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Model Forum / Radio Controlled / Air Models / April 2004nitro model engine factors
Hi, I'm trying to maximize performance of my engine and would like to hear some input on selecting the best fuel, plug, prop and muffler. Here are some things I've came up with: Plugs ----- cold weather - hot plug hot weather - cold plug if running high oil content (18%) then use hotter plug than normal Fuel ---- US Market engines - 10->15% nitro Europe, Asian engines - 5% nitro high reving engines - more nitro small carbs on big engines - more nitro (don't have enough air to combust) small engines and big carbs - less nitro (don't have trouble getting enough air) higher oil content fuel reduces performance somewhat, use hotter plugs (plug is cooled by oil) Props ----- for better idle use higher diameter and heavier props (flywheel) pitch speed should be at least 3X stall speed larger diameter lower pitch props give more thrust but less speed; good for 3D and aerobatics (also pulling out of trouble) smaller diameter higher pitch props give more speed but less thrust; good for pattern aerobatics and flying around Mufflers -------- Expansion chamber mufflers can increase rpm by 500 over stock MCP mufflers typically add 1000 to 2000 "Kraut" double chamber MCP can add somewhat less than MCP but costs $3 Background: I'm tuning my Leo .37 and now I'm running on Wildcat 10% nitro, 18% 80/20 oil blend. Switching from medium to medium-hot plug increased the top rpm by 400. Switching from 9x7.5 to 11x4 reduced the idle from 2,900 to 2,500. Currently the engine is turning APC11x4 at 12,000. There seems to be _too_ much oil in the exchaust so my next fuel will be 16% oil. I will also try less nitro as the engine has a huge .46 sized (7.5mm) dual needle valve carb. Next mod will be a 46 size expansion chamber muffler since the one shipped with it looks like 25 size. Can you suggest other mods? rules of thumb? Has anybody built regular MCP and "Kraut" MCP mufflers for the same engine and compared performance? I'm really curious about the numbers. Thanks, Michal  Signatureu*x sys admin, programmer, aviation enthusiast, photographer I don;t quite understand the theory about carb throat size and nitro content.. A small carb gets enough air for the fuel entering it, if you set the needle(s) right. A large carb gets enough air for the fuel entering it, if you set the needle(s) right. A small (i.e. smaller than typical) carb for a particular engine may increase fuel draw and improve overall manners (i.e. throttle response, linearity) over the throttlaeable range at the expense of top end. A large carb can improve top end (within limits) but you may suffer from some temperment throttling up and down. Also, FAI speed and FF engines run happily at 30k rpm plus on fuel with no nitromethane. High compression ratios call for lower nitromethane content. Low compression ratios call for higher nitromethane content. Smaller engines - higher nitromethane content (larger cooling area relative to displacement) and vice versa, other factors being somewhat equal. Also, I have my doubts about the smarts of reducing the oil content. What do you mean by "too much oil" in the exhaust - there's always going to be some. It's been argued pretty strongly that for normal size engines much below 18% oil invites premature wear. While 16% is not "radical" I personally wouldn't go that low for a .37 unless I was willing to accept a potential compromise in life span for a small gain in performance. If you want more horsepower, why not increase the nitromethane content, shim the head if necessary to prevent preignition, and keep your oil content up where it should be. Mike D. > Hi, > [quoted text clipped - 55 lines] > > Michal > I don;t quite understand the theory about carb throat size and nitro > content.. Nitro is used in engines to reduce the amount of oxygen needed to combust a given volume of fuel. High reving engines need lots of nitro because they can't get enough oxygen to burn all the fuel. > A small carb gets enough air for the fuel entering it, if you set the > needle(s) right. If you want lots of power you need to burn more fuel, that requires delivering a lot of oxygen to the engine. If the carb is small, then you can't get enough oxygen, but if you substitute methanol with nitromethane, you can burn more of that mixture. > A large carb gets enough air for the fuel entering it, if you set the > needle(s) right. In this case you have plenty of oxygen to burn so you can use close to straight methanol for more power. > Also, FAI speed and FF engines run happily at 30k rpm plus on fuel with no > nitromethane. FAI speed and FF run happily only when throttling isn't required. They are optimized for high rpm only and suck at everything else. > Also, I have my doubts about the smarts of reducing the oil content. What do > you mean by "too much oil" in the exhaust - there's always going to be some. > It's been argued pretty strongly that for normal size engines much below 18% > oil invites premature wear. While 16% is not "radical" I personally wouldn't Seemed like more than my other engines have in their exhaust. Maybe because it was a cool day the exhaust oils felt thicker. I wouldn't go radically lower, just 1-2%. Thanks for oil info. Michal