Hey guys, I went and spoke to the machine shop yesterday and got some advice for my new motor, I asked what compression ratio he would recommend (Forced Induction with LPG EFI) and he said 8.5:1
I said really even with the LPG? he said yea, reakons he knew a guy who worked for BP and altho sometimes LPG is up to 105ron the Australian Standard minimum is only 91 so if you build and tune a motor for 105 and get a poor quality fill ya will likely pop ya motor...
As far as I know, there is no Australian standard for a minimum RON rating for lpg, so take any advice from that direction with a grain of salt.
The reason there is no minimum standard is that its basically very difficult to end up with a blend of lpg that has a RON less than 98.
As has been explained LPG is just a mixture of gaseous petroleum, could be any of the many isomers of propane or butane. The RONs of these can vary from as low as 94 to 112. However, that doesnt stop non gaseous fractions of fuel ending up in there, like butanol etc where the RON is even lower.
Overall, though, at pumps in Australia, you are unlikely to have a mix that goes below 30% propane, otherwise the stuff just wont come out of the tank at low temperatures. Actually determining the RON for a mixture is tricky and isnt simply taking the % mix of the fuels and applying that tow the RONs. However, it is safe to say that the value of the RON of a mix will not fall outside the min RON of either of the mixes.
Based on a worst case scenario of a 30% propane content, my best guess would be the RON would still be as good as ULP ~98.
Of course, if you were building a motor right on the edge and wanted to ensure a high RON, supagas supplies a 100% propane mix. Suppliers to service stations will be able to tell you the relative mixtures of their supplies, but dont expect too much if you ask the attendant!
MONs and RONs? there is plenty on the internet about it.
Bascially there needed to be a way of establishing the quality of fuel compared to "pure" petrol which would be 100% octane and have a 100% octane rating. The detonation quality of fuels is measured relative to 100% octane, a fuel with a rating of 88 would mean it has the same qualities as 88% octane 12% n heptane(typically what makes up the rest of petrol)
The R in RON stands for research where the detonation of the fuel under test is compared to 100% octane in a test engine.
MON is similar and stands for Motor where the intake temp of the fuel is heated to a higher temp and the test motor is run at a higher speed.
The MON rating is typically lower than the RON rating for the same fuel, the MON rating perhaps being more relevant to today's engines.
There is much misinformation out there, certainly anyone claiming to be an expert on the matter who simply refers to the octane rating of lpg is out of their depth as there is no octane in lpg, they should be quoting RON or MON.
I think its a fair call for an engine builder not to recommend going beyond a compression ratio of 10.5:1 if you are just going to run normal lpg, rather than propane.
though you havent mentioned here whether you are going straight lpg or dual fuel, you still have to build the motor for the lowest quality fuel that may end up in there.
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