Nitro Engine Tips
Basic nitro engine information, by Ron Paris
IDEAL ENGINE TEMPERATURE:
There is no ideal temperature for any engine. There are many variables that affect it such as ambient temperature, fuel type and nitro content, altitude & barometric pressure, pipe/manifold type and setting, clutch setting, glow-plug, gear ratio, available traction, how hard you’re driving, on road or off road, etc. All of the following conditions must be met before any temperature can be considered correct (assuming the engine is in good shape):
1 - There must be a visible trail of smoke when accelerating from every corner.
2 - The idle must be stable.
3 - The glow plug wire should stay somewhat shiny, and the coil should stay round (un-distorted).
4 - The performance must be good.
COMMON ENGINE QUESTION - HOW MANY PORTS DOES AN ENGINE NEED?
More is not indicative of being better. If more ports were the only solution to more power, all engines would have maximum number of ports. Simply put, there is only so much room for ports in any given displacement engine. Some engines with twice the ports can have less total port area. This is not good or bad - it is just different. Every thing is a combination of many variables. Some of the most powerful engines I have built were three ports! We top qualified the world championships in Holland with a three port engine wile many of the competitors were using 5, 6, 7 and even 8 port engines and we had the fastest top speeds displayed on the board. It is true in most cases that within any brand of engine their power ratings go up with the amount of ports, but this is due to more than just the quantity of ports. Generally there are many other components in the engine that have more refined or modified parts than those with less ports, complementing the combination. You can only compare the quantity of ports between engines of the same manufacture. They all have their own combinations. In other words, brand A’s 17 port may or may not run equal to brand B’s 29 port.
COMMON ENGINE QUESTION - HOW MUCH RPM and HORSEPOWER?
There is no industry standard for hp or rpm testing like the full size automotive SAE (Society of Automotive Engineers) and there are so many things that can dramatically effect readings such as nitro, altitude tested at, with or without a tuned pipe, absorption unit differences (or prop size, brand and manufacturing tolerances) etc. You can not under any circumstance compare hp or rpm ratings between manufactures. You can however use these ratings to compare relatively between different engines in a manufacturer’s own product line.
HOW TO SET CARB NEEDLES
New racers often ask where to set the needles on their carb. There is no such thing as a definitive universal setting for any engine. Every application will have its own unique requirements. Even two identical set ups can (and most likely will) have at least slightly different settings.
Some racers make the mistake of setting the idle speed opening to wide (high idle) and set the bottom end too rich. This will give a false normal idle speed even though the speed is set to high because it “loads up” the engine with excess fuel causing the idle to be lower than set. The end result is a very unstable idling engine that surges and may cut out as full throttle is applied because the over rich bottom end can disguise a too lean top end setting!
It is possible too set the idle screw adjustment in to far, but yet the idle speed is not high! Even though the air regulation (carb barrel or slide) may be set to a position that would normally equal a vary fast idle, the idle is low because the low speed mixture adjustment is set so rich that the engine loads up with excess fuel, and the engine goes into what’s commonly called a four cycle idle. One tell-tale sign of this is if after revving up the completely warmed up engine, it tends to idle fast for a few seconds, then drops to a lower idle speed. In other words, it’s something like da..da..da..da..da..da..da..da then it drops to da….da….da….da. If you start leaning the bottom end a little at a time (then repeat the revving up and idle test) and it takes longer before the idle drops, you’re going in the right direction. Eventually as you keep leaning the bottom, the idle will stay too high. Now it is time to lower the idle to where it belongs by re-adjusting the idle screw. Now that you have the idle set correctly, the top end may be too lean! Keep in mind the fuel does not directly enter the cylinder area like a 4 stroke engine; it enters the crankcase area first, then is transferred or pumped up to the cylinder area by the piston movement. Simply put, the crank case volume can hold much more capacity than the cylinder, so it takes some time to burn off the residual fuel. If the bottom end is too rich, the engine will be supplied by this residual fuel briefly, and depending on the demand you may be actually be experiencing a lean condition on the top end that can range from:
1 - Seems to run well, but engine life is short
2 - Seems to run well, but car continues to get hotter the longer you run to the point of overheat!