Signatureu*x sys admin, programmer, aviation enthusiast, photographer On 4/12/2004 1:01 PM Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge: > Hi, > > I'm trying to maximize performance of my engine and would like to hear > some input on selecting the best fuel, plug, prop and muffler. What are you trying to maximize the performance for ? Racing, pattern flying, etc ? For instance, For racing, you are not concerned about engine longevity, just maximum rpm/speed. For pattern, you are looking for consistent, reliable performance. > Here are some things I've came up with: > [quoted text clipped - 3 lines] > hot weather - cold plug > if running high oil content (18%) then use hotter plug than normal Not necessarily. So far the general idea is correct, however, the humidity on any given day will also effect the performance of the engine (see comment under fuel). I have had excellent results with OS#8 plugs for general sport and fun fly flying at sea level. > Fuel > ---- > US Market engines - 10->15% nitro > Europe, Asian engines - 5% nitro You said you have a Leo. This is primarily a European engine. If memory serves me correctly, European engines are designed to run on FAI fuel (0% nitro) to a MAX of 5% nitro. To use a higher percentage nitro, you will most likely have to add head shims (extra head gaskets) to lower the compression (or risk burning a hole in the piston). You also need to be aware of your altitude. An engine that performs a certain way on 5% fuel at sea level will require 15% - 20% nitro at 3,000' - 4,000' (and usually a different plug) to perform the same way due to lower air density. Humidity also plays a big part. In humid conditions, not only is the air denser, but the moisture in the air helps to cool the fuel mixture. > high reving engines - more nitro AND/OR a smaller diameter and pitch prop. > small carbs on big engines - more nitro (don't have enough air to > combust) [quoted text clipped - 11 lines] > smaller diameter higher pitch props give more speed but less thrust; > good for pattern aerobatics and flying around Still on track (in general terms) > Mufflers > -------- > Expansion chamber mufflers can increase rpm by 500 over stock > MCP mufflers typically add 1000 to 2000 > "Kraut" double chamber MCP can add somewhat less than MCP but costs $3 I am not familiar with MCP or "Kraut" mufflers, but this is pretty much correct. You do need to be careful that you maintain adequate back pressure for pressurization of the fuel tank, or run a fuel pump. > Background: > [quoted text clipped - 3 lines] > to 2,500. Currently the engine is turning APC11x4 at 12,000. There seems > to be _too_ much oil in the exchaust so my next fuel will be 16% oil. There may SEEM like excessive oil in the exhaust, but it is a safety measure. In case of a "lean run" it will help to minimize or eliminate damage to the engine. The manufacturer usually states the minimal percentage of oil the engine should have. I have found you can SAFELY go 2% or 3% less. I would stick with the manufacturer's suggested percentage of oil, at least until the engine is completely broken in. > I will also try less nitro as the engine has a huge .46 sized (7.5mm) dual > needle valve carb. Next mod will be a 46 size expansion chamber muffler > since the one shipped with it looks like 25 size. If noise is a minimal concern, remove the internal baffle in the muffler. That will usually provide extra power while still maintaining a good back pressure for the fuel tank. > Can you suggest other mods? rules of thumb? There is an excellent article in Fly RC (Premiere Edition) on tuning your engine. It covers several of my comments in detail. > Has anybody built regular MCP and "Kraut" MCP mufflers for the same > engine and compared performance? I'm really curious about the numbers. > > Thanks, > > Michal > What are you trying to maximize the performance for ? Racing, pattern > flying, etc ? I would like to optimize for thrust without spending too much money (and keeping it light). >> Fuel >> ---- [quoted text clipped - 7 lines] > conditions, not only is the air denser, but the moisture in the air > helps to cool the fuel mixture. So during hot and humid midwest days I should see higher rpm? Of course in those conditions airfoils generate less lift so actually overall performance actually decreases. >> I'm tuning my Leo .37 and now I'm running on Wildcat 10% nitro, 18% >> 80/20 oil blend. Switching from medium to medium-hot plug increased the [quoted text clipped - 8 lines] > go 2% or 3% less. I would stick with the manufacturer's suggested > percentage of oil, at least until the engine is completely broken in. It seems excessive compared to my past engines (all lasted 5+ years). The manufacturer recommends 17% oil, so having 80/20 synthetic/castor 18% blend seems much above recommendations. I'm going to use it for the first gallon and then switch down to 16%. If I pick 200rpm for each little optimization it will add up to something noticeable. That is my strategy. >> I will also try less nitro as the engine has a huge .46 sized (7.5mm) dual >> needle valve carb. Next mod will be a 46 size expansion chamber muffler [quoted text clipped - 3 lines] > muffler. That will usually provide extra power while still maintaining > a good back pressure for the fuel tank. This one doesn't