Just what exactly is a "looper"?

[KilroyJC]

OK - I realize this is a type of J-E Outboard, but what exactly does it mean??!? What does that do as opposed to a non-looper? And is that even the correct term for something that isn't a looper?<br /><br />Just wondering...

 

[Dave Abrahamson]

Re: Just what exactly is a "looper"?

It has to do with the way the air-fuel mixture is introduced to the combustion chamber, I think.<br />Some are crossflows and some are loopers. I run small two stroke r/c aircraft engines, and some are what they call schneurle scavanged. This means that there are multiple (usually three) bypass ports in the cylinder which allow fuel up into the combustion chamber. On a non schnuerle ported or "loop scavanged" engine there is only one bypass port which is opposite the exhaust port. This may result in some of the fuel passing over the piston and right on out the exhaust port. With schnuerle porting the fuel arrives from different directions and converges at the center of the cylinder which allows more fuel to be burned, hence more power. Not totally sure if the same applies to outboards but I'm sure someone more knowledgable than I will chime in here shortly.

 

[rodbolt]

Re: Just what exactly is a "looper"?

pretty much.<br /> the incoming charge is desined to enter from a point acrossed fron the ex port and usually has a cutout in the piston skirt as well. the pistom domes are usually flat or lighty crowned and the fuel charge is directed more towards the combustion chamber insted of towards a piston deflector.<br /> but in the words of the jap engineers, why you need to know?

 

[Paul Moir]

Re: Just what exactly is a "looper"?

Pictures explain it best, so click here. <br /><br />Loop scavenging is a more efficient way to fill the cylinder with fresh fuel/air, so it tends to be more powerful and/or better on gas & emissions. With the big ramp on the top of the piston, the piston is pretty darn heavy so you usually don't see them in high revving 2-stroke engines.<br /><br />Loopers require a bit more unusual methods to make where building crossflow engines is dead easy. There's some advantage to crossflows that I don't quite understand: I've heard people say they can be easier to make idle and such in smaller HP engines. Perhaps it's for this reason most the smaller Johnny/Rude 2-stroke carbureted engines were cross-flows (even up to today). There are some small loopers out there, for example the 4hp 2 cylinder merc.<br /><br />EDIT: Gotta type faster...<br />Oh, and also - Scavenging: the removal of exhaust gases from the cylinder. On a two stroke this happens at the same time as the fresh fuel-air charge is sent in.

 

[TOHATSU GURU]

Re: Just what exactly is a "looper"?

I thought it was when my wife smacked me on the head.

 

[KilroyJC]

Re: Just what exactly is a "looper"?

Thanks for all the info!<br /><br />As need-to-know...after seeing the term repeatedly used, with no explanation - especially since it was used to describe my motor in another thread - well, I NEEDED TO KNOW!<br /><br />So my '75 70HP is a looper, and my '52 3HP is a crossflow.<br /><br />Now it makes sense.<br /><br />Thanks again,<br />Matt M

 

[Scaaty]

Re: Just what exactly is a "looper"?

Originally posted by KilroyJC:<br /> '52 3HP is a crossflow.Thanks again,<br />Matt M

Cool ...I have a 52 3hp Johnny too. Little beat up, runs perfect

 

[KilroyJC]

Re: Just what exactly is a "looper"?

I got my 3HP from a friend who got it while cleaning out a basement. Did a basic overhaul (didn't open the crankcase) for about $160 in parts. Now, every time I start it up, I can't help but laugh because the thing runs SO WELL!

 

[Scaaty]

Re: Just what exactly is a "looper"?

I can top that! I advertise free on our local "Freecycle" for old dirtbikes, motorcycles, and outboards. Guy emails ne about the 3hp. Runs, needs a carb clean. Ask $50, I say $40. Good enough, and to make sure I don't get away, he delivers it 45 miles! I'll die with it and the 12 foot aluminum, as both will outlast me.

 

[Chris1956]

Re: Just what exactly is a "looper"?

Back to the original topic. It would appear that a loop charged engine requires multiple transfer ports. My '73 single cylinder motorcycle has a 7 port, loop charge motor. One intake port, one exhaust port, four transfer ports and the seventh port is an elongation of the intake port, which allows a direct charge into the combustion chamber (bypassing the crankcase). This direct charge occurs because of the partial vacuum created "behind" the transfer port "blasts" of fuel mixture. This technology was state of the art for 2 cycle motorcycles for many years in the 70s and 80s. <br /><br />Did the Outboard manufacturers ever graduate to something like a 7 port loop charge engine? I know it would be hard to cast a multicylinder block, but a single cylinder wouldn't have been that tough.

 

[KilroyJC]

Re: Just what exactly is a "looper"?

OK - Chris, what kind of bike? That sounds quite complicated! Not to mention that just whe I think I get the concepts down - here comes another one! It also sonds llike a multicylinder app would require a modular block instead of a single casting.<br /><br />Paul - the pics ake it much clearer & I have that bookmarked in my "Reference Materials" folder now. Thanks!

 

[rodbolt]

Re: Just what exactly is a "looper"?

actually vacum has noting to do with the transfer<br /> the transfer is created by the primary or crankcase compression when the piston is on the down stroke. a major difference in two and four stroke rings is 2 strokes MUST seal both ways. the vacum created in the crankcase when the pison is traveling up is what draws air into the crankcase, normally but not always through a set of reeds.<br /> loop charging has been around for 40 years or more. as casting technology got better you see more sophisticated porting.<br /> most BRP and all V yamahas are now loop charged as well as V mercs and suzukis. makes for a better scavenge and better airflow. all in all loop charging makes for a more efficient operating engine.however the old crossflow motors chug along daily as well.<br /> in years past the blocks were cast seperatly from the cylinders as common belief was you could not cast them together, kinda like the physisist that said in the 1920's that an automobile could not accelerate past 120 MPH in the 1/4 mile.<br /> now most are going to a lost foam casting thats can make incredibly complex castings from a foam core. its actually quite amazing.<br /> but the primary compression on a 2 stroke is just as critical as the secondary. thats why one of the first indications of a piston or ring failure is loss of idle quality.if the ring fails to seal combustion gas contaminates the next fuel charge, if the ring/piston skirt fails to seal on the down stroke it will not properly compress or scavenge the next and last charge.

 

[Chris1956]

Re: Just what exactly is a "looper"?

Kilroy, The bike is a '73 Yamaha RT3 (360 CC Enduro)which used a single reed valve between the carb and the cylinder neck-mounted manifold. This design was labled "Torque Induction".<br /><br />Rodbolt, On the newer Outboard "loopers" how many transfer ports are they able to include?

 

[rodbolt]

Re: Just what exactly is a "looper"?

about as many as they wish.<br /> most now have one big port, occasionally a model will have 2. dont need anymore with todays technology. although some do have finger ports. most use cutouts on the piston skirt to aid air transfer.<br /> lost foam casting can give incredibly complex casting with reliability.

 

[noelm]

Re: Just what exactly is a "looper"?

I remember doing a tour of the OMC plant in Sydney Australia in 1980 and the tour guide was praising their "lost foam" casting procedures they were using in the foundry at the time.