What are "Pumping Losses", and why don't we want them?

[ace.cafe]

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.

[23hp]

Ace

Excellent post.  Very descriptive and I can actually visualize what your talking about.  Helps me understand what is happening or should be happening with every thud.

Thanks

[Cabo Cruz]

"Ace

Excellent post.  Very descriptive and I can actually visualize what your talking about.  Helps me understand what is happening or should be happening with every thud.

Thanks"  23hp

Ditto and vale!!!

[Rick Sperko]

Great post. So basically a straight through pipe is better than having any kind of baffle on the bike. For some reason I thought you wanted some small amount of pressure in the exhaust, but by your description I wouldn't.

Maybe I will reopen that hole where the baffle meets the header pipe. My neighbors wont like it, but their teenagers used to wake us up too.

-Rick

[ace.cafe]

Great post. So basically a straight through pipe is better than having any kind of baffle on the bike. For some reason I thought you wanted some small amount of pressure in the exhaust, but by your description I wouldn't.

Maybe I will reopen that hole where the baffle meets the header pipe. My neighbors wont like it, but their teenagers used to wake us up too.

-Rick

Rick,
There is always some "back pressure" in a exhaust system, so you'll be having some there, even with an open header pipe.
The racers use open pipes like that for performance, but often they don't need to have mufflers for meeting regulations, and they don't use any.
However, on the street, you may or may not be able to get away without using a muffler.
In either case, the lowest restriction at the tail end would be the best.

When you make changes like that, you need to watch your jetting, to make sure that you haven't created any lean spots in the power curve.

The use of megaphones and reverse-cones on the end of the header, are sometimes used to counteract the effects of wave-tuning concentrating power into a narrow rpm range at the high rpms. And they also serve to "impedance match" the exhaust flow into the outside air, and actually cause an improvement with that.
All very loud, of course.

I would recommend that some sort of "muffler body" be used on the end of the header pipe. A straight pipe, or straight pipe extension on the end of the header, is a "tuned pipe", and it will cause "peaky" results in a narrow rpm range.
Personally, I really like the Goldstar system, because it is sort of a reverse megaphone type body, without a baffle inside.
I use my Goldstar system with no baffle in it. Basically an open system.
However I do have the removable baffle for winter when I put it in to make the bike run a little richer for cold weather.

Basically, the "rule of thumb" for street performance is to lower the internal restrictions as much as possible without getting a ticket, and use some sort of resonator body or muffler body on the end of the header pipe to avoid the inevitable wave-tuning results that straight pipes have. And don't make it too short.

[Chuck D]

Ace, thanks once again for a fascinating (and for me), timely treatise. Timely because I am in the midst of a stage 2 head conversion with 32mm Mikuni and K&N filter. I already have the freeflow exhaust, albeit with baffle. So if I understand you correctly, the peashooter muffler shape is enough of a restriction to create the correct backpressure without the baffle, and is not what is referred to when we say "open pipe". I guess what I'm asking is , does it make sense to go the whole hog and remove the baffles and just tune the carb accordingly?  Thanks again.   Chuck.

[ace.cafe]

Ace, thanks once again for a fascinating (and for me), timely treatise. Timely because I am in the midst of a stage 2 head conversion with 32mm Mikuni and K&N filter. I already have the freeflow exhaust, albeit with baffle. So if I understand you correctly, the peashooter muffler shape is enough of a restriction to create the correct backpressure without the baffle, and is not what is referred to when we say "open pipe". I guess what I'm asking is , does it make sense to go the whole hog and remove the baffles and just tune the carb accordingly?  Thanks again.   Chuck.

Chuck,
Yes, as long as the carb is tuned properly for that arrangement, so that it isn't too lean.
And, as long as it isn't too loud for your area.
The baffles do nothing positive, except quiet it down.
However, the jetting must be right for it.

If it's a bit too loud, a "straight-thru" glasspack muffer, like a Cherry Bomb or something like that can quiet it down a bit, and still provide the unrestricted flow.

