IM CONVERTING A SHINDAWA 24CC TO A LITE WEIGHT GLOW ENGINE. I MACHINE THE TOP OF THE CYCLIDER TO ACCEPT AN OLD KB40 HEAD. I HAVE 2CC COMPRESSION CHAMBER AND ID LIKE TO HAVE AT LEAST 12:1 COMPRESSON. DO I TAKE THE VOLUME FROM THE POINT WHERE THE PORTS CLOSE OR DO I USE THE WHOLE 24CC. IF YOU PUT 24 INTO 2 IS THIS 12:1 OR IS IT 24+2 INTO 2 OR IF ITS FROM THE PORT CLOSING WHO KNOWS WHAT IT IS. I CAN STILL DROP THE CYC DOWN IF I NEED TO GET MORE COMPRESSION. ID LIKE THE COMPRESSION HIGH ENOUGH TO RUN NO NITRO.
FARMER TED

Two stroke compression is generally figured from the exhaust port closing. However, it is consequently therefore a lower number than we are used to seeing in four stroke engines which are of course calculated full stroke. If you have 12:1 compression figured from exhaust port closing, I don't think the engine will last very long!!

AV8TOR

FROM THE EX PORT UP I HAVE 18.5CC INTO 2CC SHOULD GIVE ME ONLY 9.25:1. IN 4 STROKES THATS NOT ENOUGH TO GET THE BENIFIT FROM USING ALCOHOL. WHAT SHOULD IT BE WITH A 2 STROKE? WHY WOULD THESE ENGINES NOT HOLD UP AT 12:1 BECAUSE THE BEARING SYSTEM IS MUCH BETTER THAN WE USE IN 4 STROKE RACING ENGINES AND WE RUN THEM AT 15.5:1?
FARMER TED

TED: What kind of racing engine do you run at 15-1 compression ratio? Capt,n

There's two schools of thought on how to measure compression ratios in 2 strokes. Personally I go by the geometric way which is the way it's always done with 4 strokes which also have a port open as the piston is rising on the compression stroke. Neither type of engine is 100% efficient at filling the cylinder anyway (although the 4 stroke is better).

CR is calculated (geometrically) as (swept volume + comb chamber volume)/comb chamber volume or for your engine (24+2)/2 = 26/2 = 13:1 which is right in the ball park for using zero nitro fuel.

I did an experiment on an Enya 60X where I gradually raised the compression and kept a check of the rev rise each time. When the increase in revs each time flattened out (it made a curve on a graph) I backed off to the previous setting which was 13.5:1 (on zero nitro fuel). This gave an increase of 20% HP over the original 9.75:1 compression. I could have gone even higher if I'd played with cooler plugs because raising compression advances the ignition point but a cooler plug retards it.

Methanol can handle a compression of around 17:1 before you run into detonation problems but compression by itself doesn't add any real loads to the engine. The real loads come from centrifugal forces and piston accelerations.

Downunder: I like your explanation real well...good job! Hey your friend in the photo is neat too!!!!!! Best Regards Capt,n

I was thinking more in terms of heat than engine stresses. Of course our gas conversion engines are a bit overcooled anyway, which will help keep you out of trouble, as well as using alcohol which also runs cooler than gasoline.

AV8TOR

339ci to 383ci small block chevy 15:1 on alcohol

compression is the cheapest h.p you can get only machine work no parts

Ted: But kinda expensive fuel. I do love those small block Chevy engines. Wish I had one!!! Capt,n

130 OCTANE GAS IS $10.50 GAL. AT THE RACETRACK. "THAT HURTS"

Hi Teddy,
I think that you will find that very high compression will destroy your glow plug. When running engines setup for FAI fuel with no nitro, on nitro blends we have to lower the compression to keep the plugs alive. You might want to start with lower compression and increase it gradually until you have plug failures or over heating issues. Being a machist, this should be easy. Good luck.

Dave

HI DAVE
IVE RUN THIS THING ON MOKI 18% OIL NO NITRO AND MY WALBRO ALKY MODIFICATIONS WORKED WELL. I HAD ADJUSTMENT IN BOTH DIRECTIONS - TO RICH & TO LEAN. I MADE A BATCH OF ALKY AND 40:1 KLOTZ TECHNIPALTE WITH 20% CASTOR IN IT. THINK ITS KL100. ON THIS FUEL ITS SO RICH I NEED TO ALMOST CLOSE BOTH NEEDLES. I GUESS ITS BECAUSE I NOW HAVE 16% MORE FUEL SINCE MY MOKI FUEL IS 18% AND MY NEW FUEL IS 2%. ILL HAVE TO DO ANOTHER CARB NOW OR MAYBE ILL JUST TRY 1 IN THE STOCK FORM SINCE CLOSER TO THE GAS RATIO IT MAYBE RICH ENOUGH WITH OUT ANY CHANGES. IVE HAVE ABOUT 2HRS ON THIS GLOW PLUG SO ID SAY I DONT HAVE TO MUCH COMPRESSION. IM TURNING AN 18-6 MASTER CLASSIC AT 8400 WITH THE OVER RICH CARB.
FARMER TED

I FORGOT
I RAN THIS ON 10% NITRO AND 18% OIL AND IT DETONATS AT HIGH RPM SO IM RITE ON THE EDGE. SO WITH THE EXTRA OIL RAISING THE COMPRESSION THE NITRO PUT IT OVER THE EDGE
FARMER TED