Compression vs. Horsepower

[Big Woody]

My cat jumped up on me and woke me up last night. Since there aren't any sheep around here I had to start counting my chainsaws. When I got to my 3120 and 090 the following question came to mind.

Since a new saw has a compression of 160+ psi (full power) and around 100 psi (no power) seems to be the bottom end for a barely running saw, what would the power be at 130 psi. I bought both used and both test at 130 psi.

I suspect the power stays pretty good initially then drops rapidly with compression. Has anyone dyno'ed used saws to know how the power drops with compression? I really want to find out at 130 psi what percentage of original power is left in these big saws?

Any expert opinions out there?

 

[Stumper]

Woody, I'm not sure in chainsaws-It always seemed to me that power was good right up to the point that it suddenly was not good at all an dthe saw would barely run. In 4 cycles many engine makers used to insist that normat compression was simply within x% on all cylinders. Porsche used to say that above 90 and even on all was fine.

 

[ErrolC]

I can't think off the top of my head what the actual expected figures should be that you would be looking for...but you can find the table in a book called "two stroke performance tuning" by A G Bell (thats right A G Bell)..he also wrote "four stroke performance tuning..good reading if you're into hotrodding anything, principles are the same..anyway I digress. "performance" two strokes can only achieve a peak performance compression ratio of around 10:1..you can bump it up higher, but you won't get performance gains because.....the stroke to bore ratio gets to long..the piston speed gets higher and you get seizure tendancies etc, or the size of the ports has to become smaller and the engines breathing becomes more restricted..unlike 4 strokes..2 strokes have holes cut in the cylinders, so compression only begins from the top of the highest hole/port till TDC (top dead center)..if you messure the volume of air in the space from the port top to TDC..then messure the volume of the combustion chamber, thats your compression ratio..10:1 x 14.7 (normal air pressure) = 147lbs idea with perfect rings..knock off about 10lbs and thats what you should expect with good rings..to see how good the rings are, messure the compression, then squirt some SAE30 oil in the bore with the plug out and see what the difference is..some engine designs/manufacture are better than others, but about 10psi is healthy..better is a bonus....I've seen a difference over 20psi between two Husk 162s same age near new as a result of carefull running in by one of them..ie good warm up periods and gradual increase of workload for the 1st hour or two when new etc

 

[sawjo]

I just rebuilt an 028 Super a friend had given me. New rings, drive side bearing and general clean up. Finished last night and it was too late to fire her up so I put few drops of 2cyc oil in the spark plug hole and attached my compression tester - 175 psi
after 4 pulls!. I think this thing is going to rock!

 

[bwalker]

ratio of around 10:1..you can bump it up higher, but you won't get performance gains because.....the stroke to bore ratio gets to long..the piston speed gets higher and you get seizure tendancies etc, or the size of the

Compression can be raised above 10:1 in a two stroke very easily by simply making a new head. This doesnt change the bore, stroke, piston speed or any other parameters you mentioned. Seizure tendencies would not increase provided the fuel was matched to the compression ratio.

 

[ErrolC]

10:1..bwalker is correct...but the gains with 2 strokes becomes signifcantly reduced and raises more issues to resolve in getting reliable returns..combustion chamber shapes and size as a percentage of the bore affect the way combustion occures..there are issues concerning the octane diluting properties oil mix has on fuel..inefficiencies in scavenging exhaust gases from the chamber (which mix with the fresh intake charge which also dilutes the fuels octane rating)..unless you use a properly tuned expansion chamber/pipe on your saw..these things all affect its design parameters..and its power delivery in narrow or broad power bands..I mean.. we started talking about compression & comp ratios...there is a whole lot more to it than this...there are good books on the stuff..read some..then go out and practice blowing a few motors up..the performance mufflers I read in the forum all open the exhaust exits up..including the noise I would suspect..what is trying to be achieved with this..some careful porting will return much better HP without making your saw unreasonably loud..I remember the like of XL12s and the old Macs..you could here them for MILEs, who wants that!

