I looked around on this topic and found no mention of anyone trying to reduce the volume of the crankcase to increase power. Is it just not worth the trouble perhaps? More likely putting things in the crankcase that don't normally belong there makes people nervous.lol Has anyone tried it?
Crankcase stuffing?
- Thread starter Eljeffo41
- Start date
It has been tried and there is evidence that it can improve power HOWEVER...
It has to be done right. Relying on cheap dime store glue and trying to do it inside a crankcase that hasn't been properly prepped (cleaned) can and usually does result in a damaged engine when the 'glue' breaks loose and ends up in places where it isn't supposed to be.
What the proponents of this concept usually fail to mention is the importance of making sure the metal is absolutely clean and free of ANY oil residue. This will require more than just wiping it with a rag or using a solvent.
The performance increase might be okay for an engine destined to be used for racing only where engine failure is expected but for a bike used for daily transportation or neighborhood pleasure riding the risk far outweighs any advantage.
Do a lot of reading before attempting this questionable method of increasing horsepower.
Tom
People have done it, but doing it right is hard on these things. The only real place for any added volume in the crank case is by the bearing holders. Some people have made Aluminum space fillers and some have used JB weld. I guess the problem is that it doesn't really provide a big performance difference in comparison to where you could throw your time otherwise. I have thought about it on my performance build, but couldn't justify my means of doing it in terms of how long it would last (JB Weld) My engine spins 9k rpm at 45mph and that is more than enough it terms of power the way along. It's really a torque monster handling hills with ease. Anyways off topic there I guess the point I am trying to put across is it isn't worth it for a possible increase, because you can do too much and have negative impacts as well. Seems like whats there already is pretty effective at doing it's job unless you were to add a big reed case.
I agree, there are few methods of case stuffing and I'm not a fan of putting JB Weld inside the case because if it does break off, the chunks can do a lot of damage in a really short time, and this stuff will crack eventually from being bonded to the aluminum that's going to expand as it heats up and contract as it cools. I was following another post where Theon made a set of plates to stuff his case which were basically press fit so they would stay put on their own and red locktite was used to fill the smaller voids between the plates and the case halves. I liked this idea so I cut out a set of plates myself out of some .080" thick aluminum but went a step further by drilling and tapping 3 holes thru the case and plates and using allen screws from outside the case to secure the plates. Red locktite was used between the plates and the case, and also on the threads. The allen screws I used were the M4 size so torque had to be very low to prevent stripping.
This worked because it didn't over stuff the case, the transfer cutouts were blended in with the casting of the case halves, and the plates were secured from outside the case. One issue I had was that the new stuffing left little room for the air/fuel charge to squeeze past the flywheels and into the transfer tunnels, then once enough metal was cut out of the way the decrease in case volume wasn't really enough to make it worth it to repeat on another engine.
Later on I may give case stuffing another chance but it'l be on a reed valved engine with a large reed box similar to the Arrow type. Since I got a CNC to make the plates, they will be cut to fit over the bearing area and secured from outside the case with small allen screws. The plate idea seemed good at the time I was doing this, but by the time enough metal was removed around the transfer tunnels the gains were minimal. I will use what I've learned that worked like the set screws from outside the case, and the idea of stuffing around the bearing area which is a good one, but billet aluminum instead of jb weld and it should work well without the fear of epoxy breaking off and destroying the engine, but be out of the way so it won't obstruct the path between the flywheels and the transfer tunnel openings.
All my research seems to suggest stuffing the crankcase will at best make the engine more powerful at higher rpm's, with a definite sacrifice in low end torque (witch in my opinion is in dreadfully low supply in these engines).
At worse you end with a narrower power band, with no extra power anywhere to show for it.
This has to do with the velocity (speed) of gasses as they mix during the transfer phase.
The higher the gas velocity, the more efficient the scavenging process is at higher rpm, with a sacrifice in low rpm scavenging efficiency.
This is because at low rpm's, there is more time between the strokes, meaning there is more time for fresh intake gasses to mix with old exhaust gasses during the transfer phase, reducing engine efficiency at that rpm. The higher the intake gas velocity is during the transfer event, the more undesired mixing you get of intake and exhaust gasses (at lower RPM's).
