Part Four: Compression

[Mastermind]

One of the first mods I ever made on a power saw was increase compression to lessen the squish band clearance with a gasket delete. Once I learned how to clean up and save wiped cylinders that I used to pitch in the trash from @Mastermind , I was using a lot of different replacement pistons. Come to find that a lot of them have dimensional quirks in the pin height to piston crown measurement. Usually if not right on the money, a little bit taller. Next lesson learned was that all base gaskets are not the same thickness. So with a little mix and match could bring squish into target range and still have a base gasket. Basically what I like to do right now is when I know I have a build coming, buy all the different kind of base gaskets I can get my hands on and a couple different pistons. While this seems to be in excess, keeping the left overs in stock for the next time around isn't such a bad thing for me. This also raises the point for guys just starting out, measuring squish with the piston you are going to use is very important, do not try and mach up your build with an old piston, when the new one may be different. While this does not replace what the pro guys are doing with machine work, it is one way for the do it yourself types to get started and make their saw run better.

Excellent post Steve.

 

[Lee H]

Yep. Lowest I had was 14.7:1 if I remember right. Started fine but woukd white smoke like crazy in cold weather.

White smoke = unburnt fuel. With that low of compression i'm sure it did smoke.
I bet it cleaned up once you got some heat in the motor.

 

[sunfish]

I remember when pop-ups' were really talked down. I have ported saws with flat tops and pop ups, hard to tell any dif. Work saws.

 

[XP_Slinger]

The highest power diesel engines are around 12.5-13:1 with crazy boost. Too much compression and a smaller chamber can lose power

Very true true but what is that boost doing, raising cylinder pressure. Now I want a turbo charged Saw...lol

 

[merc_man]

Compression and too much timing advance is evil..

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So if you had higher compression what would it do if you tried retard the timeing rather then advancing it?

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[t4driller]

I think it would make the saw lazy to rev up and loose power. Sorta fighting against itself. If it fires to soon the flame front would be pushing down on the piston before it gets to tdc. That's the way I see it in my head.

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[merc_man]

I think it would make the saw lazy to rev up and loose power. Sorta fighting against itself. If it fires to soon the flame front would be pushing down on the piston before it gets to tdc. That's the way I see it in my head.

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So when you advance timing does it spark a little later.

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[t4driller]

I think my last statement I have it backwards. Advanced is degrees before tdc, retard is degrees after tdc. So advancing it would make it spark sooner... I think..

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[XP_Slinger]

I think my last statement I have it backwards. Advanced is degrees before tdc, retard is degrees after tdc. So advancing it would make it spark sooner... I think..

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You are correct. Advance fires the plug earlier before TDC and too much advance has the affect that you mentioned, piston fighting combustion. Retarding timing too much ignites the mix too late and the result is no mo powa

 

[t4driller]

You are correct. Advance fires the plug earlier before TDC and too much advance has the affect that you mentioned, piston fighting combustion. Regarding timing too much ignites the mix too late and the result is no mo powa

Thank you. I started typing before my brain was hitting on all cylinders. Lol

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[force10powertools]

e85 ???.... please expand on your meaning . As far as i know the "standard unleaded" fuel we have in the UK is 95 ron with up to 10 % bio fuel (ethanal ) in its content unless disclosed . There is 98 ron "Super unleaded" available as well. I use standard unleaded for all 2..and 4 stroke engines and work on a max 60 day age in 4 stroke and aprox 30 days in mixed 2 stroke

Evenin' Simondo.
Right then.....Most of Devon, Somerset and Cornwall has been a test bed for e85 fuel for a few years now. A pal of my dad runs a garage and petrol station and is hugely into his vintage Cadillacs (has a load of the big pink buggers) Anyway, he's one of only a couple of places i can think of that still sells genuine leaded 4 star petrol, contrary to common misconception, they diddnt ban it they just put the price up and taxed it to death. Monty keeps it on one pump, sells it to the classic car guys, does a roaring trade. Anyway, i got chatting to him about fuel one time and he shows me an article in a trade newsletter about what areas the government are using to test high ethanol content fuels and how high a percentage they could get away with, sure enough, most of the SW. Whereas the rest of the country gets 5% or "up to 10%" all of the major fuel suppliers are using a 15% ethanol content fuel. this was a couple of years back now and it may have changed. just about every pump in my neck of the woods has e85 marked on it though and the dates tally up with when i started seeing a rise in the number of carb kits, hoses and breathers i was fitting and how many stale fuel issues i was dealing with.
The 98ron super unleaded is still blended with ethanol, it just has a higher octane rating.
I've just taken on Aspen in the shop, as i'm getting sick of explaining to customers that fuel goes off. most of them understand but some just don't seem to believe that its going off from the time it was refined not the time since they poured it out of the jerry can into the strimmer.

hope this explains what i mean.

