Unsaid info about high compression heads

Discussion in 'Performance Mods' started by jaguar, Mar 30, 2015.

  1. jaguar

    jaguar Well-Known Member

    squishband.jpg
    High compression heads are made with an additional feature which allows a minimal squished area between the piston and the head around the periphery of the piston which keeps the piston cooler at top RPM for less possibility of seizing and less possibility of detonation (which eats away at pistons). It is the increased compression which is responsible for most of the power increase, not the squish band. Modifying an engine without a squish band to having one is is a technical venture that has to be done right or else the engine will lose power. Most of the high compression heads with a squish band are made for engines whose cylinder height is equal to the height of the piston edge when it's at the top of its stroke. The GT-5 engine is not one of those engines. If the cylinder is higher than the piston then that allows extra space at piston-top-dead-center (TDC) that hides the fuel mixture that's there from the main combustion area at the critical beginning of the flame creation (from 10 degrees before TDC to 10 degrees after). So if 20% of the mixture is in that excessive squish area then its like having reduced your engine size by 20%. If your piston is lower than the cylinder top (at TDC) then you have to take it to a machine shop to have it lathed down that same distance. Unfortunately this info is mostly sidestepped by the sellers of these heads. What is the right squish clearance (between piston and head)? It is .012" to .024" (.3 to .6mm) for these small engines. (Less than .3mm allows the piston to hit the head when the engine gets hot due to lengthening of the connecting rod due to heat.)
    Here's how to measure it:
    Buy solder from Radio Shack. take off the head. position the piston so that when you push the bike it will rise. set it just a bit down (before TDC). put a length of solder on top of the piston from the left side to the right side. put the head back on and then push the bike. if you feel no resistance from the engine having to squish the solder then your squish clearance is ridiculously too much. try not to turn the engine over much past the TDC point. that gives less chance for the solder to fall into a port. take the head off and use a dial caliper (#142 $24 in Home Depot) to measure the squished thickness of the solder. that thickness is the squish clearance. If it isn't squished then coil two strands of solder together to make it twice as thick and retest. If it's more than .6mm and the cylinder is flush with the piston top then you can plane down the head by using sandpaper on thick glass.

    Here are the sellers, their high compression heads, and what they do or don't say about the squish band. None of them fully explain like I have here:

    Bicycle Motor Works
    6.0cc High Compression Billet Aluminum Head
    "Deck height of zero to .03″ (.8mm) [needed] for maximum compression. NOTE: GT-5 engines have a higher deck than most others. You will need to bring the deck height down to fully appreciate added power."

    CRMachine
    Stage 2 [6.0cc]Cylinder Head 66/80cc:
    "Please be aware that in order gain the maximum benefit from this cylinder head, you will need to bring the outer edge of the piston to (or close to) the top of the cylinder. A close "squish" clearance has better combustion efficiency. If you have a GT-5 engine, using a shortened cylinder body will produce optimal results. Check out our cylinder bodies and complete top end kits for the GT-5 engine."

    Jake's
    Billet High Compression "Diamond Head"
    "Squish-band design seats the head perfectly into the cylinder, funneling
    gasses precisely to the combustion point at the spark plug, increasing
    efficiency and reducing potential detonation; a condition where hot
    spots in the engine cause the fuel to combust in unwanted areas such as
    under the piston, or at various undesirable points inside the chamber. "

    KingsMotorBikes
    High Compression Cylinder Head 80cc 60cc
    "This cylinder head incorporates a squish band which has been proven to allow increased compression while reducing the risk of detonation. Please be aware that in order gain the maximum benefit from this cylinder head, you will need to bring the outer edge of the piston to (or close to) the top of the cylinder.
    *The GT-5 engine has a different crankshaft assembly and piston than most other engines, and has low compression. Adjusting the compression by decking the cylinder jug is necessary for the GT-5 engine in order to gain proper compression ratio."

    Arrow Motorized Cycles
    Stock Rechambered Cylinder Heads
    [nothing said]

    Rock Solid Engines
    Billet 70cc Cylinder Head
    [nothing said]

    PedalChopper
    "HEMI- SPHERICAL" HIGH COMPRESSION HEAD
    [nothing said]

    GasBike
    Racing CNC Cylinder Head 66cc / 80cc
    [nothing said]
     
    Last edited: Mar 31, 2015
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  2. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Hi Jaguar!
    That's a really important point! Not only is it neccesary to bring the cylinder deck height down, it may be neccesary to adjust the head somewhat. For example, My high performance head came with a squish band the was 6mm smaller in diametre than the diameter of the cylinder. With that 3mm edge all around, there would be alot of mix trapped in a corner around the edge of the piston at TDC since it is a dome shaped piston after all. So I took a piston and old pin I didnt need and glued peices of sand paper to it and bolted an old cylinder to the head and used pliers to turn the piston while applying pressure. This way I widened the squish band to match the cylinder diameter and most importantly, the piston diameter!
     