At higher RPM's there simply is not enough time between strokes for this mixing to occur, so scavenging efficiency is increased (with a higher velocity transfer event). A higher velocity transfer event becomes necessary at extremely high RPM's in order to get all of the cylinder filling, and removal of exhaust gasses done in the tiny fraction of a second it has to accomplish this task. The increased bulk flow of the the higher velocity gasses is also important for high RPM power, as it helps to completely fill the cylinder with fresh intake gasses in the shorter time period available at higher and higher RPM's.
In order to preserve low rpm scavenging efficiency it is necessary not to go to far with crankcase compression. Low end torque might even be improved by adding volume to the crankcase, in an effort to further reduce this efficiency robbing intake/exhaust gas mixing.
In order to achieve efficiency at higher rpms with this setup, it becomes necessary to increase the "time area" of the transfer and exhaust ports in order for the lower velocity gasses to be able to completely fill the cylinder in the shorter time available at higher rpms.
In this case a "boost port" is worth its weight in gold, along with widening (not raising) the exhaust port.
Sean Davis
Apollo Moto Racing
https://www.facebook.com/ApolloMotoRacing/
At worse you end with a narrower power band, with no extra power anywhere to show for it.
This has to do with the velocity (speed) of gasses as they mix during the transfer phase.
The higher the gas velocity, the more efficient the scavenging process is at higher rpm, with a sacrifice in low rpm scavenging efficiency.
This is because at low rpm's, there is more time between the strokes, meaning there is more time for fresh intake gasses to mix with old exhaust gasses during the transfer phase, reducing engine efficiency at that rpm. The higher the intake gas velocity is during the transfer event, the more undesired mixing you get of intake and exhaust gasses (at lower RPM's).
At higher RPM's there simply is not enough time between strokes for this mixing to occur, so scavenging efficiency is increased (with a higher velocity transfer event). A higher velocity transfer event becomes necessary at extremely high RPM's in order to get all of the cylinder filling, and removal of exhaust gasses done in the tiny fraction of a second it has to accomplish this task. The increased bulk flow of the the higher velocity gasses is also important for high RPM power, as it helps to completely fill the cylinder with fresh intake gasses in the shorter time period available at higher and higher RPM's.
In order to preserve low rpm scavenging efficiency it is necessary not to go to far with crankcase compression. Low end torque might even be improved by adding volume to the crankcase, in an effort to further reduce this efficiency robbing intake/exhaust gas mixing.
In order to achieve efficiency at higher rpms with this setup, it becomes necessary to increase the "time area" of the transfer and exhaust ports in order for the lower velocity gasses to be able to completely fill the cylinder in the shorter time available at higher rpms.
In this case a "boost port" is worth its weight in gold, along with widening (not raising) the exhaust port.
Sean Davis
Apollo Moto Racing
https://www.facebook.com/ApolloMotoRacing/
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The effects of stuffing the case are extremely minimal; (so minimal that I don't bother doing it anymore).
As mentioned above, there's more chance of doing serious damage to your engine from the epoxy stuffing coming loose inside than gaining any real performance from it.
The reason for stuffing the case is because air is compressible & the less air being moved, the more efficient it will be.
Picture a large air-tight room with a small straw through one wall & a 2nd straw at the other end of the room... blowing air into one straw will barely move any air out the opposite end straw because of the large volume of air in the room.
If there's only one long continuous straw going across the room?, you'll actually blow a lot of air out the other end of the straw! (it's the same principal with stuffing the case except the effects are minimal).
As mentioned above, there's more chance of doing serious damage to your engine from the epoxy stuffing coming loose inside than gaining any real performance from it.
The reason for stuffing the case is because air is compressible & the less air being moved, the more efficient it will be.
Picture a large air-tight room with a small straw through one wall & a 2nd straw at the other end of the room... blowing air into one straw will barely move any air out the opposite end straw because of the large volume of air in the room.
If there's only one long continuous straw going across the room?, you'll actually blow a lot of air out the other end of the straw! (it's the same principal with stuffing the case except the effects are minimal).
This is exactly what you get if you port out your reedbox and intake.
Me? What do I do? I fill the reedbox with JB Weld...
Wait until you see my crankcase stuffing picts (not a Blaster) , coming soon.
Sheesh, Sorry guys, thought I was posting this to a Blaster forum. Duh! Forgive me?
I have stuffed HT bike cases coming...
Steve
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