2 strokes


why do 2 strokes have more power in their powerbands? is it because of the expansion chamber?


No, it’s because every down stroke is a power stroke. It’s every ohter down stroke on a four stroke engine.


so, how much does a expansion chamber help


The expansion chamber works like a super charger. It harnesses some the energy of the escaping exhaust to cram more mixture back into the cylinder just before the exhaust port is closed. The problem with the expansion chamber is that is works best at only one RPM so you have to design the exhaust pipe to fit the RPM range you will be working in. The expansion chamber dimensions get too large for low end pull. So, you are more or less restricted to medium to high RPM ranges. If you have driven a two cycle motorcycle or gocart, you will notice how peaky the torque is in a specific range and the need to be in the correct gear all the time.


The DKW automobile that was imported to the United States in the late 1950’s and early 1960’s had a 3 cylinder 2 stroke engine. The emblem on the grille had “3 = 6” in the center. The implication was that since every down stoke was a power stroke, it had the performance of a 6 cylinder engine.


This is not necessarily true anymore. The newest four-stroke engines are so sophisticated that they are catching up with the power output of two-stroke engines. Pretty soon two-stroke motorcycle engines will be history because of new technology.


Heck, they have been history on the highways since the mid to late 70s. I would think that with significant development investment 2 strokes would be making advancements as well. There has not been as much investment due to the smaller market though I suppose racing and the off-road market might be a significant pressure. Another advantage of 2 strokes is the relative simplicity. They are less expensive to build, maintain and rebuild. When you think of how complex an efficient, powerful 4-stroke engine is it is getting mind-boggling. Variable intake length, variable valve timing schemes, 4 valves/head,?.

Expansion chamber? It is kind of a misnomer. The exhaust is designed to scavange the combustion chamber at a particular time after the previous opening of the exhaust port. It does not have so much to do with the exhaust gasses expanding as with pressure waves reflecting back and forth in the exhaust system.


I am not much for 2 strokes, ive ditched the xr75 and am getting a crf 230, 2 strokes are huge in the dirtbike racing industry because of their lightweight, very easy teardown/rebuild, and low maintainence. i dont like them because they have such big powerbands that there is no power outside of it, and keeping one in the powerband is hard to do when you are trying to navigate your way through the track, 4 strokes on the other hand, have more mods, DONT SMOKE, and have tons more low end torque! to me, its worth it!


The technology now exists to build clean 2-strokes. Evenrude & Johnson outboards use it. No fuel is injected until the ports are closed so no fuel (or oil) gets out the exhaust port. They are light-weight, powerful and reliable and meet the latest emissions standards…Maybe Detroit will notice…