Author Topic: trapped compression ratio vs. 'regular' comp  (Read 190 times)

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Offline 1manband

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trapped compression ratio vs. 'regular' comp
« on: April 04, 2015, 09:51:17 pm »
made a little chart to help me choose a comp ratio for a jonsereds 52

interesting just how little a change in combustion chamber volume affects things.

-joe

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Offline aclarke

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Re: trapped compression ratio vs. 'regular' comp
« Reply #1 on: April 04, 2015, 11:06:08 pm »
That's interesting .thk

Offline 1manband

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Re: trapped compression ratio vs. 'regular' comp
« Reply #2 on: April 05, 2015, 09:00:11 am »
for this motor....just doing a gasket delete (0.5mm gasket)...changes the volume 0.76cc.

if i were to start from a 6cc combustion chamber volume.....

the trapped comp goes up from 7.3 to 8.3

the regular comp goes up from 9.2 to 10.4

-need to get a smaller syringe that has 0.1cc graduations.  the one i have is 1.0cc graduations.  not good enough for this kind of thing.
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Offline 1manband

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Re: trapped compression ratio vs. 'regular' comp
« Reply #3 on: April 05, 2015, 09:11:23 am »


-the way i did this is approximate, because i filled the chamber and cylinder up to the exhaust port roof with liquid.......without using the piston at all.

-since the piston has a raised crown, thought it would give a similar result.  the difference from edge to center was 0.038 inch.

-next time will install the cylinder over a greased piston to see just how close i was.

-also will try using a plate.

photos to follow.

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Offline 1manband

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Re: trapped compression ratio vs. 'regular' comp
« Reply #4 on: April 05, 2015, 09:41:29 am »

-did all this to see just how small the chamber could be after machining....to be able to pick a safe comp ratio

i would not use low test gas based on what was found after deleting the gasket.  after carbon buildup, things would quickly get worse.  even without deletion of the gasket in a 6cc chamber.
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Offline Cut4fun .

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Re: trapped compression ratio vs. 'regular' comp
« Reply #5 on: April 05, 2015, 10:48:25 am »
Pretty interesting stuff.  Always wondered about the actual compression ratio like I was used to seeing in cars etc.  Is that what geometric  compression ratio is?

Dang way over my pea brain.  Helpppppp LOL  Figuring Compression Ratios   http://www.hydrostream.org/ArticleArchives/Compression.htm
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Offline 1manband

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Re: trapped compression ratio vs. 'regular' comp
« Reply #6 on: April 12, 2015, 07:53:44 pm »
somehow this changed into a how to.  i can't change the title at this point.

the first part is how you pick a chamber size/trapped ratio/regular ratio.......this part a an approximate way to measure.
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Offline 1manband

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Re: trapped compression ratio vs. 'regular' comp
« Reply #7 on: April 12, 2015, 07:55:38 pm »
more
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Offline mdavlee .

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Re: trapped compression ratio vs. 'regular' comp
« Reply #8 on: April 12, 2015, 08:11:43 pm »
Cool stuff.

Offline 1manband

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Re: trapped compression ratio vs. 'regular' comp
« Reply #9 on: April 12, 2015, 08:35:12 pm »
edit: added the chart above to this post.

in the chart above....i wanted to pick a safe 7.3:1 trapped comp ratio;  with combustion chamber size of 6cc.
(for you regular comp folks 9.2:1; 6cc)
those two comps are considered equal in this motor, same thing.

-after measuring the comb chamber volume.......without a gasket........came up with approximately 4cc.

looking at the chart, you can see that for a 4cc chamber......the trapped comp ratio would be 10.5:1 trapped comp......(13.3:1 regular).   pretty high!!!

i figured the gasket volume to be 0.7cc.

...so out the dealer door....the volume of the chamber was 4.7cc.   this equates to 9:1 trapped.....11.5:1 regular.  how this thing stayed together is beyond me.

imo, this would not be a good idea in a larger bore. 

this saw was made in the 1970's i believe.  when i got the saw......2nd ring was ****, locating pin broke off and embedded in the bore; top ring stuck in groove; piston gouged.

.......the putty mix was homemade.

hope it helps.

-joe












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