Octane for Mercury inline 6 owners

[what200]

JB- I know you recommend 87 octane, but for those of you who own a merc inline 6, please use 93. These engines are prone to detonation, and running 87 risks burning a piston. This is especially true for the late seventies models.

 

[mellowyellow]

Re: Octane for Mercury inline 6 owners

have a '70 merc 1150 and this is a new one on me.<br />can you please explain your position? would also<br />loke to hear from others on the subject.

 

[what200]

Re: Octane for Mercury inline 6 owners

Inlines were designed in a time when octane at the pump was higher. When I last took a compression test of one of my inlines it had 135 lbs in a couple of cylinders which is really pushing it for 87 or even 89 octane since 145-150 lbs is generally accepted as the max for 91-93 octane fuel. I've been told by a few that have a lot of experience with inlines that they already run fairly high cylinder temps so that they are already more prone detonation. If you run 87 maybe you could make the motor live by backing way off the timing but that would make it a slug.

 

[JB]

Re: Octane for Mercury inline 6 owners

Well. <br /><br />What does Mercury say? I've heard this nonsense about Mercs needing high octane fuel for years, but never have I seen in print that Merc recommends anything more than 87 octane unleaded regular.<br /><br />I don't know who it is that "generally accepts" 145lb as compression that calls for higher than 87 octane, but I am not one of them.<br /><br />Any engine with the ignition timing advanced beyond a safe setting risks detonation, whether the compression is 100psi or 160psi. <br /><br />Again, What does Merc say? If Merc says 92 octane IN PRINT I will go along.

 

[ODDD1]

Re: Octane for Mercury inline 6 owners

JB, What200, merc has never spec'ed anything more than 87 octane as a minimum requirement...but you have to remember, the last of these motors was built in 1988, and merc like most manufacturers do not adjust specs or recommendations on out of production hardware...2 things have changed since then...the motors have gotten older, and in alot of places, the 87 octane isnt as good as it used to be....before you get excited and say octane is octane, and why do we bother to rate fuel by a number.......the method used to rate fuel is posted on the side of most pumps...research method octane number + motor octane number divided by 2 = your averaged octane number....nowadays the research number is generally the larger of the 2, and 2 strokes like high motor octane numbers....so there is a difference from what we had in the past...<br /> over the last four or five years the inline six was built there were a number of service bulletins regarding 'long term durability and performance'....timing was dropped on earlier versions, and current production stuff had main jet increases to help control combustion temps...I believe there was even a recommendation to use mid grade [89] fuel in places that doped the fuel with large amounts of alcohol ...all that being said, yeah I would use higher octane stuff in an inline six...but you do whatever you want...<br /> as far as those comp numbers, I routinely see older crossflow motors with cold cranking numbers on the high side of 150 psi...

 

[what200]

Re: Octane for Mercury inline 6 owners

JB- I'll run 87 in my suzuki all day long, but as for the inline mercury used to say set the timing at 23. A bulletin came out that said set the timing 21. I later heard that if you really want it to last back off to 20. I stayed 21 but I use 93 and nothing less. Seems to me that I wouldn't want to risk it for a $1 difference every time I go boating. One guy I know, who used to race inlines, mixes a few gals of 100 race fuel in his tank for insurance.<br /><br />I believe that mercury later started putting low dome pistons around the time they switched to ADI ignition to make live on the gas at time, but the earlier distributor type 140 and 150 need all the help they can get.

 

[sho305]

Re: Octane for Mercury inline 6 owners

Interesting.<br /><br />I put midgrade in my 1973 150 Merc. It has 120psi on the starter all the way down. I set the timing at 23 on the starter with the trottle buried, but not running at 5k rpm as instructed so who knows. The other day I gave the timing screw a turn and because I had put premium in there so what the heck. It had more holeshot power and about 2mph gain on top. I ran it with cover off and heard no pings. I'll keep running premium in there for the minimum fee of it. I had a similiar experience with an 80hp 1976 Merc.<br /><br />However, I was under the impression that in like motors the faster/higher HP one would have lower compression due to the larger/higher ports in the cylinder. No?

 

[ruthherm]

Re: Octane for Mercury inline 6 owners

reply to sho305. It would depend on piston dome configuration.

 

[what200]

Re: Octane for Mercury inline 6 owners

You are correct in that higher ports lower the static compression in the inline, but at higher rpms the cylinder efficiency will be greater compared to the 115 (lower ports and no power port). <br />I get 125 psi on my 1500 but 140 on my 900 (pretty much the same motor). But the 1500 has high dome pistons and puts out a lot more power so it also has a lot more heat to dissapate. This makes the 1500 more prone to detonation problems. Mercury put out a bulletin way back that said to re-time to 21 degrees. <br />BTW- all 1500 and 1400 came from the factory with high dome pistons. <br />One recommendation I've been told is to jet up one number on the middle carb since the #3 pistons is usually the first to go.

