I have a Hobbico compression gauge and I'm wondering what kinds of readings I should be getting on a .46 size engine.

I realize that engines will show different readings, however they all should roughly be in the same ball park. (.46FX)

What is the general area I should expect to see?

Also, does anyone have experience with the .12 and .15 size engines pertaining to compression.
I have 2 engines that I'm seeing 30psi on and cant tell if something is wrong with the gauge or how I am turning over the motor.
I'm using a starter box to test them and they both hit around 30psi.

Any thoughts ideas on using a Hobbico compression gauge would be appreciated

Dbow

Dbow, I'm not too sure what reading you should get, but I do know that when using a compression gauge the throttle should be wide open. Our engines are air pumps and without having the throttle open, it can't get any air to pump to the gauge! I have seen lots of people get low readings and were wondering why their engine was so low on compression, when the only problem was the the engine wasn't getting any air. This goes for cars, motorcycles, glow or gas engines...anything!
I wouldn't worry to much about what it reads either, it's more to keep tract of how your engine is doing. Sort of like cycling batterys, you keep tract of the discharge time and when it decreases, you know you have a problem. Same thing with the gauge. I would also tell you to use an electric starter to spin the engine over faster to get a more reliable reading. (like it's running). I hope this info helps.
Suptter

Like Suptter say's

Record your readings and keep a log of them for each engine. Always do it the same way like with the starter and with the throttle wide open.

When you start getting lessor readings, then you know something is happening.

When you read in an automobile repair book about compression readings, they don't always mention what pressure you should be getting. They will just say that all the readings should be within 10 lbs of each other.

Enjoy your gauge,

Jim

I have the 0-160 version, and have tested with a 46fx. I don't know if I had the throttle open. I only given it one quick try so far.

If I flip the prop by hand 15-20 times I get about 40 psi, as I recall.

If I give it a blip with the starter, I get 110 psi.

That is a much bigger difference than I expected, I don't quite understand it, and dont' know which end of the spectrum is accurate.

In another thread there were reports of 80-90 psi as a good number, so I really don't know what to conclude.

I think you need to develop your own technique an stick with it. Then use the number for comparison only, as mentioned by wy8e.

Actually, the Bentley manuals for Audi, VW, and BMWs, for example, list the cylinder compression specifications. I remember testing the compression on my '89 VW GTI and gotten very close to the specs.

However, many things could produce false compression reading. Having too much carbon deposits, for instance, could raise the effective compression ratio such that you get a high reading on the compression test, but in actual running the engine EFI controller would retard the timing to compensate for the higher CR to prevent detonation. The result being substantially reduced power.

Then there are the typical difference between static compression and dynamic compression. Air is a low viscocity fluid, but it does have a viscocity. This means that at higher RPMs the combustion gasses cannot escape through the tiny leak around the piston rings as easily as with the engine running at low RPMs. This is why a tired old engine might have almost as much power at the top end, but just wouldn't idle worth a s**t.

Then there are those pesky dykes rings that rely on combustion pressure to achieve an effective ring seal. With those, you won't see much compression just spinning it with a starter.

Our RC engines are typically too small and not manufactured to high enough of a tolerance for the manufacturer to publish a factory spec, which can be used as a reference. Plus, if we are talking about ABC engine, forget it. Unless it is at operating temp, the compression gauge readings are just about meaningless.

But the values are still useful for comparison, no?

Okay.... I double checked.

I get 40 psi from my 46FX with the throttle open. I use about 20 quick hand flips.

The engine is at room temperature.

There is some after run oil in there sealing the piston/sleeve gap.

Incidentally, with the throttle closed, I see 36... indeed lower.

dbow, what number do you see? I am curious.

This particular engine was broken in too rich and at too low a speed. It runs fine, but I see less performance from it then many others report with the 46FX. So I'm interested an what your compression number is.

Jim, yes, the readings are good for comparison purposes, throughout the lifetime of the engine, as per suggestions above. Just be sure the readings are taken in the same manner under similar conditions.

Still, ABC engines need to tested at very near operating temp - 250~300F is a good start.

