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Old 06-06-2003, 06:38 PM
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Downunder: I agree that the ratio of methanol to nitro remains the same if the oil percentage is raised, but I also think that the added oil will 'take up space' in the cylinder and reduce the amount of air available for combustion. Ergo, if you open the needle to admit the same amount of burnable fuel into the cylinder to get the same power as the fuel with less oil, there won't be enough air (oxygen) in there to burn it all, will there?
Old 06-06-2003, 07:19 PM
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Originally posted by downunder

Same thing if you've got nitro in the fuel. Say you had 10% oil and equal parts of nitro and methanol (so it would then be 45% methanol and 45% nitro or 45/45/10). Now double the oil to 20%. The TOTAL methanol/nitro is now 40% of each but there's still an equal quantity of methanol and nitro (40/40/20).

That's how I'm looking at it. I realize that the nitro/methanol ratio is still the same. But wouldn't adding more oil to the mix somehow affect the fuel(nitro/methanol)/air ratio and engine performance to some degree?

BTW, not trying to bust anyones cajones here. Thanks for the insight.
Old 06-08-2003, 04:56 PM
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Hmmm... After giving it a little more thought, I think maybe downunder's assertion that you can just open the needle a little is pretty close to being correct. The volume of oil in each combustion cycle is so small compared to the volume of air, that an increase of a 2 or 3 percent of oil in the fuel wouldn't displace enough air to matter much. I doubt that you would be able to detect the difference in performance.
Old 06-09-2003, 02:36 PM
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There's a reasonably simple way to get some idea of how much volume the oil takes up in the combustion chamber but this would be an absolute maximum possible amount because obviously the piston/liner is coated with a film of oil as would be the piston crown and cylinder head to some extent. But so long as the oil doesn't get burnt then what goes in must come out sooner or later

I'll work in metrics because they're so much easier.

Say you have a 10cc (.60) engine with a 300cc (10 ounce) fuel tank that runs for 10 minutes at 10K revs. That means it used 30cc (1 ounce) of fuel every minute. In that minute the engine, theoretically, would draw in 10x10K or 100,000cc of air. In actual fact it would draw in less air because it's not 100% efficient so we'll guess it's 70% efficient. That means it draws in 70,000cc of air and 30cc of fuel every minute. To find out how much fuel goes in every rev we just divide both figures by 10,000. So every rev the engine draws in 7cc of air and .003cc of fuel.

Now this is a very small volume of fuel but only part of that is oil. If you were using 20% oil then only 1/5 of that .003cc is oil. What you have is .0006cc of oil passing through the engine every rev. As a ratio (by volume) it's approaching 12,000:1 which will be much the same for any size engine.

To go a bit further with this example, the average .60 engine has a combustion chamber that's around 1cc in volume so it's about 2000 times larger than the oil passing through each rev. So adding a few percent more oil to the fuel makes no discernible difference to compression ratio. The oil doesn't vapourise like the methanol/nitro does so it can't displace any air other than it's fixed volume.
Old 06-09-2003, 04:29 PM
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Downunder: I did that exact same calculation, only I did it in fluid ounces, and used a 40 size engine! I guess "great minds run in the same channels" or is it "fools think alike". Its one of those.

I also calculated the air to methanol volumetric ratio for a stoichiometric mixture of the two. (For those who aren't familiar with the term, a stoichiometric mixture is the "ideal" fuel/air mixture in which both the fuel and the oxygen in the air are completely consumed in the combustion.) The mass stoichiometric ratio, air to methanol, is about 6.5:1. Converted to volume, the ratio of air to liquid methanol is over 4000:1. In other words, according to my quick calculation, which probably ignored lots of important factors that I don't know about, you need over 4000 times as much air as liquid methanol by volume. Since we run our engines on the rich side, I'd guess that the actual ratio in an RC engine is probably something less than 4000:1.

By your calculation, that ratio in an RC engine is about 2300:1 air to fuel. Since that fuel contains 20% oil, the actual air to methanol ratio would be close to 3000:1 by your calculation. That we both came up with ratios in the same ballpark while approaching the problem from completely different directions is interesting, no?

