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  1. #61
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    Quote Originally Posted by baja200merk View Post
    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

  2. #62
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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?

  3. #63
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    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.

  4. #64
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    Quote Originally Posted by dwilfong View Post
    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.

  5. #65
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    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.

  6. #66
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    Quote Originally Posted by dwiforgotten View Post
    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.

  7. #67
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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

  8. #68
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    I have noticed that there is a lot of talk about the ex chest mods with theses eng and different tuners. I have some soft where that I will start playing around with the 3 in to one setup and see what it shows.
    Scavenging mean a whole lot when it comes to the ex system. It will pull the fuel air mix from all the way back to the carb if done right. That is where I have found the most power is in the way the air moves threw the case and forgetting about piston pumping.
    I call it strait shot porting. imagine the air running strait threw the case with the piston at BDC.
    Finger ports cut in the rod slots? The reason for this?

  9. #69
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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...

  10. #70
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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..

    Jay
    Jay @ JSRE

  11. #71
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    Quote Originally Posted by Jay Smith View Post
    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.


    98 Laveycraft Sebring 20.2 Tunnel 280 "catchin" motor. 90s? It's enough for me!

  12. #72
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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.

  13. #73
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    Quote Originally Posted by Onetime View Post
    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?


    98 Laveycraft Sebring 20.2 Tunnel 280 "catchin" motor. 90s? It's enough for me!

  14. #74
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    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

  15. #75
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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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