4 2 4 , what's the key?

I'd like to know what are the main requirements for achieving the 4-2-4 run and some of the tuning techniques to correct problem runs. I appreciate this is not a simply answered question, but I'd really like to get a better idea of what is supposed to be happening.
I'd like to know where to start with or what are considered the optimum port timings in degrees of crank rotation for inlet, transfer and exhaust for baffle piston motors.
How do you size the head volume initially. Not just talking shims here but head shapes and even plug placement. Where do you start with the nitro content and other what other variables are used to make adjustments to the break. What are the symptoms of underpropping or overcropping.
I've probably gone beyond what you could expect anyone to answer in depth on a message board and I haven't even raised the subject of tanks here, but can anyone point me to any articles that may have been published
While I've gathered a tremendous amount of information from some of the previous threads posted here they haven't answered all my questions.
I know that this is almost the how long is a piece of string question, but I'd really appreciate it if anyone can throw a little more light on this subject thanks.
I'm mostly interested in baffle piston motors.
Cheers John

>I'd like to know what are the main requirements for
>achieving the 4-2-4 run and some of the tuning techniques to
>correct problem runs. I appreciate this is not a simply
>answered question, but I'd really like to get a better idea
>of what is supposed to be happening.

Nobody is answering your question, John, so I am going to jump in. And I am going to start by saying your question is far too broad. If there were a simple answer we would have one simple engine to fulfill all these requirements. There were engineers involved in designing (most of) our engines, and a lot of trial and error. Even when Brodak came out with his 40, or talking with Randy about the development of is latest, there was a lot of flight testing going on to see if they got the right combination. Even then it is a compromise. It is always a compromise. You will never get a simple answer of "follow this formula to get the perfect result."

>I'd like to know where to start with or what are considered
>the optimum port timings in degrees of crank rotation for
>inlet, transfer and exhaust for baffle piston motors.

Me to. Oh there are people who will do a lot of "hit or miss" guessing, but that is what it is. And beware of the person who simply cuts the ports at an angle without doing any measuring--or testing. But the bore and stroke, the shape and size of the ports will all have a factor. There is a ratio between intake and exhaust duration, and this will depend on the exhaust duration itself as well as the individual preference.

Oh, and I have mentioned "ports" in the plural. Even with a baffle piston engine, there are some engines with multiple intake ports, and some with a single intake port. Even the crankshaft timing can affect things to some degree, and the volume of the case, and...

>How do you size the head volume initially. Not just talking
>shims here but head shapes and even plug placement.

Hit and miss. You test. Then you try something different and see if it is better or worse. George Aldrich and "Big Jim" are two who like a huge hemi-shaped hole in the head. "Big Art" made a wedge shaped head for his FP 40. Lew Woolard cut the head to a 100/1000 wide squish band on his Fox 40 and then reshaped the "innards". Both Lew and Marvin Denny developed "Double Bubble" shaped heads. Some have experimented with moving the glow plug, angling the glow plug, doubling up the glow plug. People take the OS head and reshape it to something else. There is a Fox group that reshapes the Fox head to look almost exactly like the OS.

Maybe you are getting the idea here, but there is no consistence here or any one correct answer. In all the head "diddling" I did with the FP, I never came up with a head shape that I felt was better on my engines than the stock head. Getting it to the right height was important. Reshaping it never improved things and many times made it worse.

Trial and error. Ruin a lot of parts. See what you like best.

>Where do you start with the nitro content and what other
>variables are used to make adjustments to the break.

You can set the engine for the nitro or the nitro for the engine. A little more nitro can make a little more power and may or may not improve the run and will cost you more. I start with 5% nitro and most of the time leave off there. In hot weather/high altitudes a little more nitro can bring back lost power. So can raising the compression. You can also run on 5% nitro, lower the compression and run on 10% or 15%nitro and not know the difference. Unfortunately, the latter is what a lot of people do, and all they are doing is wasting money.

>What are the symptoms of underpropping or overcropping.

