Rapicon 20% from 30%

[Telehandler]

Before i buy a box of fuel just wanted to ask what difference will i see if i drop to 20% from 30%? i expect none whatsoever for my level of flying but really dont want to be messing with the tuning as there all running spoton at the moment touch wood,To be honest the only reason im gonna change is the cost!

[jimmyhorns]

Things may run a little hotter

The oil content is the same

You will see a small drop in performance, I doubt I would see any at all with my flying, infact I didnt when I moved from 20% to 30% (because it was cheaper in bulk that way).

[Si_B]

Nothing changed when I dropped to 20%. Wanted to run the same fuel in my planks and the heli's and 20% fitted the bill. Haven't noticed any dip in performance.

[Steve]

I did this quite a few years ago now. Yes, there's a drop in performance, but it ain't much unless you're really ragging it on the edge of the envelope. By the time you've flown the 20% for a weekend, it will feel totally natural and you will most likely have adapted your collective/cyclic control to suit.

You'll soon forget what all the fuss was about.

And, your bearings will last a lot, lot longer.

[Andy from Sandy]

Things may run a little hotter

That would be down to tuning. It should be possible regardless of nitro content to make the engine run as cool as you want.

[jimmyhorns]

That would be down to tuning. It should be possible regardless of nitro content to make the engine run as cool as you want.

Sure, but with a similar tune, you will run hotter with less nitro.

Less nitro = slower fuel flow = less oil for cooling = hotter engine

(In general)

[Andy from Sandy]

Why would you change fuel and then not retune the engine? That just doesn't make any sense to me at all.

[jimmyhorns]

Im not suggesting you wouldnt re-tune

[Andy from Sandy]

Then why would the engine run hotter?

[jimmyhorns]

The engine would run hotter because the fuel flow is slower.

For an engine to be 'in tune' with less nitro, you have a slower fuel flow. Nitro needs much less oxygen per ml than methanol to burn, so as you reduce the fuel nitro content, you have to reduce the fuel flow through the needle valves to compensate, unless you want the engine to be running richer and making less power.

As nitro content goes up, methanol content goes down, but by and large the oil content stays the same.

If the fuel flow is slower, the amount of oil (which has a signficant cooling effect) is less per unit time, and so its cooling effect is less.

[Andy from Sandy]

unless you want the engine to be running richer and making less power.

But that is the whole point and determines what percentage of nitro you need to run in a particular engine to get the required power.

[jimmyhorns]

Thats not the whole point at all.

In an ideal world, you want your engine producing as much power as you can for a given fuel, whilst not running it so lean that you get a failure of lubrication, or a runaway temperature (usually as a result of a failure of lubrication). This requires proper tuning of carb/engine, and not just running it on the rich side to be safe...

If you tune the same engine, with the same method, so that you get good performance out of it, you will be running hotter on less nitro. If you are willing to run an engine that is tuned richer to keep it at the same operating temp then so be it, but all that means is you havent tuned the engine properly.


I was pointing out the issue of a raised temp not as a problem, but as some of us run temperature sensors that either govern the tuning (such as a multigov pro) or serve as a warning against a lean run. If my temps went up 20 degrees it would raise an eyebrow and I would want to know why, which is why I mentioned it. An engine running hotter doesnt always equal a problem, thats all.

[Telehandler]

Thanks for the advice, so if anything ill be going a couple of clicks richer ??

[Andy from Sandy]

Thanks for the advice, so if anything ill be going a couple of clicks richer ??

No you actually might need to go a couple of clicks leaner. The nitro is an oxygen carrying agent and allows for more fuel to be burnt. By removing the nitro you are removing some oxygen so you have less to get a complete burn so you need less fuel.

[jimmyhorns]

actually its the other way, which sounds counter intuitive.

You will need to turn in the main needle most probably, rather than turn it out. The act of going from 30 to 20% will richen your fuel mixture in effect.