Crankcase compression for experts

[powerabout]

I think I read on here one of the merc guys who experimented with primary compression said too much of it is not a good thing for rpm. Cant remember who it was.

nosub said with bigger revs its parasitic loss and heat
( just like huge comp at big revs ( 12k plus) doesnt work on a 2 stroke either lots of SAE papers on that from our Japanese friends

[powerabout]

dwilfong

I dont think removing the reed or rotary valve in a HP 2 stroke outboard will make much difference at WOT

Most people on this site will be interested in the science of our vee engines that have to have 2 parts to the crankcase, the part where the crank is and the part on the other side of the rod slot which is where the pumping force comes from.
We want the inlet charge to go through it as fast as possible but on the compression stoke ( talking crankcase here) we dont want it to go back into the crankcase we want it to go into the ports in/behind the liner

I wonder how the Jap 500cc Vee engined GP bikes dealt with this?

[powerabout]

I have just started to study the out board block lay out. It is much different from other 2 strokes as thy separate the cyl from the crank cavity with a slot the rod goes threw. from what I have seen this is to separate the cyl from one and other in this tight V layout. no more no less.
There is a eng designed by Russell bourke that uses the crank case pressure to supercharge the cyl. http://www.bourke-engine.com/
A bike eng also use a tune pipe to draw the cly and case down. A tune pipe eng is a much different animal as compared to a none piped eng.

Dont forget that any 2 stroke pipe, Expansion chamber or megaphone, is also there to ram the charge back into the cylinder.
Outboards dont have gears so a perfect pipe is only uselfull when you have lots of gears or on a jet and with centrifugal clutch to get it going.
Outboard has a unique job as mentioned above.

[powerabout]

I have designed my own tune pipe for my 1cid eng running in my toy boat. I can crawl at 5mph and pop of to 90+ with this set up. It is direct drive with a flex shaft. So it is 1 to1 drive. The design of the pipe dose many things but to interject that here would not be of any help as there is no pipe on a 2.5.
My understanding is there is some scavenging from the ex but it is a whole different setup. I will investigate more on the ex side but that is for a different topic.

Most outboards have very specific designed megaphones, just ask anyone that has altered one.
The 2 cyl etecs have a computer controlled flap valve to change the length.
Its not a forgotten area on any big outboard.

[pointer]

As previously said the modern outboard is many things to many people - meaning the manufacturer must make the outboard without knowing the hull, load or intended speed. The hobby engine ( I run 2 stroke gas ) is much the same and yet very different. Most do not have the rod slot to deal with, and the load is for the most part constant ( your not loading in 4 or five extra bodies and gear). As such my V Hull 26 cc is a different engine than one for a catamaran. Modern snowmobile engines are absolute marvels when it comes to HP, but they have the clutch system to keep them is peak power / torque band. 160 HP out of a 2 cylinder 800cc is phenomenal compared to 200 HP out of a V6 2.0, 2.4, 2.5 or 2.6. But that snowmobile engine would fail badly mated to a outboard lower on a traditional boat of today.
Way off topic now but wanted to put "2 stroke" in perspective.
Some very knowledgeable outboard folks have posted on this thread. Keep it coming!

pointer

[CadCam]

I am new to this group and studying outboard mods. I have lots of experience with two strokes, especially twin cylinder 350cc Yamaha Banshee motors. There is a good gain to be had on them by spacing the reeds OUT from the block. This would of course add crankcase volume. Just my 2 cents...

[Jay Smith]

99% of every motor I build I cut the front case half to increase , crankcase compression ,torque, grunt on the bottom and mid range. My customers rave about the new found bottom end torque these mods bring..

Jay

[luv2gofast]

99% of every motor I build I cut the front case half to increase , crankcase compression ,torque, grunt on the bottom and mid range. My customers rave about the new found bottom end torque these mods bring..

Jay

So what does adding a 1" spacer do, add top end and hurt the low end torque? I see that pop up frequently in discussions on mods.

[Onetime]

The one inch spacer doesn't change crankcase compression. The reeds end up in the same place. Increases runner length in front of the reeds.

[luv2gofast]

The one inch spacer doesn't change crankcase compression. The reeds end up in the same place. Increases runner length in front of the reeds.

Thanks...so what effect does it have in performance?

[mrcrsr]

It sounds like you are comparing a 2 stroke motorcycle engine to an outboard as far as reed spacing placement. They try to increase crankcase volume on the bikes because the expansion chambers pull the mixture (or scavenge) through the engine. Outboards don't have that benefit and depend on crankcase compression to force the mixture into the cylinders hence higher crankcase pressure help performance. I find it makes the engine snappier. The 1" spacer straightens the air flow and creates a tunnel ram effect helps lo and midrange torque

[powerabout]

I think crankcase compression is another tuning tool as if it adds to the low end its not helping the top end and vice versa
there will be a best compromise somewhere...

[powerabout]

Every mod depends on what you want from the eng. You can't have it both ways. are you looking for the family truckster or a top end beast?
A small case will most definitely give great low RPM response. This also depends on the port volume and timing. Port volume means a lot in velocity in to the cyl. To big of a port runner and you will lose velocity at low RPMs. But there is a trade off in top end flow. Transfer timing mans a lot as to where the power band will be best. High transfers will give good timed area but will reduce velocity. with out a pipe to draw the charge high transfers will not work to good. I see that tune pipes are not used on large out boards as the rules for racing do not allow this. Would be hard to have proper fuel delivery with out some very expensive testing to get it right. Could you imagine the power when it hits the pipe!!!

sure but without 10 gears its useless

[olboatman]

OK Guys please help me out! I have read over the years that tuned pipes on a 2 stroke produced more power buy using sound waves to ram part of the charge back thru the exhaust ports to in affect "supercharge" the cylinder--- not suck out the exhaust. Have I been misled ? Gary

[Onetime]

People are mixing up two stroke and four stroke scavenging theories. Interesting.