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Carlos, I was doing some more research on two stroke compression and found that many people include exhaust port height ATDC, I was wondering why you don't use that formula? I'm not saying one way is right or wrong but when I use the torquesoft two stroke calculator it's almost 4 points difference.
between the two methods. With your formula I was getting 17.6:1, and with torquesoft I got just about 14:1. I'm just wondering when ty the difference between the two.
I already accounted for the piston dome displacement, I am going to re-measure my exhaust port height tonight. I'm wondering If that software should really have the exhaust port width too, I'm assuming it's only calculating trapped compression when the port is closed.
 

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Discussion Starter #22
Yes, two schools of though, one calculates UCCR (Un-Corrected Compression Ratio) and the CR (Corrected Ratio). The CR method is what you utilized to get 14/1...known as the 'Japanese Method'. Without going into a lot of detail, the CR method sounds good, seems to make sense, however; in reality is not a good measure of 'true' compression. In 'layman' terms, CR defines cutting your exhaust port roof higher as efectively reducing engine displacement (less trapped volume), when in reality a 250CC is a 250CC at 170 degrees or 200 degrees exhaust port timing.

UCCR is a 'truer' measure of compression and a much safer number to work to IMO.

Carlos
 

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38.5 x 38.5 x pi x 72 + 19 ÷19=17647.2÷1000= 17.647
19 is the volume at tdc with a 23cc dome, but it's had bad detonation and 290psi static compression. I was running c12.
I used this website. TorqSoft - Compression Ratio - Calculation Tool
Cylinder bore-77
stroke -72
exhaust port height-52
cylinder head volume - 19
=13.8
I am thinking I may have not tared my calipers so I need to re-measure the exhaust port height. Unless there is a way to figure it out by it opening at 85° ATDC..
I switched to a 33cc dome but I am thinking I went too far as the quad runs strongest in 6th gear at WOT.. but all detonation was eliminated..
 

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Your right, I'm sorry. I added the 19cc before I devided by 1000. I updated the last post with the rest of the info you asked about+ some.
 

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The gauge is not bad, it first I had an 18cc dome in it, did a static compression test on it and pegged the gauge out at 250psi after 4 kicks. Then I changed to dome to 23cc and used a different gauge that went up to 300psi. After 14 kicks at WOT it quit building pressure at 290psi.
 

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Discussion Starter #26
27CC-28CC Dome for a 330CC with a 77MM bore, set at .045"/.050" squish clearance with our Dome design will end up at around 14.5/1 to 15/1 on our builds. I like to set 330R's at 14.5/1 to run on C12. While C12 is rated to 15/1 max, as long as your bore size is under 80MM, you can push to 16/1 safely.

Carlos
 

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This topic comes up almost weekly, I will attempt to clarify it as much as possible...



· Take the filled syringe and fill through the spark plug hole up to the bottom most thread (the thread deepest inside the head closes to the Dome bowl looking from the top). Look at how much oil went into the cylinder-head by subtracting the amount of oil left in the syringe from the amount you started with. This is your Volume of Combustion Chamber at TDC…record this number.

· Add ½ a (.50)CC for the spark plug volume

UCCR = Volume of cylinder at BDC + Volume of combustion chamber at TDC divided by Volume of Combustion chamber at TDC)


Carlos.
Just out of curiosity why would you add .5 cc to the volume of the combustion chamber? In reality when you insert a spark plug you would probably lose .5 cc of volume at TDC. I know it's a small number, but just curious none the less.
 

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Discussion Starter #29

"Take the filled syringe and fill through the spark plug hole up to the bottom most thread (the thread deepest inside the head closes to the Dome bowl looking from the top)."

Add .5 after filling to bottom thread, meaning up to the bottom of the spark plug, not including the air gap between the porcelain and the outer steel thread area.
Yup...the .5CC is to account for the volume found inside the Spark Plug body; the "air gap between the porcelain and the outer steel thread area"...the .5CC volume is an average of '7', '8' & '9' series spark plugs.

Carlos
 

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I hope I did not miss it, but is there a chart that will give an indication on what UCCR you have to what octane of gas you should run.
 

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Discussion Starter #32
I hope I did not miss it, but is there a chart that will give an indication on what UCCR you have to what octane of gas you should run.
I would go with the Race Fuel manufactures recommended CR; for C12 VP is rated at an MO of 108 with a max CR of 15/1. VP110 is rated at 107 MO with a CR of 13/1. Sunaco Std is equal to VP110 and Sunoco Supreme is = to C12...

Carlos
 
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