Making third intake on 66cc.......................

Discussion in 'Performance Mods' started by geebt48cc, Oct 30, 2012.

  1. geebt48cc

    geebt48cc Member

    I'm running a 2010 Skyhawk 66cc slant. I've got RockSolid reed valve on its way here. I've been reading that the only really best way to get this valve to work, is to port drill into piston in front of intake ? The main problem is I really need to know where to drill in well as best size?

    I don't want to make engine lose any down low torque, because this is the reason I got the reed valve in the first place. Can you give me some ideals here........... I also need to mark & trim piston base off on piston skirt when at TDC. I've heard that if you trim it all off, it can hurt low end??

    PS- It's got high compression head (milled),Jag's CDI,SBP's expansion, Speed carb, Port matched all, and soon RS reed valve.

    Appreciate your time.


  2. jaguar

    jaguar Well-Known Member

    Glen, the thing about cutting the piston skirt only has an effect on powerband when the intake is only piston port, without reed valve.
    As far as where to drill holes and what size, you can do it exactly the same way I did with my 48cc and 55cc. Read the specifics at
  3. Fabian

    Fabian Well-Known Member

    With the reed valve intake, the engine is not susceptible to how much the intake timing is changed through removal of material from the intake side of the piston, as the reed valve petals determine when air/fuel mixture enters the intake port and when air/fuel mixture stops entering the intake port - crankcase vacuum determing when the reed valve petals open and close.

    To the point where the piston is critically weakened by large amounts of removal of material, intake port timing can be changed (through piston modification) to where the engine would not normally be able to idle or run smoothly at lower rpms without a reed valve.

    The only potential downside to wild piston modification, beyond where it would normally idle smoothly, is that you're relying on piston porting for intake timing, should a reed valve petal fail; not enabling the engine to idle or run smoothly, until revving fast enough for intake flow velocity to overcome significantly higher levels of reversion.

    One of the big advantages of the reed valve intake is acceptable transition from idle to high rpm with wild intake timing - something that would otherwise make it a pig to operate, outside of it's peak power curve; like a highly tuned piston ported or disk valve engine.

    My advise: use the piston as a secondary backup for intake timing, should a reed valve petal get a fracture, thereby allowing normal use independent of the reed valve, although this scenario can be covered by carrying a spare reed valve if wishing to set your engine up with significantly extended intake duration.
  4. geebt48cc

    geebt48cc Member

    Ok, Jag, I read everything and saved on the piston porting. I wanted to ask if you think it's necessarily a most to drill a total of five- 5/16th holes in piston, or could I drill fewer? See, I know your the man with these little fellows, ..............but on other site, I've seen people cut just one larger piston port hole?

    PS-Just drill the five holes that you've posted with following close attention/with measurements............?
  5. jaguar

    jaguar Well-Known Member

    One big or five small ones, makes no difference to me. The advantage of doing multiple holes is that the whole distance of the intake skirt can be "holey" whereas with one hole it is either high up or low down, neigher of which are advantageous.
    I once had a piston I had drilled 27 holes in it to lighten it as much as possible when I was experimenting with balancing the crank. I kept it in my engine to see if it would last, and it did although the engine reved up to 8000. With no piston slap and no high horsepower there is nothing to break a holey piston.
    Fabian your advice is a bit skewed. If you leave the piston as it is as a "piston port intake backup" then all you have done is added a reed valve as an intake flow restrictor and it will run like ****. If you put on a reed valve you absolutely have to put holes in the intake skirt if you want it to run right.
  6. geebt48cc

    geebt48cc Member

    Fabian, that's a very good thought. Yes, I just mailed Jag to see if it would still perform as sufficiently with a few less piston holes?
  7. geebt48cc

    geebt48cc Member

    Well, I guess will just need to wait and see?!~I know, that everyone is just a bit apprehensive to how much punishment these pistons can take. Sounds like a plan though.................:)
  8. jaguar

    jaguar Well-Known Member

    low engine power = low stress on these pistons. worse you can do to them is let them get too hot due to overly advanced ignition or too high compression or too lean fuel mixture.
  9. Pawnrae

