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Home › RC & Power › ✈️Aircraft › 🚁Helicopter › Engines Plugs Mufflers Fuel › How common is the need for head shims?

07-28-2008 05:09 PM 12 years ago

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Jlerch

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Parrish, Florida

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How common is the need for head shims?

When and how much to shim a glow motor appears to be one of those black arts, from what I can tell. So I am not asking if I should shim, but I am curious how common the practice is?

I fly YS-80-ST in one bird and a YS-91-SR in a second, both birds are flown down near sea level in Tampa Florida. Over the course of 6 months, I've fractured a ring and destroyed a piston on both. Both have been reassembled with new sleeve, piston, ring and continue to run. Both motors were used and abused when I got them, so the damage may have been done before I put my hands on them or after I got them, don't know. Its also possible that it was just their time to die..

When I built the YS-91-SR, I put an extra 0.007" head shim in it, and it runs as well as it did before. Honestly, It may make more power now, but I really can't tell a difference between before and after the rebuild, other than the needles ended up a few clicks leaner after a few gallons.

So, is it pretty common to shim heads in high density altitude environments?James Lerch - Tampa Bay FL

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07-29-2008 05:33 AM 12 years ago

jongurley

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North Carolina

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I beleive the compression issue or shims, are more for adapting to the high or lower nitro content of the fuel, I know that on some of the litte super tigre .32 engines we used to run on our little delta planes,, if the nitro content was 10% or lower you ran it stock, if you were going to run 15 to 30 percent, they sent a .004 shim to lower the compression a little,, with less nitro you will have to have higher compression,, it is always better to have a lower compression with high nitro,, so if when in doubt shim the engine slightly,,HeliWerks Inc. Team Factory Pilot
Team Thomco Hobbies
Thompson Heli Consultants Inc.

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07-29-2008 06:04 AM 12 years ago

jvraptor

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ca

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yea it seems that every time people say to shim they forget to put in the little detail about how u lower the compeshion u lose power. if in dought dont shim. if it ran great and u messed with something. and it dosnt ran as great. get it back to the way it was befor befor u start changing things. that is one of the common mistakes with this hobby. something isnt performing like it should people think that adding things to it will make it run better. and then u have this other crap now in the problem and then its just a mess. im not baggin on anybody but i see this all the time just becafull with the shims.

jordan

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07-29-2008 12:27 PM 12 years ago

Jlerch

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Well, I do run 30% nitro, either Cool Power or CYB. I'd mix my own if I could do it cheaper than buying it

So far, both motors run fine, I've never killed the same motor twice. (subject to change with out notice!)

My concern is, I'm just not smart enough to notice the sound or symptoms of detonation. I think if I were more aware, I might have been able to extend the life of the of both motors prior to rebuild.

Also, I'm not looking to make more power, both motors seem to have more than my skill set can support. I'd just like to extend their service life, or at least not shorten it any!

So let me ask this, other than a lose of power output, are there any other downsides to lowering the compression ratio via head shims? I thought I read somewhere that it could cause high head or cylinder temps do to a reduction in thermal to mechanical efficiency (IE, the motor burns fuel, but makes no power, so the energy lost has to go somewhere)

Also

head shims = lower compression = reduction in effective ignition timing and lower chance of detonation, yes?
leaning the fuel air mix = higher temps = more chance of detonation
I'm guessing there is there a 'sweet spot' where the 'proper' compression ratio and the fuel / air mix cross?

All this reminds me of a saying we have at work "If it were easy, everyone would be doing it!"James Lerch - Tampa Bay FL

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07-29-2008 12:55 PM 12 years ago

jsenicka

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The YS 90 did not seem quite as over compressed as the OS91SXH CSPEC out of the box, although I still wound up shimming mine.
You are correct that shimming reduces compression and power, but at same time, if over compressed and detonating you have to run so rich you will never make power anyway, so you have far less available than a properly compressed motor.
A detonating motor is pretty easy to spot. You go from wet, snotty rich to cackling lean in just a few clicks of the needle.

My take on the 91 CSPEC and 91SZ is both need .016 to .020 to run 30% at sea level. A buddy and I both needed to shim a YS91ST as well. I take same engines home to fly with my Dad's club in Wisconsin at 1700 feet and cooler, and I think I could get away with less shim (my buddy up there has a stock shimmed 91ST that makes great power).

