This is very true, Chuck.

This is why I wrote what I did about the recommendation of MVVS head engineer, to add a single extra head shim, to accommodate 15% nitro.

In recent past, I was strongly opposed to it, but I guess the knowledge and experience of MVVS head engineer, should not be reckoned with.

When it is broken in perhaps, although I am not convinced that all engines have the proper taper to be straight at operating temperature. I have had many engines that would not idle smooth during break in and would stop with the piston near TDC when idled low. They will also shake, but this goes away when broken in.

Sport_Pilot,


Tapered-bore engine are not supposed to idle at all, during the break-in.

You are supposed to start at about 1/4 throttle and advance it to full, as soon as the engine starts. You are also supposed to lean it ASAP, to 400-500 RPM rich of max with the recommended 25% oil (half of which is degummed Castor).

The engine must be hot enough during the break-in, for the piston not to catch at TDC. That is the point of this technique.

Please read [link=http://www.rcuniverse.com/forum/Tapered%2DBore_Engine_Break%2Din_%2D_Upgraded/m_1850473/tm.htm]the tapered-bore break-in thread[/link] again.


And you are right about the piston catching at TDC, during starting attempts.
This makes the first few starts a bit difficult to go through (more difficult for some tighter engines), but it goes away as soon as you advance the throttle and the engine's temperature rises.

It never seizes while going full bore, so the added vibration at start-up is a short term issue. This is another reason for doing the break-in on the test-stand; not on your precious, sensitive model.
Since this does not happen on the model, it becomes a non-issue.

Dar,
I don't consider any engine even ABC broken in after a few tanks. I have seen many that would not idle well below 2800 till running in the air for several tanks. I recall an early Super Tigre ABC that took almost as much fuel as a ringed engine, it wouldn't idle well till I ran almost a gallon of fuel through it. It would shake a bit when idleing below 3000. Onr of my TT .46 engines took over a half gallon. Both would stop near TDC if idled too low.

This is incorrect. The mixture should be set to a point just beyond where it begins to 2 cycle.

Please read your own [link=http://www.rcuniverse.com/forum/Tapered%2DBore_Engine_Break%2Din_%2D_Upgraded/m_1850473/tm.htm]tapered-bore break-in thread[/link] again.

there are a lot of in betweens but basically,you bring the head closer to the piston you increase compression because you are making the combustion chamber smaller.when compression is higher more heat is produced,as was mentioned here nitro has some very good quality's for making power.i ran an asp and in summer used two head shims because of the ambient temperature.in winter they can tolerate 5 to 10%.as far as hearing detonation in an engine i honestly can say i have never herd it in one of these small engines and partly due to the fact that we fly them such a distance away.there is a distinct difference in pitch when wide open and leaned out properly i depend on a tach to make sure the engine is running close accordingly to prop rpm's.if you have a high compression engine and can get no % nitro use it.in my case i cant find in the Hobie shops in this area o nitro so i added shims to my engine as recommended (asp90)and run 10% due to the fact that our club buys it in bulk and we get a good discount.

Brian,


I just did.
Please read post #7 in that thread:


As you lean the mixture; breaking from a definite four-cycle to a clean two-cycle mode, the engine will immediately gain 1,500-2,000 RPM.
If you lean it further to peak it, you will only gain about 500 RPM more, before the mixture becomes too lean.

This is typical in sport engines.

Some very high performance engines that cannot be made to four-cycle cleanly (Bob Brassell says most Jett engines will not four-cycle), can be made to run at two-cycle at all RPM levels. So they can be slowed more by enriching the mixture.

Furthermore, Bob wrote that the 60 second very rich initial run, in the Jett web site break-in procedure, is not a four-cycling run, but a wet, unsteady two-cycle.

I know this is wandering off topic but hey...that happens all the time

Dar, I suspect you used that 1,500-2000 rev increase from your prop charts on the mistaken assumption that breaking into a 2 stroke doubles the HP. It doesn't.

