Hi All,
I was out tinkering some with my DA 100 today just seeing what would happen. I'm one of those people that like to play with things a little to see what effect they have and alot of times I stumble on things that work better doing that. With aerobatic planes I've found that tinkering with things can really improve performance and the way the airplane flies.

Anyhow, what I did today was loosen the timing sensor and advance it all the way and run it a little to see what the effect was, and then I retarded it all the way to see what the effect was. It didn't change much, but with it retarded all the way, the engine seemed much happier. It took the throttle a *little* better. I say a *little* because the DA engines are known for great throttle transition and it was fine before...it's just it seems to be a little better now with it retarded. It also seemed to have more pull and run stronger even though the rpms were the same on the top end. I also noticed that it idles lower and smoother...it idles so slow you can count blades now...I mean ***really*** low. It will hold that idle and take the throttle too even after idling it for extended periods which normally would not be done. Can anyone explain to me why this is so? I'm not sure I understand why on a 2 stroke engine. I assume that it's because it is firing later thus it is firing at a higher compression and is also allowing more fuel air mixture to be compressed more which causes more power. Is this so? Or is my thinking not quite right? I would appreciate hearing from anyone who understands this as I'm not sure my thinking is right on this.

The timing on an engine is critical. And can cause a motor to run hot and pre ignite gases on the compresion stroke, wich will destroy cylinders and piston sometimes you wont even here the knock thats destroying your engine so make sure your engine is in the operating range of timing.

Aero thats some interesting tinkering you are doing. I have a DA50 that does not idle as low as I would like, and if I lower the idle it will cut off after landing. I think I'll try a little retarded timing.

Thanks for the idea, Kent

If you got the same top rpm and yet a better idle and better acceleration that's where I'd leave it. Many engines have a range of 28 to 32 degrees BTC and you set it where it runs the best for you. If you change to a smaller prop some day you may find that it runs better with a few degrees more advance. I put a CH ignition on my Maloney 125 and made a bracket so I could adjust the timing between 28 and 33 degrees BTC, it runs best at 28, if you advance the timing to 32 BTC the rpm does not change but it vibrates my picnic table something awful.

How well does the picnic table fly


Kent

Hey Kent, I gave up trying to fly it , my wife made me sand it down to new wood and repaint it. Something about Diesel and glow exhaust not looking good on it.

I bet that gave the burgers a niffty taste!


Kent

OPPS I have steered this off topic, a little fun.

Hi Planecracy,
I have talked with DA about doing this and they said that the sensor position wasn't critical on the DA's. They said you could run it all the way back or all the way up and it can't hurt the engine. Too much advance is what will kill one but there is enough movement in the sensor to do that on the DA's.

Kent,
You may want to wait a while before tinkering as the 50 is not broken in yet and the idle will get much better as the engine breaks in and you go leaner on the mixtures. It takes a whole year of flying to get one of these engines really fully broke in. The initial breakin is just to make sure the rings seat good and to wear them in so the cylinders don't get scored, but after that it takes a long long time for the engine to loosen up on the bearings and for the cylinder to get worn in real good. That 50 is one honking engine! Before you do anything I would talk with Dave or somebody to make sure it's something you really want to do. I don't wanna suggest that this will work for everybody as I don't really know why it's working so well for me. I also have pipes installed in my airplane and am running premium fuel so all that combined could effect the results of what I'm seeing. I have read where Dick Hanson has run alot of engines fully retarded before and had good luck. I'm hoping he will See this and reply. Really what I'm looking for here is ***why*** this is happening so that i understand fully what is going on. I also want to make sure this is working as it should, and continue to work as it should. I really recommend that anybody who reads this not do this until we understand fully why my engine showed what was posted in my last reply. These engines are expensive and I wouldn't want anyone to do anything that could be detrimental to their engine. There are many variables with these engines. I have a high load prop also on my engine which could have an effect on what I'm seeing. I'm running a 25-12 3 blade which is alot of load at 5800. I would like to say that the engine did not seem to run hotter and there was no hint of detonation as this engine runs very quiet and I'm feel certain it could be heard. The engine, pipe, and prop combo I'm running is as quiet as they make at this point in time. It has tested quietest of the meet at just about every meet I've flown in except for one where there was a 1.40 sized airplane which was obviously quieter. Normal ground tests come in at 87dB and the highest recorded was 89dB...on the 89dB reading there was a tree line at that particular field that I feel certain was reflecting some sound back at the meter. Just thought I would give a little more info on my setup, and thought I better warn against this until we understand more about the why part.

Well after 22 gals of fuel I think its broken-in. Also have 24x8 prop on it , a pretty good load, but the engine don't care. The idle is not reliable on the ground, in the air has never cut off, go figure. Lean the mixture for better lower idle and the transittion is bad. What you think abot dat.

