Hi, I just read https://stunthanger.com/smf/engine-set-up-tips/engine-tuning-tips/
by Randy. I have a few questions I think will merit a new thread. There are a lot of things I'm wrestling with in my mind. I'll try to be clear.

Question 1: Is the advantage of a piped engine over non-piped, that a piped engine will keep constant speed better without the surges? If not, what makes the pipe such a radical improvement? Should the piped engine still 4-2-4?

Question 2: Can any engine run a pipe, or are 40vf, and 46vf (in my new ship), and PAs designed just for pipes? Example, I have a K&B .61, which is the clone of the old Merco .61. I also have a header for it. Would it run on a pipe or are the older motors not designed for this?

Question 3: Some engines without pipe are happy running 2cycle the whole flight. My O.S. 25sf is happy in a rich 2 cycle.  It seems my Enya .29 (though still new) doesn't like 2 cycle too much without overheating. My old McCoy .35 will purr along 4 cycling, 4-2-4, or even 2 cycling the whole flight. Whatever I ask of it. Is that true for all engines?

Okay, let's start with that. Thanks again.
David

Hi, I just read https://stunthanger.com/smf/engine-set-up-tips/engine-tuning-tips/
by Randy. I have a few questions I think will merit a new thread. There are a lot of things I'm wrestling with in my mind. I'll try to be clear.

Question 1: Is the advantage of a piped engine over non-piped, that a piped engine will keep constant speed better without the surges? If not, what makes the pipe such a radical improvement? Should the piped engine still 4-2-4?

    The advantage is being able to harness the tremendous power improvement and the great performance improvement from running low-pitch props possible running schneurle engines, and able to use the pipe tuning to control them.

    The "surges" you are referring to is not really the issue. What you are trying to accomplish is holding the airplane speed more constant, or let it drop less, in the corners. The "surges" you refer to are the reaction of the engine to the airplane slowing down.

     If you need it to you can run a 4-2 break, but I try to avoid running across the break if at all possible, because frequently the power difference is excessive, and if you try to reduce it (by piling in head gaskets), all you do is kill the overall power. How the engine reacts across the break varies - the 40/46VF are pretty smooth, the RO-Jett 61 BSE "mistake"  version is very mild. The PA series reacts extremely strongly, particularly the original PA40, and the RO-Jett 61 BSE or cast-case with the 136 degree exhaust is similar, although not nearly the same degree. The pipe causes the engine speed to vary in the right direction (on a good setup) without requiring a phase change.
   
    Note that on a 4-2 break, when it breaks into a 2-stroke, it's actually running *slower* than it was the instant before.

    In any case, if you are running a piped schneurle 61 or larger in a 650-square-inch 4 lb airplane, why in the world would you *need* to push it into a 2-stroke?

Question 2: Can any engine run a pipe, or are 40vf, and 46vf (in my new ship), and PAs designed just for pipes? Example, I have a K&B .61, which is the clone of the old Merco .61. I also have a header for it. Would it run on a pipe or are the older motors not designed for this?

  You can run any engine on a pipe, the issue is, why? What effect are you trying for?  A baffle-piston 60 from the 60s/70s doesn't need a pipe to tame it, it will run out of power at high revs anyway, pipe or no pipe, so it doesn't need one. Of course, it doesn't work as well, either, because they are so feeble that they have to run 6" of pitch. You can try it with a 4" pitch prop, but you will be nearly peaking it out, which means you are going to get nearly nothing in the corners, and the performance will be dismal. I know, I tried stuff like that with a large variety of engines.

   The pipe is not there to Get More Power. The pipe is to prevent you from getting Too Much Power.

   You run good engines like the 40VF, etc, to get the high power at moderate revs you need for a 4" pitch prop (figure 11,500-12,000 in the air), and then shape the engine response with exhaust tuning. You are not trying to Get More Power - it already has far more power capability than you could possibly use. 

     The problem without a pipe is that if you want it to go 12,000, and it wants to go 15,000, at some point it will get unloaded and go there. A merco 61 isn't going to go 15,000 on a flywheel, practically speaking, if you are running it at 8500 in the air, that's probably faster than ideal. That means using a lot of pitch, which means poor speed stability, break or no.

     The K&B 61 was a very good 4-2 break engine in the day, at least as good as the ST60. Almost no one uses either one any more because *they are not competitive* compared to a similar-skilled pilot with a 40VF, PA40, Jett 61, etc.

Question 3: Some engines without pipe are happy running 2cycle the whole flight. My O.S. 25sf is happy in a rich 2 cycle.  It seems my Enya .29 (though still new) doesn't like 2 cycle too much without overheating. My old McCoy .35 will purr along 4 cycling, 4-2-4, or even 2 cycling the whole flight. Whatever I ask of it. Is that true for all engines?

     Your Enya 29 is not broken in completely, or you have way too much prop (which is one and the same, effectively). We are stunt people, we have a lot of strange ideas, but essentially ALL 2-stroke model engines are intended to run at a peaked-out 2-stroke on an appropriate prop *all the time*, that is, indefinitely. If it doesn't, it's not broken in.