Bottom line, if the ratio is in the range of several thousand to one as we've calculated, then adding a little more oil to the fuel won't reduce power any measurable amount, just like you said in the first place.
Old 06-09-2003, 07:16 PM
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Default Oil volume and RCV

So, judging by what you guys are saying, whether the fuel is 18% lube or 16% lube would have a very minimal if any effect on the engines performance. The "supposed" resident expert on RCV engines has been discouraging people from using fuel with 18% oil in RCV engines. I have run WildCat 16% lube and 18% lube in my RCV .90 and .58 CD and there is absolutely no difference except that a little more oil comes out the crankcase vent with the 18%. I had thought that line of reasoning was pure unadulterated bunk and now I'm convinced of it.
Old 06-09-2003, 08:59 PM
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Hobbsy: I'm saying that a couple of percent extra oil won't screw up the fuel/air ratio enough to matter. There may be other reasons that RVC engines run better on 16% oil that they do on 18%. I know zero about those engines except that they are darn interesting! Maybe the "expert" just doesn't want to see people waste oil. Personally, I consider a little extra oil in the fuel to be cheap insurance. But then that's just my opinion, I could be wrong.
Old 06-10-2003, 12:48 AM
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hauckf...I did the calculation once using the usual air/fuel ratios based on mass but decided against it this time because there's so many variables it gets overly complicated. Especially when you add nitro to the fuel the stoichiometric ratio becomes almost anyone's guess

hobbsy...theoretically the amount of oil should have no affect on performance because you set the needles to get the right amount of methanol/nitro into the engine each cycle. For instance, on one of my engines (an ST G51) I upped the oil from 20% to 25% castor and the needle had to be opened maybe a 1/4 turn.
Old 06-10-2003, 02:59 AM
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Default Interesting

Wow that's some interesting information. I'm not a rocket scientist but the cool part is that you guys put it in a way that I understood what you were trying to convey.

I found some other interesting reading about glow fuel at http://www.powermasterfuels.com/facts.htm . The article was written by a former owner of Powermaster fuels. It's interesting that in the article it states that reducing 1% oil is like raising the nitro 1%. I wonder if the opposite holds true.
Old 06-10-2003, 02:57 PM
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squareloop...I've got a bit of difficulty believing that. It sounds like what's normally said about pre-mixed 2 stroke fuel for a motorbike except in that case they say every 1% of oil in the mix reduces the octane rating of the petrol by 1 point.
Old 06-10-2003, 04:24 PM
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Regarding that comment on the Powermaster website, the writer was quoting someone else. There is no explanation or supporting data, just the quote. Perhaps it was taken out of context, who knows.

Here's something to think about: A gallon of 15% nitro, 17% oil fuel contains 68% methanol. Ignoring the oil, which doesn't contribute a whole lot to combustion, we have 18.1% nitro and 81.9% methanol in the part that burns to produce power.

A gallon of 15% nitro, 20% oil fuel contains 65% methanol. Ignoring the oil, we have 18.75% nitro, and 81.25% methanol in the part that burns. The effective % nitro is actually higher in the fuel with the higher oil content! (Credit R.W. Stuart, Lorain County R/C Club for that little factoid.)

This assumes, of course, that the manufacturers blend their fuel just like we would if we home-brewed it: 15% * 128 = 19.2 fl oz nitro, 20% * 128 = 25.6 fl oz oil, 65% * 128 = 83.2 fl oz methanol. Also, adding oil to the gallon of 17% oil fuel, of course, won't change the relative percentages of nitro and methanol, just as downunder expained above.
Old 06-10-2003, 08:38 PM
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Default I can see clearly now...

thanks guys!

With respect to Wildcat fuels and since this thread originated as a Wildcat Fuel thread, I also found some good reading material on their site too. I like the article under their Helimix fuel regarding the oil in our glow fuels. A different angle than what is said in the Powermaster article. I guess there are no absolutes just theories.