Kent

Hi Kent,
Be sure to let the engine warm up good. I have noticed that my DA is kinda cold natured which is good, but it has to warm up to set the carb right. I set mine just a bit rich on the bottom where transition is good and there are no dead spots. I noticed that there was one place in there that it didn't want to go clean through...a little over a 1/4 throttle and after the timing was retarded it went through cleanly. I had to readjust the bottom a hair richer also. This is what ***I think*** is causing the engine to idle lower and more reliably. I really hope the very experienced engine gurus get in on this thread as I really need to understand the way part of this. You may want to try it on your engine. You can always put it back where it was. Be sure and mark with a real sharp pencil where the edge of the sensor was so you can put it back exactly where it was if need be. Just be careful and if it don't sound right, put it back. I really don't wanna recommend this though until, like I said in the above post, we understand this more. It's probably not a real big deal, but I don't recommend things until I understand why, and am positive that what I post is something that won't cause ill effects. These gas motors get into the high dollar category and I wouldn't want anybody to have trouble with something they spent their hard earned money on because of something I started.

Aero, the reason you are getting a performance boost is due to several factors.

First, you are spinning a larger and draggier propellor, that makes top rpm at a much slower speed than a 2-blade. In most cases, given the exact same engine, the engine will actually make better "power" at a certain, "Lower than peak" rpm if the timing is retarded slightly. This is due to the burn speed of the mixture in the combustion chamber, and how much average pressure is created over the burn/power cycles time period. The technical term for this is BMEP. . Brake Mean Effective Pressure. . or the AVERAGE of the pressure inside the cylinder as it first fires off and then the piston descends.

If the piston is moving slower, due to slower rpm, having the BMEP "spike" later is actually advantageous to increased overall "torque" at a specific rpm. For instance, if you are running 6500 rpm, you might want the BMEP to spike at about 4-5 degrees ATDC, but turning slower rpm, like about 5800 where you are turning it, youwant the BMEP to spike a little later (in theory) at (for argumetns sake) 8-10 degrees ATDC. Of course, these are just numbers to show a reference.

The reason for this is several fold. . first off, if you maintain "advanced' timing with slower rpm, the BMEP will spike closer to TDC, which is undesirable, since the pressure will degrade more quickly as the piston descends. It can also lead to pre-ignition or unstable burn from the mixture because the pressure builds too fast and the mixture will spontaneously combust (pre-ignite) instead of burning smoothly. By retarding the timing slightly at lower rpm, you move the BMEP spike farther back in the stroke of the piston, keep the pressure at a higher average level throughout the pistons descent, help remove the possibility of pre-ignition since the BMEP is spiking later, and gain an overall TOTAL BMEP for the burn/power stroke of the engine. If you gain 1% total BMEP increase while the piston descends, you gain 1% greater useable power during that power stroke. . and VOILA. . more useable power. This will help, usually, in the mid-range of an engine, since the timing sensors on the DA stop "advancing" the timing around 4000 rpm or so, and this is the critical area where BMEP pressures can soar due to over-advancing the timing at the same time you are stuffing the cylinder as full as possible, which tends to cause an unstable, out of control, mixture burn.

Conversely, for a higher rpm or lighter weight propeller, such as a 2-blade, you would want to advance the timing to take advantage of the shorter burn cycle as the engine is turning up easier, and transitions through this rough area more quickly, with less tendency to be held back and start running into unstable mixture burn conditions.

As for why the idle is smoother. . the same principles apply, and when spinning the heavier 3-bladed prop, you need a smoother power delivery with less static timing, to help smooth it out. Retarding timing moves the BMEP spike back a good distance during closed throttle/idling conditions, which makes the engine "Hit" softer and with a smoother power delivery. I'm sure if you advanced the timing fully at idle, it would become a bit rougher, noticeably so.

Hope this helps answer your question.

Kris and others have most of the info above -
Without getting into a wee wee contest - what we want to have happen --inside the engine --is for the fuel charge to burn at the right time and as completely as possible -in an even fashion- at the best time -- to push the piston AFTER the connecting rod is at the "best " relationship with the crankpin.
Inertia (heavy flywheel ) allows this position to be earlier (carries the engine thru.
also inertia ( speed squared x weight ) greatly affects where this magic point is located.
Some manufacturers have an internal geometry of stroke and conrod length which makes all of this better -
sometimes noted as rod to stroke length.
this also has a profound effect on how well the engine tolerates timing .
The DA has a very good setup in this respect -
basically - it allows the piston to rise and fall LESS during the "over the top" portion of the stroke.
If you draw on a piece of paper - a basic two stroke engine -- then change ONLY the con rod length -- you will see a different acceleration and deceleration curve of the vertical travel of the piston -- during a complete revolution.
(each degree of crank rotation, yeilds a different amount of piston travel.)
anyway - what is full retard or full advance - can be all over the place - depending on engine design which has this critical period figured right - and make little difference - as Dave Johnson stated.
That keeps things simple.
and you stay out of trouble.
The ZDZ engines run best with the timing at full advance - -at least here at 450 ft altitude.
adding a tuned system -- that is, a TUNED system---will increase piston pressures and in some cases - retarding does increase top speed -
the "why" here -is that the critical start time of a more dense charge - can atually try to kick the engine backward (extreme case ) or detonate -increasing heat. and ultimately bustin the engine
Ideally the big push starts at the right time and increases as the piston is going down.
Thre is no magic timing number - just the best position for your engine -prop - exhaust setup.