Jim, 20 quick hand flips are not enough. Your only spinning the engine over 1 to 2 times at best. All readings should be made at cranking speeds with the throttle open. A warm engine will give a better reading also because the clearances are closer to what a running engine would be.
Also, what about ringed engines? (your's isn't) Volfy is right about dykes rings, so it would have to be checked while spinning over at close to running speeds just to get a reading. What I'm saying is: If you check every engine the same way, you will get a more consistent readings from engine to engine.
Suptter

At 20 hand flips, the pressure stops increasing. So I settled on that.

But, I'd accept that spinning at cranking speeds gives a more accurate results. In which case, I'd say 110 psi is the answer for my 46fx - the result of spining with the starter (at room temp).

I would have to agree that the engine would produce more realistic results at running temps.
I also agree about the carb being open, which in my previous testing the carbs were closed. I didnt even think about opening them.
My current desire is to be able to uncover a defective engine based on compression. I have a new MT-12 that did not have the tight fit that my previous MT engines have shown. While it seems to run fine, it bothers me that the fit is not very tight.
I do also have a O.S .46fx that fails to prime and hold fuel in the carb. It has had about 3 gallons through it and some lean runs, I suspect the piston and sleeve to be worn out, it will soon be on its way to Hobby Services. I do have a new .46fx to compare it to but have not broken it in yet.

I am just in from work and about to sit down to some dinner, I will do some more testing later tonight and post my findings.

Thanks for all the info,

Dbow

I did some additional testing on this engine.
Flipping the prop by hand Im seeing 38psi, when I throw the starter on it Im seeing 58psi.

I want to go ahead and break in my new .46fx and test it but I wont be able to until later in the week.
It looks like the old .46fx is toast though, and I did subject it to a few lean runs when I changed props and did not re-tune.

My other .12's and .15's all tested the same with the carb open and so forth.
The .12 is going back because I have owned several MT-12's and they would still get stuck at the top of the sleeve with almost a gallon through them.
THis new MT pushes over with no problem and I only have a few tanks through it. The first tank I ran with needles set for blubbering did not go well, temps got up to 220 to 230 on idle, obviously something wrong with the engine.
Anyhow I described the problem to Mugen and they said to send it on in.

Thanks

Dbow

A compressin gauge for model engines. I'll have to look into that, I didn't know they made one.

What about a leakdown test? That is a common and useful diagnostic with larger engines, and I can see it's applicability with model engines.

--Bill

It might tell you something on a large 4stroke ringed RC engine. The leak down test is really geared more toward 4stroke engines with all the valves, valve seats, valve stem guides.

The absence of valves and the interference fit at TDC of the typical ABC 2stroke don't lend themselves well to this kind of testing.

That's right, I was thinking 4- stroke engines. Wouldn't tell anything about 2-cycle engines.

--Bill

Off topic, but:

What did you use to measure the temperature?

What temperature is normal, i.e. were you expecting?

Jim,

I use a Raytek temp gun which some have said is not very accurate.
I tested it against a Exergen (Known to be Exact) and it consistantly showed to be about 1 degree off...Not bad.

I was expecting to see between 160 and 180 at idle as seen on previous MT engine break ins.
My last MT engine broke in at about 180 degrees with about 80 to 85 degrees ambient temperature. The piston and sleeve fit was so tight that I had to heat it up with a hair dryer several times because it would get stuck during break in. I always made sure at the end of a tank that the piston was at the bottom of the sleeve, but when starting it would sometimes get stuck.

The new engine was running 230 on same break in needle settings and around 50 degrees ambient temp. I did also notice that the piston and sleeve fit was not very tight.
It has about 15 tanks through it now and I can flop it over with my fingers, could never do that to any of my previous Nova Rossi engines.

I did go through all the normal troubleshooting when I observed that the engine was running hot. I installed brand new fuel lines, new tank, checked needles, and checked that everything was sealed and tight.
I am sending this one back to Mugen, hopefully they will replace the piston and sleeve or just send me a new engine.
I will update everyone on how it goes.

Dbow

Shouldn't the pressure be relative to the compression ratio? With gasses, if you 1/2 the volume, you double the pressure. I would think that different sized engines with the same CR, would have similar readings?