I've heard running a 7cc compression head after tricking ure motor out can produce better gain's then running a 5cc head can any one confirm this??? i imagine running the 5cc would be hard on the engine and shortin it's life span compared to the 7cc but buy how much can any one say ?
compression questions
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hahah yes it is thats what i run , i was mostly just wondering what the effects of using low vs high compression are on the motor
BMEP Brake Mean Engine pressure or PSI exceeding right around 120lbs engine warm, or more is when you need to consider using fuels other than pump gas, say 140-150lbs or more.. . Cylinder Pressure of 90lbs or higher is good up to 115-120lbs or so. Keep in mind these are engine warm measurments. I have found that a 120 warm can be 150 cold. When you get to these higher pressures, under detonation you can put more than 400lbs of pressure on the wrist pin and crank pin, crank ect.. This is a lot ! These little F50 and F80 engines are really not made for such pressures. You can bend wrist pins and cause bearings to fail under such pressure. You can increase power by adding compression.. But as with many things, there are limits.. If you keep engine BMEP around 100lbs or so, and open your exchange ports, port match the intake and Exhaust to exhaust pipe, you will gain amazing power. All with out adding compression.. Another aid to power is oil. More oil adds power. More 2 stroke oil in your fuel will add power. The extra oil helps to seal imperfections in cylinder wall, helps cool better, which all adds up to more compression and more power.. Here again.. There are limits.. Once you exceed 18:1 or there abouts,. Say.. You go to 15:1 or 12:1, you start to get too much oil and not enough fuel to burn, and you begin to lose power.. But on the positive side, you will not hurt your motor because of too much oil. Never had that happen where I hurt a 2 stroke engine because of too much oil in the gas.. Currently on my KTM and KX 2 stroke race bikes, the factory states that mix on these race bikes is 32:1.. And these are expensive race bilks to be sure.. And so..With that, I also tell my customers to never run less oil than 32:1 mix. Also.. I have spoken to racers in Canada that use the Opti lube and they never go leaner than 60:1 using that stuff and they educated me on the process of getting to that mix ratio of 60:1. Yes.. It is a process.. Anyway.. Enjoy the ride..
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You may need to raise primary compression to get the most from a smaller combustion chamber?
I find 150 Psi measured on my compression tester at a slow pedal works good for me.
There is a definite improvement in low down with higher compression, however I believe it can be better not to over do compression if looking for top end.
I find 150 Psi measured on my compression tester at a slow pedal works good for me.
There is a definite improvement in low down with higher compression, however I believe it can be better not to over do compression if looking for top end.
wow !! You are running 150 BMEP on an F50 or F80 2 stroke Engine? Wow !! Wow !! that is unreal.. Typical 4 Stroke will run with that kind of pressure but they have valves. I have some 2 stroke Race bikes that run with that kind of pressure, but they are water cooled. F1 formula cars run 175-200 BMEP making around 800hp with their little engines.. How do you keep it cool? Do you have water cooling?
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If you use Valvoline 2 Stroke Oil you will not foul a plug because of 18:1 mix.. 18:1 is pretty rich with oil for sure.. Try 25:1.. That should be plenty.. 25:1 is 5 ounces of oil per gallon.
It will surely run hot with 150 Psi and an angle fire head.
The Puch head shows no problems, neither does a straight fire.
The only motor I don't run that sort of compression on is the one with the pull start.
150 Psi is likely to shorten the life of the bearings and it's best to stuff the cases or at least use a 5 piece crank for better primary compression.
The Puch head shows no problems, neither does a straight fire.
The only motor I don't run that sort of compression on is the one with the pull start.
150 Psi is likely to shorten the life of the bearings and it's best to stuff the cases or at least use a 5 piece crank for better primary compression.
From my Testing so far on the motors we build , it seems that 110-120 BMEP seems to be the ticket. Anything above 80-90 BMEP the engine will run ok and provide a longer life.. But if you want to become a tuner and make the engine perform and still have good reliability. First do what is needed to get the BMEP Aperture in the 110-120 BMEP range.. Once you get this, then you are ready to tune !! Ways of getting this number vary .. Depends on the motor parameters.. For Example, the Slanted Spark Plug head or Slant head is typically 15-20 pounds BMEP higher than the Straight plug head.. There are many other ways to up the BMEP.. Finding them is half the fun.. keep in mind these measurements are engine warm. .. Enjoy the ride.. 
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