Do slant heads really make a difference in power?

Discussion in 'Motorized Bicycle General Discussion' started by fall_down_stand_up, Dec 5, 2009.

  1. fall_down_stand_up

    fall_down_stand_up New Member

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    I have always thought it was for a tight fit in a frame,but have heard it gives better performance....Is there any truth to this????I want your oppinions....
    John-John.shft.
     
  2. fall_down_stand_up

    fall_down_stand_up New Member

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    There has to be someone out there that has some facts on this....

    John-John
     
  3. momentummotorgroup

    momentummotorgroup New Member

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    allegedly the slant head has a smaller compression area resulting in a bigger "bang".. at least that's what I was told..
     
  4. Dan

    Dan Staff
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    Dunno John-John. Never got around to trying one but they sure is sharp looking.
     
  5. fall_down_stand_up

    fall_down_stand_up New Member

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    They are one sharp looking head thats for sure....
    John-John
     
  6. stv1jzgte

    stv1jzgte New Member

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    I have crudely cc'd a std head, slant head and a grubee (skyhawk) head.

    the slant head i have took 6 more drops (of water) than the std head and the grubee took 2 more than the std

    What i have notices is the grubee has massive ports compared to the std and slant head motor i have.
     
  7. george_n_texas

    george_n_texas New Member

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    I just got a slant head from Zoom and it sure has more power than the grubee did it isn't broke in yet though. Both are 48cc...I did also notice the Zoom kit has larger intake, same size carb mount at the end. If it fits I may try the slant head on the grubee.
     
  8. bseelbach

    bseelbach New Member

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    I put a Zoom aftermarket slant head on a Grubee 66cc. I wanted to see a performance difference based on all of the hype but to me it is the same. The slant might be a bit quieter but it could just be me. I never changed back because frankly I'm lazy.

    I've never taken the time to really compare volume of the combustion area but plenty claim the slant have more compression. Personally I think that the "perceived" performance increase is really attributed to the maintenance factor of changing the heads...New head gasket, proper torque on all bolts.

    I'm sure this will rile some but its not my intention, just my peronal observation and thoughts.
     
  9. Riding Rich

    Riding Rich New Member

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    What i notice about the slant head is this.....
    Th PK80 that i have with the slant has a dome shape piston.
    The piston travels .145 inches above the top of the cylinder at TDC.
    So it is inside the head.
    Combination of the head and piston and stroke = higher compression.

    The proper way to check the combustion volume is to put the piston at TDC.
    Use a graduated dropper and fill the head with oil till it reaches the plug threads.
    That's the volume.

    I will say even milling down a std head on sandpaper and glass and replacing the gasket with a really thin one creates much higher compression.
    And a huge power boost.
    Also runs much hotter and harder on the bearings.

    But i like it.
     
  10. Humsuckler

    Humsuckler New Member

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    i was blown away by the low end te zoom pk 80 had over my first two unknown 80's

    and its dead reliable too. it only complains if i hit too hard....
     
  11. Pablo

    Pablo Master Bike Builder & Forum Sponsor

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    I'm still running the slant head. Compression is increased over stock, therefore slightly more torque. I guess it depends on the shape and size of the original head you are taking off.
     
  12. Riding Rich

    Riding Rich New Member

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    Yup.

    The PK80 i have will need both head gaskets for clearence.
    Without them the piston hits the head.

    Have to mill the head to get the proper squish.
     
  13. Riding Rich

    Riding Rich New Member

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    With a flat top piston you wouldn't have this problem.
    But the dome shape piston travels .145 inches int the head.
    That gives increased compression too.
     
  14. fall_down_stand_up

    fall_down_stand_up New Member

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    My base gasket blew out on one of my motors,so I pulled it and used hi temp silicone for the bottom gasket and the piston hit the head so I put 2 head gaskets on it and it fixed the problem....If I buy a slant head(being it is a dome shaped piston)I will put 2 head gaskets on it....
    John-John.shft.
     
  15. Flattracker

    Flattracker New Member

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    Compared to the stock old head, the inner combustion area of the slant head is smaller which results in higher compression. After I installed mine, I noticed the difference while starting my engine. I pedaled a few times, popped the clutch, and almost went over the handlebars, as the piston compressed fully, and the thing stumbled to life! I wasn't expecting THAT much of a compression increase!
     
  16. xlite

    xlite New Member

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    I have installed a few dozen old style engines and several slant heads. I've also updated a few of the old ones with Zoom slant heads. The difference is not placebo effect. Most of the "upgrades" claimed on this forum are but not the slant and not the SBP pipe. They both added a couple MPH on "dyno-hill'. Also low-end acceleration.

    The difference in compression shows up as a major increase in starting resistance. A couple of the people I did bikes for who did not weigh much had to go back to non-slant to keep the rear wheel from skiding,
     
  17. Humsuckler

    Humsuckler New Member

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    when i read the post regarding the starting i said exactly. i noticed immiediatley on "break in run" when i left the gas station :D

    and close to 2000 kms, it still starts as easy :) i find my centerfire to be gutless lol
     
  18. Riding Rich

    Riding Rich New Member

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    Here's some numbers for ya.
    But they apply directly to the PK80-J Slant Engine.

    The stroke length will detirmine the swept area of the piston which is the volume being compressed into the head.

    As far as i know there ar two different happy time strokes out there.
    Different location of the piston wrist pin.

    The voulme of the head is approx 8cc / same as 8ml

    The volume of the compression chamber with the piston at TDC = 5cc / 5ml with 2 head gaskets.
    This is possable because the piston dome is above the top of the cylinder inside the head.

    the swept volume of this engine is 47.1291 cc.

    47.1291cc / 5cc = 9.43 : 1 "SOUNDS LIKE TIRE SKIDDING TO ME"
    I wouldn't reccommend going much higher without some serious octane.
    This is also about the same as the Rock Solid Billet head.

    Even at a 6cc / 6ml combustion chamber the ratio = 7.854 : 1 "not bad"

    now say the piston was a flat top.
    the combustion chamber would be 8cc / 8ml.
    The ratio = 5.89 : 1 probably typical for the Happy Time.

    So if you put the PK slant head on a motor with a flat top your not helping much.

    But that dome shape piston and the head combo make a real winner.
     
  19. xlite

    xlite New Member

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    Yes. I was trying to explain to someone last week that the Zoom slant was near or beyond the limit of practical compression and the billet had no advantage in that dept..

    Are we saying the PK80J "80cc" is really 50cc?
     
  20. Riding Rich

    Riding Rich New Member

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    No the PK80-J is allmost 69CC total. like 68.8 something. "stroke X bore dia."

    But to find compression. "Corrected Compression"
    We use the swept volume of the piston above the exhaust port.
    Compression dosent start until the exhaust port is closed so we use the volume of the cylinder above the top of the exhaust port.

    So thoose numbers at like 15 :1 or 16:1 compression are UNcorrected.
    They take the complete volume of the cylinder and use for calculation but with the ports open there isn't much compression yet.
    Other than the PSI in the CrankCase and PSI caused by Expansion Chamber slammin fuel back in the chamber.

    Which will add some PSI to the actual calculated compression.

    But lets see.
    69cc / 5cc = 13.8 :1 Uncorrected Compression Ratio.

    So be careful when you talk compression and make sure you know if were talking Corrected or UNcorrected compression.

    It makes a big difference in the numbers.

    Going much higher than 9:1 Corrected Compression probably won't help much.

    at a certin point you actually have a detrimental effect.

    Make sure you cange that caged needle bearing if you go high comp.
    It puts much more strain on things.
     

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