Originally Posted by 54
I need help!!!!
I run in Mexico City on an altitude of 9,842 ft above sea level an Inferno GT from Rally Game with 40% nitro, my problem is the glow plug blow up, and I see a lot of carbon deposits in the chamber and head top.
I hear that the head clearance is calculated according to the amount of nitro used, in this forum most of the categories run at sea level and 25% nitro. My question is: The higher the altitude more head clearance?? the more nitro more head clearance?? or Do I have to change the Glow plug?? I use an .21 O.S.Max with an #8 O.S. Max glow plug.
Thank you very much.
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Yes your engine comes shimed to use max 25% nitro at sea level, but as you move to higher altitute you loose power at 10% for every 1,000mts, because you loose compression witha less denser air, so you have to compensate that either by removing shims accordingly but keeping same nitro% or max 30%...and the other way is by leaving shims the same but going to a higher nitro content, which is what you are doing, but you need to change the glow plug to a colder type as suggested previously and to lessen carbon deposits maybe lean the engine a bit more.
If it helps following are some articles related to tuninmg at high altitude (here in Peru we have cities similar to Mexico's altitude and higher)
Tuning For High Altitude
The general formula for power loss with increases in altitude is 10 percent for every 1,000 mts above sea level.
At high altitudes we face a problem: how do you maximize engine performance and speed in the thinner air? Internal-combustion engines require three things to produce power - fuel, oxygen and heat -- and they need them in specific ratios. Running good is nothing special at low elevations; the denser air at sea level contains more of the oxygen an engine needs to produce power. At higher altitudes, the thinner air means less available oxygen, so an engine will not be able to burn as much fuel as it does at lower altitudes. Less fuel means less power, and that means slower speeds.
What can you do to minimize the effects of altitude? Some equip their cars with a higher gear ratios to recover some of the rpm lost because of the reduction in power. Others improve airflow, by lengthening or shortening their tuned pipes until performance improves.
But can you do anything to the engine itself to compensate for thinner, high-altitude air? Compression creates power, and because the air at high altitude is less dense, there is less fuel/air mixture available to compress.
To recover some of the lost compression, you can:
• Use a fuel containing a higher percentage of nitromethane.
• Reduce engine-head clearance to increase the compression ratio.
To compensate for higher altitudes, you need to adjust your needle valve to a leaner setting to maintain the proper fuel/air ratio. The opposite is true for lower altitudes. The idea is to maintain the optimum ratio of fuel and oxygen by adjusting the fuel volume to compensate for ambient air pressure.
Elevation complicates the situation. A venturi such as a model engine carburetor's works on the vacuum principle: as air rushes through the venturi, it accelerates and creates a vacuum at the spraybar. This vacuum draws fuel from the spraybar and fills the crankcase with a mixture of air and atomized fuel; at very high altitudes, less air accelerates through the venturi and past the spraybar. This reduces the vacuum at the spraybar, so less fuel is drawn through it, and the mixture becomes too lean.
There is a way to compensate for this:
• Switch to a venturi with a smaller area.
When you have a smaller venturi area, the airflow velocity through the venturi is increased. This also increases the vacuum at the spraybar, which, in turn, draws more fuel.
If you don't want or can’t buy a separate carb just for high-altitude running, the easiest way to increase fuel flow is to open up the needle valve to richen the mixture setting. A richer needle-valve setting requires less of a vacuum to draw in a given amount of fuel. Ultimately, this won't be quite as effective as having a smaller venturi area.
Obviously, you have to experiment to learn which combination of these high-altitude tuning techniques best suits your model. With a little testing, you can have your car running at the head of the pack.
An engine's compression ratio (CR) is the comparison of cylinder volumes prior to compression and after the piston reaches TDC. As the compression ratio is increased, the air-fuel mixture is squeezed into a smaller volume prior to ignition, resulting in a greater heat release rate after ignition. Within limits, high compression ratios produce enhanced cylinder pressure, torque and power.
Compression Ratio is the amount of volume the piston compresses the fuel air mixture to. So an 8 to 1 compression ratio would compress a volume of 8 into a volume of 1. Plainly said, it is like taking the volume of 8 bottles and squeezing them into one.