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    Last edited: May 23, 2016
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  3. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Hi Furry!

    I got it from a dealer here in Russia. He bought it from a Chinese source together with engine kits, and other custom parts like jackshaft kits, Chrome plated parts and chromed rear gears of different sizes. So it's not exactly clear to me the name of the Chinese manufacturer, but it seems to me it's not from Grubee. I asked him and he had never heard of Grubee.
     

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  4. jaguar

    jaguar Well-Known Member

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  5. Steve Best

    Steve Best Active Member

    Jaguar, I disagree with you that it is only compression that is the power factor, not squish.
    I did a lot of work on various 2 stroke head shapes that lead me away from that conclusion.
    Compression mainly affects low rpm power and tends to drive heat into the piston.
    Pipe and port is the main contributor to top rpm power and will deliver even with poor compression ratio.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    Squish and resultant squish velocity have a big effect on burn speed and timing requirement.
    Increase the squish velocity (up to a point) and you can (must!) reduce timing lead,
    More squish are is mainly helpful at low rpm. 10,000 rpm motors need very little squish.

    [​IMG]

    Even the chamber shape is important. Deep is torque producing but limiting on rpm.
    Ironically, so is a flat torroidal shape, but better on both ends, high and low.
    High rpm favours small squish area and a shallow hemi chanber.

    [​IMG]

    Otherwise an excellent article and very good write ups on your web pages.
    You are reading and studying and experimenting. Wonderful stuff.

    Steve
     
    Last edited: May 29, 2016
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  6. jaguar

    jaguar Well-Known Member

    Well we all give credence to our experiences more than to unbiased research. In my case both pointed to the same conclusion. When I replaced a normal head (shaved down for more compression) to one with a well designed squish band I only gained 1mph which may be because a squish band protects some of the piston from heat. (A too-hot piston robs power)
    The research paper definitely shows the difference in horsepower graphs and they concluded that squish bands aren't for power increase. They are for reducing piston heat.
     
  7. Frankfort MB's

    Frankfort MB's Well-Known Member

    How would you modify stock heads squish?
     
  8. jaguar

    jaguar Well-Known Member

    For one, if your compression isn't rally high and you don't rev up to 9000 or more then you don't need squish.
    Just use sandpaper on glass to grind down the mating surface of the head for more compression. Don't go over 135psi.
     
  9. Steve Best

    Steve Best Active Member

    I think the included pictures show my "experience" is very deep.
    I read those research papers (from the 1970s) and considered them very carefully.
    Understand there have been "unbiased" research papers on eggs, coffee, oatmeal and alcohol are both very good and very bad for you. Read carefully for understanding. My guru, Gordon Jennings, wrote a very well researched article that more oil makes more power. He recommended 16:1 ratios. Why don't we do that now? We might for the same circumstances.
    I am biased! I want more power! I'll follow the results that get to it.
    Read and think. Experiment to find truth.

    I'd have hard time explaining it any better than alazylightning@mail.r earlier in this post.
    [​IMG]

    [​IMG]

    I did the similar thing with my slant plug head:

    [​IMG]

    Stock head in the back, modified in the front, aftermarket to the left.
    Open up the squish to the full diameter of the cylinder/piston bore.
    Set the squish up to 0.010" to 0.030" with either the base gaskets or head gasket surface.
    Expect noticeable improvement and no more detonation.

    Steve
     
    Last edited: May 29, 2016
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  10. Steve Best

    Steve Best Active Member

    I believe the best head for our single speed bike would be a shallow torroidal shape with about 50% or less area squish.

    [​IMG]

    This is what most motocross engines are running these days. Broad torque, moderate rpm peak.

    Here is an example from a KTM 125:

    [​IMG]

    The head on the left is high rpm hemi, to the right and center is torroidal.
    Note too the effects of oil and mixture. Left is synthetic but rich jetting. Center is cheap oil.
    Right is 40:1 Ipone synthetic with lean jetting on a street driven 125. Seven years and 1000s of kms of riding and that is the limit of carbon buildup.

