oil mix ratio for "High Performance"

**Krazymaan** · 09-09-2016, 10:28 PM

My 1984 Oval Port Mod VP is suppose to be between 18 to 1 and 25 to 1 With Aviation only fuel.

**powerabout** · 09-10-2016, 01:42 AM

there is a dyno test of a bike engine where they got to 16:1 oil ratio and it kept making more power the only reason they stopped there was they couldnt jet it any larger as it was getting leaner and leaner as you replace fuel with oil.

**olboatman** · 09-10-2016, 05:51 AM

Originally Posted by Krazymaan

My 1984 Oval Port Mod VP is suppose to be between 18 to 1 and 25 to 1 With Aviation only fuel.

I think this statement sums it up....Merc does NOT want oil to be a problem( or lack of it) in their Hi-Po motors. They must know something!!!

Gary

**outlaw 10.5** · 09-10-2016, 08:21 AM

Do I need to jet up to run 40:1 ratio? It was jetted perfect for 50:1.

**FMP** · 09-10-2016, 08:43 AM

Factory jetting is over safe so we hope . Run it and check but I would say probably not.

**powerabout** · 09-10-2016, 09:53 AM

Omc are happy to run 25:1 with stock jetting

**ChrisCarsonMarine** · 09-10-2016, 11:26 AM

Originally Posted by flabum1017

Normal use is fine with whatever the manufacturer suggests..... A lot of the Lemmonrudes went to 100:1 in the eighties. Personally, I like 50:1 for fish/stock motors and 40:1 to 32:1 for hard running, high rpm motors...... I'd rather replace plugs than pistons and bearings

You're completely correct,and the only reason the small omc's went with 100:1 was to get them to pass emissions tests,and they couldn't afford the cost and space to incorporate the VeryReluctantOiler.Yes the quality of the newer oil is better,but we didn't turn our motors as hard then either,Chris

**Mark75H** · 09-10-2016, 11:28 AM

I did some scientific flow testing with ratios from pure gas and 50:1 down to 16:1. I could not measure any flow decrease until the ratio was below 20:1. The restriction was much greater than a carb jet, so the results were amplifed toward the cautious side.

I can say the answer is DEFINTELY no jet change requirement in the range that would actually be used.

The next thing people freak out about is possible octane rating suppression. Convert the ratio to percent (50:1 = 2% ... 25:1 = 4% ... 20:1 = 5%, etc) ... you'll get over the octane suppression fear even if the oil's octane was ZERO, which it is NOT.

**flabum1017** · 09-10-2016, 11:50 AM

Originally Posted by Mark75H

The next thing people freak out about is possible octane rating suppression. Convert the ratio to percent (50:1 = 2% ... 25:1 = 4% ... 20:1 = 5%, etc) ... you'll get over the octane suppression fear even if the oil's octane was ZERO, which it is NOT.

oil burns slow, that's why there's smoke. if it burned as fast and completely as the gas, there would be no smoke and octane would be a concern.

**pointer** · 09-10-2016, 11:52 AM

^ In my testing of RC boat engines, 20:1 was optimal. Totally different application but same determination.
More oil didn't produce any measurable on the water increase in performance.
Less oil has less performance.
This was 25cc single water cooled spinning ~ 12500 RPM. Tiny tach and onboard GPS, recorded results, prop switches to load engine more and also to load engine less. I liked Klotz Super Techniplate at the time - 15 or so years ago.

pointer

**olboatman** · 09-10-2016, 06:43 PM

This thread is becomming very interesting...... I have aways believed the how 2 stroke oil is used is far more important than the brand or type as long as it is high quality and has the proper rating (TC-W3 etc.) As far as rejetting I have never seen a problem using more oil as long as the jetting wasn't already on the edge. I remember using Chemlube (one of the first 2 stroke synthetics) back in the early 70s in my sled at 7k plus rpm at 25 to 1 with no problems or plug issues even with stone age ignitions.

**FMP** · 09-10-2016, 07:31 PM

I used to think a bit less like 60:1 was good enough with todays oils. Now I still use a very good syn oil but find 40-45:1 gives me peace of mind when turning 6000+. ONly the cowl is 105.

**King Dad** · 09-10-2016, 07:35 PM

I remember in the late 80's or..... whenever it was, .... when OMC went to 100/1 on small two strokes. 35 hp and down, I believe.Supposedly, according to the service clinic notes I received, they "silver coated" the main bearings to help with heat dissipation and the leaner mixture. I worked at a very large Johnson dealership then and all our white engines had much, too much black exhaust residue from 50/1. We also sold Yamahas and they didn't have that issue due to 1) grey color vs. white and 2) they already were pre-mixed leaner. OMC said unless you ran for prolonged times at WOT, 100/1 would be fine for 90%of the people out there. I can't say I had any running issues with customers running at 100/1 but we had a zillion powerheads fail in the spring. Alot of customers didn't "fogg" their motors in the fall and the leaner oil residue wasn't enough to protect rod bearings over the high humidity winter storage, which is what we had in the Puget Sound region. 1st or 2nd time out in spring the engine would seize. OMC went back to 50/1 after a few years of 100/1. Another failed attempt to beat the Japanese....... Sad!
We had a few fishermen I called "professional fishermen" as they were retired and lived on the water day in and day out. They caught so much fish I swear they depleted the winter salmon runs each year. Those guys could wear out a set of rings faster than anyone we knew. We told them they would have to stay at 50/1 as we were'nt gonna keep repairing their engines for free. By then Yamaha and Suzuki had oil tanks mounted on their engines and OMC came out with Auto-Blend tanks... Still sad!! .

**powerabout** · 09-10-2016, 07:40 PM

Originally Posted by Mark75H

I did some scientific flow testing with ratios from pure gas and 50:1 down to 16:1. I could not measure any flow decrease until the ratio was below 20:1. The restriction was much greater than a carb jet, so the results were amplifed toward the cautious side.

I can say the answer is DEFINTELY no jet change requirement in the range that would actually be used.

The next thing people freak out about is possible octane rating suppression. Convert the ratio to percent (50:1 = 2% ... 25:1 = 4% ... 20:1 = 5%, etc) ... you'll get over the octane suppression fear even if the oil's octane was ZERO, which it is NOT.

this was a viscosity test
How about when oil replaces 5% of the fuel can we assume that all the oils have a different octane rating and burn differently so some oils might lean it out more than others?

**Mark75H** · 09-10-2016, 09:56 PM

Originally Posted by powerabout

this was a viscosity test
How about when oil replaces 5% of the fuel can we assume that all the oils have a different octane rating and burn differently so some oils might lean it out more than others?

5%? who is running 20:1 and having trouble? Merc ran the T2's there with no problem.

Its just not related. Fuel ratio to air is fuel ratio to air, whether the fuel is gasoline or tar.

Compounded by ... the tiny amout of oil involved. I challenge anyone to see a 2% change in mixture strength without a pyrometer or O2 sensor ... and even then I'd put the chances at 50/50 accuracy at 2% change. And as I said, the oil is not zero on any scale. AND this theoretical 2% mixture strength change would depend on the oil being 100% non combustable; this is not the case either.

If the octane rating of the oil was zero, the maximum octane change of changing from 50:1 to 25:1 would be slightly more than 2 numbers. Some synthetic oils actually claim to be (and probably are) octane increasers rather than neutral or decrease. Oil is not zero octane, so there is NOT a 2 number drop in the rating of a stronger oil mixture.