   By peaked out, I don't mean a rich 2-stroke, I mean, lean it out until it sags, and then back off just enough to avoid the sag. That's what they are intended to do, it won't damage them to do it, it won't "burn them up".  If it's not broken in, it will not hold that setting, and sag off and either damage itself or require you to open the needle to cool it back off. The classic method for determining if an engine is broken in is to look for that point at which you can start it, peak it out, and have it hold the setting indefinitely.

     Old iron-liner Enyas are notorious for taking eternities to get to that point, at least several *hours*, which is a lot of flights at 6 minutes a flight. And if you always run it backed-off, it will NEVER break in no matter how long you run it.

  Sometimes, we run engines with props far too large to do that, but that requires more break-in still.

   Most of the failures I see of people running 20FPs, 25LA, etc, are that they are unwilling to lean it out sufficiently to provide power, and if they try, their buddies will run up to them screaming at them to "richen it up, it's going to BURN UP!!!"

    They are intended to run that way, thats how you get power. If they are unwilling, what they do it try to "run it like a stunt engine", which means grabbing a 10-6, because some moron on the internet told them it was more powerful than a Fox 35, it won't run properly ("runs away" or won't draw fuel).

     My recommendation for setting a 20FP is to start it, peak it out lean (that is, AS FAST AND AS LEAN AS IT WILL GO before it sags), back off just enough to get a distinct drop, then go fly it. It will run like that for many hundreds/thousands of flights with no damage.

   Of course, your 25SF is capable of *vastly* more power than an Enya 29 even in the best of conditions.

   As always, 99.99999999% of what "stunt lore" says about stunt engines and props is complete and utter nonsense.

     Brett

Thanks so much Brett and Tim...and Randy for the first post.

Hi David

Brett pretty much covered it, except  I can run the  PA in all  4 cycle, and run them in a 2/4 with top 5 results, just takes a  different setup, The  K&B  can be  run with a pipe, as can most any engine, I have even ran a FOX 35 and  a ST 46 on a pipe, both in a  4/2 cycle,  The  ST 60 runs very well on a pipe, we ran it at about 9800 RPMs, It runs well but  beats itself  up pretty good, rods are not happy,  The  ST  G 51  is  a great engine to use a pipe setup.  There are a huge amount of rear exhaust engines  that run well on pipe  stunt setups, The ones Brett mentioned and  The  Irvine REs, The  MVVS low time  40s, 49s, the  K&B 40 will  also work, 
I sold many engines setup for piped stunt, maybe 3500 of them and  many of the Japanese  flyers  said the  liked the  OS FSR  sde  ex  40 and  45 engines,  according to them , they ran better than the  VFs,  The  VFs   were  great engines, so  that was a  good look for the  FSRs,  The  Royal and  TT made  Magnums  run very  well on a pipe too, The  TT 36 is  wonderful on a pipe, Dondi Garrison and  other  won a lot of  Titles using  one.
To answer  the  pipe  question,  I make  pipes  for  32  to 85 size  engines, ALL of  mine  are  hand  made  Carbon Fiber

Regards
Randy

My PA 65 and 75 currently run 4-2-4 break on a pipe setup.  The 65 is very sweet running that way.  The 75 is getting there but at the moment the engine could use a little more break because it is almost as sleepy as Joe Biden. 

My 61 must have been a sibling of the one Brett liked because it really wants to stay in a super strong 4 cycle everywhere

    The advantage is being able to harness the tremendous power improvement and the great performance improvement from running low-pitch props possible running schneurle engines, and able to use the pipe tuning to control them.

    The "surges" you are referring to is not really the issue. What you are trying to accomplish is holding the airplane speed more constant, or let it drop less, in the corners. The "surges" you refer to are the reaction of the engine to the airplane slowing down.

     If you need it to you can run a 4-2 break, but I try to avoid running across the break if at all possible, because frequently the power difference is excessive, and if you try to reduce it (by piling in head gaskets), all you do is kill the overall power. How the engine reacts across the break varies - the 40/46VF are pretty smooth, the RO-Jett 61 BSE "mistake"  version is very mild. The PA series reacts extremely strongly, particularly the original PA40, and the RO-Jett 61 BSE or cast-case with the 136 degree exhaust is similar, although not nearly the same degree. The pipe causes the engine speed to vary in the right direction (on a good setup) without requiring a phase change.
   
    Note that on a 4-2 break, when it breaks into a 2-stroke, it's actually running *slower* than it was the instant before.

    In any case, if you are running a piped schneurle 61 or larger in a 650-square-inch 4 lb airplane, why in the world would you *need* to push it into a 2-stroke?

  You can run any engine on a pipe, the issue is, why? What effect are you trying for?  A baffle-piston 60 from the 60s/70s doesn't need a pipe to tame it, it will run out of power at high revs anyway, pipe or no pipe, so it doesn't need one. Of course, it doesn't work as well, either, because they are so feeble that they have to run 6" of pitch. You can try it with a 4" pitch prop, but you will be nearly peaking it out, which means you are going to get nearly nothing in the corners, and the performance will be dismal. I know, I tried stuff like that with a large variety of engines.

