# Yamaha 701 PWC engines -- why not?

Discussion in '2-Stroke Aircaft Engines' started by Andy Garrett, Jan 5, 2020.

1. Jan 5, 2020

### Andy Garrett

#### Active Member

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I did a search for posts and general web sources for PWC engines in airplanes. There is nothing here that I could find and precious little on the web. That probably means it doesn't work, but let's take a look at this option.

As I continue to research the engine options for a small plane build, I keep circling around the usual suspects and am routinely shut down by the usual ridiculous prices. I can buy a 350hp LS1 Corvette crate motor for less than a 50hp Hirth. It's ridiculous how much the word 'aviation' costs.

This frustration leads me to the larger world of engines outside those marketed for aviation. And while I'm not stoked about 2-strokes, I dislike the power/weight ratio of half VWs and the like. But, I trust the name Yamaha, having owned and ridden many of their motorcycles and PWCs over the years. As one of the most prolific engine manufactures on Earth, their merit is proven.

Enter the Yamaha 701 2cyl/2stroke
Stock horsepower is 63 to 73 depending on year of manufacture
with popular mods the hp is up to 80-85
Max HP achieved at around 6200RPM
Weight is 36kg/80lbs

Even the older but very similar 650 gets 50hp stock.
The newer 790 gets 90hp stock.

These of course, are liquid cooled/open loop systems, so we would need to convert to a closed loop and add the weight of a radiator and coolant. The advantage may be far better cooling than high output, air cooled 2-strokes are known for. I'd be happy with a little better.

We would also need a PRSU which adds some weight - common in 2-strokes.

To my mind, we have an engine with some of the same considerations as a Rotax 582, but we can get it rebuilt for $595 (plus core)..., a tenth what a rebuilt 582 costs. I'm certain I am wrong and that plenty of dialog exists to prove it, so please educate me so I can move past this 'too good to be true' flight of fancy. Thanks! 2. Jan 5, 2020 ### Bill-Higdon ### Bill-Higdon #### Well-Known Member Joined: Feb 7, 2011 Messages: 473 Likes Received: 145 Location: Salem, Oregon, USA The late George Spratt had some successes in converting outboard 2 cycle engines. You might want to search for the old Sport Aviations that talk about what he did. Although some of the PWC engines I've seen are more like snowmobile engines Andy Garrett likes this. 3. Jan 5, 2020 ### poormansairforce ### poormansairforce #### Well-Known Member Joined: Mar 28, 2017 Messages: 699 Likes Received: 194 Location: Just an Ohioan The cooling system will need to be well designed since the engine's cooling is designed around lake temp water. Radiator return water tends to be higher so will need some thought. Last edited: Jan 6, 2020 Andy Garrett likes this. 4. Jan 6, 2020 ### proppastie ### proppastie #### Well-Known Member Joined: Feb 19, 2012 Messages: 3,958 Likes Received: 1,165 Location: NJ My belief is most any engine can be converted if you are willing to spend the time to test out all the potential problems. No it is not too good to be true but rather a fair amount of work to make it happen...With the state of aviation today one could buy an air-frame very reasonably and have lots of fun making that conversion. here is a search I just did. so others are thinking like you. https://www.google.com/search?rls=o...