compression ratio for FAI fuel
12-03-2017, 12:58 PM
#1
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Join Date: May 2011
Location: INVERCARGILL, NEW ZEALAND
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compression ratio for FAI fuel
Hi people.
I am in New Zealand and at sea level presently running all my IC engines on 85% methanol and 15% castor with OS#8 plug for 2st and OS-F for 4st.
I do find that the 2 strokes are more difficult to start and I have to use an electric starter as hand flipping is futile...also the tuning is quite sensitive and I was wondering if I tinkered with the head shims (and therefore the compression ratios), would it make a difference in starting, idling and tuning.....?
I am basically being a ninny and do not want to damage any of my precious engines so maybe some else reading this who is braver than I that has experience with compression ratios can help me out?
Many thanks in advance and kindest regards.
I am in New Zealand and at sea level presently running all my IC engines on 85% methanol and 15% castor with OS#8 plug for 2st and OS-F for 4st.
I do find that the 2 strokes are more difficult to start and I have to use an electric starter as hand flipping is futile...also the tuning is quite sensitive and I was wondering if I tinkered with the head shims (and therefore the compression ratios), would it make a difference in starting, idling and tuning.....?
I am basically being a ninny and do not want to damage any of my precious engines so maybe some else reading this who is braver than I that has experience with compression ratios can help me out?
Many thanks in advance and kindest regards.
12-03-2017, 01:23 PM
#2
The ideal compression ratio for FAI fuel is around 13.5:1. The ideal head spacing is .4mm or so (about .015”). If you can get close to this with just head shims, you’re in good shape. I would suggest a better glow plug (hotter) and also adding 2-4% acetone by volume which will improve idle quality and starting.
12-03-2017, 11:24 PM
#3
What engines are you using?
Some of the smaller two stroke engines are set-up for the US market that seem to use nitro as a fuel rather than a fuel additive. Other engines, from eastern europe, are set-up for low nitro from the beginning and will not like high nitro levels without lowering the compression ratio.
I regularly use FAI but prefer to use 5-10% nitro as that helps a bit with the running in cold weather, i.e. around the freezing point (0°C). I hand start all my engines (non-ringed) it is mainly about getting to know the engine and priming it just enough.
What fuel are you using? FAI fuel is 80% methanol and 20% castor oil. The fuel used for four stroke engines will tyically not have enough oil for a two-stroke engine to run well. The optimum compression ratio depends on many things, and the oil content is one of those variables.
Some of the smaller two stroke engines are set-up for the US market that seem to use nitro as a fuel rather than a fuel additive. Other engines, from eastern europe, are set-up for low nitro from the beginning and will not like high nitro levels without lowering the compression ratio.
I regularly use FAI but prefer to use 5-10% nitro as that helps a bit with the running in cold weather, i.e. around the freezing point (0°C). I hand start all my engines (non-ringed) it is mainly about getting to know the engine and priming it just enough.
What fuel are you using? FAI fuel is 80% methanol and 20% castor oil. The fuel used for four stroke engines will tyically not have enough oil for a two-stroke engine to run well. The optimum compression ratio depends on many things, and the oil content is one of those variables.