    Pawnrae New Member

    Jaguar, a moment

    Jaguar can you email me sorry for random post about to hit the sack
  10. Fabian

    Fabian Well-Known Member

    Not true Jaguar - a reed valve intake with an unmodified piston will still gain more low and midrange torque due to less reversion and consequently greater crankcase pressure and better atomisation of the air/fuel mixture when the transfer ports open, which mirrors my experience with the reed valve intake, not to mention a slight increase in combustible mixture contained in the crankcase by the positive closing action of the reed valve.

    Later i modified the piston and port matched it to the roof of the intake port. It gave a slight increase in torque but peak power remained similar below 5,000 rpm and it made no extra power above 5,000 rpm with a low compression cylinder head and the Jaguar CDI set to the highest advance curve, which worked best on the low comp head.

    Drilling out the piston may allow greater air/fuel mixture into the engine and greater power but it may not allow the engine to idle if a reed valve petal fails - this might not be a problem for short trips, but on longer journeys, it's handy to know the engine will operate properly, when relying solely on piston porting.
  11. Fabian

    Fabian Well-Known Member

    I will agree that at rpms over 5,000, the piston (intake side) face will become an obstruction to the incoming air/fuel stream; being advantageous to hack away as much of the intake side face as possible, but the consequences of such action will eliminate redundancy if a reed valve petal delaminates or suffers a longitudinal fracture, though carrying a spare reed valve covers this scenario.
  12. jaguar

    jaguar Well-Known Member

    Please take a look at the dyno graph on my reed valve page showing a 25% loss of power with addition of reed valve. After holing the piston and making boost port the power increased by 50% over stock. (test was on a Zundapp 125cc)
  13. Fabian

    Fabian Well-Known Member

    That may be true on a Zundapp 125cc engine.

    It must be remembered that the Chinese 2-stroke bicycle engine is power restricted to begin with.
    On a standard engine and standard exhaust, the reed valve intake and a completely standard piston will hardly make any difference to peak power which is around 4,500 rpm, but the reed valve intake will and does improve low and midrange torque - this improved midrange torque allows the engine to keep chugging away and it just wants to hang onto 3,200 rpm with surprising strength, whereas without the reed valve intake, the rpm would just fall away and require down changing to easier gears.

    At those lower rpms, life is so much better; so much smoother, so much less vibration, so much less noise, and a much more pleasant riding experience.
  14. jaguar

    jaguar Well-Known Member

    You may be right because the Zundapp graph doesn't show below 4000rpm. I will test your theory since I am testing a new 60cc cylinder and upgrading it in stages. I'll let you know how it goes.
  15. Fabian

    Fabian Well-Known Member

    At the end of the day the Rock Solid Engines reed valve has been designed as an in-port reed valve system, which has advantages and disadvantages, the advantages are that it is extremely compact, which is very beneficial for engines in space restricted bicycle frames as well as having the advantage of reducing the total crankcase volume (because part of the reed valve is inside the intake port), thereby boosting crankcase compression over an externally mounted (voluminous and physically large) reed valve system.

    A large externally separated reed valve body has the net effect of decreasing crankcase volume and reduces total crank case compression (in comparison to an in-port reed valve) when the piston moves towards bottom dead centre through the closing period of the intake port, which is now experiencing reversion and balancing crank case pressure between the crank case and the internal volume of the reed valve body; having an even greater effect with the increased intake timing caused through a heavily windowed piston.

    Having said that the larger bodied separated reed valve provides less intake flow restriction on modified engines with aggressive port timing and focused power bands; tuned for high rpm and maximum peak power.

    Unfortunately with legislation throughout the western world becoming tighter and tighter against the use of motorized bicycles with internal combustion engines, the concept of setting up an engine (on public roads) for maximum peak power at high rpm and high speed doesn't really work when you're trying to quietly fly under the radar; to attract as little attention as possible.

