cylinder compression question

[blockmanjohn]

Hi, I am building a 1/4 scale Galloway hit or miss engine. I have the piston and the head installed and I am trying to get an idea if the compression will be adequate. The piston is 1.3 inches with two cast iron rings.

Here's what happens with my crude test. When I turn the flywheel slowly I can here air escaping from the back of the piston and there is little compression felt. However, when I flip the flywheel over quickly there is a great deal of compression to the point where it is difficult to get it over the top. I hear no escaping air either.

Does this sound normal? This is my first IC engine so I have nothing to compare it to. The piston has .002 clearance and the rings are a good fit. I honed the cylinder with a 320 grit flex-hone. The kind with the abrasive balls on it. The finish is very smooth, and I am wondering if it may be too smooth so as to not allow the rings to seat.

Again, this is my first try and any remarks or advice would be greatly appreciated. Thanks, John.

 

[Brian Rupnow]

That sounds good to me. If you get "bounce back" from good compression when you flip the flywheel by hand, that will be good enough to run your engine.--Brian

 

[BaronJ]

Agreed !

 

[mcostello]

Did any one ever measure the compression in PSI to give others an idea of how close They are to aid in troubleshooting a nonrunner?

 

[gbritnell]

Compression whether it's on a full size engine or a miniature needs to be enough to burn the fuel once ignited. That being said I have run lawnmower engines, weed eaters etc. with as low as 75 lbs and they still run. Ideally and engine should be anywhere from 90 t0 130 lbs. I have made up a miniature compression gauge for my small engines but I don't know how accurate it is. On a full sized compression gauge you want a check valve right at or near the tip of the fitting or probe otherwise you're adding the volume of the tube to the combustion chamber. I tried to replicate this in my miniature one but like I said I don't know how accurate it is. You shouldn't be able to hear air escaping from around the piston if you turn it over slowly but if you flip the flywheel like when trying to start the engine and it kicks back then generally the compression is good enough to start and run the engine.
gbritnell

 

[blockmanjohn]

So if I turn it over to TDC should it hold the compression for a certain amount of time?

 

[XD351]

I use a leak down tester for this because it give a percentage of leakage .

 

[Brian Rupnow]

It would be surprising if a brand new engine held compression for any measurable amount of time. On a brand new engine, the valves always leak a little bit, and the rings will pass a little bit of compression because they are not fully seated yet, and they do have a gap in order to get them on the piston. After an engine has ran for an hour or two, and everything has seated properly, the compression will improve, but may or may not hold measurable compression at top dead center.

 

[blockmanjohn]

Thanks for all of the information. You have all been a great help. As an after thought, I made the valves for this engine out of 12L14. Could I have made a better choice? It would be easy enough to remake them. Thanks again, John.

 

[Brian Rupnow]

Run it and see John. I make my valves from 1018, one piece. others use drill rod for the stems with 1018 heads soldered on. Others swear by using only stainless for their valves. Suck it and see.---Brian

 

[WOB]

You say the rings are a "good fit". How good is "good"? A good test is to insert a bare ring in the cylinder and hold it up to a bright light. If you can see any light leaking between the ring OD and the cylinder wall, the rings are defective. Some light will come thru the ring gap and that's OK. If you do see light, make sure the ring OD is burr free. Otherwise, break-in will take forever. The ring gap should be no more than .005". Flex-honed with 320 grit is a proper finish for the cylinder wall.

WOB

 

[chris]

I agree with Brian you do have a good fit if the flywheel bounces back when given a quick flip.

 

[dkwflight]

I have heard of model engines that were difficult to start because of too much compression.
Keep it in mind.
Good luck
Dennis

 

[WOB]

I have heard of model engines that were difficult to start because of too much compression.
Keep it in mind.
Good luck
Dennis

It can definitely happen on home built glow-plug engines. I upscaled a Colin Jones .605 from .965" bore to 1" bore and did not pay attention to the combustion chamber size leaving it approx. as originally designed. Could not get it to start even with an electric drill. It would stall and slip the rubber cup drive on the prop spinner After exhausting all the other possible solutions, I finally understood that it was firing early due to too much compression. Essentially it was suffering from severe pre-ignition. Calculation showed that the compression ratio was over 12:1 and should have been approx. 8:1. A few thou. enlargement of the combustion chamber did the rick and it immediately sprang to life running strong (and loud).

