After reading that Edward buys his fuel in a drum that is delivered to his door, and is presumably a largish quantity, I thought the following article might give pause for thought.
I hasten to add these are not my words but come from someone who is a 24/7 practitioner and has real knowledge of two stroke engine function where design and preparation are his living.
In non-techno terms, RVP is a measure of gasoline volatility.
“Reid vapour pressure is something of a hobby-horse of mine at present. It seems like forever, so many people have had big issues with detto in KZ2 kart engines, and after reading and hearing of these issues I decided to do some tests of my own.
I ran our SKUSA 125 Honda on the dyno one morning using fresh Avgas. From a huge number of track tests where I have recorded Relative Air Density, and Exhaust Gas Temperature, I have a straight line jetting chart that I know is reliable and accurate. The base line run gave exactly the expected result, with RAD at 97% and EGT at 725*c and the detto red lights just flickering at peak torque. I left the fuel in the tank and repeated the test the next morning, fortunately the RAD was still 97%. I ran the same test, EGT dropped 100*c and power was down by 2hp in 40. So I leaned it down, as is normal practice, trying to regain the power and the EGT number. But as I leaned down the power just dropped, the detto lights went off, giving their warning, so I stopped right there.
The light front ends had all but flashed off the fuel, and the fuel simply refused to vaporize when entering the crankcase, it simply flowed in as big gobs. The gobs don`t burn as efficiently so the EGT dropped, and power simply fades away, badly!
At the SKUSA World Finals this year we had the option of using 98VP race gas or VP C12, a way better high octane fuel. For some reason we could not get the C12 to work for us on the track, so I went to the VP race truck, explained what was happening and asked if they would open a brand new, sealed drum of fuel. They did so and the reverse of what should have happened, did happen, nothing! C 12 has a documented RVP of 9psi, and there should be an audible woooosh as the drum was opened, it didn`t happen. The fuel was degraded and rendered useless, red faces all round at VP!
The short story is, not only that fuel does degrade but it is the light, low end fractions that initiate vaporization, starting and throttle pick up that are noticed first. If the fuel doesn`t vaporize then the engine has nothing to burn properly and it just drowns on gobs of gas.”
A couple of lessons can be drawn from this experience, buy your fuel from a busy air field that replenishes it`s supplies frequently and buy in “just enough” quantities, and use fresh for each race meeting. The surplus can go in the lawn mower, don`t put it in the car as lead and cat converters don`t mix too well!
Trevor
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Fri Mar 11, 2016 4:02 am