So here's a hands on test I just finished. First, a Rossi 45 side exhaust with standard muffler gave me about a 500-800 rev rise from the highest speed I could say it was still 4 stroking to a point where the 4-2 break was virtually finished (just the very occasional 4 stroke hic). Further leaning gave an extra 2000 revs. Just for interest's sake (although I wasn't really trying) I had it growling nicely at about 5000 revs at full throttle

Next up was an OS 40VF with the pipe lengthened so it was just a huge muffler (well maybe, at the revs I was turning it could have tried getting on the pipe ). Almost exactly the same results as for the Rossi. I thought about trying it with a couple of my CL engines but that wouldn't have come under the heading of a typical sports engine.

To bring things slightly back on topic, I guess running them in wrong is one way to lower the compression

Brian,


I would guess it has to do with what the user defines as a "definite four-cycle" and a "clean two-cycle".

No, I did not use the "charts" to estimate this .


Also, both engines that you mention are high performance engines; not the typical 'sport' engine.

When I began to break-in engines, as described in that thread, I took some tachometer readings, just out of curiosity and these were the typical RPM gains, when breaking into two-cycle (under my definition and hearing).

An engine doesn't double its HP, when going from four-cycle to two-cycle, probably because when four-cycling, the firing revolutions produce a higher-than-normal cylinder pressure. I am not sure about the reason for that.

And yes, with a tuned exhaust, looking for max RPM, an engine may gain as much as 2,000 RPM; more perhaps...


I credited you in the Diesel forum (the 'ether' thread).

I'm puzzled...you said most Jett's won't 4 stroke because they're high performance...now you're saying I can get mine to 4 stroke because they're high performance? In other threads you've called the Rossi a high quality sports engine (no argument there BTW). Which way does the wiggle factor go? Believe me, there's no trick to it even with the zero nitro fuel I was using which you always say is hard to tune.

Your comment about the tuned pipe was something of a diversion too because I'd said I'd lengthened it so it wouldn't come into play. In fact it was 21 1/2" from plug to first baffle so it would have been tuned for around 8000 revs. This was done to make sure it'd have no affect on whatever rev gain the engine made when leaned out from initial 2 stroking.

You're on the right track with the higher than normal BMEP but I'll leave you to think some more on it. I have no idea what you meant about the Diesel forum though.

Brian,


You connect between unrelated things that I write.

Normal Rossi engines and OS VF engines are high performance engines, but not even close to most Jett and Nelson engines (and Profi and other state-of-the-art racing engines). So they can be made to run at a steady four-cycle, but their behavior, RPM gain over the step and other traits, are very different from 'sport' engines, despite the fact that they are able to four-cycle.

I have never actually seen a Jett. The fact that they cannot be made to four-cycle, is from what Bob Brassell wrote to me, 6-8 months ago, when I disputed the very rich 60 seconds, in Jett's break-in instructions, as four-cycling.


When a tune-pipe is adjusted too long, the engine to which it is connected is said to be 'always on the pipe' (Copyright Macs). It will get very little RPM boost, if any at all, as a result.

An RC engine with a pipe, should be adjusted so leaning the mixture very little, beyond the two-cycle/four-cycle step, will get the engine on-the-pipe, with the whole RPM gain that results and one should not lean it any further.

The RPM gain from leaning a piped engine, will be much greater, in most cases, than 500 RPM.
But without a pipe, on a sport engine, you should not expect to see much more.


The 'harder to tune zero nitro' issue is, as I already addressed previously, from what a magazine engine guru wrote. I personally have zero experience running zero nitro, despite my 25 year experience with model engines.

I just never had a chance to do it.

As to the Diesel issue, please read post #9 in [link=http://www.rcuniverse.com/forum/Too_much_ether%3F/m_2650652/tm.htm]this thread[/link].

Most stunt engines only pick up about 1000 revs from the four stroke to two stroke break. I would think they would have a stronger break. To break 2000 revs it would have to be piped where it also gets on the pipe when it breaks. As far as I know all high performance engines will four stroke. The reason they are not used for C/L stunt is because tipping the plane up is not enough to get them to break, or they often will not return to a four stroke mode after the initial break.