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Engine basics => Engine set up tips => Topic started by: Chancey Chorney on August 11, 2016, 03:06:57 PM

Title: How Much Castor To Add????
Post by: Chancey Chorney on August 11, 2016, 03:06:57 PM

Hi there. So, I got me a bit of castor oil to mix up for my Fox 35 engine. But now is where the confusion comes in. How much to add to the fuel I have on hand. Specs are Sig Champion 5% ALL Castor fuel. I did the calculations 4 ways and each gets a different answer.
1. 128 x .25 = 32
    128 x .29 = 37.12

    37.12 - 32 = 5.12
 
    So, in this case, I would add 5.12 oz of castor oil.

2. 128 x .25 = 32
    128 x .05 = 6.4

    So, in this case, I would add 6.4 oz of castor oil

3. Nitro RC Online Fuel Mixture Workshop Says 6.6 oz of castor to be added

4. Off the link from the Cox Engines Fuel Guide, the program tells me to add 7.2 oz of castor oil.

This to me is easy, in my mind, but, it is apparently not that easy. I personally feel that method 1 or 2 of calculations would be the best, but after getting 4 different answers, I am no longer sure. Thank you in advance for any help you may be able to provide.

Title: Re: How Much Castor To Add????
Post by: Tim Wescott on August 11, 2016, 03:24:28 PM

I gather that your starting point is 25% castor, and your goal is 29%?

I hate to throw math at the innocent, but all of those methods seem to be neglecting the fact that if you start out with 128 ounces of 25%, you're going to end up with MORE 29%.

So let's work backwards: you have 128 ounces of stuff, of which 25% (32 ounces) is castor, and 75% (96 ounces) is other stuff.  You want some amount of stuff with 29% castor, of which the other stuff is 96 ounces, and adds up to 71% of the total (because 100% - 29% is 71%).  The ratio of castor to other stuff is 29% to 71%.

(96 ounces) * (29 / 71) = 39.2 ounces of castor.

You already have 32 ounces.  So, 39.2 - 32 = 7.2 ounces of castor.  Note that the total amount of stuff is 135.2 ounces.

And, there you are.  As a check, 39.2 / 135.2 = 0.29, or 29%.

Title: Re: How Much Castor To Add????
Post by: Steve Helmick on August 11, 2016, 03:33:59 PM

You're ignoring the fact that you're increasing the final volume of fuel after you add "X" amount of castor. Add "X" to 128 oz and solve for "X". It's basic HS Algebra. You can do it! Basically, 128  + 7 oz = 135 oz fuel x .29 = 39.15 oz. castor. 39.15 oz - 32 oz = 7.15 oz., so you're real close to 29% if you just add 7 oz. Measure it out as well as you can using your wife's 1 cup sized Pyrex pitcher and don't worry about the decimals. Chaff in the wind...or prop blast.  

But the calculator deal is probably just fine...either one. It seems to me that Tulsa Glue Dobber's site had one on their website. Variations are probably caused by the way the program rounds off the decimals.   y1 Steve

Title: Re: How Much Castor To Add????
Post by: Dane Martin on August 11, 2016, 04:02:02 PM

Yep yep to Tim and Steve. I add 7 oz and call it good.
If that .2 is going to bite me in the @$$ later, so be it..... lol

Title: Re: How Much Castor To Add????
Post by: Chancey Chorney on August 11, 2016, 04:12:19 PM

Ok. Well thanks a lot then all for the help. I just took something easy and mad it more difficult for myself. It looks like 7 ounces it will be. Thank you again.

Title: Re: How Much Castor To Add????
Post by: George Truett on August 11, 2016, 06:05:48 PM

Here's one of the online calculators:

http://www.nitrorc.com/default2.asp?Introduction=http://www.nitrorc.com/fuelws

Title: Re: How Much Castor To Add????
Post by: Chancey Chorney on August 11, 2016, 07:26:22 PM

Quote from: George Truett on August 11, 2016, 06:05:48 PM

Here's one of the online calculators:

http://www.nitrorc.com/default2.asp?Introduction=http://www.nitrorc.com/fuelws

Yep, that was #3 on my list, and where I had gotten one of my figures from.

Title: Re: How Much Castor To Add????
Post by: George Truett on August 12, 2016, 05:35:36 PM

I see that now, I should have read more closely.