    Steve
     
  11. jaguar

    jaguar Well-Known Member

    I've not found a single case of a person saying his new squish band head didn't gve more power but I believe that is because it's near impossible to make one without drastically raising the cranking pressure. The increased pressure raised the power, not the squish band. The beauty of that research paper is that they kept the cranking pressure the same between all heads tested. Here is the horsepower graphs of the 4 heads. Do you see much difference? One of them has no squish band.
    [​IMG]
     
    Last edited: Jun 3, 2016
  12. Steve Best

    Steve Best Active Member

    It is not impossible to change squish area while keeping CR the same.
    Far from it. Just make the chamber deeper.
    How many people do you know doing serious investigation into squish?
    It's been one of my 2 stroke and 4 stroke power secrets since reading Jennings book in the 70s.

    Comparisons. Change one thing while keeping others constant to see the effect.
    This is how we learn. I'm no scientist, but I do get paid to do this sort of stuff in industry.
    Results. Find what works and go with it. Chase the real results that you find and demonstrate.
    Listen and read the other's work but question it. Test it. Peer review.

    Note the 3 KTM heads below, all are the same cc displacement but slightly different chambers to test effect.
    [​IMG]
    These two heads are the same cc displacement, just looking at chamber shape:
    [​IMG]

    These below are both the same displacement (within 0.5cc), although smaller displacement than the ones above to investigate the effect of increased compression ratio.
    [​IMG]
    I did earlier experiments with squish area that push me to 50% area being an important dimension.
    Compression the same, more squish has a huge effect on power, not always good, but a huge effect.
    It also affects timing.

    Hey Jaguar, conflicting ideas and theories are a very good thing. They push the envelope of learning.
    Just don't shake a 44 year old graph at me and wish me to accept it as holy gospel. Test it yourself.
    These engines are so easy to work on, they are the perfect experiment media for these ideas.
    That is what I am doing here, and I enjoy reading your work, your sources and your ideas.
    If I disagree, I'll tell you why, where it comes from.

    And best of all, we can test these things on these engines quickly and cheaply.

    Steve
     
  13. jaguar

    jaguar Well-Known Member

    your right about these engines providing a virtual 2 stroke classroom. Most of what I know about 2 strokes I learned while riding a motorized bicycle and experimenting with it.
    It doesn't matter how old the research is. They used modern test methods and their test results were not in any gray area.
    But of course you are free to believe what you want to. What you were inadvertently doing was changing the timing (because of squish velocity) and seeing the power difference.
    I was one step ahead of you in that I was using the Jaguar CDI and adjusting its timing to match each head I tried. That way each head had its optimum timing.
     
  14. Daverouse92

    Daverouse92 Guest

    So you guys are saying that a high comp head that I buy from a dealer will have to have some work done to it to achieve maximum output?
    If so how about a video that SHOWS what you mean for us VISUAL LEARNERS
     
  15. jaguar

    jaguar Well-Known Member

    save your money and just modify the stock head.
    buy a good iridium spark plug, tougher nuts/bolts/studs, a better carburetor, a better CDI.
     
  16. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Hi Steve,

    Wow! More oil, more power? Sounds quite interesting! I will have to go back to more oil rich mixes and give it a try. Have you gotten into trying racing coatings on the parts? There is a special dry lubricant that is baked onto all the moving surfaces and a ceramic thermal barrier that is baked onto the inside of the head chamber and on top of the piston crown. It completely turned my engine into another speciaes of animal. It's really worth trying. I ordered the stuff in the internet, and they have a line for amatuer enthusiasts in small bottles. First the surfaces have to be sandbalsted with aluminum oxide that has a certain grit size. I bought two peices of special stainless screen with specific hole sizs from a scientific supply to get the proper fraction from some typical aluminum oxide powder also available at a scientific supply store. After washing with acetone, the coatings are carefully applied with and aerographic paint pen so thin that it doesn't affect the size of the piston diameter of the upper or lower pins. When it is really good and dry, it is then baked at at least 150C for an hour and a half. Some green scotch bright is used on the dry lubricant surfaces to make sure they are even and a final powder lubricant is rubbed onto the dry lubricant surfaces for extra super slipperiness. It's a mega game changer. I highly recommend it.
    In the photos, you can see how the thermal barrier was defective after baking it. I had to redo that piston. Maybe I didn't dry it good enough, or it absorbed moisture from wet humid weather. I will dry them in jars with calcium chloride next time just in case. The thermal barrier was a few years old when I finally got around to applying it to an engine. It may be just old. I just applied some more coatings to a new engine that I'm building and the dry lubricant still applys and bakes on perfectly. The thermal coating was still flaking off in places after baking, now four years after the purchase of the bottle of thermal barrier coating. As I see on their website, they have a new type of thermal barrier for amateurs and the previous one that I bought is discontinued. So perhaps it was a combination of circumstances and product development flaw. Still yet, well worth it. As you can see, I coated the gear teeth as well. Don't forget to do that. If the entire power train will have this coating on it, WOW!

    Best Regards,
    Paul
     

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