Compression

coxaddict wrote:Thank you Mark.  Another question that's on my mind is the need to occasionally reset the ball socket in the piston.  If the piston is being pushed up on the compression stroke then pushed down on the power stroke how is it that the socket becomes loose?  Is the piston sticking in the bore at some time causing the rod to pull on the socket? Does a loose fitting piston/cylinder need less resetting?  
Thanks in advance
Agustin Jr.

Rather than opening out the swaged joint as you suggest is the reason for ball socket loosness, I would think that the loosness in the joint would be caused by wear to the upper part of the socket and ball.

The socket itself is copper plated so that during the heat treating it remains malleable. The swaging that retains the ball socket wouldn't be possible to so so without cracking. The rod itself is very hard which leads me to believe that the loosening  occurs within the socket and not the rod. If in the event the rod is showing signs of wear, it's more than likely due to the hammering that it goes through from the socket opening up. If the ball socket play continues without reset it will poke a hole through the top of the piston crown.  Once the socket is set it initially work hardens the socket which greatly improves it's ability to stay put. It will loosen over time but generally it only takes 1-2 times to maintain it's proper set. Using the engines hard for racing will certainly accelerate the loosening and also require more reset intervals. Cox used this method due to it's low cost and speed, two major benefits to business. They love those two words. This however is somewhat archaic but it worked.

For any given engine:
1. Placing a shim/washer under the barrel will change the port timing AND reduce the compression. Vica versa if removing a shim.
2. Placing a shim/washer under the head will reduce the compression. Vica versa if removing a shim.

For any given fuel and port timing combination:
1. There will be an optimum compression.
2. Too much or too little will be detrimental.

As others have said, the trade-off is:
1. A tighter engine - more compression - more friction
2. A looser engine - less compression - less friction.

Glow fuel engines are NOT compression ignition engines. See how long a well running engine runs when the glow plug dies. Also, try starting one without the glow plug lit.

There many other factors at play, but the question was about compression.

This thread has been very informative for me. It seems that having the best piston to cylinder fit on a Cox engine.. is not all that important in the scheme of things. I had always considered engines that felt "loose" (even when well-oiled).. were tired; before even giving them a chance to prove themselves. This thread kinda' disproves that theory..

This forum is somewhat like a "Science class" for me! Running Cox engines is a science in itself!

Roddie,

a loose piston/cylinder fit can be a good trade-off with good compression provided by tight fit, but when you see the castor varnish building up in a greyish/brownish layer already on the piston skirt, that already indicates the engine will soon want to retire.

Otherwise I agree the diagnostics, theories and trouble shooting we do on these critters are really enjoyable and mind-opening.

I've had them start without the plug lit... Well, not a .049, I have seen it happen a number of times. The platinum in the element does affect this, and for a glow engine to start without power takes some conditions being correct, but it surprised my Dad a couple times priming and setting up more than once.

Generally a hot day (over 80F, more likely in the 90's) and reasonable humidity are conditions this can happen.

Phil

I have started a Cox Black Widow on glow fuel without the plug powered. It was a nice (20C ish) day, and we'd just disconnected glow for a few minutes after becoming frustrated with starting the thing. We reprimed, flipped it a few times to distribute, and the bugger fired!

Flew nice too.

J

lol, that happened to me once with a nice O.S. .46 LA, it always started with one or two flips, but once on a warm summer day right after a flight I refueled it and hooked it back up to the stooge for another practice flight...it wouldn't start! I flipped and flipped, probably 6-8 times, finally it fired up and when i went to unplug the glow driver it was still laying on ground on the other side of airplane!

Dieseling! yup that can happen when combustion chamber and ambient air temperature is high this has happened to me as well with my Saito FA.91S in my Bingo when I was flipping it over just after shut down on a hot 94 deg day Big bruse!! *Youch* This tends to happen more with larger engines but can happen with cox engines too. (Bandit)

I believe the engine must have still been hot when you managed to restart it without the plug glowing. Stuff like methanol and nitro have some ignition temp that can only be reached without the glow plug if the engine is hot.