3 - Seems to run ok on the bottom, but sputters starves or strains to gain rpm
4 - Seems to run ok on the bottom, but when you give full throttle it cuts out or stalls
An overly rich top end can act the same as 3 & 4, but excessive smoke and oil are usually present with a distinct blubbering sound.
There is no reason for an engine to continually get hotter unless the tune is wrong (classic #2 symptom) or there is a mechanical problem causing more load or drag on the engine as the run continues. An exception is if the weather or track conditions change dramatically - for example, a light drizzle starts and the off road track goes from a very dry loose to high traction condition, or during a race a rapid weather front happens, causing something like a ten degree change in temperature.
It is very important to fully warm up your engine, clutch and chassis before making final adjustments. The chassis in most applications also works like a heat sink to the engine so it is important to fully saturate the chassis!!! I like to start the engine at least three or four minutes before the qualifier to get some heat in the engine. If you don’t yet have your radio, you can operate the throttle by hand. Remember though, it will still take at least two to three minutes of hard running on the track to fully saturate the chassis!
Carb Needles
There is a series of restrictions to control fuel flow at different throttle/air flow positions called needles These are the five basic parts of the carb to concern yourself with.
The slide or barrel regulates the amount of air that can enter the engine. It is controlled by the servo. It simply blocks off the airflow to the engine, proportional to how far it is open or closed.
The idle/air speed screw sets the absolute minimum air the barrel/slide can control to maintain idle speed. It simply is an adjustment screw that comes in contact with the side at the nearly closed/idle position.
The high speed needle regulates maximum fuel flow allowed to enter the engine at any throttle position. It is simply a tapered needle that screws into the fuel flow orifice (an adjustable restriction). This maximum fuel flow ideally is adjusted to the correct mixture ratio for the surrounding conditions when the throttle is wide open.
The low speed needle regulates fuel to engine at idle. The low speed adjustment simply restricts the flow at idle speed. If you look down the bore of the carb you will usually see a long tapered needle. When the carb barrel/slide is closed the larger part or diameter portion of the long tapered needle is inserted into the spray bar, this is what’s adjusted when you turn the low speed/minimum adjustment. It literally moves either the tapered needle or the spray bar farther in or out, changing the restriction independent of the barrel/side position. This leans (more restriction) or richens (less restriction) the flow from the spray bar at idle. On some carbs the spray bar is moved and on others the needle assembly is moved. Both have the same effect.
The mid range needle regulates fuel to the engine after idle and before full fuel position. As you open and close the carb the tapered needle (mentioned above in item 4) enters into a small tube this is called the spray bar/jet. This spray bar is where ALL the fuel enters the airflow stream regulated by the high speed, mid range and idle/minimum adjustments! Normally at somewhere between ½ and ¾ throttle open position the needle is completely out of the spray bar, This is what is called full fuel position or FFP. At this point 100% of the mixture is controlled by the high speed needle. On many carbs the low speed and midrange are not independently adjustable, so the mid range is a factor of the needle taper and is engineered by the factory.
If there are adjustments on both the slide and the carb body, one is an adjustable mid range, and the other the low speed!!! Check with the engine manufacture before attempting to adjust these types of carbs! It is very important not to use the mid screw to adjust the low end by mistake; it is very easy to get the carb way out of sync, and the gains are VERY small and mostly limited to minute midrange drivability/economy changes that only the most sophisticated driver will recognize. If you do this wrong, you might end up with a carb that is so screwed up only an expert can get it back in tune!
HOW TO “READ” YOUR GLOW PLUG
When a new plug wire just goes slightly gray after a 5 or 10 minute hard run it means your very close to an optimal horsepower tune, but be careful - the next step is TOO LEAN!
If the wire and surrounding bottom of plug is wet, with like new shiny wire, you’re on the rich side of optimum power.
If the wire and surrounding bottom of the plug is starting to dry and the wire starting to gray, you’re very close to optimum power.
If the wire and surrounding bottom of plug is dry, and the wire is totally gray but not distorted, you’re at optimum power.
