psycho1122
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posted September 08, 2006 09:29 AM
 Edited By: psycho1122 on 8 Sep 2006 10:44
BTW, since the 12 has a bore / stroke ratio of 1.498 (or 1.5), it seems to like total advance in the 35-40 deg. range. Most of us 12 owners who have added advance have seen an overall torque improvement at peak VE.
In comparison, the 14 has a bore / stroke ratio of 1.377. this engine due to the natural increase of combustion pressures at 30 deg. BTDC up through TDC and down 30 deg. ATDC may require as little as 25 deg. of ignition advance to produce peak Torque!
In summary; the higher combustion pressues in a longer stroke with a poor quench area (not tight enough) could result in damaging detonation of the gasses on the outer edges of the piston. I think trying to get the quench tight enough on a stroked 12 could be a tough thing to accomplish. Never tried it........Jonny??
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Y2KZX12R
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posted September 08, 2006 10:11 AM
Edited By: Y2KZX12R on 8 Sep 2006 11:19
Good thread here.
Just a few points. Piston speedis defined by distance over time not rod length. The piston speed halfway down the bore is the same no matter what rod length. and the piston speed (0) at tdc and bdc is the same. However, the piston acceleration rate with a longer rod is higher. A longer rod keeps the piston at or near tdc and bdc longer so once the crank angle starts to increase the swept volume increases at a faster rate.
Seeing as how swept volume increases (initially) at a slower rate with a longer rod vs a shorter rod with the same bore and stroke, and then the rate starts to increase at a faster rate untill the swept volume increase rate is the same half way down the bore.
So typically an engine with a bore stroke ratio of 1.5 will build cylinder pressure faster than one of 1.4 an there fore can produce more hp at higher rpms when there is less time to build cylinder pressure. It also makes an engine more suseptable to detonation and is most likely the reason kawi was a little extra conservative with base ignition timing on the 12r from the factory.
As far as piston rock and skirt loads? Its true that a longer rod reduces skirt loads and less piston rock. But skirt loads are so rediculously low on these short stroke bike engines to begin with its not really a concern. Plus the factory pistons are hypereutectic pistons with a very high silicon content to control expansion and are able to run super tight skirt clearances.
Tighter squish area helps reduce detonation by increasing the turbulance in the combustion chamber. The air gets squeezed out and tumbles as its squeezed into the chamber. Turbulance helps reduce hot spots that can cause the fuel to pop and also increases the burn rate and reduces the time needed to burn the fuel 100%. It also burns the fuel more complete making more power and less emitions.
Thats why the chambers are getting flatter and flatter with bigger and bigger squish areas.
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psycho1122
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posted September 08, 2006 10:29 AM
This is probably why Doug said "perfect' when I discovered that my piston was close enough to the head in the quench area to "kiss" off the carbon, but not HIT.
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osti33
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posted September 08, 2006 12:54 PM
Good info guys. 
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entropy
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posted September 08, 2006 02:03 PM
great info, and i am glad that psycho is having good luck with .027 quench, but I guess I am conservative, and I would not set it up that tight...
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psycho1122
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posted September 08, 2006 02:25 PM
Edited By: psycho1122 on 8 Sep 2006 15:35
entropy; back to my earlier question.....
Could you tell me (with the info discussed) what my CR could be close to?
Thanks!!
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Y2KZX12R
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posted September 08, 2006 05:25 PM
Karl, .027" is fine with the factory pistons. Not a big deal. I ran that for 4 years. But it leaves almost no room for wear or carbon. If you take your engine apart every year or pour a cleaner or water down each throttle bore once a year or so and dont beat the engine untill its hot you can get away with very tight squish. I dont recomend it but you could most likely go as tight as .020" on a race engine and stock HU pistons.
But forged pistons run with much more skirt clearance to alow for expansion from heat. Cold they are loose and can even rattle. The rocking motion can allow the piston to get closer to the head above the skirt area than over the pin. Its best to do a mock assembly without rings and set the piston to head clearance factoring this motion in.
I dont remember off hand what the total rock was in the bore of my engine. Its written down but not handy right now. Generally the tighter the squish the better, untill the piston hits the head, and then you have big problems.
It takes circle track guys a lot of trial and error and development to find the sweet spot for their given piston rod combo. Change to a different style or make rod or piston and you start over again finding the tightest squish possable for that combo.
This winter with the FORGED JE pistons, i'll bring the squish down to about .030" by removing one of the .010" block shims. I wanted to keep the cranking pressures down so
I didnt have the hard starting issues and have to go with 24vdc starting system.
Well... I had to go 24v anyway so now it doesnt matter.
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entropy
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posted September 08, 2006 06:11 PM
Anyone know what is the squish on the stock 12?
a lot of experts don't like much less than .040 for drag bikes; maybe street use is different???
psycho,
i don't have a computer program for CR, just empirical results from my many (too many) "experiments" on my 1427. So i don't have any info on stock 1270/1290.
do you know yr cold cranking pressure???
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osti33
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posted September 08, 2006 06:15 PM
quote:
Anyone know what is the squish on the stock 12?
Don't know what psycho's was but mine was .042" stock.
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psycho1122
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posted September 09, 2006 05:18 AM
entropy; 235 psi per hole at 30.08 barometer.
osti33; never checked mine stock. Since I did not machine any surface and you add the .010 base gasket, that should get me close to the stock #.
Do the JE's Muzzy uses in thier 1270 kits have more hight above the pin than stock?
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Y2KZX12R
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posted September 09, 2006 08:48 AM
My 12 from the factory had a deck height of .018" and a pistont to head clearance of .041"
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