sredish Posted September 9, 2004 Report Share Posted September 9, 2004 Out of curiosity. When looking at race fuels and other doodads, a lot of times they reference the compression ratio, like this or that is for a compression ratio of blah blah. Well, I'm wondering how you figure this, or maybe someone has a table of lbs. compared to the ratios. Like how many lbs. is a 13:1 ratio? You get my drift. Scott Quote Link to comment Share on other sites More sharing options...
ledofthezep Posted September 9, 2004 Report Share Posted September 9, 2004 I dunno man, see if any of these links help you out. Static Compression Ratio Calculator Another calculator Quote Link to comment Share on other sites More sharing options...
nickxc Posted September 9, 2004 Report Share Posted September 9, 2004 I been wondering this myself. It seems that whenever they are talking about 4 strokes they refer to ratios... and 2 strokes lbs/in Quote Link to comment Share on other sites More sharing options...
John19 Posted September 9, 2004 Report Share Posted September 9, 2004 I don't get the mix ratio everyone is talking about on mixed fuel either, I run 1 12oz bottle of yamalube to 5 gallons of gas. Some peope say like 32:1 ect.. ect.. How do they get that ratio, didnt mean to hijack ur thread but it made me think of other ratios Quote Link to comment Share on other sites More sharing options...
rebelbanshee2 Posted September 9, 2004 Report Share Posted September 9, 2004 an atmosphere is about 14psi so a 10:1 ratio is about 140psi 14 x 10 = 140 for fuel/ oil mix 128 oz. to a gal. so 4 oz. per gallon is 32:1 since 128/4 = 32 or to figure about of onces for a ratio divide 128 by the ratio so 128/32 = 4 oz. your 12 oz bottle to 5 gals. is 53:1 Quote Link to comment Share on other sites More sharing options...
roosthrower Posted September 9, 2004 Report Share Posted September 9, 2004 from what I understand, static compression changes with altitude while compression ratio doesn't. To find the compression ratio on a two stroke, you have to find the volume of the cylinder after the exhaust port closes and divide that by the volume of the combustion chamber with the piston at tdc. and also from what I can gather, static compression has little to do with preignition because a running banshee motor puts out somewhere above 1000 psi with combustion taking place. This number will get higher when the motor is on the pipe as well. (pipe sonic wave forces extra gas into the cylinder before the exhaust port is covered) It's all about the dynamic compression. Quote Link to comment Share on other sites More sharing options...
Ducman Posted September 9, 2004 Report Share Posted September 9, 2004 Ah Ah me me pick me, I know this one! You have to use the good Ol' PV=nRT equation to figure out a relation between compression ratio and PSI. For your application you want to re-arrange the equation to P = (nRT)/V Pressure = (#moles x Universal gas constant x Temperature) / Volume You need to find the n (# moles of the gas present) Calculate n for the volume of air in the cylender when the piston just closes the exhaust port at a nominal temperature. Next take that same # of moles and put it into the equation using the volume of the cylender when the piston is at the top of the stroke (the volume of your domes) and that should give you the Pressure in the cylinder. To answer your question, you could create a chart showing Cylinder cold cranking PSI vs. compression ratio. Its just a PITA to figure out the compression ratio on a 2 stroke. Why compression ratio isn't commonly used in 2 strokes is because it is hard to figure out the distance from the top of the exhaust port to the top of the cylender which is needed to calculate the total volume of the cylender. On a 4 stroke you can just use the entire stroke length to calculate the total cylinder volume. Also 2 strokes have an extremely dynamic compression when running under heavy load due to the huge supercharging effect of the exhaust so it is hard to compare one motor to the next even with the same cold cranking cylinder PSI. Naturally asperated 4 strokes with a similar bore sizes have a much more similar dynamic compression under load so a fuel of a certain octance that works for one with a particular compresion ratio will typically work for another with the same ratio. Thats why on a banshee with a ported motor where the exhaust port is raised you cant reliably and accuratly compare octance requirements vs cold cranking PSI with a stock port banshee. Quote Link to comment Share on other sites More sharing options...
sredish Posted September 10, 2004 Author Report Share Posted September 10, 2004 Damn. Never mind. Quote Link to comment Share on other sites More sharing options...
The Rebel Posted September 10, 2004 Report Share Posted September 10, 2004 Damn. Never mind. Quote Link to comment Share on other sites More sharing options...
FadedDreams Posted September 10, 2004 Report Share Posted September 10, 2004 I don't know if this will help,but my motor is 15:1 and has to be ran with race fuel. Quote Link to comment Share on other sites More sharing options...
fixitrod Posted September 10, 2004 Report Share Posted September 10, 2004 (edited) I don't know if this will help,but my motor is 15:1 and has to be ran with race fuel. My corrected compression ratio is 9.45 to 1 at 192lbs. This is just proof that compression and compression ratio's are two different animals. And that's at 700ft above sea level. On paper that would mean my compression ratio would yield 16lbs higher at sea level. I got this info from a good builder. Don't ask me how. lol Edited September 10, 2004 by fixitrod Quote Link to comment Share on other sites More sharing options...
sredish Posted September 10, 2004 Author Report Share Posted September 10, 2004 I don't know if this will help,but my motor is 15:1 and has to be ran with race fuel. My corrected compression ratio is 9.45 to 1 at 192lbs. This is just proof that compression and compression ratio's are two different animals. And that's at 700ft above sea level. On paper that would mean my compression ratio would yield 16lbs higher at sea level. I got this info from a good builder. Don't ask me how. lol Wha?? 9.45:1 at 192 lbs.? That'd kill my crank in about fifteen minutes. Quote Link to comment Share on other sites More sharing options...
ledofthezep Posted September 10, 2004 Report Share Posted September 10, 2004 Ducman,...I didn't know you were Greek. You start mixing numbers & letters other than 350 t w i n I start going . Quote Link to comment Share on other sites More sharing options...
Ducman Posted September 11, 2004 Report Share Posted September 11, 2004 Ducman,...I didn't know you were Greek. You start mixing numbers & letters other than 350 t w i n I start going . I can crunch some #'s but hell would freeze over before I could ever get a program writen in C+ to compile! Quote Link to comment Share on other sites More sharing options...
NugShee Posted September 11, 2004 Report Share Posted September 11, 2004 I can help you better understand fuel ratio's. Maybe, hehehe. Ok here goes. Basically you are talking about parts of each (fuel, oil). when someone says 32:1 the first nuber being 32 is referring to the fuel, the second number 1 is referring to the oil. If you had a ratio of 1:1 then it would be one part fuel and one part oil mixed together which is the same as half fuel half oil. A part can be anything, cups, ounces, liters, etc. If using ounces as your means of measurement and 32:1 as the ratio you are trying to achieve then you will need to add 32 ounces of fuel to every ounce of oil. the ratio measuring cups you can buy at cycle shops already have the ratios marked on them so you just add the right amount of oil in the cup and then dump it in your gas can with your gas. Its pretty simple once you get it figured out. Quote Link to comment Share on other sites More sharing options...
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