is 100 octane good or bad?

[Hugh Davis]

#261105

Good morning!

I have a  1939 MD39 1.5hp and 1950’s 5.5hp johnson that have comeback to life and need to know if 100 octane fuel is good or bad for them.

I have access to leaded and non leaded, leaded is better, correct? My gut says get a 92 Octane, add some lead and Seafoam but I’m not sure. Now that I’ve got them back in working condition, I just want to use what’s best for them for the long term. I will run 25:1 gas:oil with a hopefully high quality 2 cycle oil I got from alhstrand.

Any other advice would be gratefully appreciated!

Thanks

Hggh

 

[frankr]

#261108

Back in “The Day” people would seek out unleaded (marine white) gas because the lead caused excessive carbon deposits in the cylinders and spark plugs.  Amoco 100 octane was an unleaded gas, so that was also desirable.   Otherwise, high octane is not needed in our old low compression outboards.  Leaded gas did provide lubrication for valve seats in 4-stroke engines, but no valves in 2-strokes, so not needed there.

[fleetwin]

#261111

High octane fuel is not needed, or recommended for most of the older lower compression OMC outboards.  Believe it or not, the high octane fuel can actually rob power from these engines.  I know it seems counter intuitive, but it is true.   Regular unleaded is just fine/mixed with the proper amount of oil.  True today’s fuel degrade sooner and damage some fuel system components.  So, be sure to drain the carburetor and tank completely if the engine is not going to be used for a few weeks.

[outbdnut2]

#261112

If you are in Minnesota like me, 100 octane is the only ethanol-free gas you can buy and not all 100 is ethanol-free, as only some stations have it.  As others said, anything ethanol-free is the gas to use regardless of octane rating, but high octane is not needed (but won’t hurt it)  if you can get lower octane ethanol-free gas.

 

1 user thanked author for this post.

ftruntz

[dave-bernard]

#261119

Are you sure 25-1 is correct and not 16-1? just asking.

[outbdnut2]

#261121

This came out in 1964 and says 25 to 1 for all the  CDs 1963 and earlier, which would be his 5-1/2 HP.  I don’t know about the 1939.  My 1947 PO says 8 to 1 on the decal.

Dave

[fleetwin]

#261136

I would ignore that foolish 24:1 recommendation for CD/AD models with plain rod bearings.  Later model CD/6hp models had actual needle bearings on the big end of the rods so 24:1 is just fine for them.

[labrador-guy]

#261141

Hugh everyone has an opinion so here is mine.   Those CD/AD motors have no ball bearings of any kind.   16:1  or maybe 20:1 with TCW3 oils should be used.  These oils are good for the motor and good for the water they are run in.  If you are afraid of alcohol fuels remember Seafoam and Stable both have alcohol in them.   10% alcohol won’t hurt but you want to drain all the fuel out of your motor at the end of the season.

JMHO  dale

get’em wet!…..don’t let’em set

[Mumbles]

#261142

16:1 for your MD-39!  Originally Johnson called for heavy #40 oil for the mix but that was probably to help seal the crankcase bearings more than lubricating the motor. Current TCW III rated outboard oil is quite thin, about 5 or 10 weight and is meant to flow thru injectors when cold but it lubricates just fine. Go with the 16:1 and you’ll be OK.

High octane fuel is meant for high compression motors and it burns slower in the cylinder than regular or lower octane gas does.  The slower burning is to prevent fuel knock or preignition and using it in a low compression motor is kind of pointless as it costs more and as already mentioned, can cause a loss of power compared to using the recommended lower octane gas. Save the high octane gas for airplanes and race cars!

• This reply was modified 1 year, 3 months ago by Mumbles.
• This reply was modified 1 year, 3 months ago by Mumbles.

[amuller]

#261240

I take it this refers to mogas, not avgas?