&ved=0ahUKEwiR84fX2e3mAhUuwVkKHZRgBwsQ4dUDCAo Andy Garrett likes this. 5. Jan 6, 2020 ### Andy Garrett ### Andy Garrett #### Active Member Joined: Jan 3, 2016 Messages: 26 Likes Received: 10 Location: Haysville, KS USA Since posting, I have been chatting with a performance engine builder/machine shop who specializes in the 701 and other PWC mills. He has converted them to closed loop and is sharing the specs he used. He used a radiator from a Ranger 800 side-by-side. This worked for a marine environment, so I have to believe a couple of narrower motorcycle radiators mounted on either side of the engine behind a propeller would be a good starting point. I need to identify optimum operating temps to determine the best amount of coolant to haul around and source an appropriate thermostat (and build a housing for it). Considering the many thousands of dollars to be saved, I find the prospect worth while. Propastie, that second video in your search of the Yamaha powered Highlander is Steve Henry. He's a local pilot and a Just Aircraft dealer. That machine is AMAZING! He can land and take off on the width of the midfield runway exit. He has a 1000cc in it I think with a clutched prop. I've never seen anything like it. He dos all the STOL comps. proppastie likes this. 6. Jan 6, 2020 ### pictsidhe ### pictsidhe #### Well-Known Member Joined: Jul 15, 2014 Messages: 7,183 Likes Received: 2,044 Location: North Carolina Stay away from power increases if you want it to be reliable. Aircraft engines spend almost all of their time at 50% power. PWC probably spend a lot too, but when they conk out, it isn't as big a deal. The manufacturers build them to be reasonably reliable, mods will detract from that. The Rotax aero engines have very close kin in seadoo PWC engines. They share many parts. The trickiest bit is likely to be the redrive. Making a Rotax C box fit is the obvious choice, though you may be pushing their torque limit. Cooling depends on many factors. It should save time to have a look at a working Rotax radiator and scale it to your horsepower, then carefully test. 7. Jan 6, 2020 ### Andy Garrett ### Andy Garrett #### Active Member Joined: Jan 3, 2016 Messages: 26 Likes Received: 10 Location: Haysville, KS USA Good advice. I have no need for mods or extra power. 73hp or even the 63hp older version is tons of power for something like an Affordaplane. The Yamaha 650 (older still) would even be fine at 50hp. Maybe limiting the 63hp to 6000rpm putting it in the mid 50s is worth a look. I know that you can really increase low end torque and sacrifice upper limit rpm with simple exhaust adjustments (common for freestyle wet-bikes). Reliability is my mantra. For the redrive, a common practice with the old Kawasaki 440s was to mount it to the engine mounting plate. That kept the relationship stable without differential movement. A PRSU with belt drive like the Valley Engineering offering might work. I still need to learn a lot about these motors. I need to see one in person--maybe buy an old rebuildable core and study it before making the call to buy a kit and do the deed myself or pay a pro for a full rebuild. I need to study the electrical system too. The best part is that these are very well known and supported motors with parts aplenty. They are known to be reliable and the price is about a tenth of a comparable 'aircraft' engine. I think I'm officially shopping. cheapracer likes this. 8. Jan 6, 2020 ### mullacharjak ### mullacharjak #### Well-Known Member Joined: Nov 5, 2010 Messages: 115 Likes Received: 28 Location: Peshawar Pakistan Do they come with integral water pump or will you have to add an external one.That would look like a major modification.The suzuki 660cc K6A 3 cylinder comes in similiar power range.It has a turbo version also but may be slightly heavier. 9. Jan 6, 2020 ### cheapracer ### cheapracer #### Well-Known Member Joined: Sep 8, 2013 Messages: 5,655 Likes Received: 4,054 Location: Australian There is a vast range of proven reliable electric water pumps around now. Suzuki K14B for the win where you are. The only surprise to me is why aviators aren't flocking to them, oh wait, I forgot, they are busy scooping up those Rotax 912iS engines at the "How can they do it so cheap!" price of$36,000.

