Originally Posted by LGM Graphix

I posted this in the pylon forum as well but it doesn't seem to be very highly watched these days so I'll post here in hopes that somebody here can offer some insight.


So recently I picked up a couple of Q40's and a quickie, all with nelson short stroken engines.
I'm not new to Nelson's, but in all fairness, I have never raced with them so never worried about a perfectly tuned engine. As such, in the past I never used a Tach on them.
Well, I burnt a lot of plugs in the past, wrecked a couple engines, and/or just plain had crappy runs so.....
I bought a Tach for use with these engines.
Now the Q500 engine is pretty consistent once needled, it generally peaks around 19,000 - 19,400. I back it off about 400 before launch and go.

The QM40 engine I'm running right now on the other hand, depending on the day will peak anywhere from about 21,900 - 23,500. I'll back it off around 500 and then go.

My question is, does air temp and humidity make that big of a difference on those engines to see a 1600rpm difference between runs? I'm using the APC carbon QM40 props (7.5x7.4 I believe), using 15% nitro with a castor synthetic blend, I add a little oil to give me around 19% oil. tanks are tetra bladder tanks etc.

I don't need every last RPM as I'm just flying them for fun, but I like to know why the peak would be so different. I want to make sure I don't lean the engine out to far and damage anything.


Another quick question. The old Nelson HD plugs with the 3/8" hex, I seem to get between 10 and 20 runs before the plug performance is degraded. I recently bought the Merlin Nelson plugs from Jett and peaking the engine the same as I did with the HD plugs, I'm only getting 1 or 2 runs before the plug is burnt out completely so I'm running the engine richer just to try to keep the plug life better. What am I doing wrong? I've never used these plugs before.

Thanks in advance
Jeremy