Why backpressure ?

[Philby]

When i run my 70hp VRO Evinrude on the muffs its sounds rough.<br /><br />When I put it in the water it runs as sweet as a nut.<br /><br />I can only presume that the backpressure on the exhaust has somthing to do with it.<br /><br />Am I right and if so why ?

 

[4link1955]

Re: Why backpressure ?

Philby,<br />The motor is basically trying to "Free Wheel" while on the water hose which affects the ideling curcuit. Go to www.overtons.com they have a muffler system that you can put on the end of the prop which makes the motor run alot quieter and puts the required backpressure on it to run smoother.<br />Hope this helps:<br />Jason

 

[Dhadley]

Re: Why backpressure ?

It's amazing what those mufflers do. I have even used the bottom of a plastic gallon jug. Cut off the bottom an inch or so up and cut an X in the middle. Remove the prop thrust washer, stick it on the shaft and replace the prop without the thrust washer. The prop doesnt have to be very tight to hold the plastic in place.<br /><br />Good luck!

 

[Beernutz]

Re: Why backpressure ?

In 2-stroke design, the intake and exhaust ports are open at the same time at the bottom of the stroke. Without backpressure, a significant amount of the intake charge escapes with the exhaust. Those "banging" sound you hear on the muffs are exhaust housing detonations, as that intake charge explodes outside the exhaust ports. If you rev it up (like the manual tells you not to), those detonations can get violent enough to blow out the gaskets between the powerhead and the exhaust housing, resulting in black gook running out from under the cowling and down usually the front of your motor.<br /><br />The power band of a 2-stroke can be raised, lowered, widened, or narrowed (with significant increase in horsepower) with different configurations of exhaust expansion chambers, which regulate backpressure at pre-determined RPM levels, and actually create a shock wave that packs that intake charge back in to the cylinder. The exhaust housing of your outboard is just such a chamber, designed to be most efficient at certain RPM's. The correct amount of restriction at the exhaust outlet of an expansion chamber is critical to proper performance. <br /><br />Next time you watch one of those lumberjack competitions (everybody watches those, right??), check out the oversized bulbous exhaust systems on the high-performance chainsaws. Those are expansion chambers. The "stinger" at the outlet is critical in both it's diameter and length. Our motocrossers on this board know all about this stuff.<br /><br />Hope this helps you understand it a little better. But, your right, it's the backpressure.

 

[DJ]

Re: Why backpressure ?

Dhadlet,<br /><br />For some reason I just can't picture what you described. Got Picture?

 

[Forktail]

Re: Why backpressure ?

Good explanation Beernutz. Hopefully someday we'll see variable exhaust on these 2-strokes. Bikes and snowmobiles have them.<br /><br />Some of what you're hearing is usually muffled by the density of the water. The water itself acts as a muffler. But the water doesn't really cause that much backpressure, the tuned exhaust system does most of that.

 

[Dhadley]

Re: Why backpressure ?

djohns19 -- I usually just cut off the bottom of a 1 gallon plastic (milk or water) jug about an inch or so up from the bottom. That dimension is not critical since the plastic is larger than the exhaust area. I then remove the prop and forward thrust washer. I cut a hole in the center of the plastic. Actually an "X". Then slip the plastic over the prop shaft. Replace the prop, spacer and nut. The prop will keep the plastic in place aginst the exhaust outlet. It creats enough back pressure to keep the idle at or near "normal" on a looper and really takes the sharpness out of the noise. <br /><br />Sort of a "poor mans muffler" but it really works. <br /><br />Hope that helps.

 

[DJ]

Re: Why backpressure ?

Dhadley,<br /><br />Now I think I get it. You are putting the milk jug on the propshaft sideways-right?<br /><br />Nifty idea. I was think about buying one of those mufflers. I'll check this out first.<br /><br />Thanks!

 

[Dhadley]

Re: Why backpressure ?

Basically I use a flat piece of plastic between the prop and exhaust outlet. <br /><br />Let me know!

 

[Philby]

Re: Why backpressure ?

Thanks<br /><br />I appreciate your considered and precise explanations.

 

[Bry21317]

Re: Why backpressure ?

Would either of these work on a 88 Force 125 HP, with the single port exhaust? It comes out of the trim tab, just above and past the prop.<br /><br />Thanks,<br />Bryan Cox

 

[Beernutz]

Re: Why backpressure ?

