Does anyone know if all the 37cc a/c Cags use the same cylinder/head? I would like to bump up my compression ratio but don't want to alter the port timing. I have access to high-octane racing gas, so 15:1 or higher should would be okay. I've looked at the combustion chamber design of my cag (Lucky 7 aka Alien) and it doesn't look very efficient, since the spark plug is off center and is in a fairly deep pocket, so if there are other cylinders out there that look the same on the outside (I don't want to be in the open class) but have a smaller or better combustion shape I'd like to try one.
once you buy a head kit you will realize that the factory dome design is shi$! the compression is like 9:1 with a .5mm squish! i am running 17:1 ada head dome on a 44mm piston engine and it sound like a yz 80 the compression is crazy high my next step is alcohol!
Yep hackmucnh is right. And just to clarify, you do want to alter the port timing. Also, having an offset dome and plug electrode is efficient. Along with a tight squish band, an offeset dome helps prevent spontaneous detonation by increasing cooling efficiency.
Yep hackmucnh is right. And just to clarify, you do want to alter the port timing. Also, having an offset dome and plug electrode is efficient. Along with a tight squish band, an offeset dome helps prevent spontaneous detonation by increasing cooling efficiency.
Ritchey, can you explain that? i'm not sure what you mean? bit early in the morning.
"Researchers have found that it is at the very outer limits of the cobustion chamber, called the "end gases", that self-ignite to cause detonation. These end gases are heated by the surrounding metal of the piston crown and combustion chamber, and also by the heat radiating from the advancing spark-ignited flame. If we move the combustion chamber down as close to the piston crown as possible, no combustion will occur around the edges of the chamber. The chamber just described is called a squish-type combustion chamber because of the squish band around its edge. The squish band was designed to squish the fuel/air charge from the edges of the cylinder toward the spark plug."
"In an effort to minimize detonation, some manufacturers have chosen to use an offset squish type combustion chamber(Figure 2.2). The exhaust side of a two-stroke cylinder and piston is always the hottest, even though cooling air flow is much better here than on the back (inlet side) of the engine. There are several reasons for this all associated with the passage of very hot (630*C) exhaust gas through the exahaust port. The escaping gas heats the exhaust port and cylinder wall as well as the side of the piston. this can cause the piston to expand abnormally and in some circumstances to sieze. To take care of this possibily, a manufacturer may choose to increase piston to cylinder clearance, but this may not be desirable as extra clearance can increase leakage past the rings and usually results in high piston wear. A safer step is to move the combustion chamber to the rear of the head. If this is done, the front of the piston crown is shielded from the combustion flame by the squish surface. Then, when the front of the piston is heated during the exhaust stroke, it will not expand so far due to it being much cooler initially."
Well spotted ritcho, I didn't really have time to ponder why they made it like that, just knew it had to go I actually had the idea they did it just to angle the plug and make the motor more compact so it can fit on the lawnmower or whipper snipper or whatever it was designed for, probably a bit of both? either way, central domes are the shiznit.
Ritchey, good research/knowledge there! The only problems I have (with the stock cag head, not your points):
1. compression is way too low, and I wouldn't mind shaving the base to up the compression, though from what I've read this would alter the port time (in a negative way? correct?),
2. I believe the "pocket" they've created for the spark plug is too deep. I agree that it should be closer to the intake side of the engine, but I think it would be beneficial if it wasn't in such a deep pocket (see blah's 1st picture, though if the individual has not seen it in person it is hard to get an ideal of the depth of that pocket from the picture). I would think that this head would be more sensitive to the placement of the spark plug's electrode much more than if the spark plug came in at more of a tangent (such as the sweet looking camber in blah's avatar pic). I'm going to experiment with spark plug indexing washers to see if it has a noticable difference, though I'm sure my butt-dyno isn't that accurate).
3. If I had to choose a head shape such as blah's avatar pic (with a couple points higher compression) and the stock head shape with its "low" compression, I would take the 1st. I could easily up the octane of the fuel to stop detonation, right?
My preference would be a combustion chamber just like the second picture from your book. A quench area round the edge of the camber, with the spark plug offset toward the intake, but not to such an extreme as the stock cag position. My hope was that one of the cag cylinders had a dome shape such as this, but that is unlikely since I assume all of them (the china copies) have similiar dome shapes for the aircooled 39cc motors. I would slap on a badazz head kit, but then that would throw me into the open class, where my modified cag wouldn't stand a chance against the "real" open class bikes.
I would get a spare cylinder and modify the stock chamber by tig welding and then machining the chamber to my desires, but I couldn't fit the tig gun and my fat hands in that itty bitty cylinder. :-(
A final note. I noticed that the spark plug when tightened leaves several threads of the chamber exposed (in other words, the plug isn't long enough to cover all the threads when looking from inside the chamber). I haven't measured, but I also think the standard plug doesn't come close to contacting the piston. Anybody have an ideal of how much of a compression ratio increase a 1/4-1/2" longer plug would give (I doubt very little, but I don't have the basic specs for the chamber volume, etc to calculate the difference), and assuming it would fit without interference to the chamber/piston?
