High altitude oil mixture

[ddrieck]

I'm running a 1978 Mercury 500 50 at 5000 ft above sea level

I know 50:1 is the normal oil mixture with a conventional oil. Could I cut back to say..60:1 --65:1 with a full synthetic to gain a little more throttle response and hp?

 

[revo4789]

Re: High altitude oil mixture

personally i usually run my engines to about 45 to 1 but yet i do live down here in the deep south of louisiana

 

[j_martin]

Re: High altitude oil mixture

I'm running a 1978 Mercury 500 50 at 5000 ft above sea level

I know 50:1 is the normal oil mixture with a conventional oil. Could I cut back to say..60:1 --65:1 with a full synthetic to gain a little more throttle response and hp?

Why would reducing lubrication increase horsepower?

At high altitude, the engine gets less air, and usually mixes a little too much fuel in that lighter air charge.

Compensation in part would be to reduce the fuel delivery by changing carb jets. You can also gain some by increasing compression if it's possible in your engine. (ie cut the heads)

One thing that might be a problem is the regular gas sold in some high altitude areas is lower octane than that sold at lower elevations. So regular isn't neccessarily regular. You might have to use a premium fuel if you increase the compression.

Any of these modifications would then put the engine at risk if operated at lower altitudes.

hope it helps
John

 

[ddrieck]

Re: High altitude oil mixture

Reason I say more HP is this....The first 2 stroke snowmobile I had as a kid ran great on the proper mix....Ran like a rabid dog for about 200 yards on straight gas, unmixed until it locked up tighter than a convict at Folsum.
My thinking may be off....reducing the amount of oil in the combustion chamber that has to burn = faster burn and explosion rate, hence more HP. I do realize the reduced oil will lead to increased friction and faster failure of the internal engine parts. I also know jetting up or down depending on altitude will increase or decrease hp. I know going to far one way or the other will also to engine failure as well.
So...to the heart of my first question. Can one reduce the amount of oil, under 50:1, if a full synthetic is used. Any thought?

 

[croSSed]

Re: High altitude oil mixture

There's an old saying, perhaps you've heard it: "Two-cycle engines run best right before they blow up." There is, in part, some truth to this. You experienced it with your snowmobile. Allow me to explain:
Two cycle, carbureted engines are set up, on purpose, to run a little rich. This keeps things running cooler and therefore keeps them from burning up. If it runs at all lean at cruising speeds or above, it won't be long until it's fried, although right before it gives up the ghost, it might be running like a violated ape. The oil in the gas is part of the combustion mixure, as opposed to 4-stroke motors where straight fuel is what is burned. In effect, you leaned out the mixture by removing the oil, and therefore allowed the engine to run more effeciently... for a time... as you said, 200 yards.

Leaning out the oil mixture in your 500 is NOT the solution. You will, at best, shorten it's life. If you are at 5000 feet, and you have the "stock" carb jets in your motor - i.e., those that are supposed to run at sea level - then you will pick up a LOT better performance by switching your carb jets for the altitude you are at. With the sea-level jets, your motor would be running SUPER rich at 5000'. The Mercury shop manual for your motor has the carb jet size recommendations in a section, but I don't have the shop manual for your model motor. But, again, I urge you to NOT lean out your OIL mixture, unless you are trying create a nice, heavy, black boat anchor.

TG

 

[j_martin]

Re: High altitude oil mixture

Reason I say more HP is this....The first 2 stroke snowmobile I had as a kid ran great on the proper mix....Ran like a rabid dog for about 200 yards on straight gas, unmixed until it locked up tighter than a convict at Folsum.
My thinking may be off....reducing the amount of oil in the combustion chamber that has to burn = faster burn and explosion rate, hence more HP. I do realize the reduced oil will lead to increased friction and faster failure of the internal engine parts. I also know jetting up or down depending on altitude will increase or decrease hp. I know going to far one way or the other will also to engine failure as well.
So...to the heart of my first question. Can one reduce the amount of oil, under 50:1, if a full synthetic is used. Any thought?

I don't know how old you are, but when I was a kid, standard mix was 16:1 30W non detergent motor oil. The gas had enough lead in it to poison you if you looked at it, and standard octane was 97 or so.


the logical falacy in your reasoning is that you assume the oil has to burn. It doesn't. Most of it does, but it really doesn't affect power, octane, etc very much. The oily mess in our prop hub is the oil that doesn't burn.

According to AmZoil, you can reduce your mix to 100:1. According to the folks I know who get their kicks running high powered 2 cycle engines on the ragged edge of destruction, AmZoil is chit, and the makers are full of it.

Best performance of any particular engine is when you match air flow, fuel mix, compression, octane, and timing to exactly what the motor wants. It'll keep performing if you lubricate it well. Their ain't no magic button, and their ain't no free lunch.

I run 35-40:1 (pre mix a little plus VRO) full synthetic. It ain't cheep, but it's a hellova lot cheeper than parts.

hope it helps
John

 

[ddrieck]

Re: High altitude oil mixture

Ok...its all starting sink back ito the gray matter. I do have bonified Mercury shop manual for this engine and after looking at the jetting sizing for my altitude, I am running way, way to rich. This explains the poor performance and my wet plugs. Thank you all for re-setting me straight. Sometimes, I just get to focused one one thing and completly over look the obvious

 

[j_martin]

Re: High altitude oil mixture

You can actually over-oil an outboard. The higher viscosity of too much oil, combined with the oil not burning, can cause the engine to lean out to the point of destruction, just because of excess oil.

Not likely in a lake motor, but it's happened at the races.

Set her up for yer altitude and yer good to go.

John