I have searched for an answer to this and not been successful so apologies if this has already been covered somewhere. The LA 46 timing is by all accounts not ideal for stunt needs, but peculiarly the engine run is perfect when the right venturi, muffler and prop are used . The one thing I noticed about the LA 46 is its lack of an opposing port (boost port?) in the liner (when you peer through the exhaust).

Is this lack of a boost port in the liner the secret to engines suitability for stunt?

NO that has very little to do with it

Randy

Thanks both of you for answering.

I will leave well alone.

The answer was: From Randy=Ports don't matter
                          From Brett= Nobody knows anything about stunt engines except Dubb Jett and I, and it's a secret

    If you got that from what I said, you are sadly mistaken. *I* know I am not an engine expert. In fact, I know just enough about the topic to be aware I am not an expert - which seems to put me in a relatively small group.  I *do* know what it needs to do to fly a stunt plane in competition, and I look at it entirely from a "user" perpective. I don't make the things, I don't even need to, they already make what I want, and I have only asked specifically for one intentional change from a manufacturer in the last 40 years (extended venturi).

     I know exactly *one* thing that I am not telling you. It's probably very closely related to Randy's comment. It's also closely related to a Fox 35.

      PLENTY of people run engines with 142 degree exhaust duration (or longer) with boost ports with absolutely no issue. My best 61 has 144 degree exhaust duration, and it certainly has two bypass ports and one boost port like everyone else. It's as perfect a run as I have ever had or seen at any time in 40+ years of serious effort.  Therefore, it is not a trick, magic, or coincidence that the 46LA runs good with 144 or whatever it is, boost ports notwithstanding.

   It frustrates me to see people struggle *continuously* with engines, and resort to fixes that are demonstrably fallacious because they are "common knowledge that everybody knows". Typically they wind up just living with a bunch of problems that they don't even know they have, struggle, and have no idea what else they could have had.    The vast, vast majority including multiple Top 5 pilots have never had an engine that was helping them, or even know for sure what I am talking about.

     I just get frustrated about the endless distribution of "information" that is so demonstrably wrong, the guys to go out and test 6 Iron-Curtain 4-2 break engines in an afternoon, then talk about "typical radio numbers", and come off as experts. Then they next week you find out "hey, I finally got through my first vertical 8 today". Grind this, drill that, add 25 pop can head gaskets, do "hemi" conversions, "what do I do with a Dremel tool to make my 40VF do a real "stunt run"?"

      Brett

   His  question  was  "Is this lack of a boost port in the liner the secret to engines suitability for stunt? " 

The definitive  answer  is   NO

It would take many many pages to go into  WHY the boost port is of little  to NO consequence .  ONE thing  I can tell you the  bore stroke ratio , is  WAY more to do with the engine success  than the lack of a boost port , and there  are dozens of things that make a stunt engine better.

Randy

Ok, just read all of this and have ask " where does one find out what DOES makes a good stunt engine then?"

If common knowledge is commonly wrong then is there some kind of uncommon Holy Grail somewhere?

A number of years agol I came across an extensive article on two stroke engines(long since mislaid) by two Japanese engineers from Kawasaki.  They did extensive tested how changes to Scneurle porting, bypasses, case volume to a fair thee well.  They discovered that for a 2 stroke to run at all there has to be a specific ranges for what they called something like peak average opening- essentially when in the cycle the average area of a port was 50% open.  They didn't say exactly what the range was, but if any of the valves(cylinder ports and shaft port) were outside that range an engine would barely run if at all.  The kicker was that any configuration within the range would run, although it might not be suitable for any particular application(motocross, flat racing, road racing, etc).  Adapting a configuration to a particular purpose required further changes to port timing, flow in the bypasses and intake valves, head shapes, bore/stroke, desired rpm,etc.

So the bottom line appears to be that there isn't any one correct answer to one, two, three, or more bypasses.  Stunt engines seem to do just fine with two or three.  Racing car engines often have five or more.  All that matters is that is does the job you need done.

And then there's John Kilsdonk's quote- "if you've got the right piston and liner fit at operating temperature you've got 90% of the horsepower.  Everything else is fine tuning."

Randy-
Is being heavy on the bore and light on the stoke better? I would think it would not be good for stunt, and it would favor high revs and less torque, which I would think would be the exact opposite of what we want in stunt (to swing large diameter props slower than an RC engine, which are more geared towards higher RPM), right?
I would think we would want long stroke engines with modest bore.
Just a guess on my part.

I hear that the LA.46 is big bore and short stroke, and that would go against what I am suggesting, if that is indeed how the LA6 is spec'd out.

Thanks,
Chris

Hi Chris

No, im  afraid  you have it backwards, small bore long stroke is good for  High RPMs, NOT  low RPMs, Big Bore engines produce their HP and Torque at a LOWER RPM, long stroke engines make better power at  high RPMs ,  example just look at the Nelson NEW 40 , it had the bore reduced and a small piston with a 50 stroke greatly outperforms the older one.

another The  OS LA 40 is the same as the 46, just a small piston difference,  ST G21 40  is  exact  same as the  46, just smaller 35 size piston , The PA Merlin 40  has a  huge  bore,   the  Aero Tiger has a large bore, the  Merlin 75 has a huge bore , The Webra line of engines that are  heli based have a small bore long stroke, the   OS , TT or  Magnum counter parts all outperform it in lower RPM  stunt ranges, the small piston Webra  does well  in high RPM  heli runs.

