Part Four: Compression

[Lee H]

Rule of thumb for making power. High compression for gas motors and low compression for diesels.
Play with a few diesels back in the day. Used to lower comp a bit so you could get more fuel without
increasing cylinder pressures. Also play with cam timing. Especially on the 855 Cummins with cam
operated injectors. Slow the cam down which is retarding. That lowered cylinder pressures a bit and
then you can add more fuel. More fuel, more air, more power. Had some mean running Cummins
back in the day.

 

[mdavlee]

Rule of thumb for making power. High compression for gas motors and low compression for diesels.
Play with a few diesels back in the day. Used to lower comp a bit so you could get more fuel without
increasing cylinder pressures. Also play with cam timing. Especially on the 855 Cummins with cam
operated injectors. Slow the cam down which is retarding. That lowered cylinder pressures a bit and
then you can add more fuel. More fuel, more air, more power. Had some mean running Cummins
back in the day.

Bet you liked Bruce Mallinson if I remember right. He did some cool stuff

 

[MG2186]

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

Multi turbo charged diesel pulling tractors are actually 11:1-11.5-1 but they make 250-300lbs of boost though


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[Lee H]

Bruce was at my house once. He dropped off a bunch of stuff for an 855
on his way to a Boston truck show. He's a very knowledgeable guy.
That motor I did was putting out about 8-9 hundred horse. I still buy
stuff from him on occasion.

 

[mdavlee]

Multi turbo charged diesel pulling tractors are actually 11:1-11.5-1 but they make 250-300lbs of boost though


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Yep. Insane really

 

[drf256]

Multi turbo charged diesel pulling tractors are actually 11:1-11.5-1 but they make 250-300lbs of boost though


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All forced induction motors want less static CR. Because the charge is compressed even before the piston moves.

The less static CR, the more you can fill a cylinder for an effective true CR near optimal.

 

[MG2186]

Yep. Insane really

Very much! Our tractor only has 230lbs cranking compression, hell some of your saws have more than that.... lol


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[mdavlee]

Very much! Our tractor only has 230lbs cranking compression, hell some of your saws have more than that.... lol


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

 

[MG2186]

All forced induction motors want less static CR. Because the charge is compressed even before the piston moves.

The less static CR, the more you can fill a cylinder for an effective true CR near optimal.

Yes, that's the point! Blowered/turbocharged gas motors are around 4:1-5:1 generally. That's the point of forced induction. You ever seen someone try to pound a bunch of boost into a high compression engine? The results aren't good, either the rods exit or the crank winds up on the grind


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[MG2186]

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

For sure, depends how far you have the timing advanced also and cam timing. Our tractor is at 49* advanced, it's a bit cantankerous, but lights pretty quickly with good ether, just pull the fuel shut off and light it with ether. Oh man I love the smell of diesels and ether.... lol


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[mdavlee]

For sure, depends how far you have the timing advanced also and cam timing. Our tractor is at 49* advanced, it's a bit cantankerous, but lights pretty quickly with good ether, just pull the fuel shut off and light it with ether. Oh man I love the smell of diesels and ether.... lol


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22-24°. It was a street truck.

 

[CJ Brown]

Just a 290 nothing special. I thought it was the baddest saw ever and only revealed it on special occasions like a samurai sword! Then I found this site and its for sale.

That's the funniest thing I have read in a long time - I can't stop laughing hahahaha you made my day and it's only 8am

 

[MG2186]

22-24°. It was a street truck.

Yea I figured that.


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[mdavlee]

Yea I figured that.


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12 valve with EEP cam.

 

[MG2186]

12 valve with EEP cam.

Who's/what's EEP?


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[mdavlee]

Who's/what's EEP?


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Enterprise Engine

 

[Dub11]

That's the funniest thing I have read in a long time - I can't stop laughing hahahaha you made my day and it's only 8am

Your welcome

 

[P.M.P.]

Back in the day say early 80's Sachs Dolmar used dished pistons in there
50 and 60 cc saws. Not sure about the larger saws

My old 85cc Sachs Dolmar 133 had a dished piston.The compression was not high but it had a ton of torgue.

 

[Simondo]

In my experience e85 is both brilliant and a nightmare. It's the only pump fuel we can get in the south west UK and as such is what almost everybody runs. Bought from a busy fuel station and used the same day i recon my 372/2165/272 etc run at their best on it, but give the fuel a couple of weeks to degrade and collect water the same saws start getting fussy, my 268 fun saw however which has considerably more compression than the previous saws seems less bothered by stale fuel and runs much the same regardless. This seems counter intuitive to me, i'd have thought it would be more noticeable in a higher compression engine.

Any ideas?

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

 

[Stump Shot]

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.