Unsaid info about high compression heads

jaguar

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Nov 25, 2010
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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]
 
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lazylightning@mail.r

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Jun 2, 2011
Messages
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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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lazylightning@mail.r

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Jun 2, 2011
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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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Steve Best

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Sep 22, 2012
Messages
1,235
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.











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.



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.



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

Steve
 
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jaguar

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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.
 

jaguar

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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.
 

Steve Best

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Sep 22, 2012
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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.

How would you modify stock heads squish?
I'd have hard time explaining it any better than alazylightning@mail.r earlier in this post.




I did the similar thing with my slant plug head:



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
 
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Steve Best

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Sep 22, 2012
Messages
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I believe the best head for our single speed bike would be a shallow torroidal shape with about 50% or less area squish.



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

Here is an example from a KTM 125:



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
 
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