forsakenrider

FINALLY got an electric starter (sullivan hornet) and got rid of the "muffler" and added my pressure tapped back plate and we got another 1400rpm out of her! 26600 steady! for a big mig that sounds pretty good. I put my unmodified AME P/L and it gave me 23300 but the sun is going down so i didnt have a chance to play with head shims, not to mentions its -19c here right now.

forsakenrider

Got some props, nelson head, and N/V assemblies from Larry and now my Toadified bigmig is complete. On a kt17 4.6X2.8 prop, 30%, and 3 shims we've got 31000 steady out of the BigMig. Now Ive just gotta tighten my shoes and get'er up in the air.....*gulp*

Thanks toad! over 30 is SCARY!

Toad

Thats really good RPM numbers on a BigMig and that prop...
You are gonna have a blast!
Stick with straight castor oil fuels, she will last longer

Toad

Going back a bit and reading this thread, I realized I didnt comment on the "circular" cut on the bottom of the piston you seen. that is there so the piston on the .061 will not hit the back cover at BDC. The .049 clears so it isnt relieved there.
On my modified pistons, that "circular cut is completly machined away. If you are kinda handy with a dremel tool, it is easy to do as long as you dont go too wide with the window you are cutting and only go as deep as the counterbore in the piston.
Great fun!

forsakenrider

Yeah I think I figured that out after a little thinking anyway.

One thing I did want to ask about the piston trimming though was, why not trim it 3/4 of the way around just leave the exhaust side? I've seen this somewhere before on a different engine, bigger cyclon maybe? I forget. Have you tried that or thought about it?

Toad

I did that, twice. On each they wore out very fast, like 4 runs on the stand. On a good look at them and the sleeves, I discovered the sleeves are tapered more than most other 1/2a's and removing the skirt minus that part needed to block the exhaust port from venting into the lower case at TDC, the piston "rocked" too much and literally wore it out in a hurry. the piston rocked back and forth from the exhaust side and the oppossing missing skirt side. So I dont do that on the Norvels anymore.
I was thinking of doing the skirt removal mod on a GZ, only to notice that the sleeve on that engine is very short. If I removed most the skirt I think the piston would rock too much and the same thing would happen, it would wear out in a hurry.
A long sleeve with very little taper in it, and a Rear Exhaust porting cinfiguration would be the ticket. Exactly how Cyclon and Profi are doing it on their 1/2a's.
Im thinkin its best to leave some skirt on the piston oposite the exhaust on the Norvels, get a bit too much rockin going on and the timming goes whacky also, like bypasses openning way too early because the top of the piston is now .010 or more lower than intended on that side because the piston rocked sideways some. it dont take much to ruin it. Too much taper in the sleeve adds to it bigtime.

forsakenrider

and heres some pics of my work

forsakenrider

and some more.

Some R/C car guy inspired crank mods.

Toad

Very nice!
Thanks for showing me that. Interesting Crank work.

forsakenrider

Ok then If we only need to prevent rocking, then what about trimming the skirt on that side to about the width of the port opposite the exhaust.. I think maybe removing a 1/4 of it on each side would leave a decent chunk and leave it stock length. Looking at it I think it would work, any ideas as to why it wouldn't?

Also I dremmeled the insides of my bigmig liner and polished it around the intakes (another r/c car guy thing I stumbled upon) and after 8-10oz of fuel I dont see any peeling or discolouration or anything, but I dont really know if I gained anything doing so.

DeviousDave

I wonder at what point with piston lightening you end up with vibration issues? It would be easy enough to calculate if you had the measurements of the counterweight, the math skills or access to a cad program that computes volume.

Toad

The cad/cam I use can calculate volume's..... I think a lot of material can be pulled off the piston before bad vibes start to arise.
I cut the skirt to open up the bypass flow, More important to me than lightening the piston, that helps also though.
I cant get myself to experiment much more with em, parts are getting hard to find.
I hope some one else trys it and lets us know soon. If it works well then I will do it on my next one.

forsakenrider

Ok ok toad, sounds like a challenge! I will try it, maybe even this week.

RocketRob

My Feedback: (1)

Forsakenrider,
Could you explain in greater detail the mods for both the crank and liner and theary behind?
Some more photos would be great also?
Thank you, Rob

forsakenrider

The liner is simple. the Idea is to create less corners for the fuel mixture flow. If theres less corners then it should flow smoother thus creating a better mix (im not entirely sold on this)

as for the crank, there are two ideas there. One is to lighten the counter balance since we are lightening the piston and the other is to mix the fuel inside the crankcase. Ive seen some people go as far as to cut turbine like fins in the crank claiming that it would "compress" the mixture inside the crankcase so you would get a bigger "bang".

All of this I found googling about crank balancing. I dont know if Im completely sold on it but my modified Bigmig is screaming so far.

Toad

I would think I would want to leave the couterbalance heavy as the piston weight is way out of proportion, to balance it correctly, I dont think you could remove enough ofF of that piston to get it to balance. (with the stock crank.)
I understood that we are suppose to balance out all the con rod weight and 1/2 the piston weight with the counterbalance, I cant see where that is possible in 1/2a size motors unless you use an ungodly huge crank, which is heavy. cant win that game.

DeviousDave

Is the crank timing stock?

Toad

I dont change the timming on mine, I just relieve and lighten them a bit.

forsakenrider

Mine is also stock. I wondered about playing with it, or maybe stuffing it, but I havent got that far yet.

RocketRob

My Feedback: (1)

Thanks for the info guys!

forsakenrider

http://cgi.ebay.com/sirio-picco-rb-n...QQcmdZViewItem

that is one place I got the crank ideas from....

unattainium

Balancing cannot be perfect in a single cylinder engine. What is involved is balancing the rotating mass, which is not so difficult because it involves the crankshaft and the rotating part of the conrod plus the impossible part, the balancing of a single reciprocating piston and the reciprocating component of the rod.

Here is an article which gives an empirical method for balancing an engine by using the crankshaft counterweight to compensate for some of the out of balance effects of the piston and rod.

I have noted that some engines increase the counterweight mass by drilling a hole in it and inserting a piece of tungsten carbide, which is very heavy because of the tungsten. Recently in an aircraft parts store I saw some depleted uranium counterweights pulled from a DC10. This material would be ideal for crankshaft balancing since it is heavier than tungsten

Tungsten carbide is brazed onto steel tools and a similar method using brass or silver solder could ensure that the counterweight would not come loose.

Finally an optimal balance for a single cylinder engine can only be achieved at one RPM and hence changes with different props.. Fine tuning can be done with deliberate propeller imbalance or crankshaft drive plate balance weights.

Read this http://findarticles.com/p/articles/m...g=content;col1

I have noted that when a crankshaft is shaped for better balance the parts removed are replaced with a light metal filler to retain the crankcase compression.

Minimizing piston weight and the reciprocating part of a conrod can dramatically reduce the inertia loads on the rod and crankpin at high RPM, and no doubt also reduce the out of balance forces inherent in a single piston engine. On that basis any piston lightening would be a very good thing.