"1/2 A" & "1/8 A" airplanes These are the small ones...more popular now than ever.

2 COX questions

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Old 04-21-2013, 12:45 PM
  #1
TampaRC
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Default 2 COX questions



1 How much play should there be in a properly set piston ball socket. I disassembled a new sure start and there is a small amount of play.

2 What happens when you put a muffler on a TeeDee sub induction type engine? Major power loss?

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Old 04-21-2013, 01:35 PM
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Default RE: 2 COX questions

(1) As little as possible is desirable-without being stiff to move. This usually requires judicious use of one of the commercial reset tools-which are available to fit all sizes of Cox from 010 to 15 (though the latter sized reset tools are pretty thin on the ground-mind you-so are the engines these days!). How much-perhaps 0.001-0.002" of movement (think of it as being no different to a conventional conrod and the ideal clearances for little end and big end on these) [also note- from recent information posted in this forum, and elsewhere, it appears the production tolerances on Sure Starts were not as good as on earlier Coxes]

(2) There will inevitably be be power loss on any SPI equipped engine fitted with a muffler (not just Coxes). How much will depend on the actual engine design, and how fast it is running. On a TD you would realistically expect a much greater loss on a 5x3 prop than on a 6x3 for example. Published power curves with and without silencer typically show an increasing divergence between unsilenced and silenced power curves as the revs go up. In some cases you simply see a lower BHP figure, in most you see a lower BHP figure at a somewhat lower peak rpm.

I don't think a great deal of experimentation has been done on Coxes and muffling-the basic Cox annular muffler design is pretty standard from 020 through to 15-it would be interesting to compare the annular muffler and the later Cox 05R/C muffler applied to the same SPI cylinder-and see which generates the most power loss [note that the Cox 05 R/C used a different cylinder from the standard TD 049/051, which did not have SPI. Presumably (because I've never had the opportunity to examine one) Cox made some compensatory changes to the transfer porting.......

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Old 04-21-2013, 04:11 PM
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Default RE: 2 COX questions

The late George Aldrich told me he had run a engine with muffler and recorded performance. He then cut the bottom of the piston to give sub piston induction. It ran the same as it had before. I do not remember which engine.
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Old 04-21-2013, 04:21 PM
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Default RE: 2 COX questions

Wow, so no difference in performance
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Old 04-21-2013, 05:37 PM
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Default RE: 2 COX questions


Quote:
ORIGINAL: Jim Thomerson

The late George Aldrich told me he had run a engine with muffler and recorded performance. He then cut the bottom of the piston to give sub piston induction. It ran the same as it had before. I do not remember which engine.
The engine wasn't using the sub piston induction, the engines that are designed for it need it, so I'm not that surprised that it didn't lose power.
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Old 04-21-2013, 05:52 PM
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Default RE: 2 COX questions

There are far more accurately documented examples of SPI engines power loss when muffled than ones where there is not-ST G20/23, DC Wasp being two good examples (and where power loss was extreme)-you will find plenty of other examples in PGF Chinn's engine tests in the pages of Aeromodeller, Model Aircraft and Model Airplane news.
I think either you or George Aldrich mis-remembered....

Consider: an SPI engine that is muffled still has SPI-and that still works-EXCEPT the engine is now taking in exhaust gas not fresh air via SPI. That exhaust gas has much lower oxygen content-and much higher temperature-than air inducted by SPI without a muffler. Two immediate effects will occur-the charge in the crankcase will have less oxygen in the mix, so the fuel:air stoichiometry will be wrong, and the hot exhaust gas being inducted instead of air will expand the charge lowering its density-both those effects will cause the engine to develop less power-it cannot be otherwise. The only debatable point is how much less..........

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Old 04-22-2013, 10:37 AM
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Default RE: 2 COX questions

The old Cox Quiet Zone muffled engines used to come with pistons with longer skirts that did not provide SPI. Testing at the time showed that if used in non muffled engines the QZ pistons produced a slight power loss. But when used with the mufflers that the loss in power due to the lack of SPI was less than the loss with SPI and sucking in hot exhaust.
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Old 04-22-2013, 12:09 PM
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Default RE: 2 COX questions

Exactly-and they also used a twin port cylinder and TD head-and the net result was a muffled engine that produced the same power as an unmuffled Babe Bee/Golden Bee. Of course this was in the mid 1960s-well before the advent of the Black Widow-which raised previous reed valve output by about 30%. I had a QZ in the early 70s but eventually left it behind with a friend when my parents moved town. I wish I still had it-so I could do some actual noise measurements on it now that I have a decibel meter...........
Whilst we're on this whole SPI thing, I ought to point out that SPI may occur either accidentally-through manufacturing tolerances, or deliberately as an intentional feature of the engine design. Most is intentional-though I have examples of the early AME 049 (the pre-Norvel type) with and without SPI. Most unintentional SPI is very small-only a couple of thou. In the cases where it is intentional it is usually at least 10 times greater-typically at least .010-.015" (and in some cases huge-such as the British Elfins, where the entire exhaust port depth is used-ie the lower edge of the piston at TDC is level with or above the top edge of the exhaust port opening.
Certainly in the early days of diesels-late 1940s-significant increases in power were achieved simply by adding SPI to an existing engine design-the ED Competition Special 2cc and Allbon 2.8 being two examples. [the corollary to this is that the greatest beneficial effect is seen on engines with restrictive intake systems-sideports and reed valves......]

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Old 04-25-2013, 12:38 PM
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Default RE: 2 COX questions

Witht the standard tube expansion or the tounge mufflers you can drill a hole on the front of the muffler extension right at the base so that fresh air is ramed into the exhaust in the areas where the exhaust port is.  It should get some of the power loss back, how much depends on the size of the hole and how much extra loudness you can stand.
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