If the wire and surrounding bottom of the plug is dry, and the wire is distorted, you’re slightly lean.
If the wire and surrounding bottom of plug is dry, the wire is broken and distorted or burnt up, your engine is extremely lean and there’s the possibility of engine damage.
You can only “Read” your plug in a nearly new state (wire like new and shiny). A gray plug can still operate well. After it has totally gone gray, performance can start to fall off. To test, just put in a new plug. If there is no difference in performance, save the gray one or put it back in. If your engine does not feel or run right, try a new plug before making major tune changes.
MANIFOLD LENGTH
A longer manifold means more bottom end torque (at the expense of some rpm). A shorter manifold means more top end rpm (at the expense of some torque). You can go to far in either direction, causing the engine to run erratic or hot! You must keep it in the “sweet spot”. There is no set length for every engine or pipe. Experiment with the length to suit your driving style or track needs.
HOW TO SHUT OFF A RUNAWAY ENGINE
If the engine is out of control and at high RPM never use your hand on the flywheel! Some racers flip the car over and stick the toe of their shoe on the flywheel, but the engine can break before it’s stopped. A better way is to hold the car down hard so the wheels can’t spin (this may waste the clutch shoes but it is cheaper than an engine). Yank the body if you can’t get to the air cleaner, then yank the air cleaner and put your finger on the carb intake. Do not just push your finger down on the foam air cleaner, which will suck all the dirt into the engine! Holding a rag on the exhaust works too if you happen to have one handy, but, be sure there are several layers of rag it will take more than a few seconds to choke it and it will be very hot!
FUEL TIPS
Always shake the fuel before using it. Remember that water absorption is the #1 reason for bad fuel. The methanol in the fuel is hygroscopic, meaning it tries to absorb water.
1 - Never leave the bottle open any longer than it takes to fill your fuel bulb.
2 - Never leave the FUEL BULB uncapped any longer than it takes to fill the car.
3 - Always rinse out you fuel bulb at the start of each day with a little fresh fuel.
4 - Never draw fuel back into your bulb or put it back in the jug.
5 - Keep your fuel out of direct sunlight and off the ground.
6 - Store your fuel in a stable temperature.
7 - Squeeze the extra air out of the bottle before tightening the lid.
WILL IT HURT MY ENGINE TO RUN IT OUT OF FUEL?
If your engine was tuned correctly, to start with it will not harm it running out of fuel. If just before running out of fuel the car was tuned slightly rich, it will speed up for a brief moment just before it dies as you pass through the “ideal performance” mixture stage. If it was tuned for ideal performance, it will fall off in power just before it dies as it passes through the too lean stage. If it was tuned too lean to start with, it will bog down noticeably and may blow the plug as it pass though the “way to lean you just screwed up stage”. This can cause some damage to the piston and sleeve if the element wire breaks and falls in to the engine. However, that brief lean moment should not hurt the bearings, bushings, rod, crank etc. if you’re using high quality fuel.
SWITCHING FUEL BRANDS
If you switch brands it would not hurt to let the engine idle a few minutes before you go out on the track. Run one half or one tank of fuel with the engine tuned on the rich side. Check your glow plug, and if it looks good, fine tune and go for it. The reason for checking the plug is that different fuels have different oil additives and detergent packages. This may wash away some of the built up oil deposits and platelets or bonding agents (as the oil companies call them) and foul the plug. After a few runs your new fuel will leave deposits of there own. With some brands you can see the residue, because of its blue or red color. With other brands you can’t see the residue because they’re a light yellow or amber color but it's still there. It is normal to have these build-ups and they cause no harm.
SHOULD I USE AFTER RUN OIL
It is always a good idea to use some after run oil. Marvel Mystery oil works great and can be had at any automotive or hardware store. Other substitutes would be automatic Trans fluid, 3-in-1 oil or mineral oil from the drug store! Never use any oils that contain silicones! To use after run oil, let the tank run completely empty at idle until it runs out of fuel (try re-starting the engine several times). Next put a few drops of after-run oil down the carb (hold it open) and a few drops down the glow plug hole. Leave the plug out and put a rag over the top of the engine. Spin the engine over for five seconds on the starter box. Put a few more drops down the carb, re-install the glow pug and spin the engine over five more seconds. You’re done!