I saw that happen in my OSMax FS56 alpha when I turned the shaft over compression right after the engine stopped, and no power was connected to the plug. Needless to say the prop almost cut my finger, this is a 1HP critter..

balogh wrote:I believe the engine must have still been hot when you managed to restart it without the plug glowing. Stuff like methanol and nitro have some ignition temp that can only be reached without the glow plug if the engine is hot.

I saw that happen in my OSMax FS56 alpha when I turned the shaft over compression right after the engine stopped, and no power was connected to the plug. Needless to say the prop almost cut my finger, this is a 1HP critter..

Yup fourstrokes are buggers about busting off unassisted when hot on hot days especially if they have been run lean. I think this is due to their usual higher compression combined with hot exhaust valves. (Bandit)

Fox 35s are famous for kicking off without a glow driver on hot days. The first time i cranked one it took the chicken stick out of my hand and threw it at me about three times lol. I quickly developed a cautious respect for those suckers.
Rusty

RknRusty wrote:Fox 35s are famous for kicking off without a glow driver on hot days. The first time i cranked one it took the chicken stick out of my hand and threw it at me about three times lol. I quickly developed a cautious respect for those suckers.
Rusty

Oh don't say that, I'm fixin' to break one in lol...Hmmm safety thong on chicken stick (Bandit)

TDbandit wrote:

RknRusty wrote:Fox 35s are famous for kicking off without a glow driver on hot days. The first time i cranked one it took the chicken stick out of my hand and threw it at me about three times lol. I quickly developed a cautious respect for those suckers.
Rusty

Oh don't say that, I'm fixin' to break one in lol...Hmmm safety thong on chicken stick  (Bandit)

No thong around rotating parts. Getting hit by the stick once or even twice is nothing compared to getting your hand pulled in, or twisted in tourniquet style.

Either tape up a leather glove, loose enough to whip off your hand still (cut the cuff off too), or use stiff rubber hose on the stick, catch with the hose that does not contain the stick.

Bare fingers are safer than a thong.

Phil

I know that I was just joking I ment to put Naa at the end but didn't but thanks anyway (Bandit)

Great question, Ron!  

It turns out that Cox .049's are a major compromise in many respects. The head & compression being one of them. In a perfectly tuned engine some parameters are designed (for good reason) to be fixed, for a given fuel.  One is plug height, one may be plug heat range, & one may be (piston to head) deck height. The "tuning for different weather" is best done by changing heads that have slightly more or less volumes. That way the plug height & other things can stay the same.
Now with our Coxes, adding head shims, adding nitro etc. ( & the piston to cylinder fit) can all cause performance to go way up or down. Essentially, since Cox's are one big COMPROMISE, many different combo's can work for a given set of circumstances.  
In your case the piston to cylinder fit may in fact be the only difference, or may be one of several factors. Without assembling an engine with precision measuring tools, (to ensure TDC & plug height are the same) one can never really be sure, what change does what...
All the above is to tell you that I'm not sure & hence the test taching run is the final arbiter of performance.

I will say however, that my experience has shown that the most important single performance item is the CYLINDER. Some of my cylinders are noticeably better than all the others. I'm only guessing that it has to do with the cyl. profile (taper). My opinion is that without piston rings, compression per se, is not a valid parameter to check for performance, as the hot (running) & cold fit may be very different. YES, playing with Coxes has even me questioning WHY?
Sadly, the original designers have passed on & so we can no longer ask them what their thoughts were at the time.

Cheers, Paul

Cribbs74 wrote:Why is it a cylinder with less compression turns over faster than one with more? I am noticing this with my unlimited entry, I swapped out a modified#1 Cox International cylinder for a Bonifide number#4 that I pulled off of a TD. I lost over 1K RPM with the #4. I swapped back and sure enough gained my 1K back.
My #4 had noticeably more compression not to mention 2 more boost ports. Anyway I just thought it was strange. Anybody else experience this?

In as much as "compromise" in this context is used as a derogatory noun, I will, with due respect, not agree that COX 049-s are a compromise. (I as a non-native English speaker may regrettably misunderstand the contextual use of this word here).

We should be fair and recognize that the smaller the engine size, the major impact its dimension variations (piston top position with respect to the glow head bottom, piston/cylinder fit, the "play" at the balljoint, addition of one single shim under the glow head etc.) will have on the engine output.

