Thursday, November 19, 2009

Crankcase compression: theory and implications

EDIT: MAY 2014:  
This is an article I wrote 5 years ago, since then I've learned a lot and realized that primary compression is a lot more complicated in its overall effect on an engine.

I don't necessarily agree with all the comments, but I don't really have time (and still probably don't know enough) to properly discuss or refute them. At face value this is a pretty basic article describing the basic crankcase pumping operation on a two stroke. I could take it down, but it helps to remember how far I've come and how much I still have to learn, so I'll leave it, but use it at your own risk.


I originally wrote this for a moped army forum thread located here:

With the increase of availability of moped parts here in the U.S., we've seen a huge boom in the technical sophistication of domestic tuners. With this availability, everyone seems to be pushing the envelope further, trying to squeeze out every last iota of power from our little machines.

A good example of this is the availability of 'stuffed' cranks. Many of us have purchased them for absurdly cheap through the US moped parts sellers, having only a vague idea of what it really means to 'stuff' a crankcase, why it can produce more power, and what the implications are for the rest of the engine.

Crankcase stuffing is decreasing the volume of the crankcase to improve what is called the 'primary compression ratio'- the compression ratio for the system comprising the bottom of the piston, crankcase, and transfer ports up to the edge of the piston.

Primary compression is defined as Vi/Vf which is:
volume final divided by volume iniitial.
final volume is the displacement volume of the piston plus the volume of the case. initial volume is just the volume of the case.
when doing this for a cylinder, the head volume is relatively small, so we can achieve 11:1 (11/1 = 50cc disp + 5 cc head volume/ 5 cc head vol.) or better. When we do this same thing in the case by either increasing the displacement, or decreasing the crankcase volume (say, by stuffing the crank) we can get better compression ratios...

so what? right? why does changing the compression ratio help us?
having a higher crankcase compression ratio lets us get more air in during the intake phase, and creates higher pressure pushing that air through the transfers.
This is obvious when we go back to high school chemistry and remember PV=T(nr): Pressure times volume equals temperature, if you decrease the volume of an area, the pressure or temperature MUST increase to compensate. In the very short time that the piston is going up and down, we can assume the temp cant change much.
This makes sense for the second part, the transfer pressure. The higher pressure from a smaller final volume will push the air through the transfer ports at a higher velocity, improving scavenging, and transferring more air in the very short time the ports are open. But this only makes sense if we are getting more air in the crankcase in the first place. The displacement of the piston isn't changing, so we cant change how much air gets into the crankcase right? Wrong.

The higher ratio means that when the piston goes up and creates a vaccum in the crankcase, the vaccum is stronger. Even though transfer pressures can be as high as 50 PSI, the intake pressure pushing air through the carb and past your reeds is only 1 bar, because it is being pushed in by the atmosphere. You want to get as close to a perfect vaccuum as possible to get air to transfer as quickly as possible. (This is why piston port engines can blast reeds at the top end, the piston port has much less resistance to flow, but i digress. ) Having a compression ratio of infinity, as in 0 crankcase volume, with no leaks, would create a 'perfect' vaccuum, This would provide 32 PSI across your intake tract from the air filter in. In reality this is impossible, the ratio drops very quickly with increase in volume in the case, and the potential transfer pressure drops from 32 even faster, by PV=T. The time your piston is going up is limited by your RPMs, and the amount of air that can transfer is limited by the pressure, restrictions in the intake, and time. Therefore, if you can make more pressure (by improving the vaccuum in the case) you can get more air in the case on each stroke, which means you get more air into your cylinder, and can burn more fuel.


  1. Oh dear, it seems the youth of today are very gullable to devised-through-ignorance, outdated and proven-wrong ideas from Victorian times. Do you think you are the first ones to come-upon and try the idea?

    Don't give up your day-job.

    Stuffing crankcases DECREASES performance.

    Any extra 'power' you may feel comes at the cost of a very narrow power-band. Only a slightly 'off' ratio will cause this, not only a too high a ratio but a too LOW ratio will also give a peaky powerband. It's this peaky kick-in-the-pants that fools you into thinking you've 'tuned' your motor. The too-high ratio just squirts the charge out of the exhaust port, LOSING power, only when the port closes quick-enough is enough charge left to produce power, hence the sudden 'zip'. You can tell if the crankcase ratio is incorrect by THE SMELL of the exhaust, the stink of unburnt petrol.

