5.8 OMC Fuel Octane vs. Performance

[raquettelaker2]

We have a 1993 Four Winns 210 Horizon. It's a gorgeous boat for its age and is powered by a stern-drive 5.8l OMC Cobra Ford V8 with "multiport electronic fuel injection."

This year we have been having an absolute miserable time getting skiers out of the water. The boat just doesn't have the balls to do it! It's a damn heavy boat but it's always been able to pull people out of the water pretty easily. We did hit a rock with our stainless prop a few years ago so we have a cheaper aluminum prop on it, which I understand the blades can flex a bit more under acceleration of the boat. But I'm also realizing we've also been cheaping out on gas. The only gas dock on our lake only sells 93 octane and recognizes that they can get away with charging over $5.00/gallon for it, so we bought a 12-gallon wheeled gas jug and fill it on land. Pretty sure we've been using 87 octane. Haven't noticed any pinging or knocking, but would this greatly reduce the power output of the engine?

 

[RogersJetboat454]

Re: 5.8 OMC Fuel Octane vs. Performance

We have a 1993 Four Winns 210 Horizon. It's a gorgeous boat for its age and is powered by a stern-drive 5.8l OMC Cobra Ford V8 with "multiport electronic fuel injection."

This year we have been having an absolute miserable time getting skiers out of the water. The boat just doesn't have the balls to do it! It's a damn heavy boat but it's always been able to pull people out of the water pretty easily. We did hit a rock with our stainless prop a few years ago so we have a cheaper aluminum prop on it, which I understand the blades can flex a bit more under acceleration of the boat. But I'm also realizing we've also been cheaping out on gas. The only gas dock on our lake only sells 93 octane and recognizes that they can get away with charging over $5.00/gallon for it, so we bought a 12-gallon wheeled gas jug and fill it on land. Pretty sure we've been using 87 octane. Haven't noticed any pinging or knocking, but would this greatly reduce the power output of the engine?

Shouldn't have any problems with the 87. That's most likely what the engine was designed to use.

Have you done the basic's to it?
Compression test? Recent tune-up (plugs, cap, rotor, wires, check timing, fuel filter, etc)?
When you replaced the damaged prop, did you match the pitch of the old prop to the new one?

 

[bruceb58]

Re: 5.8 OMC Fuel Octane vs. Performance

Absolutely no difference in power between octane numbers.

 

[raquettelaker2]

Re: 5.8 OMC Fuel Octane vs. Performance

Shouldn't have any problems with the 87. That's most likely what the engine was designed to use.

Have you done the basic's to it?
Compression test? Recent tune-up (plugs, cap, rotor, wires, check timing, fuel filter, etc)?
When you replaced the damaged prop, did you match the pitch of the old prop to the new one?

I think it may have the original plugs. :facepalm: It's been in the family since 1993 and now that the youngest generation (me) is employed I'm starting to take over some responsibility for maintenance. Not sure about prop pitch either. I would assume my grandfather had the brains to match it.

Absolutely no difference in power between octane numbers.

Okay thanks. I know a lot of modern cars these days (mine included) have knock sensors, etc. and can adapt to lower octane fuels without damaging the engine by retarding timing, reducing boost, etc, but at a cost of power. I figured a 20 year old marine engine wouldn't care, but wanted to check. Sounds like we need to be focusing on a tune-up though.

Thanks!

 

[HT32BSX115]

Re: 5.8 OMC Fuel Octane vs. Performance

Okay thanks. I know a lot of modern cars these days (mine included) have knock sensors, etc. and can adapt to lower octane fuels without damaging the engine by retarding timing, reducing boost, etc, but at a cost of power. I figured a 20 year old marine engine wouldn't care, but wanted to check. Sounds like we need to be focusing on a tune-up though.

Thanks!

Howdy,


According to the OMC service manual, Your 5.8L EFI engine is "OK" with 86 AKI fuel and they say 93 AKI fuel is "Ideal".

Also, although you didn't indicate your engine model number, there's SEVERAL OMC EFI 5.8L models that are essentially the "same".

They ALL have Knock Sensors and YES they do adjust (the ECM retards) the timing when they detect "knock". It's unlikely you would hear it at all.

Because of that, low octane fuel WILL result in lower performance.......not because it's inferior fuel etc, but because the engine ECM will retard the timing to protect the engine (which reduces performance)

Evidently OMC changed the knock sensor back then

All 1993 (JV) - 1996 (NC) 5.0, 5.8 Litre EFI Ford Engines

Research has shown gasoline that starts out as 89 octane can quickly deteriorate to a lower octane. Degraded octane or inferior quality fuel can cause undetected detonation (spark knock) in these engines.

A new two-wire Knock Sensor Kit, P/N 3857030, has been released for these models to improve monitoring of engine operating conditions. The new knock sensor is more sensitive and will detect a wider range of detonation.

The knock sensor is located at the rear of the port cylinder head, near the intake manifold. The wiring harness must be modified to install the new two wire sensor. Follow the instructions that come with the sensor kit.

Submit a warranty claim, along with the old knock sensor, to the OMC Warranty Department.

You are at somewhat of a disadvantage though with your 20 year old OMC system.............some or all the EFI parts are no longer available, anywhere.

If you cannot find the parts to fix it, your only option may be to build another engine (with a different cam, intake manifold etc) and use a newer distributor and carburetor.


Regards,


Rick

The only gas dock on our lake only sells 93 octane and recognizes that they can get away with charging over $5.00/gallon for it

btw, Just got back from Shasta lake, gasoline in Redding is approx $3.90-$4.25/gallon. Gas on the lake is $5.50-$6.50/gal

There's a long and winding road to each marina at Shasta. They really HAVE to sell it at a higher price just to pay for all the equipment, permits, storeage, etc.

It's sure easier to just pull up to the gas dock and fill up and yes it IS cheaper to bring your gas but to go get it it's a 1 hour round-trip drive and a LONG walk dragging the gas-buddy up and down to the water.....

I donno.......I might do it......but I only filled up about 30 gallons while I was there. If I used more gas I guess it might be willing to do it but I would want a 100 gallon tank in a pickup Then it would be worth it!!

 

[jkust]

Re: 5.8 OMC Fuel Octane vs. Performance

Right so that boat is just under 3200lbs dry. I'm going to say you need to look deeper than octane. I'll also ask then if you have replaced the impeller if some of the other wear items are original.

 

[bruceb58]

Re: 5.8 OMC Fuel Octane vs. Performance

Okay thanks. I know a lot of modern cars these days (mine included) have knock sensors, etc. and can adapt to lower octane fuels without damaging the engine by retarding timing, reducing boost, etc, but at a cost of power. I figured a 20 year old marine engine wouldn't care, but wanted to check. Sounds like we need to be focusing on a tune-up though.

Thanks!

Yes, I spoke too soon without looking to see what you have. The difference in power is still going to be negligible between the two octane numbers as far as your problem is concerned.

 

[raquettelaker2]

Re: 5.8 OMC Fuel Octane vs. Performance

Right so that boat is just under 3200lbs dry. I'm going to say you need to look deeper than octane. I'll also ask then if you have replaced the impeller if some of the other wear items are original.

I have no idea. I'd be inclined to say that it has. My grandfather is very particular about care/maintenance/cleaning/winterizing. Doesn't explain why the spark plugs haven't been changed though. Maybe it hasn't gotten to the specified hours yet for plug replacement. The boat doesn't get a whole ton of use. It was in a few years ago for U-joints and gimbal bearing, and it's in this week to have the bellows replaced (idiots didn't replace it with all that other stuff). Is the impeller involved in any of those projects? I'm borderline boat-illiterate.