I have an NIB FP 40, to possibly go in a Vector.    Does adding an extra head shim (I have one original) help tame the FP 40's runaway characteristics?

I have another one and I try to use 11/3 props or something like that.  Using those props does help.

No not really, that is not the problem people had with them, problem one was the 20 size Needle vavle assem.  in them, it was near .30 thou smaller diameter than the old OS 40 bar, that made a .275 ventuurie closer to a 310 in area, big problem with trying to get used to calculating area, which most no one did, second problem was the ting hole in the stock muffler, this cause heat and slow cycling...ie runaway.  adding a head shim just leans the motor out and causes more heat,most any time you decompress the engine it runs leaner ..hotter.
So use a 240 to 250 with the stock NVA or a 265 275 with a NVA that is .157 (PA  ST  Old OS  Enya), then use an open muffler, you can also run in a wet 2 if you have needle set problems

Randy

  In fact, adding a gasket when not necessary might enhance the tendency to run away, since it has to run hotter.

     But, absolutely, to give yourself a chance you have to put a very small venturi on it, or an ST needle. I still think it's mostly a waste of time to run 40FPs when you can get a 46LA for $85, and that runs right with no fiddling.

    Brett

Guy's
       Keen to learn, why adding a shim makes the engine run hotter?
Thanks
Paul
In OZ

When you add a shim or decompress the head, you will have to turn the needle in...leaner... you will run leaner use less fuel and the engine will normally run hotter, adding nitro "can" cool an engine because you can then both run more fuel and oil for an exact given time, and you can turn the needle out and run the engine richer... richer is almost ALWAYs  cooler.

Randy

<
When you add a shim or decompress the head, you will have to turn the needle in...leaner... >

Randy, why would the fuel/air ratio change?

   I'm not Randy, but you lose power by reducing the compression, which will require you to run closer to the mixture closer to the ideal to compensate, hence, leaner.

    Most of what I hear people talking about with head shims is backwards, and this sort of stuff is becoming a lost art to some extent, since 4-2 break motors are rarely used for serious competition any more. You can still use the compression to adjust a piped engine, but you don't necessarily have to, so people have sort of lost the bubble.

   more compression: more overall power, bigger power difference between 4 and 2, faster transition from 4 to 2 and back, cooler (until you go over the top...)
   less compression: less overall power, reduced power difference between 4 and 2, and slower or delayed transition from 4 to 2 and back, hotter. In particular, it will tend to hang in a 2 longer due to thermal effects of running hotter in the first place. The ultimate expression of this is putting in so many head gaskets that it never transitions back to a 4, hence, runaway

I have found most of the piped engines I have used to lose power rapidly with compression changes (leaving everything else the same) but don't gain much in terms of 4-2 power difference, basically, you are only reducing the 2-stroke power only slightly faster than the 4-stroke power. That has made me try to control the 4-2 power break with the pipe and the prop, or, once we got 61s and up, run it in a 4-stroke all the time, pump it to the gills on compression, and not worry about the transition effects.

    That's not what generally happened with many 4-2 break motors. Reducing the compression tended to reduce the 2-stroke power and either do nothing, or increase the 4-stroke power, so slight compression changes would significantly effect the 4 to 2 difference but not hurt the overall power very much.

     Brett

   I'm not Randy, but you lose power by reducing the compression, which will require you to run closer to the mixture closer to the ideal to compensate, hence, leaner.