The compression ratio on model engines actually controls the ignition timing which is also affected by the nitro content of the fuel.
A glow engine is similar to a diesel engine. The compression ratio in model engine can be from as low as 7.5 to 1 to a high of 10 to 1.
Raising an engine's compression ratio advances the ignition point timing; lowering it retards the ignition point timing. Raising or lowering the compression ratio by removing or adding head shims (gaskets) is the most common method of achieving this change.
Removing a head shim (gasket) increases the compression ratio; adding a shim reduces the engine's compression.
Considered a radical adjustment, compression change is usually reserved for significant changes in altitude, atmospheric conditions, or nitromethane content in the fuel.
The compression ratio varies with the type of fuel, altitude, port timing, and exhaust configuration. Higher performance engines hold very close tolerances to assure the proper combustion point.
Nitromethane is a powerful fuel ingredient. Under ideal conditions, increasing the fuel's nitro content (% by volume) will increase engine power. Because nitro is very slow burning, the ignition point timing must be advanced to maintain the correct pressure peak sweet spot after TDC. Raising the fuel's nitro content often requires adjusting one or more of the other variables to obtain satisfactory performance, as we will discuss.
Tuning For High Altitude Engine Operation
You've decided to attend a race in another state. Your engine has been running great at home, but you're concerned about the altitude at the new site, it's 2000 feet higher. All non-supercharged internal combustion engines lose torque and horsepower performance when operating at higher elevations. The air contains less oxygen per volume and is less dense. Since all reciprocating piston engines are essentially air pumps, their ability to transfer and trap air in the combustion zone is reduced when operating at higher altitudes.
Check your notes to see if rpm has dropped. On arrival at the new site, run your engine at and set the needles as usual. Compare the rpm and head temperature with your records from back home—same gearing, fuel, and plug. Depending on the weather conditions, you will probably notice that rpm has dropped and power is lost.
If your engine is overheating... It could be due to the compression ratio being too low for the fuel you are using. Lower nitro content require higher compression ratio. If the compression is too low the firing of the combustion charge will be too late or retarted creating poor burn and thus allowing a burning fuel charge to be released out the exhaust. This elevates the temperature of the engine and lowers the performance.
Experienced tuners have learned that some of the oxygen can be replaced by increasing the nitromethane content of the fuel; the nitromethane molecule contains oxygen that is released during the combustion process. If you use 20% nitro fuel at home, bring along some 30%. Keep in mind, higher nitro content reduces the run time for a tank of fuel.
If you increase the nitro content the combustion will be advanced and can produce a more powerful and cooler running engine……..or
You can increase the compression ratio by removing head shims appropriately
Reduce the load by gearing lower, and run the engine again. If it still runs hot, reach into your bag of tricks and try a colder plug, or more oil (%) in the fuel blend.
Once you have achieved acceptable head temperatures with the high nitro fuel, additional experimentation with plugs, gearing and oil content can further optimize the engine's performance.
On the other hand if your compression ratio is too high the engine will detonate. This is the same as the ignition being too advanced in you car or if you run too low of an octane rating. Like when you car engine pings with regular gas., so you may have to lower the compression ratio. You may need to add head shims. Detonation is basically the fuel charge exploding before the piston reaches the top of the stroke so the piston forces the explosion to be contained without expanding. It has no place to go because the inertia of the crankshaft is forcing the the piston to move thru its proper course. Thus you get a ping which is the sound of all the clearances in the connection rod being driven together squishing the oil film out of te bearing. If you have detonation you can destroy your piston as the crown (or top) of the piston is subjected to exrteme heat and pressure because of the trapped exploding gasses.
Engines are designed to have the combustion expansion drive the piston down the cylinder, not have it trapped.
Note the higher the nitro content is not like a higher octane gasoline rating. Higher octane gasoline requires higher compression ratio as it actually burns slower than regular gas and requires more compression to ignite properly. Nitro actually lowers the compression ignition point of glow fuels.
With all this extra oxygen and fuel going into the engine, more power is available, as mentioned before, So for every 5% nitro, a power increase of about 3-4% might occur if everything is adjusted correctly.