Hi from Calgary, Canada

I am researching using a big triple for conversion to an Ultralight plane. I still ride borrowed and rented sleds in the foothills but am out of touch with the big triple tech stuff. Go to the dealer and mention airplane and it gets real silent! I have narrowed my choice down to 700 Yamaha's or the big Triple cats. Want triple torque and smoothness, I've flown behind the Rotax twins, but never again! Will buy a used sled from the mid to late 90's. Would you guys be interested in discussing your experience with the big 900 or 1000, please?

Criteria
Bulletproof
Detuned for monster torque and best power at 5200 rpm range. Is this too conservative of a range for these engines?
Full power for takeoff 2-3 minutes max with 75-80% after that usual flying day 2-3 hours max.
Lots of cooling air flow in a plane
Operating altitudes 3000-5500 feet, do these engines have efi or altitude compensation, heated carbs, do they perform ok at varying altitude with baseline set?
Is this engine oil injected?
Simple single pipe for weight and simplicity.

Thanks Bob

I'll throw my two cents in. The Cat triples you mentioned were all triple piped. Yamaha made a single piped 700, but there would be a big power difference between them. If you want more torque and lower operating RPM's you may want to look to watercraft engines over snowmobile engines as they are designed for different power curves. The cylinder porting and the exhaust determine the power curve of the engine and in a sled you use the clutch tuning to keep the engine within the power curve RPM's.

The Cat triples were all carbed engines and snowmobile carbs are all fixed jet set ups that do not compensate for temperature and altitude. You can get aftermarket devices, like the Holtzman ATAAC that provides altitude and temperature compensation for snowmobiles, so that might work for your application. The later model, rack style, flat slide carbs usually had coolant passages for heating the carbs. Check the online parts micro fische at Brown's to get help on Cat parts.

I believe there was someone one here that used a Cat 700 twin for their ultralight, so you might want to try a search and see if you can find the topic(s) that dealt with that.

Thanks,

Sled engines were chosen because of excellent availability and cost. I looked at the marine engines but the used mkt. is small in Alberta.

I realize that I am thinking way out of the curve and powerband of the average two stroke. My idea is to use the low end of the power output and have a huge built in reliability factor. The power valve engines have some advantages here.

A speed reducer will take prop speed down to 2500 rpm so the ratio will be decided once top engine speed is decided.

How would you guys feel flying low and slow at 1000 feet above ground with a Cat Triple up front? From the big Cat I hoped to approach 85-100 HP with monster torque. Would a higher rpm range be reliable, say 6200?

If you can deal with the mess of triple pipes it should be made workable. Peak RPM on a stock T-Cat engine should be around 8500 +/- a couple hundred. Engagement RPM was probably around 4000 - 4500. I don't see why these engines couldn't run happily at a constant low RPM, 4 - 6000, if properly tuned. The gear reduction for the prop multiplies the torque, so it should provide good power. There should be dyno charts available on the web somewhere for the T-Cat engines, which would show you the power curve. I think peak power is around 160 -170 HP at peak RPM and peak torque might be in the 90 lbs/ft range at a slightly lower RPM. There will be some people here that will know the real numbers and can correct any number I put down.

If the engine is in front, then it will be real loud, but it will sound great.

we have looked into this,but the gearbox mounting is the problem on the cat triples there is no room around the case where the pto shaft exits to bore bolt holes,there was a rumour rotax made a few triples for gear box but they are almost non existant,polaris you say? nope not enough crankcase material around pto shaft,of course you could rigg a detached gearbox but the extra weight we encountered really sucked and it looked bad,having two stress points [engine+separate box] was just to much.we settled on rotax 670 with power valves some of these motors have the bolt holes already there for the gearbox and lots of good torque down lower rpm easy prop dial in these engines made 128hp and detuned with pipe these make a very reliable 100hp and light,you could mod this engine for more power very easy but modded engine 500ft in the air no thanks. these engines are in a terra