Re: Sub induction

PlaidHatter Sun Jul 19, 2015 12:46 am

That's a pretty precise tight gap!
Razz

SkyStreak Mon Mar 12, 2018 1:29 pm

Sorry for rehashing an older thread but I am fairly new to the forum.

Can you take a older non slit, non SPI piston/cylinder set and shorten the piston and get the benefit that the SPI provides.
I would just try it but I don't want to render a piston (in my limited stock) un-usable for something that may be pointless.

KariFS Mon Mar 12, 2018 2:20 pm

I think non-slit, non-SPI cylinders are at least somewhat rare. They were used in QZ and QRC engines. I am all for experimenting, but if you just want SPI, it should be easy get a correct ready-made cylinder.

Of course, trimming the piston skirt will make the piston lighter, and you may gain some rpm over a regular SPI cylinder.

fredvon4 Mon Mar 12, 2018 2:24 pm

1/2a nut thanks

I missed Megan Fox way back when...

**Admin** Mon Mar 12, 2018 5:33 pm

fredvon4 wrote:1/2a nut thanks

I missed Megan Fox way back when...

yeah, I think that was GermanBeez playing around. I haven't seen him in a LONG time. Same with nitroairplane, last I heard (several years ago now) his shed burnt down which had all his planes and engines in it.

**RknRusty** Mon Mar 12, 2018 8:10 pm

SkyStreak wrote:Sorry for rehashing an older thread but I am fairly new to the forum.

Can you take a older non slit, non SPI piston/cylinder set and shorten the piston and get the benefit that the SPI provides.
I would just try it but I don't want to render a piston (in my limited stock) un-usable for something that may be pointless.

My educated guess is yes. I think you'd get about the same performance as a slit exhaust with a short skirted piston. I think the location of the bottom of the old open exhaust port is the same height as the slit ports. It's been said by experts that I trust that the area of the old wide open ports was overkill, and the change to slit with its reduced open area caused no appreciable restriction.

Plus you'd gain the benefit of a lighter weight piston, even without thinning its wall, as some tuners do.

Rusty

SkyStreak Wed Mar 14, 2018 9:54 am

I might try shortening the piston skirt on one of my twin slit sets to see what that does.
If one does not have access to a machine shop or equipment how would one accomplish shortening a piston with basic resources???

**RknRusty** Wed Mar 14, 2018 10:42 am

You could try wrapping tape around the piston and chuck it in a drill press. Use the tape edge to expose the metal to be removed, and carefully use a small file to skim it off.

MauricioB Wed Mar 14, 2018 10:48 am

Admin wrote:
fredvon4 wrote:1/2a nut thanks

I missed Megan Fox way back when...

yeah, I think that was GermanBeez playing around. I haven't seen him in a LONG time. Same with nitroairplane, last I heard (several years ago now) his shed burnt down which had all his planes and engines in it.

sorry when it says nitroairplene refers to this person ?:
https://www.youtube.com/channel/UCD70RDxxXd10PmmFx0JQRqg

roddie Wed Mar 14, 2018 1:09 pm

SkyStreak wrote:I might try shortening the piston skirt on one of my twin slit sets to see what that does.
If one does not have access to a machine shop or equipment how would one accomplish shortening a piston with basic resources???

Here's a photo of a simple tooling setup that is pretty-much self-explanatory.. although I can't take the credit for the photo.. nor have I performed the procedure.

The piston is held in a chuck with the connecting-rod's eye "wired" to a stand-off out of the way. The ball-socket joint makes this possible. A rotary tool is then used to remove material from the piston-skirt. If desired; removing material from the inside of the skirt can result in "lightening" the piston without shortening the skirt.. BUT it's a precise operation that's impossible to perform accurately without the use of a lathe.. and precision measuring instruments. Shortening the skirt to add (or increase) sub-piston induction would again; require the use of a vernier/caliper to accurately measure "how much" of the skirt is being removed in the process.