    Most of what I hear people talking about with head shims is backwards, and this sort of stuff is becoming a lost art to some extent, since 4-2 break motors are rarely used for serious competition any more. You can still use the compression to adjust a piped engine, but you don't necessarily have to, so people have sort of lost the bubble.

   more compression: more overall power, bigger power difference between 4 and 2, faster transition from 4 to 2 and back, cooler (until you go over the top...)
   less compression: less overall power, reduced power difference between 4 and 2, and slower or delayed transition from 4 to 2 and back, hotter. In particular, it will tend to hang in a 2 longer due to thermal effects of running hotter in the first place. The ultimate expression of this is putting in so many head gaskets that it never transitions back to a 4, hence, runaway

I have found most of the piped engines I have used to lose power rapidly with compression changes (leaving everything else the same) but don't gain much in terms of 4-2 power difference, basically, you are only reducing the 2-stroke power only slightly faster than the 4-stroke power. That has made me try to control the 4-2 power break with the pipe and the prop, or, once we got 61s and up, run it in a 4-stroke all the time, pump it to the gills on compression, and not worry about the transition effects.

    That's not what generally happened with many 4-2 break motors. Reducing the compression tended to reduce the 2-stroke power and either do nothing, or increase the 4-stroke power, so slight compression changes would significantly effect the 4 to 2 difference but not hurt the overall power very much.

     Brett

I wish I had seen a description of shim effects this succinct about a dozen years ago.  When I get home I will print this out and tuck it into the pipe article in my Special Edition SN.

As a bit of a deviation from the original topic, something else that I have seen is that people want very small 4-2 power difference, and run the compression way down. That loses a bunch of power, so they open the venturi up to compensate - which works for some engines (Fox) and puts you in a death spiral on others (40FP).

   George Aldrich was one of the competent engine guys who took this approach to just about every engine. So he would greatly decompress, say, an ST60, then put on a massively large venturi to recover some of the power. In many cases I saw, this resulted in a pretty wimpy engine with a Fox-like 4-2 - lots of sound difference and negligible power difference. In thick air, and on light airplanes, it also sometimes resulted in the engine quitting during the takeoff roll because the fuel draw was so weak. It's a pretty sad state of affairs when an ST60 isn't enough power for a kit-sized Shark 45. Lew flew the +10% version with an ST46 with no real problems.

    It ran exactly like he wanted it to, like a big Fox - but just not much usable power. Maybe you didn't care in the 50s but you sure weren't going to be too competitive in the modern era if you were giving up power on your (already feeble) 4-2 break motor.

     A quickie field solution to this is to add copious amounts of nitro which helps get the power back and permits it to run richer on the ground, so it will get through the takeoff roll. I can recall have good results going from 5% to 17.5% nitro on a GMA ST60 at one of the NWR in Roseburg, but it would have been even better with 25% but I couldn't talk the pilot into it. The real solution is to put the compression back up, put on a smaller venturi, and regular nitro, then adjust the break with nitro, small compression changes, or prop fiddling like normal.

     Brett

Most all European and Asian engines were designed to run on FAI fuel, no nitro, hence, higher compression. Engines designed here in the USA were designed to run with nitro, hence, lower compression.
An engine that 'runs away' can some times be corrected by using FAI fuel, or lowering the compression in order to use nitro. Some years ago, Duke Fox did some extensive work on developing engines for FAI fuel usage. Those engines wouldn't run worth two cents on any nitro fuel but ran fine on FAI fuel.
BTW, My .40 FP has one additional head shim, a very free flowing tounge muffler, and three layer of pantyhose material on the venturie.
11x3 props and ten percent nitro. 22 percent all castor.and no problems.
Regards, Phil

Hi Phil

Nothing wrong with doing it the way you have listed if, it works well and your happy, However I have setup hundreds of these, and the High comp with proper venturie size, good NVA, and open muffler just works better, more thrust, better power delivery etc...  That also goes the same for the FP clones such as the TT and Mag GP 40 and 44

Also  European ST 46, 35, 40, 51, 60 Super Tigers work best on NITRO fuel, So did Como engines, as well as HP engines, and the Asian Flash, OS engines, Thunder Tiger, Magnum , Royal, Blue Bird, and many others. So I don't think the majority were made to run on just FAI No nitro fuel.

