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hauckf -> RE: Changes? (9/18/2004 11:30:12 PM)
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I read fuelman's reply. and when all that smoke cleared, the fact still remained that the "general rule of thumb" of adding 1.28 oz of oil for each percentage point increase you want is incorrect. A much more accurate amount to add for each percent of increase when the target is in the 18 to 22% range is 1.6 oz. To illustrate this, let's look at the origingal question that BAS asked in post #126, which was "how much oil do I need to add in ozs to bring some 16% oil up to 20% oil".
BAS is trying to increase the oil content of a gallon of fuel 4 percentage points. 4 times 1.28 oz, the "ballpark" figure recommended by fuelman, is 5.12 oz. Adding 5.12 oz of oil to that gallon, would bump the oil percentage up to 19.2%, not the 20% BAS was aiming for.
The standard formula for adding oil to a gallon of fuel, the one that fuelman included in post #127, gives the correct answer, which is that BAS needs to add 6.4 oz of oil to the gallon of 16% fuel to get it up to 20%. Not coincidentally, 6.4 is 4 times 1.6, 1.6 being the more accurate "rule of thumb" when adding oil. Imagine that! And yes, I do think that there is a difference between 19.2% and 20%, and I'm pretty sure that most of us can see the difference between 5.12 oz and 6.4 oz on our measuring cups.
Looking again at fuelman's post #138, in both examples #1 and #2, if fuelman had added 1.6 oz X 2 instead of 1.28 oz X 2, he would have ended up with 20% oil exactly, not 19.6%.
OK, now lets now look at it from another direction. The standard formula for adding oil is:
Ounces of oil to add to a gallon of fuel = (D%-S%)*128/(100-D%)
where D% is the desired final percent oil, and S% is the starting percent oil. Dividing both sides of the equation by (D%-S%), we get the formula for determining how much oil to add to a gallon of fuel for each percent increase. That formula is:
Oz of oil to add per percent increase = 128/(100-D%)
Plug in any percent between 18 and 22, and you will get a number very close to 1.6. In fact, I challenge anyone to to come up the a D% (other than zero) that will give an answer of 1.28. It can't be done!
Bottom line, if even when advice is called "ballpark" or "rule of thumb" it should be best advice possible. If 1.6 oz gets you not only into the "ballpark", but in for very close to the right seat, why would you want to recommend or use 1.28 oz, a figure that is about 20% low? If fuelman wants to add 1.28 oz per percent increase, so be it. The rest of us should use 1.6 oz.
As for the discussion of the nitro content, if BAS wanted to duplicate 10/20 factory made fuel made from a gallon of 18/20 fuel, which is a question he never asked, he would indeed have to add both oil and nitro. The exact amount he would have to add to the gallon of 18/20 fuel would be 3.29 oz oil, and .37 oz of nitro. If that information is in fuelman's dissertation on this topic, I missed it. Actually, all he had to do was plug the numbers into the Excel spreadsheet I gave him, and he could have given us those numbers! [:D]
PS For those of you who use the metric system, the formula for the amount of oil to add to a liter of fuel for each % of increase is: ml per percent increase = 1000/(100-D%). I think. Let me know if I'm wrong. (Ya, like I have to ask! [:D])
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