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Old 06-16-2005, 03:47 PM   #1
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Default To shim or not to shim that is the question

When you use high content nitro fuel is there a method to use to determine how many and how thick the shims need to be?
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Old 06-17-2005, 06:41 AM   #2
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What I say here is for Novarossi based engines shimmed from factory. It is a general rule of thumb on .12 engines that for every 5% nitro increase you add .05 of head shim.

However if you are running 30% nitro a .1 shim will work perfectly. 20% no shim needed.

There is a head clearance that you want to maintain however this involves actually knowing how to PROPERLY measure this.

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Old 06-17-2005, 08:56 AM   #3
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How much shim is based on the density of air. And the density of air is base on altitude;barometric pressure;weather. More close to sea level you need more shims. When atmospheric depression coming you may need to take some shims off. Sometime you may run 0.4mm in the day but 0.5mm in the night. Air temperature also has effect on the air density....etc. There is no final answer. You need to try different setting yourself.
When the shim setting is too many(low compression),the engine will get bogged down in low end. When the shim setting is too less(high compression),the plug's coil will be pushed in then burnt.
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Old 06-17-2005, 08:59 AM   #4
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So since we're doing the question game, couldn't running a colder plug in this case work instead of adding a shim? Maybe some of the engine gurus can answer.
Having done my research I have gathered the following:
Adding NItro = Advance Timing
Adding Shim = Retard Timing
Colder Plug = Retard Timing

Since you're adding nitro the timing is advanced. You want to add the shim to bring back the timing. But wouldn't the colder plug do the same? Adding a shim, will reduce compression which should reduce power. Why would we want to do that?

Just random thoughts and questions that run through my mind, thought maybe I should throw them out there and see what others think. So rant away
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Old 06-17-2005, 09:21 AM   #5
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Quote:
Originally Posted by gonzoY5
So since we're doing the question game, couldn't running a colder plug in this case work instead of adding a shim? Maybe some of the engine gurus can answer.
Having done my research I have gathered the following:
Adding NItro = Advance Timing
Adding Shim = Retard Timing
Colder Plug = Retard Timing

Since you're adding nitro the timing is advanced. You want to add the shim to bring back the timing. But wouldn't the colder plug do the same? Adding a shim, will reduce compression which should reduce power. Why would we want to do that?

Just random thoughts and questions that run through my mind, thought maybe I should throw them out there and see what others think. So rant away
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AFM

Engine Shim Tuning By Dennis Richey

Experience dictates that just by lowering the head (more compression) you gain more power especially in lower rpm range, idle quality can suffer, but the engine also runs cooler . Also, that a higher head (less compression) will increase top rpm speed on bigger tracks.
A decrease in head shims (an increase in compression ratio) will increase torque because as the compression ratio goes higher, the actual ignition timing occurs sooner. However there is a point of diminishing returns where detonation occurs or engine temps can soar, and if this happens a colder plug can help.
A colder plug will also increase torque, except in the instance of a colder plug the ignition is slowed until a greater point of compression build occurs.
When you increase head shims (a decrease in compression), top end is enhanced as the ignition timing is retarded and occurs later. Generally a hotter plug is needed to advance the ignition cycle so that timing does not occur to late in the cycle, but at this point you end up over leaning the engine to get it to rev properly and the engine life will suffer dramatically.

On a .12 engine we would only advise going 0.10mm over or under 0.46mm, so that is 0.36mm or 0.56mm, of total head clearance
Generally on a .12 engine 0.10mm will change the compression ratio about 3/4 to 1 point..

We should also state that the comments with regards to plugs and head shimming are when both are used together. Used alone a hotter plug will rev harder and a colder plug will make more torque.

One last comment. never assume that the head shim that is on the engine is the actual head clearance. in many cases you will find that the engine actually has 0.20mm to 0.15mm without any head shim. Novarossi's are 0.20mm without one, and the factory installs a 0.30mm shim, giving the engine 0.50mm stock head clearance. This has not always been the case but 99% of the time it is. When in doubt measure the head button register and the piston to the top of the liner at top dead center and subtract, to determine proper shim. This is the only way to really know.

You can safely use 30% Nitro on the stock head clearance. Using a C8TGF you can go up to 40% Nitro, and If you donít over lean it, the engine life will not suffer dramatically.

Someone asked about plug fatigue the other day, plug fatigue can occur under high sustained RPM's, and this is where the plug stays shiny and the wire just fractures for no reason. This is why we have always run Novarossi plugs, as they donít do this. When this happens it is usually not a carb. tuning or head clearance problem. If you see the wire start to pull out of the hole this means that the engine is scavenging really hard and when this occurs the engine is making serious power.

There you have it.
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Old 06-17-2005, 09:25 AM   #6
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Quote:
Originally Posted by dino.tw
How much shim is based on the density of air. And the density of air is base on altitude;barometric pressure;weather. More close to sea level you need more shims. When atmospheric depression coming you may need to take some shims off. Sometime you may run 0.4mm in the day but 0.5mm in the night. Air temperature also has effect on the air density....etc. There is no final answer. You need to try different setting yourself.
When the shim setting is too many(low compression),the engine will get bogged down in low end. When the shim setting is too less(high compression),the plug's coil will be pushed in then burnt.
GPP Model Racing
AFM

Weather Makes The Biggest Difference!

