bogateer

Okay, so I've seen advertised a Subinduction Lightweight Piston. It appears to be shorter and thinner than a standard piston. Just how does this work? Does it substantially increase power? What does it do to the longevity of the piston?

Thanks,

Rob

ffkiwi

And it's currently the subject of a heated discussion on the 1/2A collectors forum right now. It won't necesarily do anything beneficial, and could well cost performance. Pistons are matched to appropriate cylinders and porting. Getting SPI in a cylinder which does not have it in its std form MIGHT show a slight performance increase-but will show a decrease if a silencer is fitted. Not might-WILL! with-100% certainty. Conversely, fitting a shortened piston to a SPI cylinder will increase the SPI period-but may not show any increase in performance if the std setup was optimal.
Two separate issues have been combined into one by this ebay vendor-lightening the piston will reduce reciprocating weight and generally improve revs, SPI may or may not, depending on the induction system and cylinder porting. Put the two together in a single aftermarket item and you have a hit and miss situation-it may work in some instances-it definitely won't in all.

ChrisM
'ffkiwi'

Tee Bee

I agree for the most part. I do have a Skyline .049 by Time Wiltse with SPI that just screams, though. I think his engines run good due to several factors, not just the SPI. I love to run it. However; I've noticed that the extra rpms require that I keep an eye on the piston/rod clearance and reset it more often. The curiousity overwhelmed me about the ones that the bayvendor is selling and I've ordered one of his piston/cylinder sets to check it out.

combatpigg

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The main benefit I see to lighter pistons is that the engine will seek a higher red line [with less internal stress] if you take the extra steps to allow it to breathe [open up the intake]. The lighter piston takes some load away from the ball socket and from the whole bottom end in general. I doubt that it will make the engine turn a 6x3 any better, that is a torque issue. The only way to increase torque is with stroke and compression, this obviously doesn't do that.
The lighter piston works in applications where you take as much prop load as possible away from the engine, but not so much that it can't fly the weight and drag of your plane. This is where the 5x3 and 4.2x4 props work very well.
All you have to do to become a believer is see Ptulmer's Killerbee run.

Tee Bee

I'm addicted to the sound of my Skydrill flying by with the Skyline engine turning a 4.2x4 prop! Several folks have been surprised to learn that it was just a Cox 049 and that they've never heard one run that good.

bogateer

Thanks for your responses...I appreciate the info

DeviousDave

You know, all this talk of lightening pistons makes me wonder if these guys ever bother to take any weight off of the flywheel counterweight....

combatpigg

My Feedback: (3)

DD, I haven't seen the need to do it yet. The engines seem to run smoother [or just as smooth] after lightening the pistons. Stock weight pistons will predictably fail the ball socket above 24,000 rpm, lightened pistons will run over 28,000 with ten times the reliability.
My experience is with TDs running 4.25x3 props against AMEs and VAs in C/L combat.

bogateer

Actually the original Killer Bee (not the recent ones called Killer Bees) does this (lightened crankshaft flywheel). I forget if it has a lightened piston. The Cox Venom racing engine has a lightened piston (and I forget if it has the lightened crank flywheel as well).

combatpigg

My Feedback: (3)

The overall smaller crank might give the KB more potential than the TD? So far the most powerful Cox .049 I've ever seen was a KB.

Luna_Rendezvous

My KB crank is slightly heavier than the standard 049 reedie crank, is thicker around the webbing and is balanced better, and does about 2K more rpms than my black widow on a apc 5.7x3

combatpigg

My Feedback: (3)

Look at the difference in size between the intake passages. The KB is the only model I've seen that allows for a .180 hole.

Remby

My Feedback: (3)

Don't forget the aluminum rod piston Cox issued early in the Baby Bee, that really helps the R's, even stock.

