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CHRISWEIGHT posted 12-07-2003 06:45 PM ET (US)   Profile for CHRISWEIGHT   Send Email to CHRISWEIGHT  
Please explain "loop charged" as applies to Mercury 2-cycle engines. Is it back pressure effectively compressing the incoming charge from crancase pressure via transfer porting ?


JBCornwell posted 12-07-2003 07:31 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Hi, Chris.

All 2 strokes use crankcase pressure on the downstroke of the piston to pump fresh fuel/air mix into the cylinder. This is called scavenging.

In cross scavenged engines the mix comes in one side of the cylinder, opposite the exhaust ports. It then is deflected up by a deflector built in to the piston top, crosses over the top of the piston and down toward the exhaust port, which is closed by the time it gets there.

In loop charged engines ("Loopers") the scavenging charge enters the cylinder at two places, pointed toward the side of the cylinder opposite the exhaust port. It then loops up to the top of the cylinder and back down toward the exhaust ports.

Loop charged engines are more efficient, make more power and are easier and less expensive to make.

Red sky at night. . .

Clark Roberts posted 12-07-2003 09:43 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
To add to JB's excellent explanation: The pistons in a looper are flat or slightly crowned and are thus regular in shape which more evenly distributes heat. The deflector top on the cross flow pistons can cause hot spots and cause detonation and other. Another benefit of loop charging is that precise exhaust tuning is effective in drawing out the exhaust gases (re:scavenging) for sort of a reverse super-charging. A looper really thrives on high rpm where as a cross flow seem better for extended low rpm ... first looper that I recall was the 1968 Evinrude Triumph 55, a 49 cubic inch three cyl engine that was way ahead of its time and that basic engine was increased to 75hp (Johnson Stinger) and survived until OMC went under recently. Blah, blah, blah...will shut up now! Happy Whalin'... Clark..Spruce Creek Navy
gimcrack225 posted 12-08-2003 11:23 AM ET (US)     Profile for gimcrack225    
Thanks for asking a question that has puzzled me for years. None of my OMC manuals offer such a simple explanation.

It sounds like the problem is to somehow keep the incoming fuel mix away from the still open exhaust port.

CHRISWEIGHT posted 12-08-2003 02:28 PM ET (US)     Profile for CHRISWEIGHT  Send Email to CHRISWEIGHT     
JB and Clark

Many thanks on your (as always) well informed knowledge.

I think long term we will all come to miss the smooth running and powerful 2-stroke. Are they really that bad or in reality are we talking hundreds and thousands or even millions of Whaler hours for 1 EXON VALDIZ/TORRY CANYON ?

jimh posted 12-08-2003 08:25 PM ET (US)     Profile for jimh  Send Email to jimh     
After transistors had been around for a while, we used to ponder what a break through it would have been to have invented vacuum tubes!

We might be saying the same thing about classic, simple, 2-stroke outboard motors after a few years of microprocessor-intensive, sensor-filled, 4-stroke outboards.

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