# Yamaha 701 PWC engines -- why not?

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#### Andy Garrett

##### Active Member
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! #### Bill-Higdon ##### Well-Known Member 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!
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

#### poormansairforce

##### Well-Known Member
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.

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#### proppastie

##### Well-Known Member
Log Member
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.

#### Andy Garrett

##### Active Member
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.

#### pictsidhe

##### Well-Known Member
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.

#### Andy Garrett

##### Active Member
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.

#### mullacharjak

##### Well-Known Member
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.

#### cheapracer

##### Well-Known Member
Log Member
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.
There is a vast range of proven reliable electric water pumps around now.

Suzuki K14B for the win where you are.

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.
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 .. https://www.homebuiltairplanes.com/forums/threads/yamaha-with-sky-trax-psru.32575/ 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.

Bizzare, just bizzare.

##### Well-Known Member
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.

#### cheapracer

##### Well-Known Member
Log Member
1) Much higher cruising load - longer periods at 2/3+ throttle than a car/bike.

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.

#### Turd Ferguson

##### Well-Known Member
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.
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.

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Log Member

#### mullacharjak

##### Well-Known Member
If someone comes up with construction drawings for a universal 100 hp PSRU I think the monopoly of Rotax etc can be broken.We amateur homebuilders actually dont need Rotax.With all sorts of CNC machinery available its not difficult to build a reduction drive.If we can build an aircraft from drawings we can also build a PSRU.