BTW, the Jones .605 is a perfect engine for the more advanced builder: http://modelenginenews.altervista.org/mecd/3584.img.html#521 There is also a twin cylinder version: http://modelenginenews.altervista.org/mecd/3634.img.html#674

I modified my version to 1" bore and rear-rotary disc valve as I was always fascinated by rear rotary-valve engines. Worked out great.

WOB

 

[popnrattle]

Consider using rubber O-rings for compression. Shown is 1-1/4 bore in the hit-n-miss engines I build. Later, RT.

 

[kenr]

It can definitely happen on home built glow-plug engines. I upscaled a Colin Jones .605 from .965" bore to 1" bore and did not pay attention to the combustion chamber size leaving it approx. as originally designed. Could not get it to start even with an electric drill. It would stall and slip the rubber cup drive on the prop spinner After exhausting all the other possible solutions, I finally understood that it was firing early due to too much compression. Essentially it was suffering from severe pre-ignition. Calculation showed that the compression ratio was over 12:1 and should have been approx. 8:1. A few thou. enlargement of the combustion chamber did the rick and it immediately sprang to life running strong (and loud).

BTW, the Jones .605 is a perfect engine for the more advanced builder: ME Vol 144 Issue 3584 Index There is also a twin cylinder version: ME Vol 146 Issue 3634 Index

I modified my version to 1" bore and rear-rotary disc valve as I was always fascinated by rear rotary-valve engines. Worked out great.

WOB

It looks to me as if the .605 article at MEN is missing the last (3rd) installment that would have been in issue 3586.

Did you build other variations of the .605 single?

Did it run well.

I'm thinking of building a bar stock version and I wonder if you might offer me some advice on PM?

 

[stevehuckss396]

Don't over think it. You have bounce back so you have compression. Once the engine has a few hours on it it will only get better.

 

[Bentwings]

It looks to me as if the .605 article at MEN is missing the last (3rd) installment that would have been in issue 3586.

Did you build other variations of the .605 single?

Did it run well.

I'm thinking of building a bar stock version and I wonder if you might offer me some advice on PM?

It might have fun on alcohol and 5% oil . We ran our supercharged hemi at about 15:1 with 30 psi boost. It started easily made about 1600 hp according to calculators of the day we ran it out of fuel one night it nicked a piston broke second ring . We didn’t have a spare so we put it back together and ran 4 more runs with no damage .

 

[mayhugh1]

As an interesting aside I recently watched a Youtube video of a guy trouble shooting a misfire in a particular cylinder in a new (full-size) engine. He was pretty experienced and suspected a compression problem in just that cylinder. The engine sounded normal and the battery current draw looked normal when he cranked it in flood mode, and his compression gauge showed similar cranking pressures among all cylinders (155 psi). He was still sure it was a compression problem in that cylinder, and his next test was more enlightening. He screwed an electronic pressure gage into that cylinder (no plug wire), attached it to a recording oscilloscope, and then started the engine. When he watched the pressure profile in that cylinder it was obvious that a valve was hanging up while the engine was running although not when cranking and causing the misfire. He compared this recording with one from a known good cylinder (with the plug wire removed, of course). What I found remarkable was how low the compression pressures are in a cylinder in a running engine i.e. in his case 50 psi running vs 155 psi while cranking.

The take-away from this is that in a compression test the cylinders are always sucking air through the intake valve and leaking it out through the ring gaps even in a perfectly built NASCAR engine. The pressure reading is going to depend on the crankshaft speed. At very low cranking speeds the cylinder has all the time it needs to draw all the air it can through the intake, but that air also has a long time to leak out past the rings. When running, it doesn't have as much time to leak air out past the rings but neither does it have as much time to suck in all the air it can.

If enough are interested, I can go back and search for the video when I find time. - Terry