Strykaas

Hello,

I'm currently wondering what is the accurate meaning of "X % of nitro" for our glow fuel.

There are two ways (quite a bit different) to go with it :
1) either you chose a volume based approach.
2) either you chose a mass based approach.

Which one do u generally use as u mix your fuel components ?

Bear in mind these two ways drive to rather different results.

Actually, I'm pretty sure this percentage is on a mass basis.

(When you think of epoxy resins, it's the same, based on masses.)

Rule : the higher the difference between densities, the higher the error is made when using volumes instead of masses

I've searched for their respectives densities :
- methanol : 0.7915
- castor oil : 0.96
- nitromethane : 1.13

This means that errors can be great, especially regarding methanol vs. nitromethane. Let's take an example.

Reminder : density for liquid stuff = mass of one liter of this stuff

We want to make one liter of : 5% nitro, 20% oil, 75 % nitromethane.


1. Let's regard this as volume proportions :

Nitro : 0.05 liter
Oil :0.20 liter
Methanol :0.75 liter

2. Let's regard this as mass proportions (a bit more tricky Come on guys !) :

Vt = total volume = 1 liter
Vn = nitro volume within Vt
Vo = oil volume within Vt
Vm = methanol volume within Vt

Mt = Mass of Vt
Mn = nitro mass in Vt
Mo = oil mass in Vt
Mm = methanol mass in Vt

Dn = nitro density
Do = oil density
Dm = methanol density

Equations :

Vt = 1 liter = Vn + Vo + Vm = Mn / Dn + Mo / Do + Mm /Dm

Mn = 0.05 * Mt (5% mass)
Mo = 0.20 * Mt (20% mass)
Mm = 0.75 * Mt (75% mass)

Which gives :

1 = 0.05 Mt / Dn + .20 Mt / Do + .75 Mt / Dm

=> Mt = 1 / ( .05 / Dn + .2 / Do + .75 / Dm )
=> Mt = 1 / (.05 / 1.13 + .2 / .96 + .75 / .7915 )
=> Mt = 0.833 Kg

One liter of our mix will weight 833 grams

=> Mn = 0.05 * 833 = 41.7 grams
=> Mo = .2 * 833 = 166.6 grams
=> Mm = .75 * 833 = 624.75 grams

=> Vn = Mn / Dn = 41.7 / 1.13 = 36.9 milliliter = 0.0369 liter
=> Vo = Mo / Do = 166.6 / .96 = 173.5 milliliter = .174 liter
=> Vm = Mm / Dm = 624.75 / .7915 = 789 milliliter = 0.789 liter

Vn1 = 50 mililiter vs Vn2 = 36.9 milliliter :disappoin
Vo1 = 200 mililiter vs Vo2 = 174 milliliter :disappoin
Vm1 = 750 milliiter vs Vm2 = 789 milliliter :disappoin

CONCLUSION :

These two approaches are quite a bit different !!!!
Roughly, if u use weight, you are going to run lower nitro and lower oil :disappoin !

Which one do u generally use ?

I'll try to contact fuel brands to check what their fuel are made of !!!!!


Regards.

Rudeboy

And what exactly is the problem now?

I mix my fuel be volume... If that means my10% nitro is more than some fuel manufacturers 10% nitro... so be it...

I run in my engines what makes 'em run best... no more, no less...
Mass or volume are just different expressions for the same thing in my mind... like expressing speed in mph or kph...

This whole thing is only valid when comparing different fuel brands.
And still, even then people choose a fuel that performs best in their experience, don't they? At least they should IMO...

Strykaas

The point is that I've received some piece of advice for breaking in, in terms of min oil percentage. I simply want to do it right ! Mass or volume ? Think i'll go for volume...

Rudeboy

Breaking in?

If it's a modern production engine put in 20% oil (by volume) and run it...

John Hawkins

My Feedback: (2)

I believe I remember reading in a Clarence Lee column that the generally accepted practice was to mix by volume; however at the time he said he understood that some fuel maker(s?) were showing nitro % by weight in order to advertise a certain % while using less nitro than others.

John

Strykaas

John,

That's a VERY interesting point !

So take care when choosing your fuel...

downunder-RCU

John is quite right that one or two companies short change you on both oil and nitro by mixing using weight. Mixing by weight is a very good idea IF it then approximates very closely the accepted values by volume. Unfortunately, evaporation tests don't seem to show this (for oil content) so you can imagine how much nitro is really in the fuel

Mixing your own fuel should always be done by volume, for one thing it's far simpler and the coefficient of expansion is so small that over any reasonable temperature range you wouldn't notice any difference.

Cactus.

My Feedback: (1)

Weston fuel is done by volume..

ZAGNUT

My Feedback: (1)

byron mixes by weight and is the only company i know of that does so. do a search here on rcu and you'll find quite a few threads that deal with this issue.

dave

Cactus.

My Feedback: (1)

yes, lots of threads exactly like this one pointing this way.