Mounting 618 and 670 to KF-IV

[Grogen]

Here it is without the cowls, just the front cowl. There is more room under the new cowl compared with the old one. I ran the 670 under both. The 670 is a little taller than the 582 (longer stroke) but will fit nicely under the original cowl, especially if you centered your prop hub in the ring cowl.

[Dusty]

Good to see some positive feedback on the 670, I'm keen to get mine going but my engineer is going a little too slow
A 670 is the only option for my 1050pound kf3 as any more weight will put my gross close to max weight on a trip!
We are taking a slightly different approach how to set this engine up with a standard exhaust but longer primary collectors creating the equivalent to a high torque 582 . Power is calculated in the mid to high 70's but a huge improvement
on the 582's 60 -65hp ,if that doesn't work we will fit a 618 exhaust but this will be a tight fit!
I will post updates when it is finally installed
Thanks for your post

[rdooley79]

Good to see another Kitfox getting a 670. I've been very impressed with both the power and the fuel burn. It's a great powerplant upgrade for us 2 stroke guys.

As for the exhaust, I am re-working it's mount on my plane. The coolant and fuel lines are just too close for comfort. I've seen the exhaust over time with vibration work it's way closer and closer to the coolant line on the pilot's side. If you look at my pic's above you'll see that it is close. Sure I could add some more brackets to stiffen it up more and probably stop the movement BUT it just makes me worry.

Decided to completely change how it's hung.

Here's what I want to do. May take a little cutting and welding but I think it'll be the safest way to mount. It solves all the problems and all the potential problems. No more worries!

[Grogen]

Wow... I like how the exhaust is hung on your plane. I think it is important to mount the exhaust to the engine itself and allow the whole unit to vibrate together. Attaching to the engine mount sets the exhaust up for structural failure. My biggest issue is noise. My plane is loud, so much so that it is hard to hear the radios. I have a muffler on my exhaust system but it is still pretty loud. How is your 618 exhaust for noise? The guy who did my exhaust got a 618 exhaust and built one from scratch. He tested the 618 exhaust with the 670 and was getting 95 HP on a dyno and he said it was pretty quiet. I would be willing to make that change if I could keep the HP and reduce the noise.

To Dusty, If you are only getting 70 HP out of the 670 it isn't worth it. The 670 will run all day at 6,500 to 7,000 and give you great performance. Everyone running Rotax two strokes is scared to go to 7,000 because the 582 "red line" is 6,500. In a snowmobile they run these engines up to 8,000 and they last for years. I run mine at 7,000 when needed then back it to 6,500 to 6,800. As I mentioned, I have over 300 hours on the engine and it is showing very little signs of wear. If I were to do it again and were in the building stage I would put a 740 big bore kit on before I put it together. That is were I am going when I rebuild this engine. Without the proper exhaust, the engine is being restricted and won't deliver even a portion of its safe potential.

[Dusty]

Fair comment on the 670hp,I would like to think I am being realistic for the rpm I run, better would be good!
Th 670 pistons are a lot heavier than 582 pistons ,my engineer calculated the load on the rod ends at various rpm with both 582 and 670( his program didn't factor in the rod length on acceleration) some truly scary numbers.
These got scarier as the revs went up,probably ok for short duration but would generate a lot of heat in our situation.
Rotax rick? Suggested full power when needed then back off when possible,which makes sense.
If these motors are doing regular 300 plus hrs then I may have to have a rethink!
300 hrs at 50 mph average speed (estimated) =15,000 miles.We don't have many snowmobiles here , but do they do that sort of milage?

The photos of the 618 exhaust mounted lengthwise look like way to go!

[Av8r3400]

High output, carburated, snowmobile engines rarely make 10,000 miles without rebuild.

[Grogen]

Snowmobiles, at least riding here in the rock mountains aren't usually ridden at 50 mph. For short bursts they will do 100 mph, but trail riding is 20-30 mph and of course when climbing you put a huge load on the engines and machine.

In my plane I take off full throttle then back off when airspeed and altitude are reached. I cruise at 85 mph at 5,900 to 6,100 at 6,000-8,000 ft (my field elevation is 5,000). The throttle is a little more than a quarter open in cruise flight. I burn around 5 gallons an hour. Even at these altitudes I am getting great performance. I can get airborne in 150' and land in 3-500' depending on winds. At cruise the engine is not even breathing hard, it feels very comfortable. The biggest thing about two stokes is keeping them within operating perameters and making corrections quickly when they aren't where they need to be. For instance, if you pull the power back to say 5,000 and push the nose over your EGTs will start climbing quickly. You are effectively leaning out the engine by "driving" the engine with airspeed and reducing the load. You can't let it stay there or you will burn it up. Either go to idle or level off and add power so that the fuel going trough the engine can cool things down. Just two strokes, if you watch what is going on and know how to bring things in limits the engine will last a very long time.


If you are only getting 70hp out of your 670 you won't know what you are missing. 20hp is a lot to give up because you didn't put the right exhaust on. My dad has a 582 model IV and my model II 670 will blow the socks off his plane. After flying mine, especially with two people in the mountains and experiencing a slight down draft that he didn't have the performance to out climb, he is moving to the 670 and wants to bore his to 740. It is the difference between being able to get out of a situation or letting Mother Nature pilot your plane. I flying in the air force and the plane I fly was build back in the 50's. The original was very underpowered and there were many accidents due to the aircrafts inability to get out of a bad situation. The planes have been re-engined with significantly more thrust. The performance accidents disappeared and made the aircraft much safer. I consider the 670 on a Kitfox a much safer and capable aircraft than one with a 582 or less. That opinion comes from direct experience with both. If you load up a fox with two people and full fuel with a 582 it's a little gutless. The 670 changes the equation and gives the pilot more options.

[Grogen]

Dusty, my wife lived in NZ when she was a teen and is planning on running the iron man in the spring in NZ. If we get to your neck of the woods I'll have to look you up!

[Dusty]

90 is better thn 70- 75hp
I suppose just because the power is there I don't have to use it.
The 618 exhaust will fit lengthwise ,if it gives stable rpm at cruise then I agree it is a must have.
I fly at 5700 70 kts 15litres per hour,I probably won't try to fly faster but. Better rate of climb is the goal as is a shorter takeoff.
On a real nice day ,not in a hurry 60kts just love the flying!
Fellow flyers are always welcome here.

[LORENZ]

Snowmobiles, at least riding here in the rock mountains aren't usually ridden at 50 mph. For short bursts they will do 100 mph, but trail riding is 20-30 mph and of course when climbing you put a huge load on the engines and machine.

Thanks for sharing. Do you have experience with the 912 vs the 670 when it comes to mes to STOL characteristics. My siezed 582 is comming out and Im debating the 670 and 912.