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06-25-2014, 07:44 AM #61Screaming And Flying!
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06-25-2014, 07:52 AM #62Screaming And Flying!
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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?
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06-25-2014, 10:16 AM #63Screaming And Flying!
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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.
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06-25-2014, 11:25 AM #64Screaming And Flying!
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06-25-2014, 01:12 PM #655000 RPM
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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
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06-26-2014, 09:48 AM #66New Member
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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...
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06-26-2014, 03:09 PM #67Supporting Vendor
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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..
JayJay @ JSRE
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06-26-2014, 10:33 PM #68
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06-26-2014, 11:38 PM #69Scream And Fly VIP
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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.
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06-26-2014, 11:47 PM #70
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06-27-2014, 06:33 AM #71
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
Last edited by mrcrsr; 06-27-2014 at 06:36 AM.
action mobile marine, 772 528 0754, dealers for aces fuel products,wolf efi, pro marine,latham and gaffrig/livorsi. we build gearcases and modify them(3 litre) to ratchet, and powerheads as well. 21 skater/3 litre wolf efi 113 mph the engine build http://www.screamandfly.com/showthre...-3-litre-build
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06-27-2014, 07:03 AM #72Screaming And Flying!
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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...
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06-27-2014, 10:28 AM #73Screaming And Flying!
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06-27-2014, 11:47 AM #74
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
Last edited by olboatman; 06-27-2014 at 11:49 AM. Reason: old age!
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06-27-2014, 11:56 AM #75Scream And Fly VIP
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People are mixing up two stroke and four stroke scavenging theories. Interesting.