If the engine is sagging, it is set too lean or overpropped (assuming everything else is right). If it runs away it may be underpropped, although one can lower the pitch and run it faster. I like to run as much prop as my engine can handle. That is not necessarily what someone else may want to do. Some prefer a smaller prop and a higher rpm. There is both a precession factor and a "P" factor to take into consideration. The larger and heavier the prop, the more it can affect precession. The lower the pitch of the prop, the more it can affect "P" factor. Fortunately the greater factor at the moment will cancel the other out, but the affects can vary throughout the pattern. Larger props can also affect turn, but they also increase stability (meaning you can bring the CG back a little further to counter this).

Basically, what will the engine turn? And what do YOU want?

>I've probably gone beyond what you could expect anyone to
>answer in depth on a message board and I haven't even raised
>the subject of tanks here, but can anyone point me to any
>articles that may have been published?

I have several books on engine theory, but I know of nothing definitive to really answer the above questions.

>While I've gathered a tremendous amount of information from
>some of the previous threads posted here they haven't
>answered all my questions.

Mine, either. Nor will they ever. Billy Graham once stated that the first thing he was going to do when he got to heaven was go to the information booth. There's probably going to be a line up. At least nobody is going to have to ask, "What do I need to do to be saved?"

>I know that this is almost the how long is a piece of string
>question, but I'd really appreciate it if anyone can throw a
>little more light on this subject. Thanks.
>I'm mostly interested in baffle piston motors.
>Cheers John

Well, John, I probably just muddied the waters. But there really aren't true cut and dried answers to a lot of these questions. There is certainly a range of things that work and don't work in tuning engines, but there is no one simple answer. And most of it boils down to trial and error until you get it to do what you want it to do. I would suggest that you start by experimenting with props, fuel, venturis, and head shims. These are things that you can go to far on and back off on without hurting things. Try the exotic stuff once you have gotten the engine to work its best for you this way. And if you do go further, be prepared to ruin parts.

Hi John:

Randy Smith has some interesting input on the 4-2-4 run and the effect of oil content, cooling, and metalurgy in his article in the May-June Stunt News.

The engine needs to be able to dump heat in order to switch from 2 back to 4. oil, and cooling effect this to a great degree.

As Leonard said ME TOO!!!

A good place to start on the timing for an engine with a good 4-2-4 run would be to get the numbers from a Fox 35 or an OS 35S. Don't know how close these two are to the "optimal" setup but both engines worked for many years without your having to rework their timing.

Thanks Leonard and Curt

I've been sitting on this question for a couple of weeks now and been struggling how to phrase it without sounding too much like one of those questions that are asked without having done at least some investigation through the search function first.

What really prompted my question has been the hint of success I'm starting to have with the Pacemaker I'm running in my OTS model at present but I was also wanting to get a better understanding of how to make the tuning changes.

You have answered some of my questions, but I think I'm going to have to organise a muffler for this engine so I can fly the model at one of the clubs that has some much more experienced flyers that will be able to help from seeing what I'm working with first hand.

One of the useful features of the Pacemaker is the rear disc configuration that makes for easy adjustment of the intake timing as new discs are relatively simple to make. One of our better know fliers had said that the intake timing was one of the more important factors in getting a reliable break and that the closing was a fairly significant which was why I asked about the timing figures.

Maybe I'm not to far away anyway as I'm getting 5 second lap times on 64 foot lines. I'm swinging a Zinger 13x6 cut down to a 12.25 daim. and the ground revs are around the 9000 rpm ( I've only tached it the once ). The run I'm getting is a rich 2 stroke run. The tank holds 5.04 ounces and I get just over 5 minutes out of it. The fuel is 12% nitro and 28% oil 50:50 castor and synthetic. There is no break or increase in power as the you pull into a vertical climb, if anything, when I do a wing over it seems as if it is leaning out but it's well past the 90 degree mark on the downward side.The model is a Super Duper Zilch and weighs 56 ounces less fuel. It's a uniflo tank and the spray bar is approximately 3/4" from the front face of the tank being a rear intake motor.

Probably still not enough info, but thought I'd post it anyway.