 

[bwalker]

10:1..bwalker is correct...but the gains with 2 strokes becomes signifcantly reduced and raises more issues to resolve in getting reliable returns..combustion chamber shapes and size as a percentage of the bore affect the way combustion occures..there are issues concerning the octane diluting properties oil mix has on fuel.

These issues where problamatic in the sixties, but not so much any more. The fuel dilluting octane thing isnt factual in every case. Ever wonder why most racing two strokes like karts and gp bikes run ultra rich ratios in the neighborhood of 16:1?

 

[ErrolC]

16:1 is leaner..lean mixtures burn hotter..& slower..& can contribute to detonation...but more importantly lower emissions from free oxygen in the exhaust gas..which can also have an adverse effect on rings & plugs from oxydising...richer mixtures run cooler through evaporation in the chamber..also burn slower and avoids detonation..fouls plugs and creates carbon build up..depending on engine design characteristics.

To get an idea of Combustion chamber shape & effects http://www.circletrack.com/techarticles/99078/..while they talk about 4strokes...the same principles apply to 2strokes..all/every engine to be modified carries its own merrits and specific changes to one engine quite likely will not apply the same to another..in 2 strokes the placement, shape and direction/angle of the charge ports will change the place & shape of the ideal combustion chamber in the head..a bit of theory, & a lot of trial..its what makes tuners expert in "their" brand or field

Quote:Combustion flame travel through a gasoline and air mixture will be fastest within a fairly narrow mixture range from 11.5:1 to 13.5:1. Flame speed will decrease rapidly outside of this range. This varies a bit from one fuel to the next, but the above stated range is a generally accepted. http://www.yawpower.com/jultech.html

It has to be agreed, particularly with small engine, no 2 are created equal..some are better than others..due to small variations (in castings and tollerances etc)..hence some engines may respond to different mixture ratios than others..nothing is set hard and fast, there is still a LOT of art in engine modifying however there are sound general rules in getting there..the principles and effects of mixture dilutions from residule exh gas, & in 2strokes - oil mix..are very real & significant today as ever..in getting good (and reliable) performance...read the books, search the web, find the competition winners & speak to them (its all out there) then put it into practice and find out for yourself what happens..the experts still bow engines up (to their own surprise & frustrations)

 

[bwalker]

16:1 is leaner..lean mixtures burn hotter..& slower..& can contribute to detonation...but more importantly lower emissions from free oxygen in the exhaust gas..which can also have an adverse effect on rings & plugs from oxydising...richer mixtures run cooler through evaporation in the chamber..also burn slower and avoids detonation..fouls plugs and creates carbon build up..depending on engine design characteristics.

The above isnt even close to being factually true. The oil concentration does not make a differance in the air fuel ratio when the engine is tuned properly for conditions. Of course you cant just change oil ratio and nothing else and expect perfect results. You also cant change oil brand, gasoline brand, or even expect the motor to run at peak hp when the humidity, barometric pressure or temp changes. As far as "richer mixtures" burning cooler. When jetted properly the differance is minimal if any dependant on oil brand. The best oils are typicaly ones that do not inhibit combustion and as such have no effect on the such things. I have ran certain engines on a variety of oils in the 20:1 range which where outfitted with digital cylinder head and egt probes. One jetted properly the emps where virtually identical.

To get an idea of Combustion chamber shape & effects http://www.circletrack.com/techarticles/99078/..while they talk about 4strokes...the same principles apply to 2strokes..all/every engine to be modified carries its own merrits and specific changes to one engine quite likely will not apply the same to another..in 2 strokes the placement, shape and direction/angle of the charge ports will change the place & shape of the ideal combustion chamber in the head..a bit of theory, & a lot of trial..its what makes tuners expert in "their" brand or field

I agree with the above except for the insiiinuation that there is a lot of trial and error involved. trial and error only comes into play with the guys that dont know what their doing. With todays simulation software and knowledge of head design parameters trial and error is a rarity.

 

[028AV Super]

Hey SAWJO,
Can you let me know dtailed info on your rebuild? Such as machine work, parts cost, etc.? My 028 Super will need a rebuild after this season. By the way, what was the compression reading prior to rebuild?