View Full Version : Fuel For My New Mojo

10-13-2013, 07:15 AM
Hello all,

I was reading my owners manual as I prepare for the 2014 summer. Our family just bought a 2012 Moomba Mojo and the manual for my new Mojo and it recommends 89 octane gas. I was talking to other people and they said you can mix and match 87 with 89 octane between fillups and to make sure you put marine stabilizer in the tank each time. I was wondering what people's experience/knowledge on this topic. Would 87 octane gas do considerable harm to the engine? Thank you in advance for any help

10-13-2013, 08:16 AM

As far as the engine is concerned, you need to run the octane rating recommended by the manufacturer or higher, if the recommended octane is not readily available. It is not suggested to run a lower octane then whats recommended, due to the risk of pre-ignition, which if prolonged, can cause internal engine damage. Using a fuel stabilizer with each fill up has absolutely nothing to with the fuels octane rating. A stabilizer is only needed for storage or layup, to help slow the spoiling of the fuel. But, many of these products do offer some fuel system cleaning benefits, and can be used with each fill up. IMO, its a bit overkill. I would consult the manufacture about the specific additive you are looking to use.

10-13-2013, 09:37 AM
Yup, use the manufacturer recommended or higher octane. What is the cost difference between 89 over 87, about a dime a gallon, so 4 bucks more on a 40 gallon fill up? Shouldn't be a budget breaker...

10-13-2013, 09:40 AM
sure indmar is the same, but pcm lists octane rating, but also 10% ethanol blended fuel is approved so no need to use a stabilizer unless winterizing or not running for a month or so as mla suggests..

10-13-2013, 10:22 AM
To expand on sandm's post a little. Even an additive that is formulated to fight the side effects of ethanol, will not change/alter the percentage of ethanol in the fuel. Even with an additive, the ethanol is still corrosive on the fuel system components, and the higher ratios just increase the deterioration. The biggest benefit of an additive that targets ethanol, is to prevent/reduce phase separation when the ethanol blended fuel reaches its saturation point. Under normal conditions and normal use, I would not expect phase separation to occur. It should only occur when the boat is stored and the fuel is left to absorb moisture. if the fuel is being used and replenished with fresh, I dont expect a problem, but regular use of an additive should not cause harm.

ive also read that testing has shown that in many station tanks, the ethanol can be well above the 10%. This is because its added to the fuel at the distribution center/tank-town, before going to the stations. Theres not regulating of the ethanol, and its great for watering down the actual gasoline, make it go further for the refiner. Gas is expensive enough, so it sux to not get what you pay for.

10-14-2013, 09:35 AM
I can't imagine why anybody would ever intentionally not use the gas that is recommended by the manufacturer on a boat like this. My new neighbor recently started surfing with me, and he now brings a 5 gal gas can that he uses in his boat (2000 Malibu sunsetter I believe) to share the love. I didn't even think to ask him at first, but after we put it in I asked him if it was 89 octane. He said no it's 87. Off I went to get a 5 gal can of premium 91 to offset it lol! I also told him he might want to check his manual and see what they say for gas recommendation.

For the pennies in difference per gallon, it's not worth it. Since we all own these types of boats, I know none of us need to fret over $4 per fill up difference.

Speaking of ethanol in the gas...if you were to store the boat over the winter and the ethanol does absorb some moisture, wouldn't the water separator still catch that moisture once the boat is running?

10-14-2013, 10:06 AM
what harm will running 87 do ?

10-14-2013, 10:27 AM
Speaking of ethanol in the gas...if you were to store the boat over the winter and the ethanol does absorb some moisture, wouldn't the water separator still catch that moisture once the boat is running?

Yes, it will. The problem is that ethanol blended fuel has the capacity to absorb way more water then non ethanol blended fuel. This could potentially saturate the fuel/water separater. At that point, fuel stops passing through and water does. This will lead to a poor running engine or one that stalls and will not restart. This can also leave water trapped in the fuel rail, depending on the design. If enough is trapped, it can cause a lean cylinder at the closest injector. This can lead to a burned valve or piston.

what harm will running 87 do ?

Running a lower then recommended octane can cause pre-ignition/spark knock. Over time, this can destroy a piston and combustion chamber, leading to a catastrophic engine failure. The problem with boats, is that the spark knock is so hard to hear as opposed to a car or truck. This leaves us relying solely on the knock sensor to detect and the ECM so make adjustments to reduce.