 

[mellowyellow]

Re: Octane for Mercury inline 6 owners

so if I run 93 octane in my '70 1150, I should<br />adjust timing? what setting do you reccomend?<br />thanks,<br />M.Y.

 

[what200]

Re: Octane for Mercury inline 6 owners

20 or 21 degrees BTDC

 

[Lark40]

Re: Octane for Mercury inline 6 owners

I had a 1977 Mecury 1500 - 150 hp inline six. Last year made. Sold it 18 mos. ago. Ran very strong.<br /><br />Never used higher than 89 octane. It was happy at 87. Would argue that using premium simply wastes money.

 

[what200]

Re: Octane for Mercury inline 6 owners

Lark40- you may have used 87 octane and gotten away with it. I'm glad you did. But there are others around with inlines in otherwise good running order that have melted pistons for either running to low of octane or too much timing. I suspect you may have been able to get by for running a light boat or at reduced rpms (but not overpropped).<br /> Anyway, I wouldn't call running 93 a waste of money. You might spend extra $1.50 to fill the tank, but I think that is cheap compared to 1+ pistons, gasket set, ring set, block hone, etc. I'm not telling you to run it in your johnson 3cyl, but it is good insurance for old inlines.

 

[Jango]

Re: Octane for Mercury inline 6 owners

I have a friend with a 115 Merc. bought new in 1979. He has it on a 17 ft Procraft. Runs it at 6000 - 6200 RPM. He says he has never used anything but 87 octane and most of the time on Canadian low Quality fuel.

 

[mellowyellow]

Re: Octane for Mercury inline 6 owners

from what little I've learned so far on the inline<br />6cyl., they were the same block from 85hp to 150hp?<br />weren't the 85's under-powered and the 150's over?<br />the 115 is a happy medium. seems like it would be<br />much more important on say a 150 than a 85, but<br />I'm always for erring on the side of caution.<br />other than costing me a few extra bucks for fuel,<br />what can it hurt?<br />BTW. my '70 115 has a sticker that says 27 degrees<br />on timing... 6 degrees just seems like a lot to me<br />but I feel like what200 is onto something...

 

[what200]

Re: Octane for Mercury inline 6 owners

I'll have to double check the timing on yours since the 72 and earlier 99ci models are slightly different than the 73' and later and yours could have different ignition. 73 and later is 21 degrees max (merc oringinally said 23 for US models but changed later) . The 85 is a four cylinder but almost the same motor as the 6. <br /><br />The 1500 is just about the max mercury could pull out of the motor and still have a practical motor. They have higher port locations compared to the 115 and ports in pistons that the 115 doesn't. This means compared to the 115 is has crappy low end torque and lower static compression. Merc put in pistons with a higher dome height to recover some of the compression and low end. 1500's make their power above 5000 rpm's. I've been told that the ideal WOT rpm is 5800-6000 or 6000-6200 for the XS model. Your 1150 will not have the same cylinder pressures or heat at above 5000 rpm, but I still feel that running 91-93 octane is a good idea especially if your running a heavy boat that loads up the motor. I'm sure some have used 87 and not burn up their motor, but why risk it.

 

[what200]

Re: Octane for Mercury inline 6 owners

Mercury book says 23 for 69 or later 1150's. I would read that as 21 or 20 to be safe. FYI- the reed stops on that motor were listed as 5/64 instead of 5/32 as on the 1350. Something to keep in mind if you ever have to crack open the motor.

 

[sbryant]

Re: Octane for Mercury inline 6 owners

I called Mercury today, after reading something along this line elsewhere on the Internet. (I have a '75 Mercury 1500). They told me 87 octane was fine. <br />Any ideas on where I can find their tech bulletins regarding this motor?

 

[Lark40]

Re: Octane for Mercury inline 6 owners

What200 - <br /><br />"I'm not telling you to run it in your johnson 3cyl, but it is good insurance for old inlines."<br /><br />Johnson 3 cylinder??? I have a 1960 Evinrude Lark II 40 hp with the "Big Twin" block.

 

[what200]

Re: Octane for Mercury inline 6 owners

Steve- I was curious as to Mercury's response about the octane, so I told the inline guru I know (he raced them for many years and he owns at least 20 inline 6's and if you have a hard time finding parts to your 1500xs he is why). His response was: "mercury likes to sell parts and I doubt there is anyone left there that still runs inlines."<br /><br />He also said if you ran it on 87, it might live if you backed the timing back to 15 or 16.<br /><br />I would think that you would lose quite a few HP by doing this though.