Several times it has been said that running more nitro will make your engine run cooler. If so, how can that possibly be if,you are generating more power? 

 Who said you were generating more power?  If you generate the same power and consume 25% more fuel, it runs cooler.

     Brett

I am interpreting your reply to mean that rpm are kept constant. Is that it?

   Unless you change something else at the same time, yes.
 
    Brett

OK, got it. In my world ( 1/2 A ) we add nitro to increase power and rpm and things definitely get hotter!

   Even on 1/2A's, its the same arrangement. I didn't run my Medallion 049/Lou Wolgast Mirage on Cox Racing Fuel to make it go faster, it was already far too fast. It was to make it run more steadily.

    Of course you *could* do that but in stunt you can always have all the power you want very easily.

   Last time I was in Tucson, I flew my RO-Jett 61 with YS 20/20 instead of Powermaster 10%, and I actually went more slowly. But, had I tried to run 10%, the thin air would have demanded that I really lean it down, probably 2-stroking all the time. It would have done it, but it would have been really, really hot because it would be the same power with maybe 5 ounces of fuel instead of 6.7. It's possible the pipe would have melted or distorted, because that's what it did the last time I tried to run 5% in Muncie.

      Brett

Does someone have test proven data of how much head volume should be changed when nito is added to fuel?
For example, with 0% nitro I use 1,4cc head volume (measured with piston @ tdc), it gives 10:1 compression ratio to my .77 engine.
If I wanted to add some nitro to make needle less sensitive, say 5%, how much should I increase the head volume?
I can of course get to ballpark by adding shims but that's not the ideal way. I rather make a new head with both volume and squish distance correct.

    I really don't know, I don't change anything at all, just the fuel. I am also surprised that you would have a problem with 5% and a mere 10:1 compression ratio, because I have run much higher compression ratios with 15 and 25% nitro and never noticed any issues.

    I did see a Discovery-Retro that required some changes to run on 5% (vice FAI fuel) but I don't understand why it so much more touchy than, say, a 45FSR, which is about 14:1 and had no problem when I ran it on Cox Racing Fuel (30%). Other than the problems it had on 10% of course!

     Brett

But my point is, that if I get the same power and process temperature with bigger head volume, wouldn't that make needle less critical?
I don't want to argue, but I kind of like 0% nitro because it forces me to really focus on scavenging & burning efficiency. I can handle it quite well but I have no idea what is normal or acceptable in real world. L

    Of course, I am not debating the point that you might need to do something if you have a problem, and increasing the head volume might be it. I don't think tuned pipe or muffler will make very much difference, and if anything, you might get less chance of detonation with a muffler, just because the combustion pressure is likely to be lower in the worst case. The issue may well be something other than the compression ratio, like, too close in the squish area. That's certainly seemed to be the case with the ST46 - you could run any nitro you wanted, but you didn't dare decrease the head clearance, even a few thousands.

  Point being, we change the nitro all over the place and hardly ever change anything else. It may be a geometry issue and you might be able to fix the geometry, then run any volume you want, even less, and not worry about how much nitro to use.

   Running some nitro, even a feeble 5%, will very likely make the needle a lot more forgiving and certainly run cooler - if you can keep if from going crazy with detonation, misfires, etc. 

Understand, though, that what you are doing is is far off most people's experience around here. FAI fuel is almost never available, in fact, sometimes it is hard to find anything less than 10% locally, and FAI is certainly a special-order item.

      Brett

I don't have much to add to this discussion, except to mention that running such tiny fuel volumes like 2 to 2.5 ounces for 6 minutes of stunt run time, has one unintended side effect that I experienced on a couple of my FAI engines and 4 strokes, and did not like at all... and that is, with the needle barely cracked open, it is much more sensitive to any, and I mean ANY tiny fuel contaminants or congealed dry fuel residue.

EricV

  Yes, that is at least part of the sensitivity issue with the needle, too, particularly with the tiny opening required to meter 2 ounces per flight on a 77! It's a lot smaller than 2 ounces per flight on a 15, since the suction is astronomically higher.

   Brett

An additional benefit from no nitro is lower noise. It's propably due to more efficient burning and the fact that the noise spectrum that comes out from exhaust port is more predictable and easier to handle with acoustic tools.

     I am not sure what it is, but I noticed the same thing, particularly back in the day with open exhaust. Even going from 5% to 10% on a Fox, there is a distinctly sharper "crack" to the exhaust. I was not expecting it and it really caught me by surprise.

    I can't say I have been able to tell much difference with piped or muffled engines, but they are pretty quiet anyway.

      Brett