Bottom line is you need to recognize the difference between just poor tuning and detonation. The quickest way to take detonation out of the picture is to try lower nitro fuel. While I was chasing tuning on an original CSPEC I tried YS20/20. Suddenly I had almost 1/4 turn between wet/rich and hot/lean. With 30% I was about 3 clicks from billowing smoke and crackling lean. I added a second .008 shim and tried again with CY30%. Now I had better power than the 20%, and a wide needle adjustment and a good running motor.

I have never had to shim 32, 37 or 50 OS motor. Only the 90's.Jim Senicka
Team Manager, GrandRC Flight Team

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07-29-2008 05:24 PM 12 years ago

Helipilot01

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James,

When I was competiting heavily in the 90's and early 2000's it was common to shim every OS 60-90 engine to about .030 clearance, or some times referred to as the squish band. This was necessary especially with 30% fuel or if running a tuned pipe.

Why? The nitro is an accelerant for the alcohol in our fuel. That means it makes the explosive power increase. In doing so, it also makes it explode faster. By exploding faster, it's doing so as the piston is traveling upwards on the stroke. The causes the flash or ignition to happen soon than it should. This is called per-ignition or detonation. There are a couple of was to control detonation. One is to shim the head and increase the clearance between the piston and head making the piston travel further before the compression is high enough to for the fuel/air mixture to ignite. There is a mis-conception that shimming the head and lowering compression will reduce the horse power. If detonation is occurring, the opposite will be true. Pre-ignition is like trying to walk through an automatic door that starts closing before you can get through. The result is the door will hit you as your passing through, slowing your time to get through the opening. The same is true with the model engine, if the ignition causing the explosion happens too soon, the piston is still trying to travel upwards in the cylinder while the explosion is trying to push it down. Another is to use a cooler glow plug slowing the ignition process. One way to think of a hot plug is one that retains it's heat from one stroke to another. A cooler plug is one that looses some of it's heat from the previous stroke and will cause the fuel/air mixture to ignite later in the stroke.

There are other things going on with the glow plug, engine exhaust timing, ect but this will give you and understanding of what to look for as you make changes. OBTW, yes, one of the quickest ways to recognize detonation is a overly sensititive needle valve as jsenicka stated.

Hope this helps,

Mike Fortune
Team JR

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07-29-2008 06:05 PM 12 years ago

Jlerch

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Jim Senicka wrote: A detonating motor is pretty easy to spot. You go from wet, snotty rich to cackling lean in just a few clicks of the needle.

LOL, Fast forward near the very end of this video as I mumble to myself on the way back to the pits after killing my motor a few weeks ago

Watch at YouTube

This is what the engine looked like when I got it home (This would be the YS-91-SR I rebuilt)

Mike Fortune wrote: When I was competiting heavily in the 90's and early 2000's it was common to shim every OS 60-90 engine to about .030 clearance, or some times referred to as the squish band.

Hmm, Sometimes it just pays to be lucky. When I rebuilt the above motor, I assembled with stock head gasket and had a gap of 0.024" (Measured with the rosin core solder in the glow plug hole trick I read of) Thinking this wasn't enough, I put a 2nd head gasket in increasing the gap to 0.032"

Somewhere on the vast internet I read something that said the gap should be more than 0.024"ish. Not having any shim material on hand, I grabbed what was handy, the old head gasket. Like I said, sometimes it pays to be lucky

Thanks gentlemen! Its good to know I'm on the right track and not bat sh!t insane. (well, at least I'm on the right track.

)James Lerch - Tampa Bay FL

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07-29-2008 06:44 PM 12 years ago

Pinecone

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Maryalnd

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Wow, while the conclusions may be valid, LOTS of incorrect information about internal combustion engines.

1) Nitro is not an accelerent. Nitromethane, when heated, liberates oxygen while the methane portion is additional fuel. More oxygen allows you to burn more fuel. Burning more fuel produces more power. The same way that supercharging increases power, or even just running a large engine, both increase the amount of air, thus the amount of oxygen, thus allowing the burning of more fuel.

