Reed Valves

Discussion in 'General Questions' started by allen, Sep 25, 2016.

  1. allen

    allen New Member

    Please advise benefits if any ,reed valves fitted to 50cc 2 stroke... They are already fitted to this motor,(from China) Thanks in advance for any information, Allen
     

  2. jaguar

    jaguar Well-Known Member

    more power, wider powerband
     
  3. allen

    allen New Member

    Thank you,Allen
     
  4. Steve Best

    Steve Best Active Member

    Jag, you should say POTENTIALLY more power and broader powerband.
    Sleds and motocross bikes stayed reedless until the early 80s because more power was available without the intake restriction.
    Advances in reed technology helped change this but it was really the broader powerband and smooth strong idle that advanced the reed valve.

    A small reed on a stock China Girl will typically reduce power somewhat, due to the intake restriction.
    [​IMG]
    The piston needs to have a window drilled in its skirt. Its placement still makes a difference.
    [​IMG]
     
  5. jaguar

    jaguar Well-Known Member

    OK lets talk specifics.
    a piston port intake has a narrow powerband (of best power) that can be made at high RPM such as what motocross bikes use. So a motocross bike can be decent without a reed valve. A reed valve gives better power below that optimum powerband and if properly designed does not give less power than without it. There are occasions when racing that you fall out of the powerband and lug the engine a bit. In those situations the reed valve gives the advantage which is why all MX bikes now have them.
    It doesn't matter that they create a flow restriction because the intake duration is 180 degrees, versus the 120 degrees of piston port (of which only 60 degrees is pure intake if it weren't for the inertia of the intake charge).
     
  6. Jonj57

    Jonj57 Member

    Can anyone point me to a picture of a china girl piston with the window already cut or some good instructions of what to cut out and where? I'm looking to go reed soon (RSE Kit) as I want that low and mid range and have no issues adding the third transfer port (even have a backup cylinder if worst comes to worst) but haven't spotted a detailed post about where/what to do with the piston window. Thanks boys
     
  7. jaguar

    jaguar Well-Known Member

  8. Jonj57

    Jonj57 Member

    Perfect, thanks again Jag!

    EDIT: On holes to be drilled, there isn't quite room to make a 4 tiered stack of holes if I follow your earlier advice on no less than 10mm from center to center. Does this not apply to this situation? Would it be fine to simply cut out a squarish hole down the length the intake side of the piston to achieve the same affect?
     
    Last edited: Sep 27, 2016
  9. jaguar

    jaguar Well-Known Member

    yes you can cut a square hole but having many small round holes gives more structural integrity which is why I did it that way.
     
  10. Jonj57

    Jonj57 Member

    Would it comprimise the structure to much with a square inlet? What about 3 thin rectangular holes with maybe 3-5 mm inbetween them? Probably just going to have to test it and let you guys know haha.
     
  11. jaguar

    jaguar Well-Known Member

    I may have been overcautious because later I drilled 27 holes in the piston to lighten it up and road it going 8500 RPM and nothing bad happened.
    So that taught me the stresses on a low hp engines piston are not much.
     
  12. Steve Best

    Steve Best Active Member

    Skirt strength is a big issue with windowed pistons. Keep it foremost in your mind.
    While the pressure load is low compared to MX engines, a skirt failure is just as catastrophic.

    These motors are so easy to take apart that I'd advise starting conservative and working up to see if there is any power gain.
    Have a look on my Facebook (it's public) of the Blaster piston I progressively opened up until it actually gave less performance. I was using an old worn piston so I would know exactly how far to take the new $100 piston. On these China Girls we are dealing with a $10 piston and 15 minute replacement time, so experiment and learn. If you take it to the max right at the start I can assure you that you will be slower and weaker than you could be.
     
  13. jaguar

    jaguar Well-Known Member

    hi Steve. I'm open to what you may reveal as most recent findings concerning squish bands. Here's what I am going by:
    1) I modified my stock head to have a properly designed squish band and it gave me 1mph more top speed on my engine that rev'd to 9000. That's better than nothing but can't really qualify as much of a performance modification.
    2) Jennings wrote that squish bands are for engines that rev higher than 9000 and/or have high compression. In both cases there is a big increase in engine heat and the band centralizes the majority of the heat so that the rings and sides of piston get less hot which helps with power and longevity.
    3) Gordon Blair wrote that the squish band helps a high performance engine run reliably at a high compression ratio.
    4) the research paper "Squish Velocity in the Combustion Chamber of a 2-Stroke Cycle Engine" stated that the best improvement in power from a squish band (keeping all heads tested at the same compression ratio) was 1.2%

