premium gas in a 140hp V4?

[tincanman]

I have a early 1980's johnson 140hp v4 that has recently been rebuild here locally. Talking to the shop that rebuild it for the previous owner while exchanging props, he told me to make shure to use premium gas in the motor. Thats the second shop that has told me this about the 140's. Really? As if boating is not getting expensive enough, now I have to run premium in this thing. Opinions from the guru's?

thanks

 

[TN-25]

Re: premium gas in a 140hp V4?

I am not a guru, but I will weigh in with my thoughts.

First and foremost, I would pay attention to what the owner?s manual states as far as octane requirements. Having said that, there are a number of schools of thought. Pre-ignition due to inadequate octane is a killer. It can break ring lands on the pistons. On the other hand, in general terms outboard motors do not have high compression ratios except for the few racing variants. The idea that higher octane is always better is not really true. Octane is a measure of resistance to detonation or explosiveness. Higher octane means a somewhat muted tendency towards explosion, which is great if your motor requires it. If your motor does not require it, that is the muting of the explosive tendencies done to thwart pre-ignition, then you at least are throwing away good money needlessly. Also a lower octane fuel may actually lead to easier starting, all else being equal.

Now beyond that, it can also be a regional thing. Where I live in southern Ontario, Canada, Shell uses 10% ethanol in the 87 octane fuel, 5% ethanol in the 89 octane fuel, and no ethanol in the 91 octane fuel. Other locals and other brands may have entirely different rules regarding ethanol. Has your motor?s fuel system & fuel lines been gone through regarding ethanol tolerance? If everything in the fuel system, from floats to lines, fuel pump diaphragms to primer bulbs are ethanol tolerant, then run whatever you feel comfortable with. Just make sure the minimum octane requirements are being met.

Motors that have had their heads & blocks surfaced may end up with slightly higher compression than when originally built. Also the presence of carbon after a period of running may also contribute to higher octane requirements. I haven?t even broached the subject of altitude & thinner air.

As a minimum you need to ensure that the proper heat range of plug is being used, your motor is properly tuned & timed, and all carbs are pulling equal weight & compression is within the proper variances between cylinders. It really doesn?t hurt to run too high an octane as opposed to too low an octane.

My 2 cents.

 

[clanton]

Re: premium gas in a 140hp V4?

There is a service bulletin that tells you to reset timing and what fuel to use.

 

[clanton]

Re: premium gas in a 140hp V4?

Orignal engine with timing retarded 4 degrees could use 88 AKI fuel. There was a kit with low compression head gaskets, then you could use 86 AKI fuel. Engine builder should provide information on head and block being resurfaced and timing specs.

 

[tincanman]

Re: premium gas in a 140hp V4?

Motor is set up for ethonol. Heads machined with low compression head gasket kit. timing set to ethonol standards, not shure what degree, but I will ask him. the engine builder is still recomending premium fuel.

 

[clanton]

Re: premium gas in a 140hp V4?

No setup for ethonol, just changes in hoses and some gasket material. The engine should run fine on 87 with 15% ethonol if setup to OEM specs, but the builder can do as he chooses, he is the one that covers the warranty.

 

[1FASTLASER]

Re: premium gas in a 140hp V4?

TN25 THAT was the best explanation and precise answer I do believe I have seen....VERY INFORMATIONAL....i might add this small tidbit do to the fact I am familiar with higher compression engines...a general rule of thumb that I go by is simply this....after building engine run a compression check...good to go with 87octane fuel with 110-120lbs...any higher compression requires higher octane to be safe. Now this comes strictly from experience with engines that have been modified from factory specs.

 

[emdsapmgr]

Re: premium gas in a 140hp V4?

The early 140 crossflows used high compression heads-unique to that hp model only. If you run stock (thin) head gaskets with the original stock heads and factory timing (which may have been in the 28 degree range,) I'd be running premium fuel to make sure that engine lasts. Possible that the original factory compression would have been over 125 lbs, even into the 130's. By 1985 OMC changed to low compression heads. That same bubble back block in 1985 was sold with low compression heads and was downrated to 115 hp. That 85 block ran on 87 octane.