I have done a search and haven’t got an answer to my question.

Specifically, with a given engine, fuel and prop, how do you determine if the compression ratio is optimum?

I have done a search and haven’t got an answer to my question.

Specifically, with a given engine, fuel and prop, how do you determine if the compression ratio is optimum?

More compression = more powerful overall, more powerful break, faster transition back and forth
less compression =less powerful overall  less powerful break, slower transistion back and forth
too much = misfire, or intermittent running, "kickbacks" when starting
too little = weak power overall, runways, hard to start because it doesn't want to fire

Larger props/lower rpm leads you to less compression
Smaller props/higher RPM leads you to more compression

More compression leads you to smaller venturis
Less compression leads you to larger venturis. Aldrich took this to an extreme, with incredibly low compression and gigantic venturis, to the point they sometimes wouldn't make it through the takeoff roll .

Too much nitro can put you over the top from very high to too much compression, but it is a *very weak* effect on stunt engines. For example, a stock ST46 can handle as much as 25% nitro or more with stock compression, but woe be unto you if you reduce the clearance even a little bit (like .005-.010). People are far too concerned about compression when adding nitro, if adding 5% more puts you over the top, you probably had too much for stunt in the first place.

Most people in stunt have somehow gotten it in their head that lowering the compression is a great thing, sometimes the less compression, the better. Lowering the compression in small amounts helps some engines (ST46/60) which are running on the ragged edge, but most current engines are perfectly fine as they come as far as compression goes. Most of the "head gasket patrol" are actually trying to fix a problem with venturi and the prop selection, and very frequently cause the problem they are trying to avoid (the dreaded "runaway).

 What you want to do is get it running normally, and then you can adjust the compression with *tiny* changes to the head gaskets, like .005" at a time. And if you get more than about .015, you are probably trying to solve the wrong problem.

    Brett

Even with standard fuel it’s quite impossible to give an universal value for cr.
At first, you need to look at trapped cr, meaning stroke minus exhaust port heihgt. That way you get a little closer.
Another thing is scavenging efficiency, it has a huge effect on max. tolerable cr. For example, with baffle-piston engines I have ended up to about 10:1, but with my 6-port Schnürle it’s about 8,5:1 if I remember right. Same fuel, no nitro. I was a little surprised too. L

I half-understood the word "ingrained". 🤔

I have done a search and haven’t got an answer to my question.

Specifically, with a given engine, fuel and prop, how do you determine if the compression ratio is optimum?

Easy, use a vernier adjustment and set it by ear.
Look at any model diesel to see the truth in this but beware the setting changes constantly.

In the ancient times I used the good old "moving tank" test bench to explore how that engine responses to the changes of the fuel pressure.
It needed only a 2' long silicone tube plus a centimeter scaled ruler, over and above the usual test bench.
And I noticed five significant points of that engine settlement (fuel, venturi dia., muffler, prop and some years later the shape and material of actually inserted deflector of the combustion chamber. I had not too much possibilities to try too much glowplugs, but mostly MOKI standard, or RC ).
These points were (from lowest to highest):
#1 The engine stops because of leaning
#2 The engine runs at the maximum RPM
#3 Minimum clear 2-stroking
#4 Maximum clear 4-stroking
#5 The engine stops because of richening

I tried to use the settlement resulting altitude difference of tankposition #1 & #2  and #4 & #5 to be maximum and #3 & #4 minimum.
In everyday's words: distance between #3 & #4 shows the sensitivity of the engine,
distance between #1 & #2  and #4 & #5 shows the tolerace of the engine against extreme fuelpressures.  Sensitivity and tolerance are antagonistic requirements and I noticed that lower compression rate can better fulfill these requirements than higher compression ratio.
I found this has to be lower than (nominally) 1:11, no more anyway. Let me say 1:9 is optimal for me.
The lost power can be recovered by less tight muffler, larger (and well-shaped) Venturi. (And -by my engines- steel deflector instead of aluminum, with narrower bridges). My .51 engine drinks 150 ccm (or 5.3 fl. oz) fuel for 390 seconds.

 

Quote from: Brett

   One thing I am 100% certain of - the compressed charge does not know or care whether it would have been compressed in some hypothetical situation that ignores the port timing, valve timing, etc. It cares about how much it was *actually* compressed in the situation at hand.

     Brett

True, but it does care about how the charge has been mixed during scavenging-compression process, and what's left bouncing about in cylinder and channels from previous explosion.

   Yes, internal ballistics is definitely a real thing, actually, one of the most important things for stunt, anyway.

   What is not real, or relevant, is as you said above - the strictly geometric compression ratio, with no consideration of the port timing or valve timing.

     Brett

Easy, use a vernier adjustment and set it by ear.
Look at any model diesel to see the truth in this but beware the setting changes constantly.

That's essentially how I arrived at my optimum compression for zero nitro. All I did was make a variable compression glow head for my Enya 60X and gradually raised the compression while it was running and checking for rev rise until revs peaked out.

how it used to work on flathead Fords, etc,

The O H V is just a passing phase . the side Valve will return .

or

Save your confederate money, boys, the South will rise again! |

Actually , the side valve is much derided . Met a type who said 27 mpg ( imperial , 21.6 US  )  600 mile trip .
Ford ' House ' ( 1947 Sedan ).

These Olde Cowes wernt a total disaster . Some Said 155 with a Tail wind . ( With fairing ) . Cal's camera bike in O A Sunday was a S V ,

incedently , from the horses mouth , the fatal Suzuki Seizure was from F'ing with methanol , against the advice of the locals .

A Side Valves power charateristics might be highly usefull for F2B , FLAT Tourque Curve .

37 60 horse is only one that appoaches THAT ! . 

http://www.vanpeltsales.com/FH_web/flathead_specs-85early.htm

That Said , one would assume a CDI ' Spark ' ignition , spark plug NOT glow plug , would provide good speed control , again for F2B .

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PS , if it trys to take your fingers off , it might be to high . 21/40 on 20 % N , 11 x 4 , youd come home with your fingers raw .
A determined belt was fine . With C'Case Pressure a 9 mm ( Combat Intake ) was fine . Sounded superbe , & on the Std. 46 intake,
suction .

If its always kicking back presumeably the plugs to hot or CR / Nitro to high . 20 % will suck the juice but the Tourque & output is impressive
A few hot shots in the antipodes dont mind 20 % , say it cools it better ? . Certainly nitro makes for less critical needling, on peasant engines .

I'd like to see how that's done.

Google FMV engine article  down load that and look at a push pull head and imagine a glow plug in the centre of a hollow vernier screw that is tapped into the contra piston.

Easier said than done, Chris.

If you think that the added complication of your precision adjustment is worth all the form discontinuations, heat gradients and hot spots, go for it.
But I strongly feel that the gadget itself will cause more instability than what it can fix.
L

Does that mean you are going to try it then?

Does that mean you are going to try it then?

   You mean, put in a contra-piston, etc, like a diesel? Why would you do that (for stunt, at least).  It's an interesting thought experiment, but impractical, since there's no reason to think a variable version would work the same as the final fixed version.  It's not that touchy to need ultra-fine adjustment, and you still have to set based on test flights  - which is easily accomplished just changing the shim or different head buttons. You aren't going to figure out the right answer with bench tests, any more than Duke's test (which would be entirely ambiguous at best, since he didn't check the flight performance). With current competitive piped stunt engines like the RO-Jett or the PA, the small adjustments tend to have small to completely undetectable effects, at least over any practical range, and you generally want as much as you can get without causing detonation or misfire.

     Brett

Nice! Did you have any trouble with galling the threads, or use some slickery-trickery to prevent it?  Steve