Post a genuine bargain worth investigating, you get 2 responses ..

Post how is it that a Rotax 912 can be $36,000, and you get them falling over themselves to get in to make excuses for the ridiculous price. https://www.homebuiltairplanes.com/forums/threads/rotax-912is-for-35-000.32834/ Bizzare, just bizzare. poormansairforce likes this. 10. Jan 6, 2020 ### AdrianS ### AdrianS #### Well-Known Member Joined: Jul 5, 2014 Messages: 420 Likes Received: 266 Location: Australia I would assume that a boat / jetski engine would be a closer fit to aircraft use than eg a bike or car engine for 2 reasons: 1) Much higher cruising load - longer periods at 2/3+ throttle than a car/bike. 2) The consequences of failure are much higher in marine use. I know you won't fall out of the water if the prop stops, but being a couple of miles out in a lake or at sea with no power is a much bigger deal than parking at the roadside. Now you only have to cool the sucker and find a psru. ps Hearing the (only) engine hiccup when you're 5 km offshore in a small boat certainly gets ones attention. litespeed likes this. 11. Jan 6, 2020 ### cheapracer ### cheapracer #### Well-Known Member Joined: Sep 8, 2013 Messages: 5,655 Likes Received: 4,054 Location: Australian Any modern car engine will perform at any chosen constant rpms that you choose within it's designated rpm band, i.e. from idle to red line. The only troubles I have seen related to what people wrongly believe, is that most car's cooling systems are undersized for maximum continuous rpms, simply because they are almost never used like that, so manufacturers save a bit of cash by supplying the minimum amount of materials required to cover targeted usage. For example, if you tow a boat or caravan at continuous high rpms up long hills, many find it a requirement to add extra electric fans and an oil cooler. These myths tend to propagate in countries with restrictive speed limits, Europeans know well you can hold cars flat out down their (previously) open speed limits highways. Some are 140kmh now (90mph), but In Germany, you still can go flat, and they do. I will have to stand on a local hill and show you what they do to Chinese commercial vehicles, and you will see why I am so impressed with their engines. 12. Jan 6, 2020 ### Turd Ferguson ### Turd Ferguson #### Well-Known Member Joined: Mar 14, 2008 Messages: 4,891 Likes Received: 1,791 Location: Upper midwest in a house That's one of many reasons why a snowmobile engine is a better candidate for conversion. A liquid cooled snowmobile is already designed with a closed loop cooling system and has all the associated components - water pump, radiator, etc. The PTO end of a snowmobile engine is usually better suited for adding a PSRU since it supports a large torque converter clutch when mounted in a sled. A PWC typically has a skimpy coupling on the PTO end of the crankshaft that only connects the engine to the jet pump shaft. I have several PWC engines and have no doubt one could be converted but that's not what I would start with. Last edited: Jan 6, 2020 13. Jan 6, 2020 ### proppastie ### proppastie #### Well-Known Member Joined: Feb 19, 2012 Messages: 3,958 Likes Received: 1,165 Location: NJ Mostly because it is "certified" .....that FAA/European Union blessing means they have jumped through all the hoops an anal governmental engineer can think of and passed the tests. All those certification tests really pushes up the costs too. 14. Jan 6, 2020 ### Turd Ferguson ### Turd Ferguson #### Well-Known Member Joined: Mar 14, 2008 Messages: 4,891 Likes Received: 1,791 Location: Upper midwest in a house A PWC typically taps the high pressure section of the jet pump for cooling water circulation. 15. Jan 6, 2020 ### cheapracer ### cheapracer #### Well-Known Member Joined: Sep 8, 2013 Messages: 5,655 Likes Received: 4,054 Location: Australian You know most Rotax 912's are un-certified, and sold as such? proppastie likes this. 16. Jan 6, 2020 ### Vigilant1 ### Vigilant1 #### Well-Known MemberLifetime Supporter Joined: Jan 24, 2011 Messages: 4,240 Likes Received: 1,973 Location: US The price of an item is determined by the market (i.e. it is determined by what a buyer is willing to pay). It has nothing to do with the costs of manufacturing. Whether it is worthwhile for a someone to make and sell an item (i.e. will the item be made and offered for sale at all) is determined by the cost of making and selling it (raw materials, labor, cost of capital, regulatory compliance, marketing, etc) and the price they can sell it for (see para 1, above). I know this is basic stuff, but it sometimes seems to get overlooked. Rotax sells these engines at a high price because people will pay it and Rotax has determined that price maximizes their total profit. If they could make more profit by lowering the price and selling more of them, they probably would. Last edited: Jan 6, 2020 17. Jan 6, 2020 ### cheapracer ### cheapracer #### Well-Known Member Joined: Sep 8, 2013 Messages: 5,655 Likes Received: 4,054 Location: Australian ... of a lack of competition. I'm sure Viking and Aeromomentum, are taking at least a few sales from them though recently. Jabiru move a few too. Viking and AeroMo are proving it can be done, and are slowly getting more and more acceptance, albeit still slow due to the myths and lores out there about auto conversions. One thing that amuses me, is when a vintage plane is mentioned, the same guys who will bag out auto conversions will suddenly jump and and down in excitement when they hear that a Model 'A' Ford motor is to be used, an horrific, heavy, gutless load of rubbish. 18. Jan 6, 2020 ### Vigilant1 ### Vigilant1 #### Well-Known MemberLifetime Supporter Joined: Jan 24, 2011 Messages: 4,240 Likes Received: 1,973 Location: US Ah, but that's already baked into Paragraph 1. The lack of competition (more completely?: "the lack of other higher value alternatives to Rotax that prospective buyers know about and include in their analysis of alternatives") increases the price buyers are willing to pay for a Rotax. The lack of competition is important, but only because it increases the amount that buyers are willing to pay. There are cases where a lack of competition does not result in sky-high prices. When the VW Beetle was introduced to the US, there were no/few other little cars like, it, and certainly none that were rear engined and air cooled. VW could have gotten more per unit by selling the cars as niche products to those who especially valued the unique aspects of the car. Instead, they determined that they could maximize overall profits and potentially reduce >eventual< competition by keeping unit prices low and selling a lot of them. With Rotax, things are exactly opposite. The total potential size of the homebuilt/LSA/experimental airplane market is small (compared to cars), so maximizing profits by selling gobs of units has limited attractiveness. Instead, they are apparently happy and successful (for now, at least) selling fewer engines to the buyers who are least cost conscious. But, yes, as a prospective buyer, I'd like alternatives so I can pay less. 19. Jan 6, 2020 ### cheapracer ### cheapracer #### Well-Known Member Joined: Sep 8, 2013 Messages: 5,655 Likes Received: 4,054 Location: Australian Because they increase dealerships, service centers, and importantly, up to a 1000% markup parts turnover over decades - and that's where the real profit for car manufacturers is. Car makers will sell at a loss to keep that side of the business up. Anyway, the$36,000 Rotax thing has shocked me into looking at alternatives, see what I can come up with over the next few months, I have to move on something soon.

20. Jan 6, 2020

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