Head combustion chamber volume.

John Goddard Mon Feb 11, 2013 1:36 pm

Can you tell it was a quite day at work today.........?
lol!

I thought I'd attempt to measure the above so those of you who attended school the day 'teach'
talked about Pi R squared and all that could perhaps work out what the compression
ratio's of your various motors are.
I wanted to discover why I've blown a plug fairly quickly and perhaps why another of you have had the same experience with the same plug on the same engine.
Whilst the below is not 100% accurate (I used a graduated .5cc insulin syringe) it's pretty damn close,-

Standard head .17cc
Teedee .1cc
Norvel .08cc
Bernie Extreme .05cc

I calculated C/r of 6.16/1 for a Bee as opposed to the 6.5/1 quoted for the Thermal Hopper
which seems to be the only documented internet c/r quoted for a Cox 049.

If my calcs are anywhere near accurate my Extreme/Bernie with 4 gaskets comes out at over 13.6/1 c/r
no wonder those heads blow.
I'd suggest starting with 6 gaskets for those guys.
Very Happy

PS
I am a big fat liar cos I used,-
http://www.csgnetwork.com/compcalc.html
not my own brain
lol!

**RknRusty** Mon Feb 11, 2013 3:02 pm

Make sure you account that the top of the fluid will form a dome due to surface tension. You want to try to read it more like you would read a graduated cylinder. Like a rain gauge where you eyeball it level and read the level of the bottom of the water. That could be giving inflated results.

**Cribbs74** Mon Feb 11, 2013 3:13 pm

John,

An unscientific method employed by me recently was to remove gaskets until my RPM numbers peaked. That way I could fine tune my CR in regards to engine performance. I ran two with a Bernie/Matt head on my unlimited. I never even considered plug life being a factor. What % nitro are you using?

I use SIG 25% on all my engines and to date have never blown a plug. Maybe I need to try harder! Very Happy

ian1954 Mon Feb 11, 2013 3:36 pm

A good compression ratio for a diesel is around 18:1, at 18:1 the fuel will definitely go bang as required.

A glow engine is slightly different and should be between 7.5:1 and 10:1 depending on the nitro content.

A higher compression ratio advances ignition.

That is my two penny worth!

John Goddard Mon Feb 11, 2013 3:50 pm

Rusty yep, I placed a sheet of perspex with a hole in for syringe needle over the heads so no surface tension.

Ron 30% but as above 13.6/1 cr Shocked

I just went up from the last of my 20% which ran out last week so perhaps that's what pushed it over the edge?

I guess Cox put 3 gaskets in each TeeDee box cos
TD with,- 1 gasket = 10.23/1
2 gaskets = 9.41/1
3 gaskets = 8.72

The above is fairly close rather than bang on cos measuring the deck height
showed 3 differing amounts (those dang ball and sockets) on three new
engines. I settled on .22mm

Laughing

WingingIt74 Mon Feb 11, 2013 4:08 pm

Did you calculate the glow wire area too? Also, the area of the coiled glow wire compared to a "W" wire may make a difference too Very Happy

John Goddard Mon Feb 11, 2013 4:33 pm

WingingIt74 wrote:Did you calculate the glow wire area too? Also, the area of the coiled glow wire compared to a "W" wire may make a difference too

Sadly
Yes

**Cribbs74** Mon Feb 11, 2013 4:36 pm

John,

Something else to consider is the wear of your cylinder/piston fit. You may not have 13.1 CR if fuel/air is leaking past the piston. I think it might be best to set up your CR based on the individual engine itself. Of course it is good to know what the CR would be in a perfect world.

Ron

gcb Mon Feb 11, 2013 5:31 pm

What has not been addressed is that the compression ratio needs to be adjusted depending on a combination of nitro content and prop load.

Some first pick the best prop for the airframe being flown, the nitro desired, then adjust head shims for best performance. Some experimentation with those factors will yield peak performance.

Fortunately for us sport flyers, ballpark results are all that is required.

Note that this is only one opinion.

George

Edit: Forgot to mention that combustion chamber shape (squish band, etc.) is also important so volume does not tell the whole story.

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