No problem, it's a great book. About the only other books I really have are my engineering text books and they are not fun to read! And yeah the compression sucks on the stock cylinder, the dome is too large, and shaving the base will hurt the rpms but increase the CR. That head in Blah's avatar is truly a peice of art. I love central domes when you can utilize them properly.
Pity my crappy camera can't see the sparkplug, it's been modded by filing the end back, have not tested it yet but it's just another thing to tweak. Probably only good for people who are right next to their octane limit when upping CR, or maybe for someone who's runnign a bit too much advance? have to see wont i.
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Roundingoff the end of the side electrode as shown above reduces the peak temperature of the electrode, causing considerably less possibility of detonation from this source. Also in the relatively swirl-free conditions that exist at low rpm, this form gets the charge burning more effectively. Be sure to cut the electrode so the end of the tip is no further then half way over the center electrode. If plug life is not an issue, cut it so it is 1/5th to 1/3rd of the way over the center of the electrode. This round cut back will insure the plug will burn within it's heat range without over heating in adverse conditions, with improved overall performance.
_______________Paul Govostes Dec 11 2003_______________
Pity my crappy camera can't see the sparkplug, it's been modded by filing the end back, have not tested it yet but it's just another thing to tweak. Probably only good for people who are right next to their octane limit when upping CR, or maybe for someone who's runnign a bit too much advance? have to see wont i.
***
Roundingoff the end of the side electrode as shown above reduces the peak temperature of the electrode, causing considerably less possibility of detonation from this source. Also in the relatively swirl-free conditions that exist at low rpm, this form gets the charge burning more effectively. Be sure to cut the electrode so the end of the tip is no further then half way over the center electrode. If plug life is not an issue, cut it so it is 1/5th to 1/3rd of the way over the center of the electrode. This round cut back will insure the plug will burn within it's heat range without over heating in adverse conditions, with improved overall performance.
_______________Paul Govostes Dec 11 2003_______________
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Hahah... I used to do that to my 280zx back in the early 90's. It's amazing how just clipping the electrode then filing it to perfection is just enough to be noticable on the butt dyno... it also helps light up the fuel quicker since the electrode isn't blocking the spark (does the same thing that indexing does but without the washers, time and trouble). I've never thought of doing it to a 2 stroke like these but I may grab a few extra plugs and experiment a little...
Hahah... I used to do that to my 280zx back in the early 90's. It's amazing how just clipping the electrode then filing it to perfection is just enough to be noticable on the butt dyno... it also helps light up the fuel quicker since the electrode isn't blocking the spark (does the same thing that indexing does but without the washers, time and trouble). I've never thought of doing it to a 2 stroke like these but I may grab a few extra plugs and experiment a little...
My old Outlaw dirt kart, yz 250 used to run great but kill $AU8 platinum plugs like crazy, put a $AU3 -es in and it would last forever? I wonder if this mod would have given me the best of both worlds, i bet the plug makers would be pizzed if it did
I always thought platinum plugs would las longer than copper. Platinum is a poor conductor of electricity, another reason why I use copper plugs instead of platinum. Silver and gold would definitely be a better alternative, at a greater cost as well.
Quote:
Originally Posted by blah
My old Outlaw dirt kart, yz 250 used to run great but kill $AU8 platinum plugs like crazy, put a $AU3 -es in and it would last forever? I wonder if this mod would have given me the best of both worlds, i bet the plug makers would be pizzed if it did
I read a review a few years back or so of mods done to plugs by some guy that had a mazda RX7. Anyway, he did quality testing rather than using the butt dyno or "it'll do 33.1 mph now!". Can't remember if he used a chassis dyno or drag strip timing, but it was much more effectual than "hey that feels more powerful". (grin)
His main research was in the "hyped" spark plugs (split fire, douple/triple/etc electrodes) and found that the less electrode blocking the spark the more power he got. His best result of all of them was a plug similiar to the one in Blah's picture, which was the least amount of electrode that would still carry the spark but expose as much of that spark as possible to the gas mixture.
Blah's cylinder doesn't need spark plug indexing, but the stock cag cylinder seems to be a prime candidate for such (so the electrode is closest to the head and not the piston.
Well the snow finally melted today and the rain stopped long enough for me to go and ride a little. After awhile I decided to moddify the plug electrode like in Blah's photo. You can see the result in my photo below. Notice in the 2nd photo how much thread is left in the plug hole and that is with no plug washer. I'm going to the auto parts store tomorrow and picking up some longer reach NGK plugs. That should give a major bump in compression. It already almost rips my arm off when I pull start it now. Anyone have the numbers for the longer NGK plugs?
ngk b6es is the long reach plug it has a 1/2" reach! i have used it in my cag before but had to use a couple of copper washers so the plug would not hit the piston!
I actually had the idea they did it just to angle the plug and make the motor more compact so it can fit on the lawnmower or whipper snipper or whatever it was designed for, probably a bit of both? either way, central domes are the shiznit.
