new gas bad for older motors with high compression?

[nathan104]

I just ran across this snipet on the mastertech marine website.

"If you have a MERCURY, FORCE or OMC motor from the early 70's thru mid-80's refer to technical bulletins from the manufacturers for timing changes and replacement head gaskets to lower compression to use today's gasoline's."

I have a 1975 60hp Chrysler that has 150 compression on both cylinders. Is this something I really need to worry about or should todays gas be just fine for me?

 

[Frank Acampora]

Re: new gas bad for older motors with high compression?

When the fuel companies changed gasoline formulations, Chrysler/Force recommended retarding wide open throttle timing to 30 degrees advance from 32. At 150 psi, you have around 10 to 1 compression but when you consider that normal atmospheric pressure inside the cylinder is 14 psi the actual compression is around 9.5 to 1. The old auto engines of the 70s ran regular gas with this compression ratio. The outboard engine will not ping on regular 87 octane at this pressure and 30 deg. advance. So head gasket changes are not necessary.
If you are really worried, waste a little money and burn 89 octane or even premium. Or buy some octane booster. I don't know anyone who does that though.

Gasoline is a long chain hydrocarbon composed of many different carbon groups--some very short like Pentane (5 carbons), Hexane (6 carbons), Heptane (7 carbons), Octane (8 carbons) and so forth up into 40 and 50 carbons in length. A long time ago, the Society of Automotive Engineers (SAE) decided to arbitrarily set the knock resistance of Octane, when burned in a test engine, to the value of 100. All gasoline is compared to that. So: a gas with an octane rating of 87 has, when burned, 87% of the knock resistance of pure octane.
Knock is the pinging you will hear under heavy acceleration. It is usually pre-ignition but can be accompanied by detonation. Most people don't hear this in their outboard because of all the noise around them.

 

[eedwards]

Re: new gas bad for older motors with high compression?

I was just read old post does this only apply to I\O?