FYI/FWIW... A worn Cox piston will still run, but you may need an electric-starter assist to get the engine running. If you go and do some successful lightening to a piston.. you'll now have a hot-rod engine-part. CEF has an annual "tach-race".. where engines are entered into different classes, mailed parcel-post "to".. and "run-by" a single contest-director for that year. Last year's contest was directed by one of our forum-moderators "Cribbs74" (Ron Cribbs)

CEF 2017 Tach-Race link below..

https://www.coxengineforum.com/t10291-2017-cox-engine-tach-race?highlight=tach

That said; there's no harm in giving it a try with an old/worn-out piston and common shop-tools.. you "might" just be able to produce a high-performance engine!

fredvon4 Wed Mar 14, 2018 1:34 pm

Sub piston induction of .010 to .015 is fairly easy to achieve ...if you want to tune some that is already there, use the cylinder base shims Cox International sells

-you can surface reduce the Crank case FLANGE on flat glass and 400 to 1000 grit oiled sand paper

BUY one or more Piston Cyl sets Cox Inter or Ex model engines sell

BUY a LOT of many Cox used engines..... "LOT" as in several junkers for one price

If you cut down pistons...very easy... do NOT go too far...More is NOT better

No matter HOW you shorten the skirt on a piston...when done Take a fine file, or stone, or Scotch Green, Brite pad, or 320 grit paper ---and Dress up the outer perimeter of the skirt to remove any burrs

Pay some attention to the porting of the cylinder.... 2 ports with twin boost ports work better with a LITTLE SPI...got one that screams with only .008 SPI
2 Ports with one Boost port like .010 to .012

I don't mess with single port cylinders so no clue

OVERLORD Thu Mar 15, 2018 2:37 am

PlaidHatter wrote:That's a pretty precise tight gap!

When reading this thread I moved the cursor by accident over the right side of your post and this red bar came up. Maybe Jacob can remove it. This was a unintentional maneuver!!

rsv1cox Thu Mar 15, 2018 2:25 pm

This thread sure weaves in and out.

Finding stand alone pistons to modify is a challenge. Only pistons I can find come with a cylinder. I had several spares when I started assembling engines from parts but ran out fast.

Bob

Mark Boesen Thu Mar 15, 2018 4:29 pm

Idk, to me it seems the subject of SPI to me is way overrated. The amount of piston gap at TDC is probably the least critical compared to compression, piston fit, etc. If you read Paul Gibeault's article on reedies, SPI is not even addressed.

SkyStreak Thu Mar 15, 2018 5:12 pm

Mark Boesen wrote:Idk, to me it seems the subject of SPI to me is way overrated. The amount of piston gap at TDC is probably the least critical compared to compression, piston fit, etc. If you read Paul Gibeault's article on reedies, SPI is not even addressed.

Ive read Paul's article several times. I have some cylinders that according to that article should be pretty good. I test front ends (crank, cyl, piston) with the same horseshoe backplate for consistency. So far I not getting the kind of numbers that apparently I should, so for me at least, I'm looking for missing links that will add up.
I just want to learn how to build a hot little reedie for a R/C Quiky 200 type plane to fly for fun and impress my fellow fliers.

Mark Boesen Thu Mar 15, 2018 11:42 pm

Start with a dual port, preferably a Tee Dee cly., loose fit, tight ball socket, 35% nitro, new hicomp head. Start with 4 gaskets and remove until unable to needle. New reed, o-ring, make sure needle is tight. A lot of SPI vs. a little SPI won't help much if compression is off or bad cylinder fit, leaky needle, worn reed, etc.
Then there's stuff like polishing crank, lightening, deck height, etc.
Lots trial and error, you'll get there.

Mark Boesen Thu Mar 15, 2018 11:52 pm

SkyStreak wrote:
Mark Boesen wrote:Idk, to me it seems the subject of SPI to me is way overrated. The amount of piston gap at TDC is probably the least critical compared to compression, piston fit, etc. If you read Paul Gibeault's article on reedies, SPI is not even addressed.
Ive read Paul's article several times. I have some cylinders that according to that article should be pretty good. I test front ends (crank, cyl, piston) with the same horseshoe backplate for consistency. So far I not getting the kind of numbers that apparently I should, so for me at least, I'm looking for missing links that will add up.
I just want to learn how to build a hot little reedie for a R/C Quiky 200 type plane to fly for fun and impress my fellow fliers.

Open up the back plate of horseshoe, make sure spray bar is centered, silicone fuel line on needle help with possible airleaks

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