 

[mdavlee]

e85 ???.... please expand on your meaning . As far as i know the "standard unleaded" fuel we have in the UK is 95 ron with up to 10 % bio fuel (ethanal ) in its content unless disclosed . There is 98 ron "Super unleaded" available as well. I use standard unleaded for all 2..and 4 stroke engines and work on a max 60 day age in 4 stroke and aprox 30 days in mixed 2 stroke

E85 is 85% corn fuel. Need more volume but it will make more power if you can get enough in there.

 

[mdavlee]

White smoke = unburnt fuel. With that low of compression i'm sure it did smoke.
I bet it cleaned up once you got some heat in the motor.

It sure did. Actually run pretty clean once warm.

 

[XP_Slinger]

Thank you. I started typing before my brain was hitting on all cylinders. Lol

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No problem brother, I think all of us have those moments while immersed in these porting threads. My brain has over heated a few times since Randy started this series...lol

 

[Al Smith]

On my old 54 Harley 74 Fl which was a kicker if you let it idle very long with the spark on full retard the first 12 inches of the exhaust pipes would glow red because it was firing with the exhaust valve nearly open .If you tried to kick it over on full advance it would try to launch you .

 

[Al Smith]

On saw solid state ignitions I'm only sure of one that has an advance that being the SEM on the 042 and 048 Stihl .Others might but I'm not sure what they are .There isn't much info on that stuff for some reason .

 

[Al Smith]

oops last two posts in the wrong subject.

 

[Simondo]

Evenin' Simondo.
Right then.....Most of Devon, Somerset and Cornwall has been a test bed for e85 fuel for a few years now. A pal of my dad runs a garage and petrol station and is hugely into his vintage Cadillacs (has a load of the big pink buggers) Anyway, he's one of only a couple of places i can think of that still sells genuine leaded 4 star petrol, contrary to common misconception, they diddnt ban it they just put the price up and taxed it to death. Monty keeps it on one pump, sells it to the classic car guys, does a roaring trade. Anyway, i got chatting to him about fuel one time and he shows me an article in a trade newsletter about what areas the government are using to test high ethanol content fuels and how high a percentage they could get away with, sure enough, most of the SW. Whereas the rest of the country gets 5% or "up to 10%" all of the major fuel suppliers are using a 15% ethanol content fuel. this was a couple of years back now and it may have changed. just about every pump in my neck of the woods has e85 marked on it though and the dates tally up with when i started seeing a rise in the number of carb kits, hoses and breathers i was fitting and how many stale fuel issues i was dealing with.
The 98ron super unleaded is still blended with ethanol, it just has a higher octane rating.
I've just taken on Aspen in the shop, as i'm getting sick of explaining to customers that fuel goes off. most of them understand but some just don't seem to believe that its going off from the time it was refined not the time since they poured it out of the jerry can into the strimmer.

hope this explains what i mean.

That situation i was not familiar with....I will look closer at the pumps when im down your way again (later this yr) . Yes..5% was normal and 10% is now the normal amount i believe is in filling station fuel (15% must be disclosed ). Supermarket fuel ..not sure !
I do get stale fuel problems but not at the speed your talking of but educating customers about fuel is a never ending thing here to !!

 

[Stump Shot]

Thinking,(I know, never stops) is there another way to look at compression? Case in point; the 266 and 281 were/are notorious for being hard to pull over just as they came from Husqvarna. Is this the result from a larger amount of trapped swept gasses vs just being squashed a lot more, because the latter doesn't seem possible to me. So in other words does the exhaust port roof play an important part of the final outcome as much as the squish band tolerance?

 

[XP_Slinger]

Thinking,(I know, never stops) is there another way to look at compression? Case in point; the 266 and 281 were/are notorious for being hard to pull over just as they came from Husqvarna. Is this the result from a larger amount of trapped swept gasses vs just being squashed a lot more, because the latter doesn't seem possible to me. So in other words does the exhaust port roof play an important part of the final outcome as much as the squish band tolerance?

I think you're right on the money with that line of thinking. My 372 at 180psi doesn't try to stretch my fingers as bad as my stock 288 did at 150psi. The volume of air being compressed must be the difference in this example.