I have written extensively on this before, I have made homemade stunt engines, some bored to 51 some stroked  to 51,  same  with 56, 65, and  74  , ALL of them  without exception were better with big bore, and outperforms the same engines that were stroked and small bore

The reason long stroke engines develop good HP at high RPMs is  they scavenge very well as the ports are opened longer, as opposed to short stroke engines

This is just  one  of the  many things that make engines better for stunt, there are a lot more

and by the way  the  LA 46 has a 35 stroke with a 50 bore

Randy

Zesus Wicky, I sound like Gordon..!

L

Never Lauri, you have a sense of humour!

Nice hearing from you.

I wonder if the "big bore is better for fast engines" idea comes from 4-stroke car engine wisdom, where a bigger bore means better engine breathing and less mechanical stress.  It certainly seems valid in that arena, with many (but not all) high-performance car engines being over square.

I am bored so will pontificate a bit as I sit here in the AC with 108F outside----and a very very cold beer or three

LA 46 ports question by BYU is a good curiosity especially for the 2 stroke motor head in me

And all the long stroke vs short is fun observation....and very relevant

None of my model experience includes 4 cycle engines....but if it did, I would look to turbo/super charging on a small scale

A LOT of my stock car pro tuner focused on small block ford and chevy...CHEHALIS, Oly Tenino, and Spanaway tracks in the 70s
All my boating experience was 2 stroke....ever witness a Slough racing in the greater Seattle area?
Half my Motor cycle Racing/ tuning (factory sponsored) was 2 stroke ...with some 4 stroke in Oakland CA ---Honda team stuff ---at Laguna Seca and Sears Point

Not bragging I was a 19~25 kid...learning and fetching tools

MY first modeling of airplanes was stunt...kid...needed/wanted speed... found Combat....learned
Never played with racing

READ a lot of, the back then, state of the art

Biggest take away back in the 60s 70s on  High rpm for combat (I assumed same for racing boats)  was mostly exotic fuels, Crank intake ramps, crank shaft internal diameter, tuned pipes, stuffer back plates, specific angles for the liner ports, Timing...mostly focused on duration that promoted great scavenging, and about 1/2 dozen other parameters 

Never did see a badly cooked engine lean run ....did see a lot of shaft runs and broken cranks

What I really now think I was understanding..back then...and realized today was;

I was amazed that they could get a .12 to .91 2 stroke into some really really impressive RPM and HP ranges with up to 65% N with additives

I suspect Randy Smith and Henry Nelson could school me a LOT about Pylon or boat racing

All fun and curious

But today my old butt--- I fly sport combat and stunt at mundane 3.8 to 5.7 sec laps

 I have ONE Cyclon 1/2A that will turn over 36K on the bench with Sig 35% and  a Galbreath Aero (car engine adaptation) Picco .050 that will get around 38K on 45%N and 18% Ucon LB 625

Alright, I said up front, I was bored and just pontificating

Thus my 1976 Kawasaki 750 (46 CI) triple cylinder 2 stroke with a set of Flying Machines tuned pipes making over 205 HP at 8400 RPM could be considered mundane today.... BUT folks, for some reason when she got to 10,200 rpm ----It seemed to me to double HP and really freaked me out to wheel stand  a 580 LB bike, on I-5 North of Tacoma heading for Federal Way at 103 MPH....wink    grin

OK Ok that is just simply bragging about my long long ago past

Note for BYU

Ever since I found out the rational  searching for stunt engines; repeat-ability, predictable fuel use, good or better props, run times...here on SH

It seems to me (((( for sport Flying))))
I did not need the expense of the Brett Buck perfect (RoJett) engine, but could easily live with the exponentially lower cost of simple (even discontinued) OS or some what easier to find, Magnum, or Stalker 45~67 engines

The nitch of a good running .20 to .67 engine, that are still available, seems to me to have a long enough half life --- still

I tend to think that trying to complete a full AMA/FAI stunt pattern in under 7~8 min is fairly easy right now to source a plane and power package...same with props and fuel

I would humbly suggest a high rate of demand for a Brodak 40 ISH engine and the various KITs ARFS and ARCs  might give us some few years of future availability of desirable KIT.... to learn with and hone skills...or just have a damned lot of fun with

Same with other combos that seem self evident...

I know we are gaining more and more better kits/arf/arc for electric every month........I may end up electrickery ----but for now I still love getting the whole plane, fuel, bleeding finger, sight, sound an smell out of every flying session

my suggested OS 40s and 46s do what this web site said they do....

I don't really give too much a rat ass what the timing and duration is...just tell me what Vent, NVA, Fuel, prop, tank, seems to be good for a Brodak ARF p40 and I can go have fun

Sincere apologies to any who read this and thought there was some point

   His  question  was  "Is this lack of a boost port in the liner the secret to engines suitability for stunt? " 

The definitive  answer  is   NO

It would take many many pages to go into  WHY the boost port is of little  to NO consequence .  ONE thing  I can tell you the  bore stroke ratio , is  WAY more to do with the engine success  than the lack of a boost port , and there  are dozens of things that make a stunt engine better.

Randy

According to The Two-Stroke Tuners handbook, and other sources including Bill Wisnewski, the boost port was added to improve high rpm scavenging.  A longer rod and a smaller bore/stroke ratio help the engine perform better at high(25,000+rpm).  The high speed car engines go below B/S of .95.  A larger bore/stroke ratio give better handling and more adaptable stunt runs.  All the "stunt" engines from the Fox 35 through the LA46 and almost everything else have B/S of greater than 1.12 or so.  The LA 46 has a B/S of 1.25.  The LA 40 which has a bit less power and not quite and nice a stunt run  has a B/S of 1.16.