QUESTION concerning FUEL OCTANE ????

[LOUIS B HOLUB]

WHAT do you MAC owners recommend in FUEL OCTANE for a 50 HP NISSAN ?? I've heard so many different opinions !!! And, if using "STA-BIL" in the fuel, does this allow storage of old fuel in tanks for several months ?? I WANT to take good care of my 50 HP NISSAN for RELIABILILTY...and not too interested in "SPEED". I just want a good dependable running ENGINE. THANKS for your SUGGESTIONs and EXPERTISE !!!

[DLT]

As to octane, I run the cheap stuff. I assume the engine was designed for that, since I've never even seen the higher octane blends at a marina...

As for long term fuel storage, why bother? I have removable tanks. So, when I winterized my engine, I just removed the tanks and brought them home (boat is in dry mast-up storage). I'll just dump the fuel into my truck.

But, yes, that's what "Sta-bil" is supposed to do.

http://www.goldeagle.com/sta-bil/

http://www.letsmow.com/sta-bil.html

[Chip Hindes]

Louis, I'm almost certain I can picture in my mind the graphic in my owners' manual which says 87 octane. I'll check my manual when I get home and let you know if it's different.

Did you not get an owners' manual with your motor from the PO? If not and you can't find one, let me know and I'll see what I can do about making you a copy of mine. Mine's an '00 Tohatsu 50D. 99% sure it's the same motor.

As I mentioned before, mine's been as reliable as can be with no professional maintenance since new, and the only maintenance from me being the winterization, which several times I've had to dig the boat out of the snow to complete, in January. Not even so much as the recommended water pump impeller. Unfortunatley it's in the shop as I write getting a new lower unit. That's the result of the accident mentioned on another thread and no falult of the motor, which BTW ran about 40 hours this summer with a broken skeg and bent prop shaft before I took it in to be fixed.

I don't bother with Stabil either. I don't put much stock in fuel additives. Instead, I disconnect at the fuel tank and run the line and carbs dry during the winterization procedure. Whatever's left I dump either into the truck, the lawn tractor or the snowblower depending on the weather at that moment. Several times it's been the snowblower

[Tom F.]

Use 87 octane as that is what should be recommended from the manufacturer. One caveat though, fuel loses octane over time so if you do not power much and go through a tank of gas in a reasonable amount of time, you may want to use 89 octane as it will eventually degrade to 87.

[baldbaby2000]

I'd go with the recommeneded and no higher. I've also heard that higher altitudes lower that requirement. Does anyone know whether this is true?

BB

[Jeff S]

I'd go with the recommeneded and no higher. I've also heard that higher altitudes lower that requirement. Does anyone know whether this is true?

BB

Lower octane combusts more easily. Higher compression high performance type engines require higher octane to prevent the fuel from igniting from the heat in the cylinders and the compression before the actual spark is applied. This is preignition, or pinging, which will cause a power loss and run more roughly depending on the severity of the preignition. At higher altitude the air is less dense, there is less pressure and the air is generally cooler. A lower octane fuel can be used at higher altitudes as preignition is less common. VERY roughly it is about -0.5 Octane/1000' altitude requirement. Higher octane won't hurt. One more thing- Research Octane Number (RON) is based on moderate driving- under load using the minimum octane requirement could still result in preignition.

I have run 87, 89, and 91 RON fuel in my Tohatsu with no detectable difference in various temp ranges (32F-100F), no preignition noted. Colder temps don't require as high an octane rating- every 10 deg is roughly 0.5 octane in either direction. I am not sure what they used as the baseline temp in engine requirements for octane, a "standard day" in aviation is 59F which all of our charts are based on and from which we apply a temperature correction, so that number is not very useful but the concept is relevant.

So, to answer the original question as to what Mac Owners recommend in a Nissan/Tohatsu 50....As a Tohatsu 50 owner I recommend 87 octane, unless you plan to store the fuel awhile then 89 for the reasons Tom mentioned.

YMMV,
Jeff S

[baldbaby2000]

Jeff,
Thanks for the info. I'm also starting to consider what I put in our Suburban with gas prices the way they are. To be safe I've been using the highest octane when we're pulling the boat and the recommended when we're not. Usually we're pulling the boat at over 5000 feet. Given the rough number of -0.5/1000' perhaps it safe to use the mid grade when we're pulling the boat.

BB

[Moe]

Octane is the fuel's ability to resist self-detonation also known as knocking, due to pressure (and hence temperature) rise. In actual testing, it was based on the compression ratio of a specialized variable-compression, single-cylinder engine. I don't know if that's still true today or whether it is now computer calculated.