10-14-2013, 10:42 AM
while were at it and somewhat topical, same can be said for running too high octane. engines and plugs are tuned to run at a certain octane and too high octane can end up with gas that is not burning properly when the plug fires.

there's a reason the engines have been calibrated for a certain octane rating. best to stick to that recommendation.

10-14-2013, 01:04 PM
what harm will running 87 do ?

Technically None. As long as the engine doesnt knock. <combust too early on the combustion stroke>

I only have the option of 87 at 350 a gal or 91 at 5 a gallon. i run 87 almost all year.

These are my findings, when i run 87 in the boat Vs 89, i have no difference in power, I also have no knocking, surging or any other type of issue. I do fill up with our 91 <non ethanol> at the end of the season and use stabil.

Now in my Vette i have found if i run 87 <recommended 91> i have a lot less power when i accelerate.
On my old Monte Carlo SS it also recommended 91 and when i ran something lower i got as much as 3mpg difference and less power.

I'm not in any way saying, recommending or promoting running less then 89 in your boats.
I'm sharing my experience running 87.

I'm sure this has something to do with your Elevation as well, and maybe the quality of fuel you buy.
I just know for me i don't want to spend Marina prices on fuel because my station only buys 2 brands.
I always have to plan to fill my Vette on my way home from work, or too work because i rather pay the 375 for 91 in town that's not Ethanol free.

10-14-2013, 01:33 PM
assuming the computer used to run these engines is similar to a car. the computer will detect the knocking/pinging and retard the timing to compensate for it. as long as it continues to detect, it'll run in a retard state and you will have some sacrifice in power. I don't think anyone will ever notice a boat running full tune and one running with the timing advance in a retard state, but as mla suggested, this will have some longterm effects on the overall wear of the rings/pistons/valves and seals. most engines today have quite a bit of "safe" room in the tune tho..

went to 2 tuners over the years to get the old mitsu evo's tuned. it was fun to watch the pc hooked up to it real-time detect and retard the ignition when picking up on knocking/pinging. one was on the interstates out by the salt lake city airport and done while driving. tuned the last one to the brink at a shop in portland on a dyno. original baseline dyno was 330hp/310tq. when I left, it was at 363hp/362tq with no other parts swaps, just an ecu tune. shows how conservative the engine tunes can be. sure mike has even bigger gains from diesels and tuners.

10-14-2013, 02:11 PM
Thank you for your help

10-14-2013, 10:09 PM
I put 400 hours on my 08 XLV and ONLY ran 87 and it never gave so much as a hic up.

10-14-2013, 11:46 PM
I put 400 hours on my 08 XLV and ONLY ran 87 and it never gave so much as a hic up.

As mike mentioned elevation plays a big factor as well. The higher up you are the thinner the air the less compression pressure the lower the octane needed. I know in Lubbock which is about 3400 ft above sea level you can't even buy 87, their low grade is 86 because the air is thinner. They say for every 1000 ft climb in elevation your engine loses 3% of its power. I don't think 87 to 89 is a big enough difference to cause any major damage. If it requires like 110 race gas and someone was using 87 then yeah, that would be a big deal. If you use 87 and don't want to pay for 89 then fill you tank half with 87 and half with 89 and now you have 88. Compromise haha

Sent from my iPhone

10-15-2013, 09:39 AM
A lot of higher elevation areas (like CO) start at 85 octane for regular...I always wondered why that was and never realized that may be because of the elevation lol.

10-15-2013, 11:18 AM
it'll run in a retard state and you will have some sacrifice in power.

I been running this way for years. Wonder if I should be drinking Monster or Red Bull instead of Yeungling?

10-15-2013, 12:21 PM
i have been told dont waste money on over 89 either as it is calibrated for 89.. if you blend higher with 87 that is worthy, but dont expect any improvements by running 93.
Long running debate on here and wakeworld about 87 v 89. Many of us have done 87 plenty with no noticeable issues.
I ran a couple of Acura TL's that demanded 91 or higher.. was a pain but its all about the compression, computer, etc.
I try to stick to 89 most of the time though.

10-16-2013, 08:32 AM
Funny but on my '08 I'm pretty sure my boat manual says run 89 but my Indmar manual says run 87.