The atmospheric pressure, temperature and humidity all affect the density of the air. On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the amount of oxygen available for combustion and therefore reduces the engineís horsepower and torque. For tweaking the fuel/air mixture and compression ratio, the air density is the most important consideration.

The Air Temperature should ideally be the temperature of the air that is going into the intake of the engine.
The Absolute Pressure (also called actual pressure or station pressure) is the ambient air pressure.
Relative Humidity is a measure of how much moisture is in the air compared to the amount of moisture that the air could hold at saturation. Relative humidity is a function of temperature and therefore changes as the temperature changes, even if the amount of moisture in the air remains constant.
The air density is the actual weight of a given volume of air. This is a key parameter for engine tuning.

Resuming:
When the air density increases, you will need to richen the air-fuel mixture to compensate. When the air density decreases, you will need lean-out the air-fuel mixture to compensate.

Use the following as a guide to correcting your setting when the weather changes:

Air temperature: When the air temperature increases, the air density becomes lower. This will make the air-fuel mixture richer. You must lean the mixture to compensate for the lower air density. When the barometric pressure decreases, the opposite effect occurs.

Humidity: When the percentage of humidity in the air increases, the engine draws in a lower percentage of oxygen during each revolution because the water molecules (humidity) take the place of oxygen molecules in a given volume of air. High humidity will make the air-fuel mixture richer, so you should lean the mixture.

Altitude: In general, the higher the altitude the lower the air density. When driving at racetracks that are at high altitude, you should lean the mixture and increase the engine's compression ratio to compensate for the lower air density.
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Old 06-17-2005, 09:45 AM   #7
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Quote:
Originally Posted by gonzoY5
So since we're doing the question game, couldn't running a colder plug in this case work instead of adding a shim? Maybe some of the engine gurus can answer.
Having done my research I have gathered the following:
Adding NItro = Advance Timing
Adding Shim = Retard Timing
Colder Plug = Retard Timing

Since you're adding nitro the timing is advanced. You want to add the shim to bring back the timing. But wouldn't the colder plug do the same? Adding a shim, will reduce compression which should reduce power. Why would we want to do that?

Just random thoughts and questions that run through my mind, thought maybe I should throw them out there and see what others think. So rant away
Very good question!
Higher compression with cooler plug V.S. lower compression with hotter plug
Under well tuning both of them may work fine. Actuality it have different power band. Higher compression has better acceleration and lower compression has better top end.
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Old 06-17-2005, 09:51 AM   #8
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I was told that if you run Byron or O'Donnell, you will not have to worry about shimming the head of a Nova .12. (30%)
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Old 06-17-2005, 09:52 AM   #9
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Here is a great simple guide to tuning your rc motor. Everyone should print these out and stick them inside your pitbox or whatever.


(right click + Print)

Last edited by Artificial-I; 08-06-2006 at 01:15 PM.
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Old 06-17-2005, 10:32 AM   #10
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See, this is what I was looking for. Keep it coming
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Old 06-17-2005, 12:18 PM   #11
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Leaning the engine when the temp get's higher
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Old 06-17-2005, 12:26 PM   #12
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Quote:
Originally Posted by _cyclops_
Leaning the engine when the temp get's higher
Read my article a couple of popts back about weather conditions

Air temperature: When the air temperature increases, the air density becomes lower. This will make the air-fuel mixture richer. You must lean the mixture to compensate for the lower air density. When the barometric pressure decreases, the opposite effect occurs.

AFM
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Old 06-17-2005, 05:39 PM   #13
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Quote:
Originally Posted by afm
Read my article a couple of popts back about weather conditions

Air temperature: When the air temperature increases, the air density becomes lower. This will make the air-fuel mixture richer. You must lean the mixture to compensate for the lower air density. When the barometric pressure decreases, the opposite effect occurs.

AFM
hmm well... here wheater temp goes up (eg. from 10 deg to 30 deg celcius), the needle goes richer...
i live in the netherlands.. and when it get's hotter, the engine simple will overheat, so we richen the mixture to keep the engine's cool....

i think the image that we all print, is rather country dependend.....
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Old 06-17-2005, 06:08 PM   #14
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Quote:
Originally Posted by _cyclops_
hmm well... here wheater temp goes up (eg. from 10 deg to 30 deg celcius), the needle goes richer...
i live in the netherlands.. and when it get's hotter, the engine simple will overheat, so we richen the mixture to keep the engine's cool....

i think the image that we all print, is rather country dependend.....
But you have to take into account Humidity......if in your area Humidity drops when temperature raises...then you must richen the mixture.

Humidity: When the percentage of humidity in the air increases, the engine draws in a lower percentage of oxygen during each revolution because the water molecules (humidity) take the place of oxygen molecules in a given volume of air. High humidity will make the air-fuel mixture richer, so you should lean the mixture. Low humidity wil make the air-fuel mixture leaner, so you should richen the mxture.

AFM
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Old 03-25-2013, 02:48 PM   #15
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Quote:
Originally Posted by afm View Post

Altitude: In general, the higher the altitude the lower the air density. When driving at racetracks that are at high altitude, you should lean the mixture and increase the engine's compression ratio to compensate for the lower air density.
So the stock shim for my Nova .12 should be fine at my altitude and 30%?
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