RON is what was used back in the days when high-test gas was advertised as 100+ octane and regular was around 95. It reflects the fuel's resistance to knock under light load conditions. MON reflects the same under heavy load conditions.

Gasoline sold today in the US is rated as the average of its Research Octane Number and Motor Octane Number, and is commonly referred to as R+M/2, and known as anti-knock index (AKI).

The latter is what you look for in your owner's manual to determine what grade to use.

Knock, also known as detonation, is not exactly the same as pre-ignition, which can occur from glowing carbon, overheated valves, etc. Both seem to fall under the generic label of "pinging."

[kmclemore]

And for the record, folks, this is what the results of continued detonation looks like (from my very own little shop of horrors):

[waternwaves]

Kevin,

Is that Wests new supersecret high temperature expoxy shown in the picture............. suitable for pistons, cylinder heads and exhaust manifold repair.......




By volume its gotta be cheaper than JB weld and the high temp equivalents.,...

LEts see......If I can pot and glue pistions, then I should be able to bend rods and cranks back to their correct shapes (A metallurgists dream ...heheheheh)


P.S.......... That one isn't off the sunbeam is it???

[Jeff S]

Yikes! I think it is important to note that detonation can occur at varying levels of severity. A little detonation (Moe correctly adjusted my reversal of the combustion problems of detonation and pre ignition) doesn't normally damage a motor. The picture must have been from some pretty bad detonation- possible timing/valve train issues?

If a cylinder detonates at the wrong time (detonation is after ignition, preignition is, ummm, before ignition ) the valve train is not lined up properly and the cylinder/crankshaft is exposed to excessive stress from incorrect sequence of pressures from the various cylinders.

I am going to guess that the cylinder in the picture was not destroyed from detonation simply due to low octane.

Jeff S

[kmclemore]

Kevin,

Is that Wests new supersecret high temperature expoxy shown in the picture............. suitable for pistons, cylinder heads and exhaust manifold repair.......



By volume its gotta be cheaper than JB weld and the high temp equivalents.,...

LEts see......If I can pot and glue pistions, then I should be able to bend rods and cranks back to their correct shapes (A metallurgists dream ...heheheheh)

P.S.......... That one isn't off the sunbeam is it???

Yeah, well, I run an all-purpose shop, dontchaknow. You name it, we fix it!

As for the popper, that came out of a 1966 Triumph 2000 that I bought years ago... fixed her up and drove her for a while, then sold it. Although it was a pretty homely car style-wise, it was one of the nicest riding and handling sedans I've ever owned. Same motor as the sporty GT6, but with the rugged gearbox of the TR4 (with overdrive) and the supple independent rear suspension of the latter TR4A, with it's own MacPherson strut front suspension.

Yikes! ... The picture must have been from some pretty bad detonation- possible timing/valve train issues?

... I am going to guess that the cylinder in the picture was not destroyed from detonation simply due to low octane.

Jeff S

Don't exactly know, Jeff... I bought the car that way. I suspect it the ignition timing was off for a long, long time, or that the carbs were running way too lean. Here's another, closer pic....



But FYI, this is what "valve train" issues look like....

[Paul Zimmerman]

Regarding old fuel, the right idea(like DLT says) is to dump it in your truck so it doesn't get old. Here's what a dummy did(read yours truly). In trying to get the boat ready for a lobster dive, I tried to fire up my Suz 70hp 4stroke with the fuel left from last year. It didn''t want to start(first time ever). Since I had problems with my 6 gallon fuel tank sucking air thru the dip tube, I assumed it was an air-in the fuel problem. I changed out the dip tubes in all 4 tanks an proceeded to bleed the air from every line and filter I could find on the engine and low and behold it still wouldn't start. So I asked Gene at Voyager Marine if he would check the whole engine out and tune it up. He was a bit busy but asked "how old is the fuel"? "Try new fuel". I did and flushed it thru and it fired on the first crack and purred like a kitten as usual. I had no idea it was that sensitive. Lesson learned!

[Tom Spohn]

The manual just says "regular gasoline".

http://www.nissanmarine.com/tech_talk/o ... nuals.html

[Catigale]

On that 'valvetrain issue'

when that happened to our old VWs (always Number 3 Exhaust valve, whose cooling air was blocked by the oil cooler until 1971) we would buff those scuffs of with a wire brush, and paint the piston blue for increased crank speed....