Uh, Dick, I hate to break it to you but. . .we both said about the same thing. . kinda hard to have a wee wee contest when people agree. . . .

You said it first Kris You Win

Yeah, but Dick has probably been saying it for a few years longer than I have. . .guess he wins cause he's older. . . .. HEY. .being a member of the AARP seems to have an advantage in things after all!!!

well if age is a disadvantage -- I will still take it ! I hate to think what things will become if the fu-- goof balls runnin the world keep on the same paths .
(no one is watchin the store!)
I just wanted to ADD the possible reason why some engines are less critical to timing adjustments- that reason being the rod to stroke ratio -- better in some designs.

Hi Everyone,
I took the plane out this afternoon and got a few flights on it. I just thought you may want to hear about a few things I noticed. The throttle transition was much smoother especially the bottom half and was much easier to manage in the air. The engine pulls like gangbusters in the verticals! Vibration is wayyyyy down...while not a scientific analysis of the vibe level the ailerons were almost dead still across the entire rpm range. Throttle is very linear as it was before with no mix and no curve set in the Tx. . I also noticed that what I call the 'ramp' where the pipes come in about 1/2 throttle was spread out over a broader 'curve' ...it was like the boost was still there, but was not just in a few clicks of throttle around mid stick...it was smooth all the way from idle to full throttle. It also starts cold much better. I noticed that after I put the new Mejzlik 25-12 on there was it took a little more to get it running without it quitting before. it would take about 2 times choking it and flipping 3 times to get it to stay going on the first run of the day full cold. Now, it's just IGN on, choke on, flip once it pops on the first flip, and flip 3 times and it running. let warm up for 15-20 seconds and it takes the throttle. Since it was fully retarded, I went back about 1/2 way in between where it was before, and where I had it fully retarded to see what effect that has, and see if I need to 'zero in' on a batter setting. Something tells me I won't though as this made an improvement in the way the engine runs. It wasn't drastic as it was fine before as are all DA's, but it's just a tad better here and there on different things. Now...If I can just find that hidden 10 thousand horsepower they put in there... I now it's in there. :greedy: :greedy: :greedy: LMAO!!! Aw, and yes it's 6:14 am, and yes I actually do get up that early to get ready to fly model airplanes. hahaha

Put a 20-8 Bolly on it and let 'er rip!!! guaranteed to see 10,000 horsepower for a moment or two. . .

Don,t you loose RPMS on the top with it retarded?

Depends on how much you back the timing off. Turning a really low-drag prop you would probably have a situation where the timing changes would be more easily noticed, but with high-drag props the difference in total top-end power is negligible, considering the amount of timing change possible with the adjustment slots on the pickup.

How bout this. . FORGET total top end power. . . . .fly the plane. Top end power is only significant if you are BEHIND the airplane, and need to play catchup.

Too much advance will reduce rpm as well as not enough advance. Too much advance is more likely to damage the engine than not enough advance. When we say retard the timing we're not talking about ATDC, we're only talking about reducing the amount of BTDC. Finding the sweet spot is the secret. The prop, gasoline, ambient temp. and even the spark plug heat range effect the timing.

I didn't notice any rpm loss on the top, but it did effect the lower half of the rpm range. I am running a heavy load prop. The reason? I am after a slow constant speed that pulls with authority in the verticals. I am also after maximum quietness. Those two things generally don'r go hand in hand. You really have to have an engine that develops alot of excess power, and then prop it in such a way that you are still in the maximum torque range but at a low rpm that will keep the tip speed down below .6 mach. This prop does just that. It turns exactly at .6 mach but is still in the maximum torque of the engine. The DA's have a aerobatic power curve that allows them to put this torque out at those low rpms. As was said earlier, they also have a long rod to stroke ratio. This long rod to stroke ratio in conjuction with the prop and pipes I am using just happens to run a little better at a slightly retartded position. Were not talking about much change timing wise, or the way the engine runs. These are very slight changes. It's only something you would want to to after everything else has been maximized. You must have the engine fully broken in, you must have the needles dialed in, and you you have to have the pipes tuned dead on...then you would want to mess with the timing as the other things have much more of an effect on the way the engine runs.