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  1. #1
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    Question Crankcase compression for experts

    1. How important is crankcase compression to performance & at what RPM is the increase most beneficial?
    2. If crankcase compression help significantly, has anyone tried welding a plate to the bottom of the piston, sealed of course, at this would remove several cubic inches of crankcase volume, & seems to be the only area to significantly increase crankcase pressure? The piston would now be hollow & if done properly would add very little weight.

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    Increasing crankcase compression would be most noticeable at the bottom end of the RPM range.
    Sealing the bottom of the piston would be a problem because the cooling effect of the incoming air/fuel charge on the bottom of the piston crown would be lost.
    Some engine builders have used stuffers attached to the cylinder floor to take up the volume. This adds no weight to the piston and isn't the same kind of cooling problem as the sealed plate that you suggest.
    Being a close fit inside the piston cavity makes most racers wary of the cylinder floor stuffers. You can imagine how much severe damage might occur from a minor piston failure.
    An extreme amount of crankcase pressure will hasten reed failure at high RPM.

    Tony
    Tony Brucato


    (919) 718-0249

  3. #3
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    Crankcase compression

    Thanks Tony. On piston temp, I just assumed racers used thermal barriers/coatings to improve combustion thermal efficiency, & temp would not be a problem. I can see where reeds would quickly fail.

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    Terry,
    I think you would want the thermal barrier on the piston top to retain heat in the combustion chamber and to shield the piston.

    Tony
    Tony Brucato


    (919) 718-0249

  5. #5
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    Cool JSRE also offers

    wrist pin stuffers that add very little weight and there are several areas in the block/front 1/2 that can be epoxy filled to increase crankcase compression as well. Using heavy race rods vs the newer lighter style increases weight but also fills some area.

  6. #6
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    Hold on there boys like everything else in life you can have too much of a good thing with crankcase compression. When you increase crankcase compression you are necessarily reducing crankcase size and hence the VOLUME of charge available to fill the cylinder. Two strokes with small crankcase volumes are acceptable for low rpm and short intake durations but they will not be effective for a race motor. The Mercury drag motor already has proportionally about 20% less volume in its crankcase than an average PWC race motor, which is where the most radical direct drive two stroke concepts are currently being put into practice.
    except of course N2O

  7. #7
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    When you increase crankcase compression you are necessarily reducing crankcase size and hence the VOLUME of charge available to fill the cylinder.
    I don't think that I entirely agree with that. You DO still have the same volume of charge to fill the cylinder. The displacement of the piston in the crankcase is the same regardless of crankcase volume.
    The larger volume crankcase will act more lazy and is less efficient than the small crankcase.
    At very high RPM there would be a substantial amount of HP lost to pumping pressure though, and the extra pumping pressure does create more heat.
    I think also that, reduction in crankcase volume on the piston side of the rod slot would be more effective than in the crankshaft area because the rod slot acts as a choke.
    Tony
    Last edited by Tony Brucato; 09-06-2002 at 01:03 PM.
    Tony Brucato


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    Well the theory that all cylinder filling is the result of piston displacement and pressure in the crankcase is pretty outdated. That theory held that cylinder filling is simply a one to one volume exchange. The problem is that theory doesn't allow for the time lag to get the charge moving, and since charge mass per stroke is essentially constant with rpm so is the time lag. This leads to progressively poorer performance as rpm increases and this time lag becomes proportionally larger with respect to the time available to fill the cylinder. Current two stroke theory holds that a properly evacuated cylinder is not just one that is blown down to ambient pressure but one that is pulled below ambient pressure by the momentum of the exhaust rushing out the port. This low pressure wave is what starts the fresh charge moving in the transfers(generic use meaning all intake ports) before any positive pressure in the crankcase could ever hope too. Similarly the columns of charge in the transfers have mass and correspondingly momentum so they do not stop flowing as soon as the cylinder is"full" or back to ambient pressure. The charges continue to stream from the transfers and collide with each other producing an area of localized high pressure while some flows out the exhaust port. This charge that flows out the exahust is to a great extent rammed back in to the cylinder by the rising pressure in the exhaust tract either from an adjacent cylinder blow down in our V6s or a reflected wave in the expansion chamber in other applications. This combination leaves the cylinder "overfull" or supercharged above ambient pressure something a high crankcase pressure design could never achieve due to lack of crankcase volume.
    Last edited by Nosubforcid; 09-06-2002 at 11:24 PM.
    except of course N2O

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  10. #9
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    Nosubforcid,
    Good explanation. I see your point.

    I think my description would only address efficiency as an air pump without regards to the dynamics of pulse tuning.

    I stand corrected.

    Tony
    Last edited by Tony Brucato; 09-06-2002 at 03:45 PM.
    Tony Brucato


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    It is not that you stand corrected Tony the theory stands expanded.
    except of course N2O

  12. #11
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    volume

    The pwc engines may have more volume but i think they have it where its benefical mostly in the port runners, large converging ducts with big radiuses the recipe for power.Granted this is hard to do with the o/bs stacked v design the cylinders are so close together they have to be rotated for any port runner volume at all between the cylinders.The v outboards end up with volume in areas not so beneficial the crank side of the rod slot.Dave

  13. #12
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    Nosub, Great explination and I agree

  14. #13
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    Sort of like this?
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  15. #14
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    OK, I've been watching it long enough and am certain he has an error, or at least exaggeration. At BDC he shows air/fuel going back out. On a reed valve motor you will not have enough reversion (reverse flow) in the intake tract outside the reeds to mention. Reeds really are the one way valve they appear to be. The proof of this can't be made by converting an outboard to piston port intake, but can be done by converting an old piston port motorcycle to reed intake.

    Piston port motors do have significant intake resonance and reversion. There are times when the flow may actually change direction 5 times in the carb area. On a dyno with no air filter some engines even make a visible mist cloud outside the carb. Not so with reed valves. When you convert a piston port motor to reeds and kill all that intake resonance, you have to increase your jet size significantly so the air can get all the gas it needs in one pass (instead of a little each time it pops back and forth past the fuel nozzle).

    Way back when I had a 3 cylinder Kawasaki that I added a reed valve kit to. Low end torque was significantly increased (you could let the clutch out without feathering the throttle as I had become so accustomed to doing and she pulled right from the bottom instead of needing to be up around 4,000), the very highest end of the powerband was decreased very slightly; the motor could no longer freely rev past it's powerpeak; the motor's low speed WOT intake roar (which was deafeningly impressive with piston port inlet) was gone. My theory on that was the sound was actually the sound of the piston skirt popping past the port and while the intake velocity was sub-sonic it travelled out, after the intake velocity went super sonic the sound was sucked in anyway. With the reed valve kit the intake sounded the same with WOT pretty much regardless of rpm.

    With piston port the jet sizes were on the bottom end of Mukini's chart......converted to reeds I had to go waaaaaaay up to the top end of the chart to get the same plug readings.


    Going back to Terry's original question: the highly criticized Two Stroke Tuner's Handbook says there is a point of diminishing return on increasing crankcase compression.....after a certain point of crankcase stuffing, the mechnical energy required is more than the power increase. Like Nosub says if you can get resonace to do it for free why waste motor power doing it?

    In the history of two stroke motors there actually was a motor that used a second piston for extra intake compression and intake port area...an actual supercharger effect.

    Note to Psyco: I like the guy's choice of carb, hint hint.
    Last edited by Mark75H; 09-07-2002 at 08:59 AM.

  16. #15
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    It looks like to me:

    That he is showing air fuel going in at BDC.

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