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compression ratio

Old 05-11-2013, 08:04 PM
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rcbence
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Default compression ratio

what do I need to measure for a glow engine. Head cc..... deck clearance? How is it done on a glow engine?
Old 05-13-2013, 08:06 AM
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controlliner
 
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Default RE: compression ratio

Easy, a good squish clearance is .019-.020 in. This will accomodate most nitro % from 20% on to the higher in the normal range for r/c cars. If you burn out plugs despite this, you are running too lean on the high end. Take the glow plug out and do this, take a length of solder and bend it to an L shape. the short part of the L should be about 3/8ths long. Place the short L into the glow plug hole and touch the end of the L against the liner. hold this as you turn the flywheel by hand so the piston passes top dead center. Take out the solder piece and measure the squished end. This will be your deck height and you can add or remove shims as required.
Old 05-13-2013, 09:01 AM
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Anthoop
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Default RE: compression ratio


ORIGINAL: controlliner

Easy, a good squish clearance is .019-.020 in. This will accomodate most nitro % from 20% on to the higher in the normal range for r/c cars. If you burn out plugs despite this, you are running too lean on the high end. Take the glow plug out and do this, take a length of solder and bend it to an L shape. the short part of the L should be about 3/8ths long. Place the short L into the glow plug hole and touch the end of the L against the liner. hold this as you turn the flywheel by hand so the piston passes top dead center. Take out the solder piece and measure the squished end. This will be your deck height and you can add or remove shims as required.
That is not what he asked...but it may be his question...
To answer the question- Usually you would be wanting to find the trapped compression ratio...so measure the volume in the cylinder when the exhaust port is closed and then measure the volume when the piston is at TDC.
Old 05-13-2013, 06:37 PM
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Default RE: compression ratio

Effective or Trapped compression is like Anthoop said. I measure the swept volume of the cylinder separate from the head volume. The swept volume and squish volume can be determined mathematically, the head volume is a little harder. I use a 1/2cc or 1cc syringe and fill the combustion chamber with a liquid that has little to no surface tension.

I would imagine you won't find higher than 9:1 effective compression ratio in the average engine.
Old 05-15-2013, 06:59 AM
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Default RE: compression ratio

There are two ways to work out compression ratios, one is called geometric which uses the full swept volume from BDC (the actual size of the engine) and the other is trapped which uses the swept volume from the point the exhaust closes. Neither method is accurate so it's a matter of choosing one and staying with it. As a general guide, trapped compression will be ~2/3 of geometric, depending on exhaust port timing. Most standard engines will have a compression of around 9:1 measured geometrically so about 6:1 measured as trapped.

Squish clearance is the closest the piston gets to the cylinder head and a minimum figure is around .005" although I prefer a little more than that at .008"-.010" to be on the safe side. Solder isn't a good way to measure this clearance because most engines have a tapered squish band so it's near impossible to get an accurate reading by squashing the solder. You need a digital caliper, which can be used as a depth guage, to measure deck height and cylinder head projection into the liner. The difference between the two is the squish clearance.
Old 06-06-2013, 05:04 AM
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Default RE: compression ratio



Squish clearance is the closest the piston gets to the cylinder head and a minimum figure is around .005'' although I prefer a little more than that at .008''-.010'' to be on the safe side. Solder isn't a good way to measure this clearance because most engines have a tapered squish band so it's near impossible to get an accurate reading by squashing the solder. You need a digital caliper, which can be used as a depth guage, to measure deck height and cylinder head projection into the liner. The difference between the two is the squish clearance.


The Squish band taper on most heads ranges from 1-5 degrees. When the solder tip is resting against the liner, the lowest part of head protrusion is impressed into the solder. This was the way I've done it since the early 1970's with Veco .19's used for on road R/C cars, we had squish band heads from Dick McCoy racing products, they had the slot milled into the SB to accommodate the piston baffle. The solder is best measured with a vernier mic, it is way more accurate than a caliper due to the different "feel" between people measuring. I have compared measurements with "head removed measurements" and the difference was .0003. This can be attributed to con rod fit on the gudgeon pin and crank pin, oil on these journals-etc. hardly enough to make a difference for shimming using .001 shims for higher nitro fuel.

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