Buying one of these this payday

ha-ha! yeah you're probably right.
People believe hype quicker than they believe scientific evidence.
 
I have been hearing a lot of differing opinions on the so called high compression heads so I tested them myself. 6 cc head on the left, standard kings motors slant plug head on the right. Cylinder decked to 2.720 approx. .020 removed and stock gasket set up. 6cc head reads 125 lbs. stock slant plug head reads 150 lbs.. I would love to hear others results if anyone has tried this themselves so we are not just guessing at the benefits or draw backs these heads provide. I should have done this long ago because it has me re thinking my cylinder work when using slant plug heads.
 
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I have been hearing a lot of differing opinions on the so called high compression heads so I tested them myself. 6 cc head on the left, standard kings motors slant plug head on the right. Cylinder decked to 2.720 approx. .020 removed and stock gasket set up. 6cc head reads 125 lbs. stock slant plug head reads 150 lbs.. I would love to hear others results if anyone has tried this themselves so we are not just guessing at the benefits or draw backs these heads provide. I should have done this long ago because it has me re thinking my cylinder work when using slant plug heads.
you mean a the stock head was higher compression than the so called high comp head?

i dont have a compression tester but think i might just get a 300psi guage and some fittings at local hardware store and make one? i have a slant head that has a massive deep chamber and another stock one has a ridge almost flush with the head , the deep one is off a so called 79cc and the other is a so called 60cc.


but would like to test myself and see what the cranking pressure is.
 
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you mean a the stock head was higher compression than the so called high comp head?

i dont have a compression tester but think i might just get a 300psi guage and some fittings at local hardware store and make one? i have a slant head that has a massive deep chamber and another stock one has a ridge almost flush with the head , the deep one is off a so called 79cc and the other is a so called 60cc.


but would like to test myself and see what the cranking pressure is.
Sorry I forgot to upload the pics. Yes I was surprised too. Check out the heads I tested. I uploaded them in post # 12
 
Well Jag, i'm a guy who has read and followed Gordon Jennings' bible (2 Stroke Tuner's Guide) since 1976, building pipes and stuffing cases and testing what ever Gord put on paper on sleds, bikes, quads, outboards and gas powered utility tools. I've cut a lot of heads and really changed the nature of a bunch of engines with sometimes nothing more than the change of a squish band and chamber shape. Here is an experiment with same chamber volume and squish area, different chamber shape:
full

Same compression, completely different power characteristics.
Head on the left is a mid range torquer. No detonation, strong below the pipe, hard pipe hit and run, then a sharp cut off. No over-rev.
Head on the right is mild under the pipe, a little more sensitive to timing and screamer once the pipe hits. Revs to the moon..
Same engine, same pipe, same quench area, same quench gap. Left is toroidal chamber, right is hemispherical.
Gordon hinted at these things in 1974 but much of this knowledge came out in the 1990s.

Two further heads cut for comparison:
full

These were fine tuning the squish band area. We (my son and I) also cut heads looking at various radii and squish band angles.
Even these China Girl engines I tested different squish band widths from 0.300" to nothing and gaps from 0.100" to 0.010".
I know what I am talking about. I read the books, know the theories and I tried it. Back to back tested it climbing a hill.

There are optimums and they definitely make a difference in both high and low rpm.
This chamber shape below is called "torroidal" and can make your engine into a low rpm (1000-4000) tractor:
full

Chamber shape is important but the squish is part of how this works.
This shape actually shuts the engine down at high rpm, preventing run-away rpm.
Through the 2000s and into the 2010s it was the MX chamber of choice.
They are now using shallower version to gain more rpm in the past few years, while still keeping the low and mid range torque.

I bring this up to show that much has been learned since Gordon wrote his ground breaking book in 1974.
There are a lot of factors involved and while my experience does not agree with Jag's opinion on squish,
I do agree with Jag that it is a whole lot more complicated than "6cc=power!"
I am also very grateful for his compendium of knowledge on his website. Truly a good place to start.

Steve
 
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Sorry I forgot to upload the pics. Yes I was surprised too. Check out the heads I tested. I uploaded them in post # 12
nice that looks like the deep slant head i have , but the other one i have think the compression is too high might need a second base gasket.

have you measured them with a syringe? see exactly what cc they really are?
 
Well Jag, i'm a guy who has read and followed Gordon Jennings' bible (2 Stroke Tuner's Guide) since 1976, building pipes and stuffing cases and testing what ever Gord put on paper on sleds, bikes, quads, outboards and gas powered utility tools. I've cut a lot of heads and really changed the nature of a bunch of engines with sometimes nothing more than the change of a squish band and chamber shape. Here is an experiment with same chamber volume and squish area, different chamber shape:
full

Same compression, completely different power characteristics.
Head on the left is a mid range torquer. No detonation, strong below the pipe, hard pipe hit and run, then a sharp cut off. No over-rev.
Head on the right is mild under the pipe, sensitive to timing or it will detonate and screamer once the pipe hits. Revs to the moon..
Same engine, same pipe, same quench area, same quench gap. Left is toroidal chamber, right is hemispherical.
Gordon hinted at these things in 1974 but much of this knowledge came out in the 1990s.

Two further heads cut for comparison:
full

These were fine tuning the squish band area. We (my son and I) also cut heads looking at various radii and squish band angles.
Even these China Girl engines I tested different squish band widths from 0.300" to nothing and gaps from 0.100" to 0.010".
I know what I am talking about. I read the books, know the theories and I tried it. Back to back tested it climbing a hill.

There are optimums and they definitely make a difference in both high and low rpm.
This chamber shape below is called "torroidal" and can make your engine into a low rpm (1000-4000) tractor:
full

Chamber shape is important but the squish is part of how this works.
This shape actually shuts the engine down at high rpm, preventing run-away rpm.
Through the 2000s and into the 2010s it was the MX chamber of choice.
They are now using shallower version to gain more rpm in the past few years, while still keeping the low and mid range torque.

I bring this up to show that much has been learned since Gordon wrote his ground breaking book in 1974.
There are a lot of factors involved and while my experience does not agree with Jag's opinion on squish,
I do agree with Jag that it is a whole lot more complicated than "6cc=power!"
I am also very grateful for his compendium of knowledge on his website. Truly a good place to start.

Steve
so what do you recommend for our china girls ? you have any pics of the ones you experimented with?
 
so what do you recommend for our china girls ? you have any pics of the ones you experimented with?

The one to the right is stock, the one to the front revs to 10,000 with strong power and no detonation.
It is a modified slant head. Cut a squish band at 2 degrees taper to 0.200" (4-5mm) or so, gap it at 0.035" (slightly under 1mm) or so.
Most important it goes out to the full 47mm!
full

The one to the left is an RSE head, starts easy and makes great torque but gives up 5mph. I like it in hills country.

Steve
 
there is no way a stock slant plug head gives 150psi if no metal was taken off the head or the cylinder. These engines are designed to be low power so they can pass most regulations of countries. That's why they come with low compression. Mine measured at 90psi.
 
there is no way a stock slant plug head gives 150psi if no metal was taken off the head or the cylinder. These engines are designed to be low power so they can pass most regulations of countries. That's why they come with low compression. Mine measured at 90psi.

Gary explained "Cylinder decked to 2.720 approx. .020 removed"
 
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