2) Fuel does not EXPLODE in an engine, it burns. It burns very fast, but it still burns.

3) Pre-ignition and detonation are two ENTIRELY different things. Pre-ignition is when the fuel-air charge ignites too soon. This can cause the pressure peak to be too early and actually reduce power, or if really early, as in before top dead center, cause serious engine damage. Pre-ignition can also be caused by carbon build up as the carbon can be glowing with the heat of combustion and provide a different ignition point. Pre-ignition can cause higher than normal cylinder pressures, but also very high temperatures. Pre-ignition melts holes in pistons, similar to running too lean. Pre-ignition in a glow engine is controlled by compression and glow plug heat range. If you put a pressure transducer in the cylinder, pre-ignition shows a higher peak pressure, but basically a smooth pressure curve, just like normal running. Of course the peak is moved earlier in the crank rotation.

Detonation is when due to the heat and pressure within the cylinder, the fuel-air charge ignites at several uncontrolled locations around the cylinder and the multiple pressure waves cause serious pressure peaks. Detonation punchs holes in the top of pistons. Detonation is a result of the chemical makeup of the fuel, load, and compression (the last two resulting in higher cylinder pressures causing the additional spontaneous ignition locations). Detonation occurs a low RPM with high throttle openings (lugging the engine). Too lean can increase the probability of detonation. If you put a pressure transducer in the cylinder, detonation shows a very ragged pressure signal.

And while it is true that reducing compression to prevent detonation will make more power (a smooth pressure curve provides a much longer duration push on the piston) more importantly, it prevents the REALLY low power condition of a piston with a hole in it.

Both pre-ignition and detonation can make a knocking sound, but totally different mechanisms.

4) The squish band is the ring around the combustion chamber that causes the fuel-air mixture in that area to squish (squirt) into the middle of the combustion chamber. This increase turbulance for a more even burn, and reduces the combustion chamber volume for more compression, while keeping the more efficient hemi shape. And yes, it is common to measure head clearance in the squish area, as this is the area where the piston gets closest to the head.Terry
Blade CP Trex 450 SE
QJ EP8v2 EX Gaui Hurricane 550
Vibe 50 Bergen Intrepid Gasser

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07-29-2008 06:58 PM 12 years ago

Jlerch

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Parrish, Florida

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Pinecone, while your are correct, I also find humor in this, because my brain automagicly replaced 'accelerant' with 'Oxygenate' and 'Explode' with 'Burn'.

James Lerch - Tampa Bay FL

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07-29-2008 11:44 PM 12 years ago

TMoore

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Cookeville, TN

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Well, I do run 30% nitro, either Cool Power or CYB. I'd mix my own if I could do it cheaper than buying it So far, both motors run fine, I've never killed the same motor twice. (subject to change with out notice!)

My concern is, I'm just not smart enough to notice the sound or symptoms of detonation. I think if I were more aware, I might have been able to extend the life of the of both motors prior to rebuild.

Also, I'm not looking to make more power, both motors seem to have more than my skill set can support. I'd just like to extend their service life, or at least not shorten it any!

So let me ask this, other than a lose of power output, are there any other downsides to lowering the compression ratio via head shims? I thought I read somewhere that it could cause high head or cylinder temps do to a reduction in thermal to mechanical efficiency (IE, the motor burns fuel, but makes no power, so the energy lost has to go somewhere)

Also

head shims = lower compression = reduction in effective ignition timing and lower chance of detonation, yes?

leaning the fuel air mix = higher temps = more chance of detonation

I'm guessing there is there a 'sweet spot' where the 'proper' compression ratio and the fuel / air mix cross?

All this reminds me of a saying we have at work "If it were easy, everyone would be doing it!"

From my vantage point I'm not sure that shimming is the issue but tuning certainly is. The photos pretty much tell it all, the engine was way too lean. The stock head shims should work just fine on the YS 91. Detonation is the real enemy here and it is not caused by head shimming but by a heavy hand on the HS needle. Besides the damage to the ring, piston and liner, it's clear that the piston and head are getting hammered and that is a sign of detonation which is clearly visible on the top of the piston and head surface.

