Tractor Gas

Probably not at that compression ratio.
The R+M/2 method is just a tricky way of posting good numbers on the pump since it's and average between two octane rating methods.

I'm not sure since it's been along time ago since my racing days but I think one test is a high speed test and the other is a low speed test and the standard now is to average them to get the bests of both worlds for the engine at all speeds.
I don't remember, it's been along time since I messed with gas octane ratings. I can't even remember what the test chemical was but I could never pronounce it anyway.

dadsdozerhd5b wrote: do yourself a favor and run the high test. it will run better and store longer if you are not working it hard. also has less or no ethanol in it to go bad faster. i have had alot of funky things happening with carburators that have never been a problem before even with stabil in  them.

Actually the opposite is true.  If you run E10 gas all the time, you'll have less storage problems.  If the old gas tank has a bunch of water and crud in the bottom of it, the first tank or two of E10 could be a problem as you clean that stuff out.  Once cleaned out, the E10 will keep your tank and carburetor free of standing water.   It's the cleaning out process of old shellac buildup in the carbs that can be an issue referred to above but, once clean, the carbs stay clean with E10.

Contrary to all the old wives (shade tree mechanics) tales, E10 is the way to go.  I've just restarted my boat, MH, 2 garden tractors, a lawn mower, and an old chain saw after they sat all winter with E10 in the tanks.  The garden tractor and lawn mower tanks weren't even full.  Everything started up and ran well.

M is for motor method. In the motor method of testing for octane they use an engine with adjustable compression ratio. They increase the compression ratio until it begins to ping, then they mix octane (a hydrocarbon) and some other standard low octane rating hydrocarbon until the engine knocks the same. The octane rating of the fuel being tested is the amount of octane hydrocarbon needed in the standard fuel to get the same ping or knock.

I'm not sure of the Research method, its probably a chemical analysis and the two methods hardly agree. So pumps tend to show the average of the two.

Gerald J.

The "R" stands for "Research", meaning that the Research octane number is found through chemical based lab testing.

The "M" stands for "Motor", meaning that the lab uses a special engine with variable compression that is mounted on a dynamometer to arrive at a "practical" octane number that is based on real-world knock tendency in a controlled environment.

The R + M / 2 method just takes the average of the two numbers to arrive at the number that is on the sticker at the pump.  In other countries I understand that it is common to see only the Research octane number, or RON on the pump.  Since the Research octane number is generally higher than the Motor octane number, you would probably be fine to use 87 or 89 R + M / 2 octane gas.

Thanks guys, looks like the 87 R+M/2 with 10% ethanol would be OK to use, and about 15 cents a gallon less costly here in N.IL. than the 93 octane. This tractor may be for sale in the future.  Runs like a top, now, everything seems to work, I even got all the lights working.  Straight sheet metal, show 93xx hours on tachometer which still works.  Has a fuel issue I need to work out yet from sitting so long, line plugs occasionally.  Rear tires like new, left front OK, right front will need to be replaced before long.  Tire guy comes Tuesday to fix a small fluid leak before it ruins the rim.  Still needs a new seat, but for now a boat cushion will have to do, got bills to pay first.