    Keeping the rpms low doesn't require the use of a reed valve system to give the maximum possible air flow at high rpm, but it does enable the engine to achieve as much low and midrange torque as possible in a compact and stealthy package, that fits inside space restricted bicycle frames.
  16. geebt48cc

    geebt48cc Member

    Very well said Fabian..............I just will see if it at least will supply the amount of low end torque that my bike gained from using the longer intake extention that Jag developed. If so, I'm not for sure if I would dig into the piston and chance my handy work!!!!~

    Very good description of how its designed to work from (RSE). I just hope that the crank can develope enough pressure to open reeds at lower RPM's?

    Glen (PS- You know the old saying......."If it ain't broke, don't fix it!!")
  17. Fabian

    Fabian Well-Known Member

    My next door neighbour has the exact same reed valve intake on his bike as on my bike.

    On my bike the lower skirt (on the intake side) of the piston has been port matched to the roof of the intake port. The net result is that my bike (prior to the reed valve and after the reed valve installation but before the piston transfer port bevels made no more power than his bike with a standard unmodified piston and has no speed advantage over my bike. Even if i did have a speed advantage, it might have only be 0.00000000000000000000001 MPH, because it felt like i had no more than 0.00000000000000000000001 MPH speed advantage.

    Having said that, my engine has been just as reliable and makes no less power with the intake side port matched piston.
    What did make a small difference to low and midrange torque was the modified transfer port bevels on the piston, though it made no real power difference towards 5,000 rpm with a standard muffler.

    A much more accurate and properly machined piston is made by this gentleman, of which i have ordered his twin spark plug cylinder head and his twin spark plug + decompression port cylinder head and a piston with transfer port bevels and exhaust port bevels and one piston with only the transfer port bevels.

    Zoom in on the picture to check out the transfer port and exhaust port bevels:
  18. Fabian

    Fabian Well-Known Member

    Looking back on my post i need to add, that both bikes with piston porting alone but a modified and an unmodified piston made the same power. Both bikes with the reed valve intake both made a similar improvement in low and midrange torque with a modified and unmodified piston.

    In saying the port matching of the lower piston skirt to the intake port roof made very little if any power improvement over an unmodified piston. It was the piston with transfer port bevels that seemed to make a small difference over an unmodified piston without transfer port bevels.

    By far it seems that the largest power restriction is the standard exhaust/muffler, but if you want to keep things as quiet as they can be, you have to accept that it's how the game needs to be played.
  19. geebt48cc

    geebt48cc Member

    That looks like good stuff.........

    Fabian, Uno, something that you need to notice is when the piston with any top areas are beveled, you will lose a bit more compression. Notice they never give info on that? Looking at that high compression head, it would have more than enough to spare though!...........

    Wow , if I really had some extra dollars bills, I would order that kit just to really see?!~ I'm going to chill though, and really just concentrate on getting reed to work out.
    Last edited: Oct 31, 2012
  20. Fabian

    Fabian Well-Known Member

    I will agree that the beveled piston will cause a small (make that a tiny) amount of compression loss. Having said that, i have two high compression cylinder heads and using a single head gasket, my engine diesels, and with 3 head gaskets fitted, the engine detonates, even with the Jaguar CDI set to the lowest ignition curve.

    I had to add 4 head gaskets to stop the detonation, using 98 octane fuel. It wasn't a viable situation because once you add multiple head gaskets to lower compression, the squish height becomes too large to add any meaningful velocity change between the combustion face of the outer piston circumference and cylinder head, thereby reducing turbulence of the air/fuel mixture in the final phase of compression.

    As things are, i need a piston that reduces the compression ratio (of the high compression cylinder head) when using a single head gasket, even though the bevels will reduce the squish band surface area, making the squish band less effective, but it's better than detonation or worse still, dieseling.

    Sometimes too much compression isn't a good thing.