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Thread: Compression

  1. #11
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    I've never seen info on deflector pistons and combustion chambers, anyone?

  2. #12
    Team Member smittythewelder's Avatar
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    Roflhat, 3mm is an enormous figure for squish. I can't imagine the factory built that much into the engine; the squish would be ineffective. For some reason, your link won't load (everthing is running real slow on this bleeping laptop, tonight), so I can't see what the insides of the engine look like, incl. the inside of the head. Are you sure you are getting the end of the solder bumped up against the cylinder wall when you run the piston through top-center? I wouldn't expect the factory squish to be very much over 1.5mm. But again, you need to find somebody who knows the engine.

    Are you (and Powerabout as well) googling those sites I suggested? You can find answers to a lot of what you're asking.

    (EDIT) okay, found the thread. You're going near 52, 52 KNOTS!!!, with an inflatable??!! And you're disappointed? Lordy!! Anyway, doublecheck the squish, figure out which way to poke the solder to get the widest area of the squishband. That looks like a well-supported and stout crankshaft, so 1mm of squish ought be plenty safe. Does anyone build those engines for racing?

  3. #13
    Team Member Roflhat's Avatar
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    Thanks again. I've been googling about "2 stroke squish" and similar, trying to take it all in.
    so 2-2.5mm is generally regarded as "loose" squish. Also apparently on a yamaha 50 3cyl (very similar to the tohatsu), the minimum clearence is 1.5mm with a shaved head. So I'd expect a value somewhere around there. I'm going to try and get hold of some thicker solder to try.
    I'm not sure if the solder was located properly, I'll try and get one of my mates to pull the engine over while I hold it in place.

    Haha yes I know! 52 knots isn't slow, but it also isn't fast enough!
    I know it's safe to take 1mm off the head and 0.5mm off the block. I didn't take any off the block, would that suggest I can take another 0.5mm off the head?

    There's guys in south africa who modify them for racing, but they mostly use the yamahas in the modified class. I've got some good advice from a couple of them, but trying to get more is like pulling teeth. They say for the head to make a real difference it needs to be 19cc. I'm trying to get hold of a billet cylinder head with a centre fire spark plug as well.

  4. #14
    Team Member Roflhat's Avatar
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    Found this from Fast Fred on another forum - "when the pistons is at TDC the distance between the piston crown from the out side edge , to the top of the head needs to be .025min should grow to .035 as it moves to the chamber." (This is about the tohatsu 50)

    EDIT: also found "settin the squish clearances in this head, a cut has been made over the deck surface of the head, .150", then the squish clearances are put back.

    on this Race Moda, the piston crown pulls out of the block, past the head gasket and in to the head"

    0.15" in 3.81mm. That's a massive amount to be taking off the head?

  5. #15
    Team Member smittythewelder's Avatar
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    Seems like a ton to me, well over 1/8". Since we're talking in inches now, I'd expect an unmodified factory engine, at least with bikes, to have a squish height somewhere in the .055"-.075" range. The engineers at the factory know all about the benefits of tighter squish, but the reality of manufacturing production line tolerances forces them to go on the loose side. If the stock engine had the squish on the loose side, say at .075" at the outer edge, and you went with Fast Fred's target of .025" at the outer edge, you'd be taking about .050" off the head, not .150". BTW, measure all cylinders, so that you don't mill the head and then find out one piston is "printing" (bumping the head at WOT). When you rebuild any multiple cylinder engine, you can measure all the parts plus actual crank stroke at each journal and then recombine short and long parts to make the finished project have the pistons all come out to about the same height. Once you start doing this, any engine you take apart, cars, tractors, airplanes, you'll find yourself measuring and matching parts in this way.

    Again, not knowing the engine and just guessing, I'd guess you could mill the head, if you had to, until the deck surface comes even with the very outermost edge of the squishband. If your pistons are so recessed into the block that you still can't get to your targeted squish dimension, then you have to start decking the block.

    Hard to tell from the photos, but it appears that the combustion chambers are as-cast, not machined. So you better take a close look, and check the volume of each combustion chamber, and then even them up as seems appropriate. Engines typically have one or two cylinders that are more prone to detonation than the rest. Maybe they tend to get a leaner mixture, maybe the coolant flow behind them isn't as good, whatever. The most detonation-prone cylinder is the one you have to tune the whole engine around.

    Keep looking around for tech and engine-building articles, by people whole mostly know a whole lot more than I do.

  6. #16
    Sam Cullis Mark75H's Avatar
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    Quote Originally Posted by Fastjeff57 View Post
    Interesting question. Take Merc 850 (fours): The book calls for 27 degrees max advance, which is a ton more than for the similar cylindered triples and in-line sixes. Further investigation reveals that the early 850s (with the 4 exhaust ports) had a higher compression ratio; and one doesn't hear of detonation problems with them (vs. the in-line sixes with only 21 degrees max advance).

    Hope that helps.

    Jeff
    That's because the 4's do not have the 120 exhaust tuning increasing the cylinder pressure.
    Since 1925, about 150 different racing outboards have been made.


  7. #17
    Team Member smittythewelder's Avatar
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    Sam just gave us a perfect example of why dynamic compression (in this case from superior cylinder filling by the efficient pulse-tuned exhaust peculiar to a modern 2-stroke in-line triple or V-6) ultimately means more than any particular compression ratio number. Don't try for what somebody tells you is the best c.r. number; your motor is different from his, with it's own "needs."

    Thanks, Sam, I wouldn't have thought of that one. A very smart engine-builder/tuner, Jim Hallum, taught me about pulse-tuned triples in about 1972 when he took an OMC 60hp triple powerhead and built it into an F engine for Gerry Walin, with porting, squish and compression, bigger reed-blocks and carbs. They tested that engine with open megaphones, pulse-tuned exhaust, and expansion chambers. The latter made the best power, but somebody stole them. This was long before there was a MOD class for these OMC triples, before there was even a MOD division. Oh yeah, the Crescent Super C motors also had pulse-tuned exhaust, I think; maybe not the system for the very best power, but mechanically simple and LIGHT.

    Another thing to google: engine blueprinting. When you google this stuff, don't start out specifying for 2-strokes. This is basic hot-rodding, applies to any piston engine.

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