This 066 Grenaded........What The Hell Happened???????

[trophyhunter]

Shouldn't it read "Saws used at mix ratios HIGHER than 40:1 will not be covered by Mastermind Worksaws" just to CYA? The way it reads otherwise 32:1 would be a deal breaker and 50:1 would be fair game.

 

[Mastermind]

Less oil than 40:1 is what it means.....

 

[shootingarts]

The "fast burn" we are hoping to see in a chainsaw engine's combustion chamber is what we are doing with the wider squish band and tighter squish area. The thinking is that with the machining process we can increase squish velocity, reduce chamber volume, have higher (and therefore faster burning) compression ratios, and still utilize a flat top piston. It has been discussed that this could make the engine more resistant to detonation by design.

Randy,

What you are talking about here is rock solid of course. Getting a little trickier but you might find the cost of a custom piston for one or two bores and engines isn't out of reach. Flat tops do simplify things but as you continue to take the changes to extremes you are going to almost have to have a custom jug cast or go to a custom piston. Back when the world was young I could have a custom piston made for only 30-50% cost over a standard configuration from one of the high performance folks. Before you ask, no I don't remember which one. I am talking early seventies, far too many years and beers ago! That was only having eight pistons made though, not a big run.

One thing that catches my eye on a chainsaw is the connecting rod. Seems like a few grams could come off, cheap horsepower savings.

Question for you: I seem to recall a saw or some saws having a separate cylinder and head. Any of them suitable to experiment with? If someone was just machining a flat head proto-typing might be easier and cheaper. I don't know if 6061 T6 would hold up as a cylinder head but I just happen to have a couple chunks of it, 12"x16" and a little over an inch thick, I think an inch and eighth, maybe. These were left over from an old project that never happened.

Hu

 

[exSW]

So what I'm getting is chainsaws don't have detonation problems because they don't have TIME to detonate.They are moving fast at a constant rate with a reasonably constant load.

 

[trophyhunter]

Less oil than 40:1 is what it means.....

I know, completely understand the intent.

 

[Mastermind]

Randy,

What you are talking about here is rock solid of course. Getting a little trickier but you might find the cost of a custom piston for one or two bores and engines isn't out of reach. Flat tops do simplify things but as you continue to take the changes to extremes you are going to almost have to have a custom jug cast or go to a custom piston. Back when the world was young I could have a custom piston made for only 30-50% cost over a standard configuration from one of the high performance folks. Before you ask, no I don't remember which one. I am talking early seventies, far too many years and beers ago! That was only having eight pistons made though, not a big run.

One thing that catches my eye on a chainsaw is the connecting rod. Seems like a few grams could come off, cheap horsepower savings.

Question for you: I seem to recall a saw or some saws having a separate cylinder and head. Any of them suitable to experiment with? If someone was just machining a flat head proto-typing might be easier and cheaper. I don't know if 6061 T6 would hold up as a cylinder head but I just happen to have a couple chunks of it, 12"x16" and a little over an inch thick, I think an inch and eighth, maybe. These were left over from an old project that never happened.

Hu

Machining the top off of the jug and making a head is very common in saws used for racing. I'm not sure how well it would work in a work saw.

 

[shootingarts]

Machining the top off of the jug and making a head is very common in saws used for racing. I'm not sure how well it would work in a work saw.

Definitely getting into play area here swapping pistons and heads.

There might be some gains to reshaping the squish area very slightly instead of just flat though. There was horsepower being found by accelerating the mixture in the chamber and controlling swirl and such instead of just squeezing it. Not having a squish area was why the 426 hemi wasn't really that potent of a street engine. While it outflowed the wedge chambered engines the flow wasn't used on the street and the conventional engines were stouter on the street. The Ford Cleveland engines were the same way, lots of flow, not really that potent on the street.

A few ideas buzzing around but I can't help thinking I'm late to the dance with two cycle engines. When I got involved in benchrest rifle competition I had some wonderful ideas. Decided to pass them by a friend of mine who had been the world record holder for a decade or so. Jef told me he was sure that when I got through reinventing the wheel it would be rounder and better than ever. I rubbed the dent in my head and took the hint!

Hu

 

[LegDeLimber]

250 cc vs 70cc and timing?Chainsaws are made to run flat out your atv has a powerband that matchs up with your gears.

While there's a multitude of other factors ( and many have already been discussed here)
the different distance across the 2 typical sized bores of those engines
plays a factor in the detonation issue.
pressure wave expanding and temp rise and snowball effect kinda stuff going on in the hole.

Anyone got a link to a better answer than my babbling??

 

[HuskStihl]

I doubt the compression ratio is that high. The calculations are assuming a bunch of stuff that probably isn't happening. Indy cars are turbocharged, which is a whole other ball game.
Engines will almost always make more power with the lowest octane rating which doesn't detonate.
I run 93 for the safety margin, if the gas sits for a bit it might drop to 90 or 88. Adding oil will slow down the burn effectively increasing the "octane". I bet if you could "Teflon" stuff so you could run straight gas, 87 would detonate in a hot saw.

 

[shootingarts]

I doubt the compression ratio is that high. The calculations are assuming a bunch of stuff that probably isn't happening. Indy cars are turbocharged, which is a whole other ball game.
Engines will almost always make more power with the lowest octane rating which doesn't detonate.
I run 93 for the safety margin, if the gas sits for a bit it might drop to 90 or 88. Adding oil will slow down the burn effectively increasing the "octane". I bet if you could "Teflon" stuff so you could run straight gas, 87 would detonate in a hot saw.