   The pipe is not there to Get More Power. The pipe is to prevent you from getting Too Much Power.

   You run good engines like the 40VF, etc, to get the high power at moderate revs you need for a 4" pitch prop (figure 11,500-12,000 in the air), and then shape the engine response with exhaust tuning. You are not trying to Get More Power - it already has far more power capability than you could possibly use. 

     The problem without a pipe is that if you want it to go 12,000, and it wants to go 15,000, at some point it will get unloaded and go there. A merco 61 isn't going to go 15,000 on a flywheel, practically speaking, if you are running it at 8500 in the air, that's probably faster than ideal. That means using a lot of pitch, which means poor speed stability, break or no.

     The K&B 61 was a very good 4-2 break engine in the day, at least as good as the ST60. Almost no one uses either one any more because *they are not competitive* compared to a similar-skilled pilot with a 40VF, PA40, Jett 61, etc.

     Your Enya 29 is not broken in completely, or you have way too much prop (which is one and the same, effectively). We are stunt people, we have a lot of strange ideas, but essentially ALL 2-stroke model engines are intended to run at a peaked-out 2-stroke on an appropriate prop *all the time*, that is, indefinitely. If it doesn't, it's not broken in.

   By peaked out, I don't mean a rich 2-stroke, I mean, lean it out until it sags, and then back off just enough to avoid the sag. That's what they are intended to do, it won't damage them to do it, it won't "burn them up".  If it's not broken in, it will not hold that setting, and sag off and either damage itself or require you to open the needle to cool it back off. The classic method for determining if an engine is broken in is to look for that point at which you can start it, peak it out, and have it hold the setting indefinitely.

     Old iron-liner Enyas are notorious for taking eternities to get to that point, at least several *hours*, which is a lot of flights at 6 minutes a flight. And if you always run it backed-off, it will NEVER break in no matter how long you run it.

  Sometimes, we run engines with props far too large to do that, but that requires more break-in still.

   Most of the failures I see of people running 20FPs, 25LA, etc, are that they are unwilling to lean it out sufficiently to provide power, and if they try, their buddies will run up to them screaming at them to "richen it up, it's going to BURN UP!!!"

    They are intended to run that way, thats how you get power. If they are unwilling, what they do it try to "run it like a stunt engine", which means grabbing a 10-6, because some moron on the internet told them it was more powerful than a Fox 35, it won't run properly ("runs away" or won't draw fuel).

     My recommendation for setting a 20FP is to start it, peak it out lean (that is, AS FAST AND AS LEAN AS IT WILL GO before it sags), back off just enough to get a distinct drop, then go fly it. It will run like that for many hundreds/thousands of flights with no damage.

   Of course, your 25SF is capable of *vastly* more power than an Enya 29 even in the best of conditions.

   As always, 99.99999999% of what "stunt lore" says about stunt engines and props is complete and utter nonsense.

     Brett

The ability to run in a steady lean 2-stroke: does that apply to old iron piston and steel cylinder engines as well?

C R

In other words, smaller diameter props and flatter pitch?  Use a 9-4 for a fox .35 rather then a 10-6, and run the smaller prop at higher rpm?

" Of course, any engine running more power at more rpm is going to wear out faster than if the same engine was running less power and and less RPM. "

dunno if this is necesarilly so,

Automotively , a reconditioner , in NZ , found the ford ohc 1600s were running higher mileages, in the sticks anyway , as the old farts had to change gear occasionally & rev them,
whereas the lugged the 2 litres in top gear. Shock load higher & lube pressure/ speed lower ? .

So presumeably theres a ' design speed ' where the engine is ' free running ', and the cycling whatsos are more complimentary rather than in opposition .

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Another auto thingamybob , The renowned two litre Corsair V4 , with a ( non stock ) Fiat 1100 28/36 Weber , with a Accelerator JET on the secondary throat , rather than a pump
running a tall diff , if you were over 80 approaching a hill ,, steady ( set )  throttle .
As the load came on the increased vacum would richen it , and youd stay in top .On a less steep incline sometimes it'd put on speed unaided .

This is with a delicate throttle that you rest at ' the step ' as it hits the secondary choke resistance ( i.e. still primarilly on the primary )

ANYWAY ;

the Result is the INCREASED DRAW under load ,

Under certain conditions some engines will ' self accelerate ' as LOAD INCREASES

which is obviously what we're after in manouvres .

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One Corsair engine picted up 5 mph on the primary down each hill on the motorway , and held it up the next ( light triffic of course )
Going unaided from 60  ' on the level ' till it was holding 90. Just as well we ran out of hills or it mightve broken the sound barrier . 

I have to say, what a difference the "modern" engines are compared to the "classic" ones!  My Top Flite Peacemaker just purrs right along with an OS FP .25 on the front (9x4 prop), steady 2-cycle the entire time!

A little off-topic, but is the Veco .19 too much for a Sig Akromaster? I've got two, both with McCoy .19's, and was wondering if it was worth switching to the Veco . . .

Thank you! I only mention it because the Akromaster is hard to loop. I thought it was under-powered, but then later discovered it was nose-heavy . . .

Actually, with the McCoy . .