I got a Bobcat 50 pusher jet for Christmas.

http://www.nitroplanes.com/usbojet5051n.html

I am looking at a Clarence Lee Modified K&B .61 engine for power. Has anyone had any experience with this engine? I need to stay around $100-$150.


Thanks,

Larry

Hi!
Why and old Lee modified K&B .61 engine? Why not a more modern, more powerful .61 engine?

He still sells them. See the classified ads in MA and MAN magazines.

Tom

The Clarence Lee modified K&B 61 is a very powerful 61. They like the 11 1/2 - 7 prop and will really make it scream. They weigh very little more than a 46. I've had four of them. Still have one. They are rather loud so if you are in an area where noise is a problem, you better be careful.

I have never seen one run but I have seen the K&B Twister 61 and it is winner.

Larry I see on the nitro web they have a 3 blade pusher on that sucker. Thats gotta be special order........That K&B will weigh less than most 61's now a days............

Thanks for the info. The engine came in the mail today...any suggestions on a tuned pipe and where to get it?

http://www.macspro.com/pretunedsystem.asp

# 1970 $110.95

Most bang for the buck.

The K&B should get me about 12,000 RPM with a 11.5x7 prop ($135.00). The tuned pipe should raise the RPM to 13,000 ($110.00). Tower Hobbies .75 ABC should give 15,000-16,000 RPM (as they state in their catalog) for about $100.00. There is a small weight gain for the tower engine but it seems to have a lot more power. I know Clarence Lee Modified K&B .61 engines have a good rep...however, I am looking for the best bang for the buck with speed being the main factor.

Which way should I go?

I think you will find that the CFL K&B 61 will turn closer to 13,500

w8ye,

Should I also use the perry carb that tower sells for the K&B .61? Would this give any extra power? I found out it is a K&B 6550...not the twister.

Larry,


The Tower .75 will not turn that RPM with this prop.

...Nor will it even be close.


I believe FBD quoted 14,000 for the MA 11x7.5, which is markedly lower in load, compared to the 11.5x7 (Bolly ??).


Jan,


Maybe because it only weighs 14 oz. (397 gr.)?

I've used all the carbs that came on these engines and never saw much difference in power. I've had several and the one I have left has the older square aluminum carb with no fins and the low speed needle inside the throttle arm. I also have a Perry pumper carb and pump but I didn't see any noticeable increase in rpm.

My favorite of the ones I've used had a Perry carb on it but didn't make any more power.

Sorry about the multiple post...I was just trying to get an opinion about the tower .75 engine in relation to the K&B .61. I should have just continued that in this thread.

My mistake

Well, there's no doubt that the Tower 75 actually has more power than a K&B 61. Even a pumped CF Lee Version of the K&B 61.

Also the Tower 75 is a rather popular engine now.

Also a 75 two stroke is not a legal size for any kind of old time pattern competition like the SPA.

Hi!
Yes Dar I know it's light but is an old design..no power compared to modern .61 engines. But he now wants speed and this he will not get with an old KB engine.
A much better engine would be an OS .

I think I have been sold on either a Tower .75 or (if I can afford it) an OS .91. The .91 is $100.00 more but I am hearing that it will scream. I think it is not that much heavier than the Tower .75.

The OS 91 fx weighs exactly 770 grams with the muffler, just forget about that 550 grams bull.... OS is advertising, so infact it is a great deal heavier than the tower 75. I am on the brink of buying the Tower 75 myself, just to replace my 91 fx to lighten up a plane a bit. From what I have read the power of that 75 is close to the 91 fx

I am still doing R&D to find out what works best so please give details on any coments and responses.

These two posts are why I am leaning toward the .91:

http://www.rcuniverse.com/forum/m_45...15/key_/tm.htm

"Hi
This is my “Nitro models pusher Bobcat”.
Check the main gear retracts. I did not sink the wheels into the wing or fuselage.
The placement of the retract did not interfere with any of the wing structure.
This let me put a trimmed Master Airscrew 14x7 3-Blade Pusher on the O.S. 91.

This combo moves out!!!!!
The comets at the field was “except for the sound it just like a turbine jet”
I also agree.
Johnny
"

"Hi
On my OS 91 with the11X7P I also estimate the level flight speed to be around 110mph. with a lot of rpm!

Raising the gear cleared a trimmed Master Airscrew 14x7 3-Blade Pusher prop. (13.5x7)
This is when the performance went through the roof.
I would suggest this setup to anyone with the power to swing the prop.
Johnny "

Thanks,

Larry

The Tower 75 has a better reliability record than the OS 91.

Many of the second series of OS 91FX was nothing but trouble from the mixture going lean.

The current OS 91's with the plastic needle valve are OK.

Would putting a pipe on the Tower .75 be worth trying...or would I not gain that much power? I know that some engines can't handle the stress it puts on them.

I heartily agree with W8ye on the OS91. Not the best effort by OS. I have a ST G90 on a Stick with a 16oz tank in it and a 13X6 MAS on it. Another fellow has the OS91 in a Cap580 with the same tank and prop. I can pull him....not by alot but I can pull him and he has a cleaner plane. He has also sent the engine back and tried it again and has now sold it. He works in a Hobby shop in Orlando and has verified that they have had many returns on these engines. Another guy just bought a Tower 75 and his Stick runs right with mine...a very powerful engine. Also have a radar gun and we checked speeds of the three planes. Stick/G90=103mph, Stick/Tower 75=101mph, and the Cap 580/OS 91=101mph. Tower has a Hell of a punch for the money in my book!

Larry,

I am confused! What engine came in the mail? How would you mount a tuned pipe on a pusher? Why buy a KB61 Lee modified special when a KB Twister head will run rings around it for less $? Or a Tower 61 or 75 - which are the best buys for the money available!

Phil

Phil why do people have old 60's corvettes in their garage when a new one is faster ? The dirty little secret is weight wise the K&B is a drop in 46 replacement and in some cases up to 10oz lighter than modern 61 and up. I just replaced a 61 in a contender with a K&B 61 and took 3 ozs of weight off the tail.

If you want to go fast, you need lots of R's and prop pitch to do so. But you probably know this already.

I like the ST G90 for this application too. It doesn't mind spinning up and it can handle a fair amount of prop pitch.

Yep, a small diameter, but large pitch prop will drag out your takeoff run, just like a real jet. Some things you just have to live with. Flaps would help get the model off the ground a bit quicker, or increase the size of the ailerons and use them as flaperons.

Performance Specialty Products (hope I got it right), otherwise known as PSP on the forum, sells an Ultrathrust muffler that provides a bit of a boost for the G90. In fact, I think Jett sells a similar item for this engine too. It simply bolts on to the engine, requires no special tuning and is trouble-free. You will have to experiment with prop sizes in order to get the engine "on the pipe".


Ed Cregger