have a baffle. >> Can you suggest other mods? rules of thumb? > > There is an excellent article in Fly RC (Premiere Edition) on tuning > your engine. It covers several of my comments in detail. thanks, I'll try to look it up. Michal Signatureu*x sys admin, programmer, aviation enthusiast, photographer Sorry Ted, but higher humidity means the air is *less* dense. Water vapor is invisible and is *lighter* than air. When the water vapor in the air condenses it becomes fog/clouds. Most think that since water is heavier than air, moist air must be more dense than dry air.but that is not correct. Increased humidity increases the density altitude and that means longer TO distances and poorer engine performance... Took me awhile wrap my brain around that fact - almost ran out of runway on a hot, humid day and that convinced me. Ed F. > On 4/12/2004 1:01 PM Ted shuffled out of his cave and grunted these > great (and sometimes not so great) words of knowledge: [quoted text clipped - 109 lines] > > > > Michal >Sorry Ted, but higher humidity means the air is *less* dense. Water vapor >is invisible and is *lighter* than air. When the water vapor in the air [quoted text clipped - 4 lines] >around that fact - almost ran out of runway on a hot, humid day and that >convinced me. The problem is heat, not water vapor. Takes more heat to increase the water content of the atmosphere, and higher total heat (latent and sensible) in turn raises the Brownian motion, pushing the individual molecules farther apart. Don't get wrapped around the humidity thing too tightly, since dew point is only part of the density altitude equation. A google search with turn up bunches of density and pressure altitude calculators, equations, and dissertations. Cheers, Fred McClellan The House Of Balsa Dust http://home.mindspring.com/~the-plumber Hi Fred, I'm not sure I understand you point. I was responding to Ted's statement "...In humid conditions, not only is the air denser..." that statement isn't correct.- Temp 90°, humidity 80%, performance will be poorer than 90°, 40% humidity. That's the way Uncle Sugar teaches it. :-). SignatureTally Ho! Ed Forsythe Maryland, USA > >Sorry Ted, but higher humidity means the air is *less* dense. Water vapor > >is invisible and is *lighter* than air. When the water vapor in the air [quoted text clipped - 20 lines] > The House Of Balsa Dust > http://home.mindspring.com/~the-plumber Here is the layman's reason for why humid air is less dense than dry air. Humid air is less dense because water is H2O which has 2 hydrogen atoms with an atomic weight of 2 each plus one oxygen atom with an atomic weight of 16 for a total of 20. An oxygen molecule on the other hand is O2 or 2 atoms with an atomic weight of 16 for a total of 32 vice 20 for water. The other molecules that make up air are also heavier for the most part and the water vapor displaces them thus making the air less dense. Less dense air makes for poorer performance for both engines and wings. Density altitude is a function of pressure altitude with temperature and humidity (dew point) figured in. Being a meteorologist, I get to play with this stuff all the time. Just ask a full scale helo pilot about PA/DA, I'm sure he/she can give you an earful. Higher temperature and humidity with a given pressure altitude equals higher density altitude and thus poorer performance. Jim W > Sorry Ted, but higher humidity means the air is *less* dense. Water vapor > is invisible and is *lighter* than air. When the water vapor in the air [quoted text clipped - 120 lines] > > > > > > Michal {:-0 SignatureTally Ho! Ed Forsythe Maryland, USA > Here is the layman's reason for why humid air is less dense than dry air. > [quoted text clipped - 145 lines] > > > > > > > > Michal > I'm tuning my Leo .37 and now I'm running on Wildcat 10% nitro, 18% > 80/20 oil blend. Switching from medium to medium-hot plug increased the > top rpm by 400. You've found one of the clues to getting the best out of an engine, and that's the plug heat range. Going to a hotter plug advances the ignition point and seeing you gained revs then you're closer to optimising for the particular fuel/compression ratio of your engine. Basically it means that with your original medium plug you could have either increased the nitro content or raised the compression with the 10% fuel. If you now tried a hot plug and saw a rev DEcrease then you'd know that the combination of fuel/compression/medium hot plug was nearing the ideal. Remember with plugs though that some manufacturers may only provide 3 plugs from hot to cold while others may have 12 or more in their range. To confuse it even more, a medium plug from one manufacturer won't necessarily be the same as a medium plug from another. As far as oils go, so long as you don't go to ridiculous extremes then oil has no affect on the power produced. Brian Hampton Adelaide, South Oz > As far as oils go, so long as you don't go to ridiculous extremes then > oil has no affect on the power produced. I agree 100% on plugs. On my old OS40fp with 11x4 APC, I got 600rpm increase by going to a hot 4-cycle plug. I know it didn't suffer from preignition sice that engine lasted 6 years :) Michal Signatureu*x sys admin, programmer, aviation enthusiast, photographer |
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