The Great 2-stroke debate : Compression vs Flow

[Fabulous]

Compression or Flow ? Or a combo of both ?

 

[59billy]

Yes.

 

[Fabulous]

What’s the “golden ratio” ?

 

[59billy]

Hopefully, an actual smart person will reply, but in the meantime time, here's my take:

Empirically, we know that increasing compression with machining, BGD, or a popup piston can increase power.

Or, we can improve flow with muffler mods, matching exhaust port to muffler, polishing exhaust port, etc.

Which one gives the greater return? Probably depends on the saw. Different makers cut different corners. 90% of the mods we do to saws are things that maker could have done, but didn't, to cut costs.

 

[EFSM]

Most of us know what porting is like before we had access to a lathe. I think it can be good to learn without the option of raising compression, because it forces you to follow Randy’s motto: less is more. But the real freight trains come with increased compression.
Also remember that properly done porting can increase the dynamic compression without machine work.

 

[farminkarman]

definitely a combo of both for most saws. I would prioritize having effective quench with a tight squish clearance over some theoretical compression target. I have done too much machine work chasing compression to later find out that I ended up with too much intake or strato duration. Is there a particular saw you are considering?

 

[Agent Smith]

A bump in compression is good. I also go for flow and angles. Maybe im not too smart or skilled but in my porting endeavors, port shapes and angles have been quite important

 

[Fabulous]

Most of us know what porting is like before we had access to a lathe. I think it can be good to learn without the option of raising compression, because it forces you to follow Randy’s motto: less is more. But the real freight trains come with increased compression.
Also remember that properly done porting can increase the dynamic compression without machine work.

I think that is an important point : static vs dynamic compression. Static being measured when pulling the saw over vs the dynamic compression when the saw is cutting at 10000 rpm in the wood. It’s been my experience that too much compression may cause too much heat and then we are going backwards. There is no magic equation afaik - without testing equipment costing thousands if not tens of thousands of dollars it’s a game of little by little until you are satisfied with the performance.

 

[ManiacalMark]

Hoping this was an oil thread, jk. Tough to answer which, varies from saw to saw. Overall I’d say flow is more important. For instance, how much compression do you really want for a work saw?

 

[huskihl]

No set rules. Compression adds torque, but it’s also hard on the crank. Many saw porting guys base their reputation on unloaded speed versus cut speed and torque in the wood. Compression hurts RPM, but if you only have enough torque to maintain those RPMs in 4 inch wood…

 

[Stump Shot]

Compression or Flow ? Or a combo of both ?

Combo+
Everything about a two stroke is duality of functions at the same time. Increases in swept compression from machining the band and base do more than just increase final compression. It also increases squish efficiency and increases the time of the power stroke coming down. The more down push on the piston the more power and torque can be made.
Note that increases in torque are generally followed by increases in horsepower, not the other way around so much.
Transfer flow is important for what is being compressed. The closer the intake fill is to 100% without spilling over into the exhaust the more power can be made.
Proper exhaust flow helps pull in the new intake charge.
Flow through the intake dictates how much venturi effect will pull fuel from the carburetor.
The less is more theory only works on saws that are already running at a better efficiency. Example- Take a MS660 and a MS661, the more efficient 661 needs to be moved nowhere near the amount of the 660 that needs a lot to get it where it needs to go. In the end they run right with each other pretty close.
No load RPM has little bearing on the measurement of power of a work saw. Some to the extent where little to no gain is found however going from let's say a stock peak load RPM of 9,000 to between 10,000 and/or 11,000 RPM and we have a saw that will cut wood for its CC size.
If the rest of our mods like timing etc. are done properly, then it will try and hold it in this increased cut RPM, too little or too much and it will let it run away from peak loaded RPM.
This is just a good start of all the things going on and the cause and effect happening.
At the end of the day how everything about the modifications working together will yield the final results. Just how everyone goes about this will never be identical in nature and give his/hers saw a unique flavor. When you think about it, just the effect of our grinding on the cylinder with each piece of chaff coming off entering into a mathematical volumetric computation into the millions...

 

[Fabulous]

Certainly a lot going on inside the 2-stroke engine! I’d say the flow > higher compression in general as far as increasing performance while keeping the fuel usage and longevity as close to stock as possible. Ideally : more wood cut, faster, and reasonably close longevity wise to a stock saw.