xTHExSA1NTx

How can you tell if your nitro engine has good compression? Also, can you tear down a nitro engine and use wd-40 to clean it with?

ThunderbirdJunkie

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ThunderbirdJunkie uses nonchlorinated brake cleaner, and reassembles everything using ATF as assembly lube.

tommygun32

Hold engine in left hand and turn fly-wheel with right hand (or vice versa). Does it get harder to push the piston through its top stroke? If so, you have good compression. But having said that, if it's easy to turn the piston through top stroke...it doesn't necessarily mean you don't have compression.

How old is the engine and is it still running good?

proanti1

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Its hard explaining how compression should feel through the internet... but it should have a nice pop, and not feel too different than a new engine. One of the easiest tests is to turn the engine over to TDC by hand and hold it there for a few seconds. Let go and the engine should pop over at least 90 degrees in either direction. Now this only works if the cylinder is lubed up. If the engine sticks... then you have a nice pinch and therefore good compression. If the engine just sits at TDC, and seems to "leak" is compression, then it may be time for a change.

378

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Best way to tell is to examine how it runs. These will manifest in any piston engine, not just our tiny ones, but because it's difficult for us to accurately measure the compression this is the easiest method to use. I also advise before using this list to ensure everything else is in good order. No air leaks, fresh plug, fresh fuel, carb well tuned, all bolts tight, etc etc. A bad glow plug, for example, will manifest as phase two.

First thing to go when an engine starts to wear out is the WOT high-RPM performance coupled with an overall drop in power. It still starts fine, it still runs fine, it doesn't cut out, it doesn't miss often, but it tends to lack power and doesn't want to rev anymore. My full scale is just getting into this stage at 275K miles.

The next thing to go is ease of starting. It will soon become a PITA to start and spin unusually fast while cranking. Once running it tends to run okay, but with severe power loss, and it may start misfiring at full throttle. RC engines tend to stall entirely due to a lack of rotational inertia.

Third stage is the idle goes to hell. Two strokes get to this stage a ton quicker than four strokes due to the blowby pushing fresh fuel back out before it gets burned and/or diluting it with spent gases. Four strokes will still run at this stage but they're all but useless, producing so little power that they can barely be kept running at all. Most engines never get past this stage, their owners either overhauling or replacing them. But if one perseveres, if one cares to force the issue long enough...

Fourth stage it simply won't run anymore. There's not enough compression to keep it turning over. It might be able to make idle speed with half throttle, you might be able to coax life out of it with copious amounts of fuel, but by and large it's done. The engine no longer produces enough power to stay running, let alone do anything. Time to rebuild it, brah.

DaveG55

Sorry, that is wrong.
Compression and pinch are two different things.
You can have excelent pinch and crappy compression.
But
You cannot have good compression without good pinch.

A knicked piston or scratched cylinder wall will still allow excelent pinch but your compression will be crap. I know this from personal experience - wanna ask me why I zip tie air filters on???

A simply worn engine will lose pinch and never be able to develop compression - except maybe while cold. On our little engines and even small gasolene powered two strokes, you may be able to get it started ok but as soon as it warms up you will lose power and the engine will probably not stay running. But I think 378 covered that pretty well above.

Argess

You make a lot of good points Dave, but I disagree with the one I quoted you on. Eventually, I seem to lose Pinch, as tested by turning by hand with the glowplug out, yet after installing the glowplug, the compression feels fine and the engine runs great. And continues to last gallon after gallon, long after that Pinch can no longer be felt.

I don't have one of these as I don't think I need it, but it might be helpful for those who want quantitative results:

http://www.dynamiterc.com/Products/D...ProdID=DYN2514

DaveG55

Just because you cannot "feel" the pinch does not mean you do not have enough for it to run. But you really cannot have compression w/o pinch. Pinch is what seals the combustion chamber in place of the rings our engines do not have. Without that seal compression suffers and so does engine performance. And of course proper break-in and tuning are huge determining factors in how the engine wears and how long it lasts.

A more definitive way to tell if the piston/sleeve is worn is to pull the piston and sleeve from the engine and see how far into the sleeve you can push the piston. A newer engine will stop 1/8" or so from the top of the sleeve. The closer the top of the piston gets to the top of the sleeve, the more worn the engine. Once the piston goes past the top of the sleeve the engine is pretty much ready for a rebuild.

But you probably know all of that already...