Compression Ratio
03-21-2007 | 11:06 AM
#1
Thread Starter
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Compression Ratio
I'm working on a 26cc Tanaka conversion. I have a used engine and a new one. The new is getting the conventional treatment as a bench mark, test engine. The used one has had the head sliced off and a new, bolt on head is being made. Several heads are in the works. Pictured is the diesel version. I'll also be making a head to take a spark plug along with a glow plug version.
A couple of issues arise.
The Tanaka has an angled spark plug and thus, the combustion chamber is oddly shaped. I'm speculating that we have a measure of inefficiency built in. Knowing Tanaka, I'm sure that this has been minimized with careful design. However, we always want more power, don't we?
Some gassers have a straight up spark plug. What does the combustion chamber look like in these? Dome? hemi? trumpet? cone? Compression ratio?
Tanaka says that the compression ratio on their 262 (26cc) engine is 7.5 to one. Is this typical, high or low, compared to the other 26cc engines out there?
As I'm making a bolt on head, I can apply any compression ratio I want. Is there any value in increasing the CR for gas operation? Would this affect the grade/brand or whatever gas I should use?
When converting from gasser to glow operation with stock glow fuel, what of the compression ratio? Note that with a separate head, I can adjust the CR without altering the port timing. When going gas to glow, do I need more compression?
The original uses a 12mm spark plug. I have a tap for 10mm so I thought I'd go to the smaller plugs for only that reason, the availability of the tap. Any consequences of going to a smaller plug?
What is the value/differences in the various plugs available. Some have longer threaded portions than others. Is this similar to long and short glow plugs? In other words, no difference. Or, are the longer plugs cooler running due to the greater contact area with the head?
A 26cc gasser typically takes an 18 X 6. When converting to glow, what prop should I use? When one installs a CD ignition to convert a glow engine to gas is there any change in prop size?
After paying under 5 dollars for props for most of my modelling life, I'm now faced with 20 and thirty dollar props. I need to make some reasonably correct choices at the outset.
When I convert small glows to diesel, typically, the engine can now handle the next one or two sizes up. A .15 will use an 8 X 6 on glow but swing a 10 X 4 on diesel and still be quite happy about it.
My thinking is that the 18 X 6 will be good for gas and glow and the diesel should take a 20 X 8.
Lots of questions there, I know. Any input will be most welcome.
A couple of issues arise.
The Tanaka has an angled spark plug and thus, the combustion chamber is oddly shaped. I'm speculating that we have a measure of inefficiency built in. Knowing Tanaka, I'm sure that this has been minimized with careful design. However, we always want more power, don't we?
Some gassers have a straight up spark plug. What does the combustion chamber look like in these? Dome? hemi? trumpet? cone? Compression ratio?
Tanaka says that the compression ratio on their 262 (26cc) engine is 7.5 to one. Is this typical, high or low, compared to the other 26cc engines out there?
As I'm making a bolt on head, I can apply any compression ratio I want. Is there any value in increasing the CR for gas operation? Would this affect the grade/brand or whatever gas I should use?
When converting from gasser to glow operation with stock glow fuel, what of the compression ratio? Note that with a separate head, I can adjust the CR without altering the port timing. When going gas to glow, do I need more compression?
The original uses a 12mm spark plug. I have a tap for 10mm so I thought I'd go to the smaller plugs for only that reason, the availability of the tap. Any consequences of going to a smaller plug?
What is the value/differences in the various plugs available. Some have longer threaded portions than others. Is this similar to long and short glow plugs? In other words, no difference. Or, are the longer plugs cooler running due to the greater contact area with the head?
A 26cc gasser typically takes an 18 X 6. When converting to glow, what prop should I use? When one installs a CD ignition to convert a glow engine to gas is there any change in prop size?
After paying under 5 dollars for props for most of my modelling life, I'm now faced with 20 and thirty dollar props. I need to make some reasonably correct choices at the outset.
When I convert small glows to diesel, typically, the engine can now handle the next one or two sizes up. A .15 will use an 8 X 6 on glow but swing a 10 X 4 on diesel and still be quite happy about it.
My thinking is that the 18 X 6 will be good for gas and glow and the diesel should take a 20 X 8.
Lots of questions there, I know. Any input will be most welcome.
03-21-2007 | 02:03 PM
#2
Senior Member
RE: Compression Ratio
Trapped compression ratio for premium gas is indeed a typical 7.5:1, and most engines use a bowler hat shaped chamber.
A normal prop is 18x8 - 18x8, for diesel about 20x8, if you can harness the high cylinder pressures.
A CM6 plug is M10x1 fine metric thread, not the standard M10x1.5
Garden plugs are M14x1.25 fine metric thread
A normal prop is 18x8 - 18x8, for diesel about 20x8, if you can harness the high cylinder pressures.
A CM6 plug is M10x1 fine metric thread, not the standard M10x1.5
Garden plugs are M14x1.25 fine metric thread