    Yes, all good engineers/'tuners'/mechanics/riders can occasionally be seen sniffing their exhausts even on a top engine, they can tell how tight the carburation and ignition is.

    Be aware that all modern (from around '70 onwards) 2-strokes already have the correct compression for the application.

    For it is around this time engineers became aware of that there was an ideal, and why. The development of MX-engines around this time taught the engineers more about 2-stroke design than at any other time in history.

    If you're nervous about trying it, put a gloved-hand over the stinger, then sniff that. It takes great experience but there's only one way to get that. I believe there are sensors on the market that analys the exhaust via computer, but haven't found any information on that yet.

    You might think road-racing bikes have a very high ratio - not so. They used to, why the fabulous 2-strokes of the 60's had extremely peaky powerbands and an astounding number of gears. Example: Suzuki had a 3-cylinder 50cc that gave around 19bhp at 17,000rpm (?) and had 19 gears as the powerband was barely 200rpm. The gearbox was 3 times the size of the engine! Luckily nobody had to ride it as the rules were changed to 1-cylinder and 6 gears. Only a few years later the singles were producing over 20bhp, with a powerband so wide the 6-speeds were enough.

    2-stroke power is produced by the expansion chamber PULLING the charge through the crankcase, hence the 'extra' inlet ports from the reed-valve connected directly into the transfer ports, or, in the case of my '76 250 HVA MX via finger-ports direct from the reed-valve into the cylinder (a better solution to the Japanese concept, that can lower the crankcase compression under the optimum). Gas-flow though the crankcase is important for lubrication as well as cooling and transferring the charge, hence stuffing modern, well-designed cases is totally the wrong thing.

  2. "Fresh charge transfer to the engine cylinder follows a trend similar to that for air flow. Fresh charge lost through the exhaust port also increases with decreasing crankcase clearance volume, owing to greater fresh charge transfer and higher cylinder pressures. Further, the rate of increase in charge loss is greater than the rate of increase in charge transfer at small values of crankcase volume. Therefore, fresh charge trapped in the engine cylinder which equals charge transfer minus charge loss, decreases below 70% of design clearance volume".

    Parametric Studies Using a Two-Stroke Engine Cycle Simulation (V. Sathe, P.S. Myers and O.A. Uyehara)

  3. also when you decrease the crankcase volume, you increase the amount of "pumping work" (compressing the charge in the crankcase) and that detracts from your net horsepower. This paper also notes that max. brake hp gains come at 86% of design clearance volume, but only 1.2% of that theoretical gain can actually be realized "at the wheel" due to frictional losses.

  4. sorry mistake I should've said that the gain over stock BHP is 1.2%.

  5. Actually exhaust sniffers have some pretty good company. In not to far past times when we were trying to keep Mig 21s off our backs on the other side of the globe it was not uncommon to see a lowly airman tip toe around to the back of our fastest birds,insert a glass tube conected to a glass hypo looking gadjet and catch a whiff. This whiff went in a 7 figure "MassSpec" and we could see where we wanted to set our dial for the next go around. Keep in didn't hear that here though...

    1. Tuned 2 stroke exhaust pipes work by packing the intake charge from SOUND waves and act like a supercharger helping compress the air-fuel mixture--- They do NOT suck out the exhaust!

  6. Actualy they do both thats what the two cones are for the first one aids in the escape of exhaust gas and the second one sends a sort of shape charge echo back to push some of the air fuel mix back in to the cylinder before the port closes .....but good luck making one the will fit and function in the space provided on a chainsaw :(

  7. I have a friend who have experience over 40 years. He said that for the race engine crankcase compression should set to the max but it will lower the rpm a little bit. I think the greater pumping work caused rmp lower. And tuned exhaust can push back fresh charge which lost to the exhaust port back to the cylinder. What if we can suck much fresh charge to the exhaust and then the tuned pipe push it back much to the cylinder just before exhaust port closed?

  8. If I have casematched the transfer ports are bigger wouldn't a stuffed crankshaft balance the lost compression
    A35 tomos 45 mm alukit

  9. If I have casematched the transfer ports are bigger wouldn't a stuffed crankshaft balance the lost compression
    A35 tomos 45 mm alukit