Author Topic: Poulan 46cc Porting Plane conversion  (Read 1323 times)

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Online 660magnum

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Re: Poulan 46cc Porting Plane conversion
« Reply #10 on: August 03, 2014, 09:22:34 pm »
With the Poulan 46 you usually have to make a carb insulator block that attaches to the cylinder with set screws
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Re: Poulan 46cc Porting Plane conversion
« Reply #11 on: August 03, 2014, 11:09:13 pm »
I have several Poulan 295's , 46cc chainsaws that I have worked on.
First I have removed the metal piece or heat shield in the exhaust and then widened the exhaust port to eliminate the step.
Like 660 said I would not raise or lower the exhaust port opening.   
On the stock muffler I usually weld the back holes up and then make a new one on the clutch side.
I then cut away about half of the center of the baffle out and leave the spark arrestor in place.
I have done plenty of cutting and with the mods and have had no problems.   
Also when ever I grind on a port I either sand the edge smooth or bevel it just a little to prevent the ring from hanging up on it.

I did bump up the compression on one but it was done in a rather unconventional method and so I will not go into that.
Like 660 said you should look for mods that give torque but not RPM.   
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Offline Spruce_Goose

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Re: Poulan 46cc Porting Plane conversion
« Reply #12 on: August 04, 2014, 09:14:33 pm »
Quote
First I have removed the metal piece or heat shield in the exhaust and then widened the exhaust port to eliminate the step.

That's about what I had planned. The only issue I see is that the top step will still be there. I assume this is not ideal for it will create eddies. Is there a way to smooth the step without actually removing it entirely causing timing changes? Maybe just a bevel. Or adding material in that little crevice even? I didn't plan on touching the intake unless it makes sense to do some sort of balancing...

Quote
Like 660 said you should look for mods that give torque but not RPM.

Does widening ports actually increase torque or is it only RPM really?
8,000 rpm isn't too far off cutting-saw peak performance is it. Don't many saws peak power at around 9,000 rpm? For planes is it that they want lower rpm simply because a super high speed prob is less efficient than a larger more consistent speed one with more load/force output?

In regards to the intake... this probably will show my ignorance, but do some saws use one way valves and others don't? As far as I can tell there is nothing stopping flow back out the intake of the cylinder through the carb. No valve that is. But I've heard of some of that physics stuff that describe why most of the fuel air doesn't blow back out...:)

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Re: Poulan 46cc Porting Plane conversion
« Reply #13 on: August 04, 2014, 11:13:13 pm »
The piston skirt closes off the intake port at the proper time. The intake port begins to open after the piston starts up and closes after the piston starts down.
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Re: Poulan 46cc Porting Plane conversion
« Reply #14 on: August 04, 2014, 11:50:30 pm »
To answer your question about the step in the exhaust port.   Yes I did taper mine, but I did not raise the port.   That will drop compression, lose torque, and raise rpm,   

The point of widening the ports is to increase flow through the engine.   If the exhaust flows out better then it will also allow the incoming fuel air mixture to increase.

  In reality I think anytime you make any changes to these engines that helps to increase the HP rating there is also going to be a small increase in RPM.   
PP 505, 475, 445.

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Re: Poulan 46cc Porting Plane conversion
« Reply #15 on: August 05, 2014, 12:32:31 am »
Though I've seen several 46cc Poulan model airplane engines, I had two different iterations of the 45cc Timberman Homelites of which I originally intended to make model airplane engines but never did. The Homelites did not turn the rpm in the cut that my Stihl 025/MS250's do. They miss the mark by a couple thousand rpm. Stihl 9000 rpm / Homelite 7000 rpm. These Homelites have never given any trouble but fuel filters over the years. One of my kids still uses them for limbing.

But if you are used to Husqvarna 346 and 357XP's, you get the impression that the Homelite is not running full throttle.

I don't really know how the Poulan and the Homelite compared in the cut?
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Offline Spruce_Goose

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Re: Poulan 46cc Porting Plane conversion
« Reply #16 on: August 08, 2014, 12:05:56 pm »
"But if you are used to Husqvarna 346 and 357XP's, you get the impression that the Homelite is not running full throttle.

I don't really know how the Poulan and the Homelite compared in the cut?
"

Yeah, for the short test I did do with the Poulan before ripping it apart, it felt a lot more sluggish than the Husqvarna XP models. Kind of like you said: not feeling wide open when it actually was. Not sure what this means in terms of airplane engine performance or how it will change if I do any porting or muffler mods, but we'll see!

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Re: Poulan 46cc Porting Plane conversion
« Reply #17 on: August 08, 2014, 09:23:16 pm »
They run pretty much like a 46cc gas model engine should. They are more powerful than the US41 and Zenoah G38's but less power than a Brison 3.2 (Dolmar 115).
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Offline 1manband

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Re: Poulan 46cc Porting Plane conversion
« Reply #18 on: August 10, 2014, 05:40:54 am »
first thing is, this is an insomiac reponse, so tread lightly.

sounds like the torque curve of the motor needs to come down in rpm, and the powerband needs to narrow up.

similar to this idea: http://www.epi-eng.com/aircraft_engine_products/ls_crate_engines.htm

maybe a smaller venturi carb, or better yet a smaller carb plus less intake port x-sectional area/duration may help?

compare the airplane optimal torque curve from the first link to page 19 of the second link here: http://www.vintagesnow.com/SledU_Folder/delivery_ratio-1.pdf

epoxy will be your friend.  don't know if rc folks do any of this?


regards
-joe


edit:  would help if i could spell and actually include the links



 
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Offline Spruce_Goose

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Re: Poulan 46cc Porting Plane conversion
« Reply #19 on: August 10, 2014, 05:26:38 pm »
For these clamshell style cranks, anything to know about resealing it? Should I be using something like Yama bond?

I also ground some of the fins down a tad bit (just to round them out aesthetically) and so now there is exposed fresh aluminum. Is there something (and should I) to cover this with to prevent corrosion. I know some people paint or powder coat the heads. I would prefer to avoid any process too involved or costly. Really if I could just throw some bluing agent, or oil or seal, etc. on real quick that would be ideal.

 

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