[NSRCA-discussion] YS 120 NC/SC prop recommendation

Sat Jan 12 15:30:39 AKST 2008

Troy,
    As my last step before re-assembling the 120 SC I checked all the parts carefully, good thing too.  The con rod is cracked where the wrist pin goes thru the bushing.  Obviously I need to replace the conn rod.  Any cavats ?  The piston and wrist pin look good.
  Thanks,
  Paul

"Troy A. Newman" <troy_newman at msn.com> wrote:
      For the 120NC just prop it down a size...maybe 2 sizes. The 15-10 might work well. You might also look at a 15-9 or 15-8 will likely be bit too light. Personally the props used are usually about the same. The 120's didn't have that huge of a power difference. The 120SC was the strongest, and the 120SF-120NC-120FZ were all about the same..the 120AC was in between the two levels. The problem with the AC was the air box and the extra weight.

  What you want to do is run the engine at about 8500-8800rpm as a target rpm. What ever fuel/prop combo gives you this rpm will work the best on the engine. Regardless of the engine turn the rpm range an it will be happy and make good power.

  For reference there were some 14in props that were really good too. The 14-12...and the 120AC liked the 14.5-14N. 

  My guess is try the 15-10 and see how it performs on the top end rpm. If you are below say 8400 then either up your nitro or lighten the load.

  As for replacing the sleeve. If there are deep scratches, or when the crosshatch pattern is completely gone. If its just smoothed out at TDC this is typical and happens pretty quickly. but when it starts to wear down about 1/3 to 1/2 way down the pistons travel then its time to start looking for a new sleeve. Any of the 120 sleeves will work. Same way for the piston....check it to see if its round in all directions. Look for play on the wrist pin. Inspect the ring groove. The best piston to run is the original 120FS as its the most robust. For maximum power and performance the magnesium pistons are awesome but they are more fragile. The dished pistons again better power but more fragile. Don't run them lean and they will be good to go.

  As for the fuel that was left in them....not sure what fuel you are using but I have not had this problem with the Cool Power. Some other fuels that are highly touted by some...well I had problems there. Another note if the engine is going to sit for a long time. I place it in a plastic bag and put it back in the box. It resides in the house. Temperature changes are minimal...you don't want to leave it where the temps will go through extremes. This will invite condensation and moisture. That is where the rust comes from.

  Troy Newman
  Team YS 
    ----- Original Message ----- 
  From: Paul Horan 
  To: NSRCA Mailing List 
  Sent: Sunday, December 30, 2007 9:31 AM
  Subject: Re: [NSRCA-discussion] YS 120 NC/SC prop recommendation

  Troy,
     Thanks for the info.  
     I have one 120 NC and one 120 SC, the NC will go on an Aires and the SC will be a backup.  Both required a rebuild, looks like they were left with some fuel in them and left that way for years.  All the bearings needed replacement, including the camshaft bearings.  The SC has a piston with a concave top, (dished ?).  Of the two engines the SC is in the best shape, the crosshatch tooling marks are visable in the cylinder.   The NC compression could be better.  Any suggestions for running the NC ?  How do I know when I need to replace the cylinder sleeve.  
  Thanks again for the info, in this case more is better.
  Paul

"Troy A. Newman" <troy_newman at msn.com> wrote:
          15-10 is an excellent prop on the 120SC, if its a strong one you can with the 15-11

  Of special note the 120SC likes Nitro. It is a fairly high compression engine and can be a little finicky. It was the strongest of the 120's In fact I had one that gave the 140FZ a run for the money.

  Being this soupped up highly tuned 120...the extra nitro broadens the mixture settings. The other thing it does is higher nitro require richer settings to get the mixture correct and as a result they will run a bit cooler and better. More fuel flow means more oil flow and better cooling also.

  I strongly recommend the Cool Power 30% heli fuel.

  By the way, on the 120SC because its more a higher compression soupped up version, don't run it out of fuel. If you are flying and too long and the engine runs dry it can backfire and can cause damage. Its never a good idea to run any of the pattern engines 2 or 4 stroke out of fuel, however the 120SC, 120AC, and the 140DZ tend to be a little brutal when this happens. Try to run the 120SC at the higher end of rpms. I would say the goal rpm for that engine would try to be up at 8600-8800 maybe as high as 9000. On the 15" props it will make noise with ease in most cases(read exhaust systems)

  The 120SC is an awesome engine. Like I said it was the king the of the 120's. There were some folks that had issues with them, but my experience was always good with the SC. There were some piston of the month changes to the 120SC. Namely they had a dished piston, and some magnesium pistons. The goals behind these were to get more power out of the 120 case. Back then the FAI rules had not changed to allow larger than 120 displacement. So YS was trying to lighten the piston, and get the engine making max power from the little box. At the time there were a few models that were getting bigger than the 2 meter box as the 2m box didn't exist. No size limit just the engine limit and the weight limit existed at the time. On the piston front just stick any ole 120 piston in it....the 120SC piston could be a little fragile if run lean (meaning running it out of fuel). The original 120SF piston was bullet proof and works in the 120SC. The dished and designer pistons are really able
 to change the behavior of the engine. I have played with this "super" or the "hyper" pistons on the current 160DZ and 170DZ engines. Its cool how changing the piston by putting a dished top and closing off the bottom of the piston can change the way the engine reacts to certain prop loads, or it will change the noise. Some of the things they can do make more power and some of the pistons make it run super smooth and quiet. Yamada doesn't do the testing in production like they once did with the pistons. I have been fortunate to try some of the super cool, "hyper" pistons they have played with on the DZ's. I wish I had the knowledge back then that I have now. Some of the smallest changes in a piston can really totally change the way an engine performs. Its kinda neat. I remember the need was for more power more power back then and now I know which piston designs would have done it better. 

  Any of the current exhaust systems will work very well on the engine. Even the short hatori 821 pipes and the NMP header will work excellent on that engine.

  I have a couple of 120AC's that we carved up the heads to get more power...I wish I still had the 120SC as it would be a really good engine for my war bird project I'm planning. I actually need a pair of the 120SC's for it. Right now the 120AC's are a little heavier than I would like and have been considering putting a pair of 140 Sports in the model.

  I know this was likely way way more information than you needed to know. But its a good engine and you'll love it.

  Troy Newman
    ----- Original Message ----- 
  From: flyintexan 
  To: NSRCA Mailing List 
  Sent: Saturday, December 29, 2007 8:53 PM
  Subject: Re: [NSRCA-discussion] YS 120 NC/SC prop recommendation

  Should be able to run a 16x8 on 30%...
    ----- Original Message ----- 
  From: Paul Horan 
  To: NSRCA Mailing List 
  Sent: Saturday, December 29, 2007 6:09 PM
  Subject: [NSRCA-discussion] YS 120 NC/SC prop recommendation

    Any suggestions for a YS 120 prop ?  It will be on a Aires ?
  Thanks,
  Paul

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