There are telltale signs when detonation is occurring, the sound of frying bacon comes to mind when the machine is heavily loaded and unloaded, lean conditions can be figured out by doing autos and listening to the engine to determine how much hang time is on the engine when you hit hold. Listening to the engine load up and how well it recovers when unloaded immediately after loading like in a tic toc situation. There have been more times than I can count when I've richened up an engine that someone was having trouble with only to be told that it now has more power than before. Hmmmmmm.

If you are not a pro and don't get engines for free, the best advice I can give is to richen up the needles and learn to fly the machine.

TMPart 107.
The only ZERO flight hour certificate in the world.
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07-30-2008 01:11 PM 12 years ago

jester4

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.....or a carbsmart

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07-30-2008 05:24 PM 12 years ago

Jlerch

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Parrish, Florida

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TMoore wrote: From my vantage point I'm not sure that shimming is the issue but tuning certainly is. The photos pretty much tell it all, the engine was way too lean. The stock head shims should work just fine on the YS 91. Detonation is the real enemy here and it is not caused by head shimming but by a heavy hand on the HS needle.

In general I agree with what your saying. Let me ask you this, would you agree that the factory needles setting for break in are targeted to be on the rather rich side of the fuel air mix? In addition, these settings should be safe from detonation?

I could never get this motor to run well at the factory break in settings, I had to go RICHER than this to stay away from the "hang on the pipe" after a short climb out and the "Frying Bacon" sound during the climb out. Its highly probable that the motor had problems other than the user. I just don't know.

Right now the motor is running very well with 0.032" of gap between the head and the cylinder. I can't bog it with 710 blades and an 8.47:1 ratio, to me that's all the power I need. In addition, my needle setting are in the range that the manufacturer recommends for normal running, which gives me that warm fuzzy feeling.

Mostly, I was looking to learn if shimming was a common practice or not. I think I've learned that it is not uncommon, but not necessarily a common practice either.

We have another club member with a YS-91-SR on a Raptor, So far he's sent his motor back twice for repairs. I tend to keep my nose out of others peoples business, so I don't know the details, but I might suggest he shim his motor when he gets it back.James Lerch - Tampa Bay FL

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07-30-2008 05:32 PM 12 years ago

Jlerch

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jester4 wrote: or a carbsmart

I had one on this airframe, but with the YS-80-ST motor in it and stock shims. I could never get the CSM to settle on a mixuture, it was constantly going full rich or full lean (full rich being factory break in specs plus a few clicks, and full lean being about 3/8th of a turn leaner on both needles)

When data logging flights, I'd get results like this.

The Blue elevator channel is the carbsmart servo position. Top of the screen is full rich, bottom of the screen is full lean. As you can see, it never found a "happy spot" (and yes, I played with the CSM gain to no avail)

I need to re-run this test, but this time with an extra head shim in the YS-80 motor and see what happens.James Lerch - Tampa Bay FL

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07-30-2008 11:53 PM 12 years ago

TMoore

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Cookeville, TN

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Jim,

The problem that I see with factory break in settings is that they are never right. They can get you close but they are based on what the factory came up with the day they wrote the manual. For instance, in the case of a YS, how much difference is the altitude and nitro content of the fuel relative to where you are running your engine and where the engine was intially tested? Unless you know those answers, it's a toss up as to what is really right. As far as assuming that they should be safe from detonation, I can't be 100% sure of that because of the muffler, plug, fuel and altitude that you are running versus what the factory ran the engine on. My guess is that YS used a prop and a muffler and give you those starting settings, which may or may not be right. In short, my method is to ignore the factory settings altogether and go by my gut.

What I do is open the needles over and above where I think they should be and tune for sound, temps and how the engine reacts when I'm doing certain maneuvers. For break in, I'm typically a little more conservative for the first 3 tanks on a 90 and just putter around either upright or inverted with limited full throttle application. I will do the usual big air stuff, loops rolls, stall turns, snakes and stuff like that to just exercise the engine and check out the fuel lines and fittings. At the 4th or 5th tank I will get the engine at about 80% of where I think it should end up powerwise and fly the machine normally. After a gallon, it's pretty much ready to fly at normal temps.

The only thing a modern engine is wearing in is the ring to cylinder fit, nothing else.

