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Engine basics => Engine set up tips => Topic started by: Bob Reeves on October 02, 2006, 04:43:00 PM
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How would one go about measuring the compression ratio of a model engine???
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This has been argued about on the RC Universe engine forum. What I would do is run the piston up to TDC and drip rislone or similar in from a finely graduated syringe. Fill it up to the bottom of the glow plug hole. Figure the volume of oil and add that to the bore X stroke volume and divide same into the volume of oil. I think this would give you what is called mechanical compression ratio. Some say you should measure the cylinder volume from the top of the exhaust port. I have never done this, so it is all theoretical. ::)
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Jim's method gives the "Geometric Compression Ratio". Another method that we used for our two stroke motocross bikes was the "Corrected" or "Trapped" Compression Ratio.
This method was favored by the Japanese bike manufacturers based on the idea that no compression starts until the exhaust port is closed so the uncompressdd volume is calculated using the cylinder volume just at the point of the exhaust port being covered.
I would think that the real condition or compression of the engine measured with a compression guage would be a more meaningful number.
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I borrowed a compression gauge from Tom Martin (It came out of GMA's estate) but wasn't sure it would really give me the info I was after.. Trying to compair heads and head shims on a couple different engines to come up with meaningful numbers.
Lets say for example I have a stock ST 46 that runs great on 10% nitro and I have a hemi ST 46 that also has a plasma P&L. I would like to add or deduct enough head shims on the hemi engine so the compression is the same and see if it will run on 10% nitro as well as the stock engine..
I suspect the compression gauge will give me a different reading between the two engines even if the compression ratio was the same due to loss of compression past the ring.
Is this making any sense or am I out in left field someplace..
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Bob, you're probably right about variables particularly the seal of the piston to cylinder. You might get closer if you use a high viscosity oil to form a seal and lube the engine and then use a starter to spin the motor while measuring the cylinder pressure.
The leak down rate has less effect if you spin the motor while reading the pressure.
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Thanks Dick & Jim.. This is a new ball game for me and am trying to understand it better than just... Oh guess I'll try 2 head shims and see what happens :D
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"Compression ratio" And measured compression are different.
Compression ratio is the volume of the cylinder head opposed to the volume of they cylinder total volume. Divide on into the other.
Cranking compression goes up with port timing and cranking speed.
Static compression is measured with a gage.
To measure compression ratio you will need to CC ( with a beret ) the head and cyl volume.
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This has been argued about on the RC Universe engine forum. What I would do is run the piston up to TDC and drip rislone or similar in from a finely graduated syringe. Fill it up to the bottom of the glow plug hole. Figure the volume of oil and add that to the bore X stroke volume and divide same into the volume of oil. I think this would give you what is called mechanical compression ratio. Some say you should measure the cylinder volume from the top of the exhaust port. I have never done this, so it is all theoretical. ::)
I did this with an engine but a little differently.
1. Move piston up 'til exhaust just closes.
2. Fill cylinder with air tool oil.
3. Pour oil into measuring container. (I used a graduated test tube)
4. Record amount.
5. Position the Piston to TDC.
6. Pour oil from container back into engine.
7. Record amount left in container.
8. Find the difference between both measurements.
9. The ratio can be determined from the results of both measurements.
10. Now clean up the mess.
Of course all of this is meaningless unless you know the compression ratio of the engine is supposed to be in the first place.
How 'bout piping in here Randy? ???
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I did this with an engine but a little differently.
1. Move piston up 'til exhaust just closes.
2. Fill cylinder with air tool oil.
3. Pour oil into measuring container. (I used a graduated test tube)
4. Record amount.
5. Position the Piston to TDC.
6. Pour oil from container back into engine.
7. Record amount left in container.
8. Find the difference between both measurements.
9. The ratio can be determined from the results of both measurements.
10. Now clean up the mess.
Of course all of this is meaningless unless you know the compression ratio of the engine is supposed to be in the first place.
How 'bout piping in here Randy? ???
Hello BOb HI Tom
I have found the only useful measurement for me is basicly the way Tom stated, Just sealing the ports with the piston and very quickly using oil to fill the volume. I use a syringe type glue gun to do this .You will need to use a thicker rather than a thinner oil if using a ringed motor.
Bob what are you trying to accomplish ? If it is setting teh engines the same, you can do this by measuring the deck height of both and using the same head, use gaskets to set each identical to the other. If this is the case I can tell you that normally the Lapped fit AAC, ABC, Anodized ALUM. will act like it has higher comp. ratio, and normally uses .003 to.005 more gaskets for the motor to "act" the same
Regards
Randy
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Hi Randy and thanks again...
Among other things that have to do with different heads for Stalker 61's...
I have an unknown hemi head for a ST 46, two engines with ceramic p&l's, and a couple stock engines. Just trying to see if I can make some sense and learn a little out of what I have discovered trying to run these engines on various levels of nitro.
You have given me the one main piece of info I was trying to come up with(.003-.005). With the other info in this thread I think I have a handle on how it needs to be done..
Thanks gang..