Chainsaw Repair

How To Basics - Carb Fixes + Mods - IPL and Service Manuals => How To Basics and Fixes => Topic started by: 1manband on April 04, 2015, 09:51:17 pm


Title: trapped compression ratio vs. 'regular' comp
Post by: 1manband 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

Title: Re: trapped compression ratio vs. 'regular' comp
Post by: aclarke on April 04, 2015, 11:06:08 pm
That's interesting .thk
Title: Re: trapped compression ratio vs. 'regular' comp
Post by: 1manband 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.
Title: Re: trapped compression ratio vs. 'regular' comp
Post by: 1manband 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.

Title: Re: trapped compression ratio vs. 'regular' comp
Post by: 1manband 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.
Title: Re: trapped compression ratio vs. 'regular' comp
Post by: Cut4fun . 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
Title: Re: trapped compression ratio vs. 'regular' comp
Post by: 1manband 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.
Title: Re: trapped compression ratio vs. 'regular' comp
Post by: 1manband on April 12, 2015, 07:55:38 pm
more
Title: Re: trapped compression ratio vs. 'regular' comp
Post by: mdavlee . on April 12, 2015, 08:11:43 pm
Cool stuff.
Title: Re: trapped compression ratio vs. 'regular' comp
Post by: 1manband 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












Title: Re: trapped compression ratio vs. 'regular' comp
Post by: aclarke on April 12, 2015, 10:40:16 pm
Great thread.  Thk!
Title: Re: trapped compression ratio vs. 'regular' comp
Post by: 1manband on April 17, 2015, 06:38:11 am
the homemade silly putty recipe can be found with a search.  the one i ended up using was 2 spoonfuls of borax; dissolved in a couple of cups of water; then drizzle in the white glue while stirring with a wooden popsickle stick.

-the putty seems to stick a little more on carbon deposits than bare metal, but the stickiness is nothing to worry about.  it is easily removed with no effort with a pencil point.

-joe