We are talking "static" measured compression ratios. While the static numbers are extremely high in the saws, I have no idea how that translates into pressures the engine sees when it is running. Burning mix in a two cycle engine probably doesn't generate the same total pressure as burning straight gas in a four cycle engine even if they both have a measured 200PSI compression and both are normally aspirated. Like you I strongly suspect at least some of the two cycle oils serve to increase octane.

Just numbers pulled out of thin air but maybe a four cycle on gas with 12.5:1 compression sees 600psi of peak pressure when the engine is running and the fuel is being ignited. The two cycle with 12.5:1 compression may only see 450psi of peak pressure when the engine is running and the fuel being ignited. Those numbers may not be anywhere close but are just put out to try to clarify what I mean about the difference between pressure indicated by a compression gauge and the pressures generated running.

Just speculation on my part but it might explain why 87 octane plus oil works for some. I'm sticking with 93 octane myself. Kind of like that stronger oil mix for lubrication, a little extra octane is cheap insurance against detonation.

Hu

 

[Andyshine77]

Lost of good info, even more BS way above ones mental capacity, education and pay grade, myself included. Let me ask this. What is the formula to determine the compression ratio of a piston ported two cycle engine? Do you use the trapped method?

Randy has a good handle on how things work in a piston ported two cycle engine, better than myself, and better than most IMHO. I see lots of copy and paste going on with little understanding of what's been written.

 

[LegDeLimber]

I recall some long ago debate in dirtbike magazines over the comp ratio.
The calculated stroke method vs the actual trapped area.
I tended to figure that the combustion chamber felt the BTUs
from the trapped area.
Now how you factor in losses through ports and mass of the intake charge
the exhaust gases... and so on....
anyone gettin a headache yet? ;^)

 

[shootingarts]

Lost of good info, even more BS way above ones mental capacity, education and pay grade, myself included. Let me ask this. What is the formula to determine the compression ratio of a piston ported two cycle engine? Do you use the trapped method?

Randy has a good handle on how things work in a piston ported two cycle engine, better than myself, and better than most IMHO. I see lots of copy and paste going on with little understanding of what's been written.

No formula at all other than converting the compression gauge reading to a ratio. One atmosphere at sea level is roughly 14.7 psi. Blow 200 psi on a compression gauge, divide that by 14.7. Since you want a ratio instead of a fraction subtract one from that.

200/14.7=13.60. Subtract 1 from that and a little rounding and it is about 12.5:1. Perhaps it should be 13.5:1, I'm working from old memory. It seems like zero on the gauge should be one atmosphere but I don't remember 200 PSI being 13.5:1. I could be wrong after forty years. 2013-1973=40 if you need the math on that. Built my gas/altered engine in 1970, started building circle track engines a couple three years later. Back then our short track cars were usually running between 12 and 13 to one compression. Any way you size it, 200PSI on a gauge is a lot of compression.

Just for the record, I never cut and paste without documentation and credit. Anything I paste in will start (included text) and end (end included text) with a note where the information came from. The way I roll and it was a requirement on the forum I co-owned and co-administered.

Hu

 

[Andyshine77]

From what I know the compression ratio in a two cycle engine or any other isn't based on psi. It's based on volumes. Bore, combustion chamber volume and stroke. 200 psi in a small two cycle engine is nothing like 200psi in a four cycle car engine. Now add the fact the cylinder bore has a hole in.

http://www.kidsatv.com/pdfs/higher_compression6.pdf
http://www.torqsoft.net/compression-ratio.html

 

[one.man.band]

'The effect of Higher Compression Ratio in 2ST Engines' link above is right in the wheelhouse of what yall are hashing out.

probably need to re-read it many more times, to get my brain to digest it.

the fuel efficiency increase thing is pretty cool.

unless i missed it, only thing that wasn't mentioned, was spark timing vs. compression. maybe the old school, "advance it 'till it pings under load, then bring it back" does not apply here? cannot tell from the atricle.

-omb

 

[one.man.band]

maybe a little off topic, because its about 4ST.

i the mid '80's there was a guy named Larry Widmer (aka: theoldone). he built heads for many winston cup teams, during that time. while everyone else was running about 13.5 to 14:1, his heads and motor combo was running 20 to 22:1. (his heads were on bill elliott's (spelling), '88 thunderbird that still holds the top speed record at talledega as far as i know).

was interesting when i found out many years later how he did it: retarded camshaft, retarded timing. the 'bang' happened many more degrees after TDC than everyne else, which makes sense, since the piston was further down the pipe, and the volume of the chamber was much greater, so it did not detonate even at 20+:1.

long story short, nascar changed the comp rules to 16:1 max. since he was the only one doing that and winning. larry got out of winston cup, and into nhra. ford screwed him over and sold his prototype heads to a pro stock driver who used them to win many championships.

EDIT: forgot to mention the most important part, the heads did not have a combustion chamber. the valves were even with the bottom of the head, basically sitting flat at the deck. Aside: his company currently is involved with aerospace, and development contracts. they do make some import heads, but they are sold out years before the heads are even carved.

-omb

 

[komatsuvarna]

So does 2 cycle oil increse or decrease the fuel octane?.

 

[Mastermind]

That is a whole nuther can O worms, as you well know Big D.

 

[HuskStihl]

So does 2 cycle oil increse or decrease the fuel octane?.

Yes

 

[Metals406]

I haven't read the whole thread yet. . . But my guess is Randy dropped a Wassabi pea in the jug during assembly -- & it wrecked the top end, just like they wreck Randy's bottom end! LOL

Sent from my Bic RaZor