Randy

Mine works well with good power for me the way that I have it set up. Any high compression glow engine will give better runs with low nitro/no nitro fuel. Reducing compression allows for the use of nitro.
Regards, Phil

   I know I am going to regret this, but none of the engines in this thread are "high compression" engines and almost all of them will run fine with any reasonable level of nitro stunt fliers put in them. A stock ST46, for example, loves 25% nitro, and certainly the FP isn't going to have any problem with it. You don't dare cut down on the head gasket or raise the compression ratio, but going from 5% to 25% is a non-problem with no changes required.

   I think the compression ratio of a 40VF is around 14:1, and have finished in the top 10 at the NATs in a 40VF running 17.5% nitro. I ran 15% in the RO-Jett at the 2003 TT, and I think I was down to about .005 head clearance to the point it was hard to choke - no issues.

     The general rule that you state is correct, but unless you are near the edge, you generally don't need to change the compression when you change nitro on stunt engines (the exception being the Discovery Retro). If you go over the top, it will be obvious as soon as you start flipping it.

   Brett

   

Almost all stock engines have a compression somewhere between ~8-10:1 but I've found by experimenting that a compression of about 13.5:1 works best for zero nitro fuel so even with a 10:1 compression there's a fair bit of leeway for using nitro. I only have 2 engines that approach the best for zero nitro, the best being an Irvine 61RLS with 13.6 followed by a Bluebird 28 with 12.5:1. In roundish figures a Brodak 40 is 10:1, Enya SS45 S at 10:1, Enya 61 CXLRS Pro at 8.8:1, 46VF at 10.7:1 and ST G51 at 7.4:1.

While playing around with my G51 trying to tame a massive runaway (later found to be simply over heating) I tried reducing compression in stages down to around 5:1 and essentially all it did was reduce the power in both 2 and 4 stroke mode by roughly the same amount.

Brett, I suggest that you may want to take a look at this page.

<http://sceptreflight.net/Model%20Engine%20Tests/OS%2040%20FP-S.html.

The .40 FP stunt engine is listed as having 7.4: 1 Compression and that that is the only difference from the higher compression RC version,
Regards, Phil

Brett, I suggest that you may want to take a look at this page.

<http://sceptreflight.net/Model%20Engine%20Tests/OS%2040%20FP-S.html.

The .40 FP stunt engine is listed as having 7.4: 1 Compression and that that is the only difference from the higher compression RC version,
Regards, Phil

I have had tons of the 40 FPs in my shop, both R/C and C/L  versions, all of the ones I have seen are  the same C/R , someone made a mistake in the measure or the article.
Matter of fact they are the same parts except for the carb  vs  venturie, and they did have 2 different mufflers,  and the Head  , which would be different for a higher C/R motor is the same on both and is the same as the FSR BB 40

Randy

Maybe I should clarify the compressions I mentioned in Reply #16 in that these are all geometric compressions, not trapped. However just for interest's sake I measured the exhaust port height on a typical engine and subtracted that figure from the stroke to get a trapped stroke which is what trapped compression is calculated from. Geometric compression on that engine is 10.75:1 and trapped compression worked out to be a tad under 8:1 giving almost an exact reduction for a trapped compression to 75% of geometric. On that basis a G51 with 7.4:1 would then have about 5.5 trapped.

The heads and liner timing is the same on both FP 40 RC and CL.....  would be extremely hard to change C/R  using the same parts, especially such a drastic change. The ones that were factory did NOT come with the extra head shims installed, they came just added to the engine box.

Randy

Yeah, the article must be wrong...................

OK let me say this, The article is wrong if it says that..!! period.  However if you want to believe that, then I am OK with, I really don't care, I just put out the info, you can get out of it whatever you want. Matter or fact anyone that reads this can decide for themselves... I am OK with whatever  they want to believe.   You can lead a horse to water............ 


Regards
Randy

Brett, I suggest that you may want to take a look at this page.

<http://sceptreflight.net/Model%20Engine%20Tests/OS%2040%20FP-S.html.