CLEANING YOUR ENGINE
First, follow the after run oil instructions above! Remove the engine from the car. You can then wash it off but be careful not to wash dirt into the front bearing (even if it has a rubber sealed type). Brush it off behind the flywheel then wrap a strip of cloth around the crank, “wedging” it between the front of the engine and back of the flywheel. Plug the carb and exhaust (be sure there is a glow plug installed) then squirt it off with methanol or denatured alcohol. Don’t use isopropyl alcohol as it is 30% water. Do not use brake cleaner or motor spray. If compressed air is available, blow of off the engine; if not, shake the engine vigorously and blow it off until dry with a hair dryer, being careful not to get it hot, just warm to the touch. Remove the strip of cloth and let the engine come to ambient temp (so there is no condensation formed inside) and put the engine in a zip lock bag or re-install in your car.
Removing the carb should not be a problem and is something that is commonly done by racers during cleaning and maintenance. If the carb seems tight compleatly remove the pinch bar assembly (screw and to clap bars) by totally removing the screw. You can then thread in the screw from the opposite side to allow you to "Pull" out the bar half that is threaded. Then you can tap out the other side bar with a small pin or punch. Drip a bit of oil down where the assembly contacts the carb before twisting out the carb.
WHAT TO DO IF AN ENGINE “LOCKS”
The most common cause is "Hydraulic lock", meaning too much fuel is getting in the engine when it is not running. Liquids do not compress. Don’t force your engine to turn, or you may damage it. You can remove the glow-plug, then crank the engine over to get rid of the excess fuel. To prevent this, always empty the fuel thank when working on or storing the car. Also, always keep the carb higher than the fuel tank when it is not running. You can use a forceps (surgical clamp) to pinch off the line between runs, or store the car standing on end, nose down.
DIRT IN YOUR PIPE CAN RUIN YOUR ENGINE
A common cause of dirt ingestion in off-road racing is through the exhaust. You crash or cartwheel the car, the exhaust stinger gets stuck into the dirt, and the engine stalls. There is always residue fuel and oil in the pipe, and it mixes with the dirt. Then you carry the car, and are not careful how you hold the car, which allows the dirt/fuel/oil to run back into the exhaust port!!!! A second cause is that the pipe has a reverse pulse action (it’s part of what make a tuned pipe work) and when the engine is re-started, the dirt in pipe ends up in engine!!
On-road racers often suffer from another variation of this problem. You’re at a race, and you blow up your old trusty engine. You bolt in your spare or even brand new back up engine. DID you remember to wash the debris out of your pipe from the exploded engine before mounting and starting your new engine? Not doing so has led to many brand new engines ruined just spinning them over on the starter box the first time!
PRESSURE LINE LENGTH AND PERFORMANCE
A longer line "can" lean out the midrange when the throttle is applied rapidly! Keep in mind pressure is self equalizing (the tank and pipe pressure will be the same). The only consideration is if the pressure feed line is restrictive, (i.e. long length or small diameter) it can delay the time it takes the tank to reach maximum pipe pressure. Pressure dictates fuel flow at any given needle setting; therefore a leaner midrange can result! There is no ideal length; this is something you can experiment with. In most cases 8" to 14" is common. This is something I would not concern my self with until you have completely mastered the art of tuning the standard low and high-end needles. Line length has a very small effect by comparison and should be considered "fine" tuning.
LOCKING THE ENGINE FOR FLYWHEEL INSTALLATION/REMOVAL
Many engines are damaged every year by "locking devices" By the way, the thread pitch on the crank is “national extra fine” and is a special aircraft pitch, which is normally cut on the lathe! A safe way to lock the engine is to hold the engine upside down (with glow plug installed), fill the cylinder through the exhaust port with after run oil (do not use silicone oils) and let it "hydraulic lock" This evenly distributes the load over the entire surface of the piston. When done simply pull the plug and drain the excess oil!!! No harm done and the cost is pennies.