the cylinder, it "blows" out the exhaust pipe by itself, without needing to be pushed out by the ascending piston. This reduces pumping losses, but it doesn't get it all out. But, as this fast moving air gets flowing down the exhaust, it is sort of like a "slug of air" that has a low-pressure area right behind it as it moves down the pipe. This low pressure area behind the exhaust "slug" pulls more exhaust out of the cylinder, and leaves as small residual gases in the cylinder as possible, so that it doesn't take much work to push them out. Thus reducing the pumping losses on the exhaust stroke. Careful placement of the exhaust valve opening event can produce full power, and give good blow-down, and result in a better performing engine.
But what happens when we have a very restrictive muffler, or even a restrictor in the back end of the header pipe, which holds up the flow, like a plug.
Well, we all know what happens, Everything is slowed-down, and blow-down doesn't work as well, and high-speed exhaust scavenging doesn't work well, and the end result is that we have pressure built-up in the exhaust system, and this causes the piston to have to work harder to shove that exhaust out the pipe. And that takes power to do. And it isn't fully successful at doing it, either. Some extra exhaust gas remains in the cylinder, which dilutes the incoming fresh mixture charge by taking up space in the cylinder with burnt exhaust fumes. Space which could have gotten filled with fresh mixture. Bad karma.
So, we call up CMW, and order the free-flow exhaust system, and the free-flow filter, and voila!
We get an extra 4-5 horsepower and also some torque to go with it, simply by releasing the engine to better do what it was supposed to be doing in the first place.
And it's not only that.
These losses take that 4-5 hp, and translate it into wasted heat that the engine needs to get rid of via the fins or the oil. That's a big extra heat load on the engine. Putting on the free flow stuff gets that power to the wheel for more gusto, AND eases the heat loading that the bike has to deal with. And with an aircooled engine like ours, which suffers from heat soaking, alleviating that extra heat and freeing it up to produce power is a very good thing for us to do.