The .40 FP stunt engine is listed as having 7.4: 1 Compression and that that is the only difference from the higher compression RC version,
Regards, Phil

Phil  you wrote that the RC version has higher compression ratio, and you quoted an article that tells you that.... please see above...

I am telling you that it is a FACT, that is wrong, the RC version  does  * not * have a higher C/R..     that was my only point, matters not if you believe the facts, if the article says  what you said it does, then it is wrong.
Or the only other option is that someone stacked up the  3  head shims  to lower the C/R on one engine over the other, since all parts are the same, that would be one of the only ways to lower the C/R, but would be a bogus test since they did not ship that way, and the head shims would have to have been installed by someone before the test.
Regards
Randy

Randy, maybe you should go back and look at parts lists for the first .40 FPs with the iron and steel setup. That is what the test was done on.
The fact that the tester devoted so much time expounding on the difference in CR between the CL and RC versions makes no sense if they were the same.
Anyway, the setup that I have on the  iron and steel .40 FP of mine runs well and is dead reliable. It was converted from RC.
Regards, Phil.

Phil

I did, I still have here both the FP iron motors and the parts sheet, Boxes, even the 3 pack of head shims included ( Separate ) in the motor box, as well as many new in pack parts for them.  ALL The parts are the same, The case is exact part number, piston sleeve..same part number, same with head, rod, wrist pin...  The C/R was identical, unless someone machined the head or stacked up gaskets, again  the article is wrong.
Or  maybe you can find a way both can have same identical head, P/S, Crank, Rod, pin.... and have a differant C/R ?? 

Randy

FP40S was shipped with extra head shims. If I am to believe my lying eyes. Saw what appeared to be more than one NIB FP40 with the extra shims put there by OS.

I don't think you are getting anywhere with facts, Randy.  Maybe it would be more believable if magic was involved.  

maybe a magic ROD... that grows and shrinks as it goes up and down..? Or maybe the CL motor had a magic flexible head that gave way under compression?

At any rate  not really worth the argument, if the test was with added head shims to one, but not the other, then it is a bogus test anyway. Then you get into how many shims? which engine? how thick are the shims? are the ALL the same size shims? did Some one  also put shims in an R/C engine that wasn't tested?

Randy

Yes. I was referring to the extra head shims in the pack. Which I assumed were there to adjust the compression ratio and alter the performance of the engine. Why were they there? Why did OS think it desirable to adjust the compression ratio of this stunt engine by installing extra head shims and reducing the compression of the engine?

Hi Dennis

You maybe want to read the second post of this thread...  it is below

What I address with you is the C/R of the 2 engines , and the fact the shims were not installed, they were just giving you ones free, maybe because they screwed up the venturie size to Spray bar ratio and muffler outlet size so VERY badly

"Quote from: Allen Eshleman on January 01, 2014, 11:21:24 AM
I have an NIB FP 40, to possibly go in a Vector.    Does adding an extra head shim (I have one original) help tame the FP 40's runaway characteristics?

I have another one and I try to use 11/3 props or something like that.  Using those props does help."

Randy Wrote:
No not really, that is not the problem people had with them, problem one was the 20 size Needle vavle assem.  in them, it was near .30 thou smaller diameter than the old OS 40 bar, that made a .275 ventuurie closer to a 310 in area, big problem with trying to get used to calculating area, which most no one did, second problem was the tiny hole in the stock muffler, this cause heat and slow cycling...ie runaway.  adding a head shim just leans the motor out and causes more heat,most any time you decompress the engine it runs leaner ..hotter.
So use a 240 to 250 with the stock NVA or a 265 275 with a NVA that is .157 (PA  ST  Old OS  Enya), then use an open muffler, you can also run in a wet 2 if you have needle set problems

Randy

Yes. I was referring to the extra head shims in the pack. Which I assumed were there to adjust the compression ratio and alter the performance of the engine. Why were they there? Why did OS think it desirable to adjust the compression ratio of this stunt engine by installing extra head shims and reducing the compression of the engine?