If you add/remove e.g just one shim under the head of a COX 049 the compression ratio will change more dramatically than the shim of the same thickness would impact a real size car engine. The "play" of the balljoint that we know is good in a range and extensive beyond may also impact the compression ratio in this small engine much more than a similar "play" of the piston in a large engine etc.

In summary, the predictability/reproducability of the performance of an engine of such a small size like a COX 049 is fairly lower, due to the impact of its geometry variations on its output, than those of the tuned, real size engines. The similarity of the mass-produced tiny COX engines amazes me and make me tolerant of the forgivable, small performance deviation between two otherwise identical COX engines.

So these engines are rather to be called vulnerable to the dimensional changes than a compromise.

The COX-advocate Balogh

Quite right Balough,

One thing the Cox reed valve designer(s) were not thinking of is those of us pushing the engine RPM band into the 20-25,000 RPM range.
So a number of us are using hot fuels & props that were never originally intended to be used at the time...especially for "normal use".

And YES, Cox .005" head shims are too large for fine compression tuning. If I was skilled, I would make one piece shims graduated in .001" increments, & only use one shim instead of a stack.  But... we have to use what we have for now as you know.

I suppose it's no different than tuning a family sedan to race on a track. Things are quickly going to fail, because they are being used for something they were never designed for.
None the less, I REALLY like my " hot-rod " Coxes (even if I only have a very few of them...)

Cheers,

Paul ( a.k.a. Mr. Mouse)

balogh wrote:In as much as "compromise" in this context is used as a derogatory noun, I will, with due respect, not agree that COX 049-s are a compromise. (I as a non-native English speaker may regrettably misunderstand the contextual use of this word here).

We should be fair and recognize that the smaller the engine size, the major impact its dimension variations (piston top position with respect to the glow head bottom, piston/cylinder fit, the "play" at the balljoint, addition of one single shim under the glow head etc.) will have on the engine output.

If you add/remove e.g just one shim under the head of a COX 049 the compression ratio will change more dramatically than the shim of the same thickness would impact a real size car engine. The "play" of the balljoint that we know is good in a range and extensive beyond may also impact the compression ratio in this small engine much more than a similar "play" of the piston in a large engine etc.

In summary, the predictability/reproducability of the performance of an engine of such a small size like a COX 049 is fairly lower, due to the impact of its geometry variations on its output, than those of the tuned, real size engines. The similarity of the mass-produced tiny COX engines amazes me and make me tolerant of the forgivable, small performance deviation between two otherwise identical COX engines.

So these engines are rather to be called vulnerable to the dimensional changes than a compromise.

The COX-advocate Balogh

Giving out greenies for just being brilliant members. Really. Don't deny it, Paul.
Rusty

Thanks Rusty, I appreciate the greeny. As long as it helps the better understanding of how our favorit little engines work, I will share my humble assumptions and theories with the community here. Occassional mistakes in my postings, however, cannot be fully excluded . I am also benefiting a lot from the discussions here with COX and modeling professionals like you, Paul, 1/2A Nut, roddie....(I think I should not try to list all member names here) and everybody else
Have a nice, breezy long weekend with lots of castor smeared on your planes.

coxaddict wrote:Hi coxaddict here,
I read that the groove on TD .051 pistons was put there to bleed power from the larger displacement .051 to get the same power as the TD .049s.  Can someone explain that concept?
If there was no groove would the larger bore produce more power?  Would a shortened piston skirt bleed off power also? Performance tuning is all so new to me.  
Thanks in advance

I have read, perhaps somewhere in this forum, that reducing the .051 power to equal the .049's was so that free-flight models would not have to be re-trimmed when the .049/.051 engines were swapped. The rules of the events put .049's in class 1/2A and .051's in class A, so that one could fly the same model in both events by just changing engines. The .051 would put out 4% more power than the .049 were it not for the groove in the .051 piston.

Thanks Rusty & Andras for the kind words. I take pleasure in passing on knowledge to others to help them enjoy this wonderful hobby.  

RknRusty wrote:Giving out greenies for just being brilliant members. Really. Don't deny it, Paul.
Rusty