I'll run a degree disc over my Fox 35 and ST 21/46 but I asked my question not out of laziness but thought that there may well have been some more up to date information or new thoughts as these are after all truly old designs by anyone's standard.

Thanks again for your replies, I've already organised to borrow a copy of the Stunt News.

>our better know fliers had said that the intake timing was
>one of the more important factors in getting a reliable
>break and that the closing was a fairly significant which
>was why I asked about the timing figures.
>
>Maybe I'm not to far away anyway as I'm getting 5 second lap
>times on 64 foot lines. I'm swinging a Zinger 13x6 cut down
>to a 12.25 daim. and the ground revs are around the 9000 rpm
>( I've only tached it the once ). The run I'm getting is a
>rich 2 stroke run. The tank holds 5.04 ounces and I get just
>over 5 minutes out of it. The fuel is 12% nitro and 28% oil
>50:50 castor and synthetic. There is no break or increase in
>power as the you pull into a vertical climb, if anything,
>when I do a wing over it seems as if it is leaning out but
>it's well past the 90 degree mark on the downward side.The
>model is a Super Duper Zilch and weighs 56 ounces less fuel.
>It's a uniflo tank and the spray bar is approximately 3/4"
>from the front face of the tank being a rear intake motor.
>

>I'll run a degree disc over my Fox 35 and ST 21/46 but I
>asked my question not out of laziness

John, assuming the Pacemaker has relatively conservative, stunt timing(Fox 35) you can think about the following things.
Load- heavy loads(large, high pitch prop vs smaller, lower pitch)or a heavy plane(56 oz. on a SD Zilch seems kinda high) will generate more heat and push the engine more towards a two stroke run. Try and see what size prop lets the motor run in a solid 4 stroke. It may not fly the plane well, but you will get an idea of what the prop limits are.

High oil(which you have) tends to smooth out the run and reduce the break between 4 and 2.

More nitro(going up from 5%) increases cylinder pressures and heat. Depending on where the motor is loaded, this may push it into a 2 stroke, or it may allow you to run a 4 stroke with more power.

Compression- as Len has pointed out elsewhere, lowering compression with head gaskets, etc. reduces the difference between 4 stroke and 2 stroke rpm. Go low enough and there is little if any pickup when the motor breaks. It also reduces peak horsepower, but may not hit the 4 stroke power much. Less surge usually makes the plane easier to fly, unless it is overweight and needs the kick to keep up speed in maneuvers.

Muffler backpressure- more tends to keep the motor on 2 stroke longer. Reducing it(removing baffles, boring the exit, etc) lets the motor go back to 4stroke quicker.

Prop carving- changing the airfoil shape by thinning or undercambering can have a big effect on the amount of 4/2 break. Two props of the same nominal diameter/pitch can vary 500-1000 rpm between 4 stroke and 2 stroke.

Venturi size- once you get over a certain size(determined by experiment) going larger doesn't buy more power, it just hurts fuel draw and makes the motor harder to set. Going lean late in the run and sagging during later maneuvers are signs of too big a venturi(ask me how I know!). Len's suggested sizes are a good place to start. The Pacemaker sounds like it might have too large a venturi, since it is leaning out up high and maybe sagging.

>A good place to start on the timing for an engine with a
>good 4-2-4 run would be to get the numbers from a Fox 35 or
>an OS 35S. Don't know how close these two are to the
>"optimal" setup but both engines worked for many years
>without your having to rework their timing.

George A. regularly stated that Duke Fox borrowed the stunt Fox's setup from the Orwick 64. It's almost always been my own contention that OS turned around 20 years later and borrowed the Fox setup for their Max-S engine. George never corrected me when I said anything along those lines around him, so I have to guess he was in agreement.

.

Kiwi

Intake timings are all over the place with the engines I've checked, there doesn't seem to be any rhyme or reason even with engines of a similar type. For instance, my 40VF has the intake opening at 35 ABDC and closing at 55 ATDC where the very similar Irvine 40RLS has 50/50. For older engines the model 5224 Enya 35 is 30/42, Merco 35 is 43/43, Max S35 and Fox 35 are both 45/40.