View Full Version : Higher Octane - Better???

08-21-2007, 04:13 PM
I know that many people claim that high octane gas is a waste of money, but boats are different deals than cars. My Outback manual calls for minimum 89 octane but I've been using 91, and always from a reputable station.
Any thoughts on this?
Will the boat run just as well on 89 as it will on 91?
Any long term issues related to 89 vs high test?

If it will help the boat I'll pay the difference but don't want to spend it if it really doesn't help or make a difference.

08-21-2007, 05:43 PM
I've used 87 octane for three years now with no problems ( 350HO carb).
Used at 400 - 1600 feet above sea level.

08-21-2007, 06:22 PM
Have used 89 octane only. 2007 Outback w/ 325 EFI engine. 50 hours. Starts first time every time. Runs flawless. :lol:

08-21-2007, 09:50 PM
I've used 87 in mine for the last several years w/o a problem. I've owned several autos and the only reason I can see to use a higher octane is higher compression, which I don't think our boats have, or higher temperatures due to turbos or supercharges. Higher octane just means higher ignition point which keeps the fuel from pre-igniting prior to spark due to pressure/heat. I don't think any of these are problems on our boats. If you're getting pre-ignition and knocking, your timing's most likely off. A high grade quality fuel is more important than the octane rating IMO.

08-21-2007, 10:31 PM
As in the manual it states mid grade or higher but I too have been running 87/89 and boat runs fine on both . We are at 3500 ft fairly high elevation and have used it at lower elevations and boat runs fine. I do prefer to fill at inland stations when possible to get gas that is less likely to have water problems. Have run into this in the past with a previous boat.Water / sand in the gas. :lol:

08-22-2007, 11:14 AM
The engine was designed for 89, Use 89.

I'm sure Larry from Indmar will be on to explain the official standpoint, but higher octanes are designed for engines that need the higher compression levels and higher octane retards combustion. (the opposite of what most people think). Lower octanes are more combustable (and better at higher altitudes ??), but can create more "dirt" in the engine.
If you leave the boat for longer periods between use, then a blend to obtain the correct level can be done - 89 in tank for 4 weeks is actually 88, so add 91 next fill to balance it out.
Engines are now designed for a specific octane and it should be used as closey as possible... To that point, does anyone have one of those stations with 5 or 6 levels of gas ??? We have three 7-11's in the area that offer:
87,88,89,91,93, deisel, clean deisel


08-22-2007, 11:34 AM
Use 87 in mine.

08-23-2007, 03:38 PM
I was told by my mechanic that because of the 9.4 to 1 compression ratio, 89 is required, It will run ok on 87, but the 87 octane combusts at much lower compresson, causing a double hit, one on compression and one on spark. I am sticking with 89 unless someone convinces me otherwise.


08-23-2007, 07:21 PM
We bought used and was not aware of the 89 recommendation. We have been running 87 for 6-7 years now with no real issues. However, recently it has been 'dieseling' when I shut it off occasionally. I have added some Sea-foam to help with that? But, as much as I hate to pay the price, it sounds like I should be switching to 89+.

Question now is - Is there any risk in going up after so long a time at lower ocatane? I thought I had heard that if you had been running your car at 87 for a long time and switched to 89+ there could be some issues. I was skeptical of that, but nothing would surprise me...


08-23-2007, 09:10 PM
I'd run 87 octane . My 98 diesels when I pull back on the throttle fairly quick and shut off the engine like picking up a skier. I now idle the engine down for a few seconds and then shut it off. I can smell the rich burnt gas that would have caused the dieseling. The world of carbs. The EFIs have knock sensors that should prevent knocking. Looks like octane is an issue in the car world also.