Advancing Ignition

[skipu]

Jak - I don't have a site that has the formula, but I can explain, I think. There are quite a few ways to formulate a two stroke engine, I know it seems like if you limit the amount of time the charge can push the piston down it would also limit the power produced. The problem is that you also limit the time for the exhaust to blow down the pipe, thus limiting the amount of charge you can introduce into the chamber. Remember the piston is not pushing the charge out, it is the pulse from the expanding gasses. Also if you are using an expansion chamber you have to wait for the ram affect to return from the pipe, so you also want to let some of the charge escape down the pipe to be returned with the shock wave. There is another reason that the ports are manipulated and that is to restrict or enhance the amount of power, this engine is supposed to be at the two HP only to meet some of the restrictions placed on the motored bikes. You can throttle a two stroke at both ends, by restricting the end of the exhaust of a larger engine you can use the torque produced by the larger bore and still limit the top end. I used to sneak a little hole in the muffler of the little Honda's we used to run around on, I never let on and the rest never could figure out why my bike would always run away from there's. I hope this helps, I have built a lot of engines, and I like answering questions. Have fun, Dave

you said it better than i ever could and that is the theory wow very clear also this is good info you should do a tutorial on two stroke theory!!!

 

[skipu]

what the heck are you using to work on the transfer ports, lil tiny guys with picks & shovels?

I have an extra cylinder here if you have something to work on the transfer ports with '
I think matching the case to the cylinder would help allot
I dont like the idea of raising the exhaust port as it shortens both the compression & power strokes
Ive heard that most pros turn down the base to effectively lower these ports
take care
JAK

itty bitty radius die grinders on a jeweler tool (patience)it is trying at times!!

 

[skipu]

ooops !!! and matching the cases can make all the difference in the world! case stuffers help considerably too

 

[skipu]

Skipu - Tell me about it, one of the things I have noticed is that as I get older the days just get shorter. I only have one cylinder extra, I have fooled with the ports leaving the exhaust alone and have found that I could not get anything out of the motor, in the end I had to move the exhaust up to the 1.1" from the top, I think that is still conservative. On the CR Honda there is only about 1/2 of the stroke for power the rest is for scavenging. On some of the newer CR's they have the variable port timing with the little rotary door they invented, that still I think does not get it to the height that we get at a full port job, ours is still lower at 1.1. I have a 29 Ford in the garage I hope to someday get finished, but you know the days that get short. LOL. Have fun, Dave

thats exactly what i want to try a powervalve low rpms port size smaller hence torq. as throttle position and rpm increase port size increases this way it keeps you at peak performance through the entire rpm range i think its worth a go i cant wait to start it but ive been hard at work on my scratch build jack shaft set up wooooo!! time together / apart together / apart repeat etc . lol have a good 1