Question:
Jack, finally reviewing this, with the shim method and raising the exhaust port in mind. The shim method should keep all parameters equal, just everything occurs earlier (equivalent to advancing the spark?), but if you raise the exhaust port only, that will lower your duration only, correct? Raising duration to a large extent seems bad due to blowback in the carb, and that makes sense, and only seems possible by changing the piston skirt. How does that translate to more power using either method? I don’t question your methods (you obviously know your shit), just trying to understand. And which do you find better?Lots of questions here, so forgive me if this is silly. Also, when are you gonna do this with a cutout of an NC50 engine?
Let’s take these one at a time, shall we? The shim method should keep all parameters equal, just everything occurs earlier (equivalent to advancing the spark?)
Adding a metal shim or spacer at the base gasket does not keep all parameters the same. By adding a spacer (I usually add a 1.5mm spacer), I am raising the cylinder up. This means I am raising all the ports up. The piston still travels in its same range. But by raising the ports up by 1.5mm or so, the piston no longer blocks the ports. The ports are almost fully open when the piston is at bottom dead center.
So take a moment and remove your cylinder head or look at the video above – don’t be thrown off by the qt50 engine if you are a honda guy. Yamaha and Honda did the same thing. Rotate the piston to BDC by using the kickstart lever or otherwise. You should see that your piston does not fully open or uncover the ports. Since the piston is still partially blocking those ports, your engine is being robbed of flow through those ports. Honda and others restricted these engines by doing exactly this – having the piston partially block the ports at BDC. Less air/fuel mixture flow and less exhaust gas flow = less power.
Let’s compare the ports in a 2 stroke cylinder to an automatic door at the grocery store. Pretend you have a line of people trying to get into the store. If the automatic door only opens 3/4 of the way, only one person can get in at a time. But if you allow the automatic door to fully open, two people can get in at the same time. That line of people is going to enter more efficiently and faster if the automatic door fully opens. If we apply the analogy to air/fuel mixture and exhaust gases, you are going to move more of both by fully opening the ports. You will also change the pressure and thus the velocity of the charge and exhaust gases by doing so but that’s another discussion.
So if you shim the cylinder by using a spacer, we have changed parameters because we have almost fully opened all the ports. Before shimming the cylinder, the ports were partially blocked. So all the ports are open for a longer duration.
We have also changed the timing by raising all the ports. All the ports open sooner then they normally would because they have been pushed up by the shim/spacer. In addition, if we add a 1.5mm spacer, we need to take a similar amount off the top of the cylinder jug so that the piston returns to the top of the cylinder at TDC and we maintain our original compression. Otherwise, TDC would occur 1.5mm below the top of the cylinder jug.
However, doing all this is not equivalent to advancing the spark. Generally when you advance the spark, you get better low end acceleration. Anytime, you raise the exhaust port on a 2 stroke, you generally raise the power band (rpm range where most power occurs) and raising the power band means your low end will suffer. You can tune this out with clutch engagement at higher rpms, for example. But, in our example, we have effectively raised all the ports (including the exhaust port) by shimming the cylinder. Raising the exhaust port = raising the rpm range where more power is created. All of this assumes, you are using a performance exhaust and not the stock exhaust.
Check out this article (especially the porting paragraph) for an explanation of the opposite of what I’m doing by shimming the cylinder at the base gasket. By understanding how to increase low end power, you can see what will happen when we go the opposite direction to increase top end power.
But if you raise the exhaust port only, that will lower your duration only, correct?
By shimming the cylinder jug, we are raising the exhaust port and fully opening the exhaust port (remember it was partially blocked by the piston at BDC before). If you haven’t done it yet, take off your cylinder head and rotate the piston to BDC and verify for yourself that the piston is partially blocking all the ports at BDC.
By fully opening the exhaust port, we are increasing the duration (length of time) that it is open. We are also increasing the duration of all the other ports because they are now almost fully open with the aid of the shim/spacer.
Raising duration to a large extent seems bad due to blowback in the carb, and that makes sense, and only seems possible by changing the piston skirt.
I think you got this from the above video and hopefully I was able to cue it to the exact point that we are talking about – 9:26 into the video. Remember, he had a piston ported engine and not a reed valve engine. He says in the video do not have your intake duration too long as that will result in bringing too much air/fuel mixture and the increased, resulting pressure will cause it to bleed back into the carburetor. To a limited extent, I would think that reed valve engines would handle this better but it still happens on reed valve engines. I would imagine it happens for a similar reason, too much intake duration.
Remember again, we have increased duration of all the ports, including the intake port, by using the shim/spacer because the piston is now almost fully uncovering all the ports. Have we increased intake duration too much by doing this? I don’t know.
Yes, you can increase intake duration by modifying the piston but you can also increase intake duration by modifying the port or by using the shim/spacer at the base of the cylinder.
How does that translate to more power using either method?
Simple answer – By unblocking the ports, I’m allowing for more flow of air/fuel mixture and greater flow of exhaust gases. More air/fuel mixture generally means more power if the engine can take advantage of it. Better flow of exhaust gases also generally means more power if I’m using a performance exhaust. A performance exhaust actually packs more air/fuel charge into the combustion chamber. If I can better the flow of exhaust gases out and and air/fuel charge into the combustion chamber, I can increase power. By using a shim/spacer, I’m raising all the ports but most importantly, I’m raising the exhaust port. Doing so, moves the increased power to a higher rpm range.
Also, when are you gonna do this with a cutout of an NC50 engine?
I don’t have any spare nc50 engines right now but I could do it on a similar engine. Almost all noped manufacturers restricted these engines in the same way by having the piston partially block the ports at BDC.