As far as shimming the heads, you will get a lot of conflicting information on that score. In the days when we were running highly compressed low nitro engines on high nitro, shimming was common for a lot of reasons. Reasons like smoothing out the engine pulses for a lower vibration setup. Lowering the compression, reduced the ignition pulses to a more manageable level and didn't hammer the machine at high revs. We would sometimes increase the head shims to lower the compression to smooth the engines once they were on the pipe in an effort to get a smoother drone in the hover.

The Webras, Rossis and OPS engines of the day, were setup for 0-5% nitro and a tuned pipe. Japanese engines are totally different in terms of cylinder timing and compression ratios, IOW Japanese engines aren't highly compressed for low nitro.

A modern case in point is a 90 size engine with a tuned muffler with big expansion chamber like the Hatori or Funtec that may hammer a bit in the aerobatic flight mode at high revs on certain 30% mixes. Lowering the compression ratio with a .004" shim reduces the detonation or hammering that occurs when you are running right on the ragged edge of lean. It doesn't change the timing of the cylinder but it will soften the blow of ignition ever so slightly. If you didn't shim the head you simply backed off the HS needle but may lose that little bit of edge that you were getting in certain maneuvers. An MP 5 likes the engine lean enough to get on the pipe and start to draw air through the engine. IOW, it has a bit of a pipe effect at RPM. If you run lean enough to get on the pipe, you may hear the sound of bacon frying in a skillet at times. A .004" head shim reduces the compression allowing you to still run lean but you may reduce the detonation to a manageable level and the added benefit is that the engine pulsing will reduce and that will show up in the tank as a lower vibration level. The other benefit is the HS needle gets a little broader and easier to manage.

When I started running C-Specs, I went immediately to Viperheads and OMI Cline Carb setups. I had seen the issues with the early 91SX engines and years ago had used the Enya 80's and first YS 80's and I wasn't inclined at the time to do my own R&D. The VH's are designed to run a certain amount of head shims to give you adjustability with different fuels. Fuel like WC, CP and others run just fine on .016-.020" shim settings, Magnum didn't like that and we use .024" shims instead. Magnum would detonate, the HS needle was finicky and as temps changed so did needle settings and it was very easy to go from just right to the ragged edge of really wrong in the same flight. Different fuels different shims. Jim Senicka is on the right track. The real key, IMHO, is to listen to what the engine is telling you. I hate to say it but that takes experience. The way to get there is to keep the engine rich enough to last but still make good power. Sometimes that means that you will have to work a little harder with the collective and accept lower power in the process. The benefit is you accrue more flight time with less trouble. There is nothing wrong with experimenting with shims. You will learn a lot in the process but don't expect it to be a panacea for all the usual ills like not running rich enough to avoid detonation.

I hope this helps.

TMPart 107.
The only ZERO flight hour certificate in the world.
It's like getting a driver's license without the driver's test.

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07-31-2008 12:07 AM 12 years ago

helical

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Nice info dude!

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07-31-2008 04:52 PM 12 years ago

Jlerch

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WOW, thanks TMoore! Let me ask one last question if I may.

Is it probable, that a motor lean enough to detonate strongly enough to cause permanent damage, may have a non overly hot head temp?

In other words, will real time adjustment of the fuel mixture based on head temp keep a motor out of danger from detonation?

Prior to the YS-91 eating itself, I was constantly checking back plate temp after a strong climb out / auto rotation landing, and I can't remember a time when I couldn't keep my finger on the back plate.

Thanks again for your time!James Lerch - Tampa Bay FL

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07-31-2008 06:01 PM 12 years ago

TMoore

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Is it probable, that a motor lean enough to detonate strongly enough to cause permanent damage, may have a non overly hot head temp?

In other words, will real time adjustment of the fuel mixture based on head temp keep a motor out of danger from detonation?

Prior to the YS-91 eating itself, I was constantly checking back plate temp after a strong climb out / auto rotation landing, and I can't remember a time when I couldn't keep my finger on the back plate.

If the engine is detonating, you can hear it. All of our engines will detonate to a certain degree from time to time. If I understand the question correctly, I wouldn't necessarily say it was probable but more likely possible that the engine was detonating and the head temp was still acceptable.