      They didn't install them, they were in a bag. Compression is a useful adjustment, but not necessarily to solve runaway problems.

     Brett

Excellent thread guys, extra thanks to Brett Buck for post #6 and #10. Very clearly explained.   

Randy, I will stick with the article.
As I said, mine runs very well set up with the added head shim, a drilled out tongue muffler, and three layers of panty hose  material over the venturi .

Regards, Phil

Randy, I will stick with the article.
As I said, mine runs very well set up with the added head shim, a drilled out tongue muffler, and three layers of panty hose  material over the venturi .

Regards, Phil

Tell me Phil,
                   if you are going to stick with the article as being gospel then its logical to assume that, by way of proof, you have the stated compression ratio from that test.

Is this so?  Just curious to see if there is any hard evidence here.

Cheers.

Chris< here is the link to the engine test and review.
The information regarding the compression ratio is in the paragraph below the picture of the engine.

<  http://sceptreflight.net/Model%20Engine%20Tests/OS%2040%20FP-S.html  >

Regards, Phil

Dennis, at some point you need to break down and get a true stunt engine.  You have no idea what you are missing....

Not enough time these days. And bad weather. Went like that last season. Having the bellcrank pull out of my best flying plane didn't help. The big PAs work great. But I am flying 40 sized planes at the moment. Most of them profiles. I have not had good luck with the ARF Vectors. Very slow handling. I have an ARC half done and will check cg and control ratios thoroughly. This could help. For years I thought it was me. Terrible corners. Then a light profile with a Tower40 flew so much better. At least with me wiggling. With all do respect engine worked very well. At Huntersville I was flying what I had left to fly. An eight year old Galaxy that had been through the wars. The FP40 suddenly started acting up. Turned out to be the nipple on the tongue muffler was loose. It would seal decently on the ground, but when up in the air it would leak and stop feeding air to the uniflo, resulting in instant lean mixture. Tricky to diagnose, easy once the nipple screwed itself out all the way. But the Galaxy was never the most solid profile mount for an engine. Always suffered some from a weird profile vibe. I usually ran the plane balls out. Not trying for a decent 2/2 run. The plane itself something of a liability to be sure when up against the sophisticated stuff in Advanced. My plan is to build a 40 sized full body with moments similar to the plane that worked for me as a profile. Will an aerotigre costing $250 or an even more dedicated stunt engine be appropriate and that much of an advantage. Could be. And I am willing to take a shot at that when I've better planes on which to mount them. Sure. I am as well considering a full sized stunt war wagon with a PA61, something like that. My run around to contests car is a Honda Civic. Not much room for a 60 sized stunt plane. I have of course my 1980 Malibu wagon. No air. For years I drove to Brodak with a gallon jug of water beside me. My sweat soaked back a joke enjoyed by all of us in our club. But. Now. Over 65, I have come to depend on air, a good car stereo, even blue tooth.

Smaller venturis consume less fuel, all things being equal. Depends on fuel, needle setting, prop, weight and drag of the plane and ambient temp, perhaps even barometric pressure. Running a large vanturi with a free flowing muffler and a prop similar to an apc 101/2x41/2, I have seen a nice Gieseke Nobler do the pattern needing 4 1/2 ounces when there was a chill in the air. This using Powermast 5%, 22% (50/50). In the heat of the summer the consumption fell to a hair bellow 4 ounces. Definitely true for this plane. Since early in the season, when practicing, my friend had to cut the pattern a bit short with a 4 ounce tank. When temps hit 70 and above, not a problem. Some of my best running profiles needed similar amounts of fuel when I used a large venturi. These days I use .272 and smaller venturis. .272 is midway between the small, .257 (kinda) and the large .283. I often use less than 3.75 ounces when set up like this.