    OK that's what I have, so what do you have? Telling me that my info is outdated isn't valid because two strokes are still basically the same as they were when Jennings and Blair wrote about them.
    These chinese engines are not high-performance. It's a cruel joke by the aftermarket guys to sell high compression heads to these engine owners (with or without squish bands).
    Our discussion about squish bands really should be on a regular dirt bike forum, not here.
     
    zippinaround likes this.
  14. Steve Best

    Steve Best Active Member

    We are jumping threads here Jag, this connects in another off-topic thread!
    1) there is no such thing as a "properly designed squish band". Different specs do different things. More squish area tends to reduce top speed unless you are experiencing impending detonation at top rpm (very common, this is why your CDI works so good). Motocross and ATV rule of thumb is 50% area. Killed my China Girl top speed by 10%. We only need 5% to 25% depending on where we want to run it.
    2) Jennings was generally dealing with high rpm motors and concerned mainly with max power and did not have electronic or variable timing on most of the motors he worked or reported on. They were using squish only to fight top rpm detonation. Jennings is right on with the cooling effects.
    3) Gordon Blair is right on and has some workable formulas for squish design. Squish works for low performance engines too.
    4) Avram Ziv wrote that paper in 1973 for a chainsaw manufacturer. Not groundbreaking research and I don't believe he ever won a race or his chainsaws were ever know for high specific output. That paper might even fall under the category of "putting the competition off the scent". Racers have reported completely the opposite.

    2 strokes are not the same now as in 1973. The tuned pipe was in its infancy back then as was so much else. Specific power and fuel economy have increased and even higher compression ratios on lower grade fuels. A 600cc 120hp snowmobile can get 20mpg running a track on snow. My 25hp 1973 Elan got 16mpg!

    Things have changed, there is new information. We can apply it to these motors.

    You are right about high compression heads being a cruel and damaging joke foisted on newbies. I totally agree.
     
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  15. jaguar

    jaguar Well-Known Member

    hmm. Blair said that what mattered was the squish velocity which should be between 15 and 20m/s at top power RPM. If that is the case then given the same squish clearance the wider the band is the faster the squish velocity. That research paper listed 15m/s as giving the best power boost although it was only 1.2%. There are a wide variety of opinions on this topic which is why I stick as close as possible to proven research. Seems like your focus is on squish area with disregard to velocity which I think is an error. But to each his own. I still appreciate your experience and willingness to discuss a topic calmly.
     
    zippinaround likes this.
  16. Steve Best

    Steve Best Active Member

    Yeah, Blair was big on numbers and they give a good place to start. Empirical evidence is undeniable.
    So you take Blair's formulae and Jennings theories as a place to start, and test them yourself.
    Yes I do focus on area and chamber shape. Those are hard things to machine. Squish gap (adjusting velocity) I can easily vary.
    Generally I find the gap quite forgiving in the 0.5mm to 1mm range, which calls in question the velocity theories.
    What do I know? I'm just an old wrench turner who likes to go fast...
     
  17. Steve Best

    Steve Best Active Member

    I was just thinking about the work Avram Ziv did, running 3 different heads on a chainsaw motor on a dyno and writing an SAE paper on it back in the early 70s. Wow. If I had a corporations finances I'd have that done in an afternoon. Any drag racer does that sort of a test over and over again to find optimum. While not a PhD, they find what works. An SAE paper is not the Holy Gospel, just a good place to start. Test your own ideas and questions. Change one thing at a time and see what works.

    Hey, this was my fun this afternoon. Bored and stroked Blaster, lovingly assembled for me by my son. What a great and loving gift. I've been working on this for a couple years now, slowly inquiring parts and putting it together from frame up. He went at it and shared his parts to get it running yesterday. Still lots of work to do, but I am grateful for his work. Gutsy, starts well. Lots to experiment with but it runs great. Head design, lower pegs, longer swingarm and it screams for gnarly tires. A good son is worth having.
    [​IMG]

    Steve
     
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  18. Fabian

    Fabian Well-Known Member

    The practical torque improvement with a reed valve intake system is improved low and midrage power, with consequent improvement in engine life as the engine can be operated with less rpm for the same torque of an engine timed solely on piston porting.
     
    marshall johnson likes this.
  19. Jonj57

    Jonj57 Member

    SO I just window'd my piston, added a boost port (small) and installed a reed valve. The thing will start but it just sort of puts around and bogs to zero basically the second I give it throttle. It idles high oddly enough and revs alright when 0 load. Any tips guys (jag haha). From my understanding people generally said I'd need a richer setup but I think I'm getting the symptoms of too rich? (haven't dinked around with jetting tho)
     
  20. Steve Best

    Steve Best Active Member

    Wow, did you even read post #12? Why did you add the boost port? Hopefully it is just mixture...
     
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