I don't worry about the head temp as much some folks might. If I take a temp at all and it isn't very frequently, I'm more interested in the back of the cylinder not the head. The head is designed to dissipate heat so by the time you take the temperature of the head, where would you do it at and still get a consistent reading? If you use a sensor to take head temps, you would have to mount it some where that is convenient, usually on the outer part of the head. The fan is running over the head, cooling it, raw fuel is entering the engine and providing some cooling effect and then you are working the engine loading and unloading it while the fan rpm changes constantly. I think the head is the part of the engine that is capable of dissipating heat the quickest because it is aluminum.

The reason that I use the back of the cylinder is that it is opposite the flow of air and the liner is made of steel which doesn't dissipate heat as quickly as aluminum. The readings are generally more consistent.

TMPart 107.
The only ZERO flight hour certificate in the world.
It's like getting a driver's license without the driver's test.

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08-01-2008 10:21 AM 12 years ago

Pinecone

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Maryalnd

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Actually the highest power output and lowest fuel consumption is right on the edge of detonation, and actually with a TINY amount of detonation.

BUT, a slight error in the wrong way, and you destroy an engine.

But modern car system are actually running the car right on the edge, using a computer, sensors, adjusting the fuel flow and timing, and in some cases even valve timing is adjsutable on the fly.Terry
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QJ EP8v2 EX Gaui Hurricane 550
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08-01-2008 01:48 PM 12 years ago

jsenicka

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Eagle River, WI

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Lots of good info here.
My very much compressed (

) thinking on the subject:

Detonation can be a symptom of overheating, or overcompression.
Overheating is a matter of adjusting the mixture slightly richer, and trade a bit of top end power for cooler temps. In an over compressed engine, you need to run so sloppy rich to prevent pre-ignition that you cannot make any real power. As soon as you lean up the tiniest bit, you start to detonate and subsequently overheat.

With this in mind, there is no way you can use needle valve settings to tune around a problem with compression too high for the nitro percentage you are running. You need to either reduce nitro (noooo!!!!!) or reduce compression to a point where you have a reasonable tuning range without pre-ignition causing issues. This is why OS ships additional head gaskets/shims with the 91SZ. The difference between 10% and 30% fuel requires you to change compression to achieve max power. The only alternate would be for OS to configure the motor for best running on 30%, and then pilots running 0, 5, 10, 15% etc suffer a significant drop in achievable power.

Bottom line is I do not in any way feel that adjusting head spacing is bad, or is a way to get out of needing to properly tune an engine. I feel that on the larger motors that are tweaked for absolute top power by the manufacturer, you need to configure the motor to match the fuel you are running.
Based on 4 years of fiddling here at sea level in Southern VA, and working with some very experienced pilots, I have proven (at least to my group here) that the OS 91 engines need at least .016 of head gasket to run reliably on 30% fuel. We have had the exact same experience with the YS91ST. With stock shims (about .008 on either motor), you cannot get a decent tuning range on the needles. It goes from sloppy wet to crackling in a couple clicks, and never makes clean power. Going to .016 of gasket makes the motor far easier to tune, and makes far better top end power on 30%.Jim Senicka
Team Manager, GrandRC Flight Team

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08-01-2008 04:31 PM 12 years ago

Jlerch

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Parrish, Florida

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When I started the Nitro side of this hobby 5 months ago, I threatened to build a Heli Dyno. The more I think about it, the more it sounds like a fun project.

Having torque, HP vs RPM charts with a slew of variables sure seems like the only reasonably scientific way of learning and solving these issues. (well, at least from my perspective, and I will admit I find odd things like this enjoyable)

I'm hatching a plan to weld up a 'chassis' dyno for heli's. The idea I'm playing with would be to removed the head block, fabricate an adapter to bolt to the main shaft a 2.5KW Generator head. The generator head would be suspended over the main shaft, lowered till the jesus bolt lines up, and secure the generator head. Next clamp the skids / tail boom in place somehow.

After that its "just" instrumentation while dumping the energy into a resistive load and measure RPM, Voltage, and Amperage. (well its slightly more complicated than that, as I'll have to have control of the field winding of the gen head, so as to achieve target RPM while at wide open throttle)

In any event, as long as were all having fun, I guess that is what counts!James Lerch - Tampa Bay FL

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