It wouldn't last long, but it did run faster.

forsakenrider

What if the engine already has sub-induction? then do you grind away the piston walls to lighten it? I would really like to see what kinda rpms i can get on my mouse's engine!

combatpigg

My Feedback: (3)

FSR, the thinning of the piston is done with a scraping [or shaving] action with the back side of a #11 Xacto blade. You can go paper thin. With enough outward pressure [use a dull tool] you can also swell a piston.

forsakenrider

WOW! thanks CP. now i have a reason to save a few of the blades. I think i might try it out right now, im not ready for bed yet!.

combatpigg

My Feedback: (3)

Well now, not so fast! I forgot to mention that you will need to make a holding fixture so that you can turn the piston with a drill motor mounted sideways [or a lathe]. The idea is to make a bushing with fairly thick walls that is a close fit to the piston's O.D.. Then cut a slit in the bushing so it can clamp a Cox piston snug,[without warping it] then chuck it up in your lathe. Use a length of music wire with a small hook in it to secure the rod to one side as the piston is turned. Now you're all set to start carving. I use a tool rest to steady the blade, but it is done freehand. If you have a grain scale you could make a science out of how far to go, otherwise just have trust in your eagle eyes to judge when enough is enough.

BTW, if you're already a machinist, sorry for the boring and crude DIY class....

forsakenrider

HA! Im far from a machinist, but seriously into DIY. Im still working on the holding fixture idea cuz i definitely dont have a lathe in my shopartment.... but maybe I could go to the university and ask my friend, but thats no fun. any info on something the right size I should keep my eye out for? Im going to throw a piston on my pocket and check the cut offs from aluminium ski poles at work today.

combatpigg

My Feedback: (3)

I've seen a guy take hose clamps, duct tape, etc.....whatever it takes to mount a standard drill motor flat on a board. You got to be able to watch the progress of the cut, that's why this operation should be done with the drill laying flat. .The leaded steel is relatively soft and it peels off nicely. I would grind a slight radius at the end tip of your cutting tool, so the upper inside corner of the piston is radiused.

DICKEYBIRD

CP describes the piston lightening thing pretty well. I never tried it with an Exacto blade but did mine with a Dremel and a little 1/8" "pea" shaped carbide burr. I searched in the wayback machine and came up with this old post that has a picture of my drill/poor man's lathe. It's in post #12 or 13.

http://www.rcuniverse.com/forum/m_68.../tm.htm#688342

I never went on as drastic a weight reduction program as CP did; just tried to emulate a Venom or KB which is around 2.8 gms if memory serves. They worked well for what I was doing at the time.

Looking at that old post makes me wonder what ol' Prole's up to these days. Rekkin' he's bought himself a Carribean island with our T-shirt money?

forsakenrider

WOW cp and db. I will be keeping my eyes out for an old 1/2" chucked drill for the poor mans lathe, looks great. For now ive come up with a holding fixture made from an old ski pole that is tapered and has 4 slots cut in it, I slide another piece of ski pole over it to tighten it onto the piston. seems to work decent but my drill is definitely not the roundest spinning machine. I should splurge and buy a scale now, this piston looks LIGHT. I might try and go thinner since this is a trial version but its pretty thin! The #11 blade works EXCELLENT. Id love to see what kinda power i can get out of a reedie....

Thanks guys! but you are giving me way toooo many projects! ha.

combatpigg

My Feedback: (3)

I'm glad you got 'er done. If there's a will, there's a way.

Honker1

My Feedback: (4)

Where is the 1/2a engine collectors forum? I would like to read what they have to say. And yes, with SPI you cannot use a muffler and get a good run. I have been flying these engines ( like many of you) for over, I hate to say it, 35 years. As easy as the larger ones are to operate I can't get away from the Cox engines. Must be my memories from childhood. I got a Cox control line airplane every Christmas and birthday for years. My dad was a pilot and he encouraged me to get into aviation. I must say though, I have more fun with the 1/2 stuff than full scale!!

If only OS would make a good .049!

Thanks,

Honker1

Tee Bee

"If only OS would make a good .049! "

I'll second that. I'm also a full-scale pilot and have been around aviation most of my life but still get a big kick out of flying 1/2A models. I still remember the short period that I owned a Cox Shrike car as a kid and hearing it fire up for the first time. Have loved the sound since then.