Forktail, I think the late 60's must have been the birth of the development of 2-stroke technology, as we discovered what they could do in motorcycle racing on both dirt and road courses. In the garage at night, we were installing sealed main bearings and stuffing crank pins with foam to decrease crankcase volume, thus increasing crankcase compression ratio and transfer port velocity. To begin with, all were piston/port timed, so we were cutting piston skirts or "windowing" pistons to add reed valves. Rotary valves were fun, but made the crankcase too wide. We raised, lowered, and widened ports, and even cut additional transfer ports into the cylinder walls with a die grinder. With all this going on, we fabricated our own expansion chambers to accomodate for the alterations in breathing and powerband. You could get it so tight that, if you fell off the band, it was like hitting the brakes. It was an exciting time, though! We were the engineers, right out there in the garage! You could get a motor almost perfect, then "just a little more twingling", and she'd just go flat, and you'd have to throw away the piston and cylinder and start over. Bultaco parts weren't cheap, either! Now the manufacturers have taken over all that fun.<br /><br />Having said that, I too have felt that the outboard engineers haven't kept up in the exhaust design department. Maybe it has something to do with the constraints of size or the environment that our motors run in. The necessary wide powerband may also have something to do with it. But, why haven't we seen any experimentation with a "power valve", as you have suggested? Even with high pressure fuel injection, some of the fresh air charge still must get scavenged out the exhaust port, wouldn't you think? Maybe the problems with weight and complexity of 4-stroke design will refocus attention upon further development of 2-stroke outboard technology. We'll see.<br /><br />Dhadley, I love your muffler design!

 

[Franki]

Re: Why backpressure ?

With regards to the exhaust in 2stroke outboards, can you imagine how big the perfect solution would be??<br /><br />take a V4 OMC for example.. start by looking at an 250cc motocross bike powerpipe... then multiply it by 4.. and have it feed into a cental collecter pipe, (imagine a set of tuned leagth headers with expansion chambers on all of them.)<br /><br />that would be huge.. fantastic, but huge..<br /><br />I'd love it, but it would make the outboards leg huge and ugly..<br /><br />but things would change alot of they added that, (or as much as possible) and AETC or similiar..<br /><br />would be facinating to see.<br /><br />regards<br /><br />Frank

 

[Beernutz]

Re: Why backpressure ?

Yeah, I'm into shapely legs, too!

 

[Forktail]

Re: Why backpressure ?

Beernutz, wish I was in the garage with you guys! I went off to college and got a Mechanical Engineering degree instead, but its just not the same! <br /><br />I'm very surprised that outboards don't use variable exhaust valve technology. Like you said, it's probably because of space limitations and the fact that outboards are used primarily in the maximum power band where tuned exhausts are sufficient.<br /><br />But variable exhaust is quite simple, compact, and light weight. It's currently found on almost every brand of snowmobile and even motorcycles. Some are even consumer adjustable, and most with single pipes. They're basically a guilotine valve that opens and closes, either by spring tension, electronically, or throttle position cable, in the exhaust outlet of each cylinder. They make a huge difference in bottom end and midrange efficiency. Check out the RAVE valves on the ski-doo's and sea-doo's, and Yamaha's powervalves.

 

[12Footer]

Re: Why backpressure ?

Well,yes and no, Forktail. The outboard factories DO make special exaust packs for racing boats,as do some of the individual teams. I've been told that Mercury Marine has a seperate division that does only expansion chambers/exaust tuning in design and racing.<br />Some of the older engines can be retrofitted with them. Those on some Hydroplanes are clearly visable when they race,and kind of look like after market headers off of a VW beetle.<br />Some of these can be purchased second-hand off ebay...For some reason, they are quite prolific there.<br />But no, you can't go into the niehborhood marina and buy them, or even purchase them from the engine maker (that I'm aware of).

 

[Forktail]

Re: Why backpressure ?

12Footer, your talking about "tuned" exhaust systems, stingers, headers, expansion/resonance chambers, exhaust packs, etc.<br /><br />This is not what a "variable" exhaust system is. A "variable" exhaust system changes the diameter of the exhaust outlet by means of a mechanical valve, giving maximum efficiency throughout the entire powerband. The "tuned" pipes you're talking about can only produce maximum efficiency at maximum hp. That's why you see them for racing. But the average Joe boat owner needs maximum efficiency throughout the powerband.<br /><br />Big difference between "tuned" and "variable" exhausts on a 2-stroke.