The muffler body is more of a "wave-breaker" and "expansion chamber", which allows the hot exhaust to expand, and therefore cool a bit, and recover some pressure(and lose some velocity) in the muffler body, which will quiet it down some, but not present a huge restriction to flow. Like a resonator. It doesn't really "muffle", but it controls the sound a little more than a straight pipe, and is less likely to give as peaky power tuning.
There are trade-offs involved in all of this stuff, and some of my recommendations are aimed at the use of the bike on the street, to get good power results, but not render the bike less useful for street riding.

For all out power, regardless of anything else, you could run a tuned-length exhaust with a megaphone tip of the proper dimensions, and get great power in the upper ranges, and it would be so loud that you could be heard a mile away. It's just not a feasible way to go on the street.

[Chuck D]

I don't think noise will be an issue. You should hear some of the bikes around here. Not to mention the sirens, garbage trucks, construction crews,  car stereos... you get the picture. 

[ace.cafe]

I don't think noise will be an issue. You should hear some of the bikes around here. Not to mention the sirens, garbage trucks, construction crews,  car stereos... you get the picture. 

Well, if you want max power, then use an unrestricted header pipe with an open tapered megaphone that has a 3.5" opening at the end, and make the entire length of the exhaust from the exhaust valve head to the end of the megaphone about 48" total length.

[Chuck D]

I don't think noise will be an issue. You should hear some of the bikes around here. Not to mention the sirens, garbage trucks, construction crews,  car stereos... you get the picture. 

Well, if you want max power, then use an unrestricted header pipe with an open tapered megaphone that has a 3.5" opening at the end, and make the entire length of the exhaust from the exhaust valve head to the end of the megaphone about 48" total length.

Ahh... I think i'll just use what i've got for now. But hey, ...   Chuck.

[Rick Sperko]

with 32mm Mikuni and K&N filter.

Chuck, is that a flat slide, and if so what are you using for jets and needle?

Thanks,
-Rick

[Chuck D]

with 32mm Mikuni and K&N filter.

Chuck, is that a flat slide, and if so what are you using for jets and needle?

Thanks,
-Rick

Rick, No it's not the flatslide. It's the Mikuni kit on page 120, 'cept with a pancake instead of a cone. The jetting I haven't sorted out yet.   Chuck.

[Chuck D]

You guys find the rest of 'em. Great stuff.

[Arizoni]

Excellent post Ace.

I'm sure the readers can visualize the effects of a wide open inlet on gaining power because of the reduced restriction on the inlet airflow but for those who are not racing and those who want to reduce the dirt that is being sucked into the engine, I feel a good air filter is a must.

This brings us to the air filter inlet design.

When a "pancake style" air filter is installed very close to the carburetor or throttle body, there is very little reserve air in the system to feed air to the cylinder as the inlet valve opens.
This can cause a starvation of air which results in a power loss.

One way to address this problem so that the system allows as much air as possible into the cylinder is to use a air filter that has very little restriction or eliminate the filter altogether.  This is contrary to having a filter that eliminates as much dirt as possible to protect your engine.

Another solution to the problem is to have a air plenum or housing that can store and supply an adequate amount of clean filtered air when the inlet valve opens.
If such a container of clean air is available there will be very little restriction loss into the cylinder.

Most modern motorcycles have a inlet filter housing to accomplish exactly this purpose.
It will provide clean air at nearly atmospheric pressures while the inlet valve is open and while the inlet valve is closed the somewhat restrictive air filter will have a chance of passing enough air to refill the housing.

This isn't to imply that a highly restrictive air filter doesn't lessen the amount of air that is available for the next inlet cycle.  It will.  But hopefully people will recognize that one can have good filtration and still have a good airflow thru the system at the same time. 

[ace.cafe]

Hi Jim,
Yes that's right, and when I designed the Ace Air Canister filtration system for the Iron Barrel Bullet, I followed the guidelines for airbox volume, so that we would have sufficiently available air inside the filter barrier.
I totally agree.