Anyway, correct me if I'm wrong but here are some calculations I made base on what has been said so far:

YS 20/20 = 20% nitro, 20% oil, 60% methanol. Ignoring the oil content I came up with a 34.5% nitro to 65.5% methanol ratio.

Helimix 30%= 30% nitro, 17% oil, 53% methanol. Ignoring the oil content I came up with a 35.8% nitro to 64.2% methanol.

A difference of 1.3% nitro content between the 20% and the 30%? Is this right? I guess you can look at it as having 3% more burnable fuel than the 20/20 mix but that's unreal! Definitely something to think about when I choose my fuel.
Old 06-11-2003, 12:59 AM
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squareloop...better throw that calculator away

The YS 20/20 has 20 parts nitro to 60 parts methanol or 1:3 which means 33.33% nitro and 66.66% methanol.

The Helimix has 30 parts nitro and 53 parts methanol or 30:51 which is 56.6% nitro and 43.3% methanol.

So the Helimix has almost twice as much nitro compared to the methanol. However it's a good example of why what seems to be a small increase in nitro is actually a large change in the type of fuel. You add nitro, you take away methanol.
Old 06-11-2003, 06:19 AM
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Default Nooooo!!!

I'm confused again!!!!
Old 06-11-2003, 07:59 AM
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Default If you need question answer about WildCat Fuel Feel free to ask.

Okay here's how I got my numbers:

128 oz = 1 gal

YS 2020
20% nitro = 25.6 oz
60% methanol = 48.8 oz
20% oil = 25.6 oz

Helimix 30
30% nitro = 38.4 oz
53% methanol = 68.85 oz
17% oil = 21.75 oz

I added the nitro and the methanol contents to get a total volume of the nitro/methanol mix. Then I divided that number into either volume of nitro or methanol to get a percentage. For example, using the YS 2020 numbers I got a total of 74.4 oz of nitro/methanol mix. Then to find the percentage of nitro I asked myself "25.6 is what percent of 74.4?" So I divided 74.4 into 25.6 and that's how I got 34.4 percent nitro in the YS2020.

Using the Heli 30 numbers I got a total of 107.25 oz of nitro/methanol mix. 38.4 oz is 35.8% of 107.25 oz according to the calculation.

Where did I go wrong? Please help me understand this. Don't give up on me! Thanks!
Old 06-11-2003, 06:52 PM
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Good grief! Looks like both squareloop and downunder need new calculators.

Squareloop: Your method is correct, but you calculator let you down. In the YS, 60% of 128 is 76.8, not 48.8. and in the Helimix, 53% of 128 is 67.84, not 68.85. Also, I think Cooper Fuels in Michigan sells a special fuel blended just for RCV engines if you're interested.

Downunder: If I have a green apple in my left hand and three red apples in my right, a ratio of 1:3, what percentage of the apples are green? Squareloop is trying to calculate what percent of the combustable part of the fuel is nitro and what percent is methanol, like I did in the example I cited. His method OK; his math was off a little.
Old 06-11-2003, 10:15 PM
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Default OMG! LOL!!!

I think I got a bad batch of pencils!

So YS 2020 actual nitro comes out to 25%; Heli 30 comes out to 36.1% nitro.

Also I think Hobbsy is the one with the RCV motor.
Old 06-12-2003, 02:44 AM
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Default If you need question answer about WildCat Fuel Feel free to ask.

hauckf...you're right, I was wrong
I got caught by the oldest trick in the book. I should have known better because if I'd been mixing a batch of fuel at 3:1 methanol/oil I'd have known straight away that there'd be 25% oil. So the same goes for a 3:1 mix of methanol/nitro...the nitro is 25% of the total burnable mixture in the YS fuel. In the same way the correct figure for the Helimix is 36.15% as squareloop said (hides head in shame). LOL
Old 06-12-2003, 01:18 PM
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Downunder: If you are old enough, you can always claim that you had a "senior moment". I use that excuse all the time, and it seems to work. Unfortunately, in my case, its usually true! :stupid:

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