Adrenalin Model Motorsport - agents for Mugen Seiki, Novarossi, Arrows Tyres, and Frewer Bodies.
Home ] [ Cars/Buggies ] [ Engines ] [ Tyres ] [ Bodies ] [ Radios ] [ Accessories ] [ Tools ]

Specialists in 1/8th and 1/10th scale nitro-powered R/C cars.
ph & fax: John Hawkins (02) 9975-3911
PO Box 157 North Sydney NSW 2059 Australia
e-mail: adrenaln@zip.com.au

The Art of Engine Tuning

Version 2.41 31-May-99

Introduction

Nitro-powered R/C cars are easy to run and race. Their big advantages are you don't have to stop the fun every few minutes for a new battery - and they're fast.

However, to get the best from them you must invest a little time in some basic education. The intent of this guide is to put all the information you need in one place.

It assumes you are familiar with how model nitro engines work, and that you're looking to understand how to tune them for reliable high performance operation.

 

Assumptions

As with any car, electric or nitro, there are a number of fundamentals that need to be in place before any progress can be made. In specific reference to nitro power, they are:

You should refer to the instructions supplied with your engine regarding the glowplug specs and appropriate number of shims for your fuel.

Fuel

Plain-bearing .10 or .12ci non race engines (eg Panda, Kyosho):

Model aircraft fuel with 20% Castor oil and 5% nitro has been used in these low power engines successfully, according to the distributors. We have our doubts, but as you otherwise risk voiding whatever limited warranty exists with these motors it is probably wiser to follow the distributor's recommendation.

Ballraced ABC racing engines, .12 through .21ci (2-3.5cc):

NEVER use model aircraft fuel! Regardless of the percentage used and whether synthetic or not, the lubricants simply do not handle the high revs and temperatures to which performance car engines are subjected and can be a short cut to an expensive disaster. It will also wear your engine out early.

The good oil on good oils.

A word of caution. There are many synthetic oils on the market, most of which make claims about better fuel consumption or more horsepower or cooler running. Some may even live up to a few of these claims.

However, it must be remembered that the inside of a model car is the hottest and dirtiest place you can put a nitro engine, and that their power output per cubic centimetre is easily the highest in R/C (eg a good .21 exceeds the output of a good .61 aircraft engine). Oils that offer sometimes spectacular success in aircraft and even boats are often found to fall short of the demands placed on them in model cars.

Protect your investment and use only designated specialist model car oils in your fuel.

Of the many available in this country only three have been found to be completely suitable. They are EDL, PB Red Oil and WB. Insist on one of these oils for your fuel. An appropriate mix is 10% EDL or WB, 2% castor, a minimum of 5% nitro (essential for ease of tuning) up to a maximum of 25% (16% under 1/10th scale rules) and the balance made up of methanol. The only reason for the small percentage of castor is to provide a smoke trail to give a visual aid to tuning which will be explained further on in these notes.

PB Red Oil is more for serious racers, who are perfectionists in the area of engine care.

99% of the major races held in Australia in the last five years have been run and won using either EDL, PB Red or WB - which should be a sufficient recommendation. However, for pull-start engines we would recommend PB Red over WB as the latter offers little corrosion resistance.

EDL and PB Red based fuel can be obtained from Adrenalin Model Motorsport, WB from Serpent agents, or alternatively from any source that stocks Magnum fuel - ask for "Automix".

Run-In Fuels?

There has been some recent discussion favouring castor based break-in fuel. We do not recommend the practice but if you choose this route it is essential that an experienced racer is present at all times. The engine should never be allowed to get hotter than warm to the touch.

It will need to be bedded in again every time you swap oils or reduce nitro, so it is better to run in from scratch using the oil and nitro percentage you will race with.

Water and fuel do mix.

Due to the moisture absorbing ("hygroscopic") qualities of methanol your fuel has a finite shelf life and should be used up no later than six months from date of manufacture.

It should be stored in a metal container only. Plastic allows moisture in and the volatiles out, sending your fuel "off" early - bad news. Keep the tin tightly closed at all times when not refuelling.

Warning!Nitromethane and methanol are both highly poisonous and explosive. Above all do not smoke in the vicinity of an open fuel container.

Setting up your engine

Engine Preparation

It is almost impossible for engine manufacturers to ensure their engines are 100% free of machining burrs and debris 100% of the time. Remove the fuel inlet and needle valve assembly carefully so as not to disturb the setting and make sure there are no burrs or debris. Blow out using a strong source of compressed air to make completely sure. If you need to blow out the carburettor itself remove it from the engine first.

Take the ball joint off the carbie slide and glue the rubber boot to the slide with superglue to ensure there are no air leaks. Secure the other end to the carbie body with a cable tie.

Keep the exhaust, carburettor and glowplug hole blocked up at all times with clean cloth until ready to go in your car with pipe and air filter fitted. Dirt is your engine's major enemy.

Air filter selection

A good, well secured air filter is critical to the life expectancy of your engine. The minutest amounts of dust will take mere seconds to damage the all important piston seal in your engine and lose you performance. A few extra dollars spent on the most effective filter you can fit into your car will more than repay the investment.

We recommend K&N air filters for circuit cars. Off road cars MUST use the dual element oiled foam/paper units. The oiled elements must be cleaned after every race day and re-oiled before being used again. When cleaning ensure no dirt gets on the inside of the filter element.

Recently, foam rubber elements have become popular. If you use these, it is absolutely essential that you use the correct oil. Filter oil with a tack element, such as that used in off-road dirt bikes, is the only type suitable. Fortunately it is widely available. We can recommend Putoline and Castrol, although the latter tends to attack some types of foam and filter boots.

Filter maintenance

K & N filters should be cleaned in warm soapy water with a soft toothbrush. Keep your thumb over the open end to prevent debris from finding its way inside. Flush with copious amounts of water from the inside out and allow to dry overnight. Re-oil per the instructions on the sachet.

Paper elements may be cleaned by blowing from the inside out with compressed air, but should be replaced after doing this four or five times. To test if its due for replacement tap with the bottom down and moderate pressure on to a hard surface and see if any particles fall out. If they do, replace it.

Whether or not to clean foam filters or simply throw them away after a few hours is a controversial subject, but we have found with Putoline filter oil that it is possible to clean properly, as the separately available cleaning solvent makes the oil and tack agent completely water soluble.

Do not use a water soluble filter oil - a trip onto dewy grass in the morning practice session or a damp track could spell disaster.

Your filter must be secured in such a way that it will not become dislodged in a crash. Cable ties are excellent for around the rubber boot and you should be able to pick the engine up by the filter and shake without it coming loose.

Conscientiousness in filter maintenance and security will dramatically improve your engine's service life.

Throttle Linkages

Throttle should only just be fully open when the trigger is fully squeezed. You will also need to adjust this from the transmitter.

A spring loaded slide setup is required to allow brakes to be operated after the throttle is fully closed, and if one is not included in your kit they may be cheaply obtained from the Adrenalin Model Motorsport. A piece of fuel tube on the side of the linkage that pulls the throttle open is recommended to allow a little over throw without damaging the servo just in case you don't quite get it right first time.

Refer the accompanying photos for a better idea of what this means.

You must also ensure that the brakes are fully released before the carbie starts to open at all - the car will be diabolical to drive if this is not the case. You should also confirm that brakes do not come back on again at full throttle. If they do, you may need to grind away one side of the face on the brake cam (disc brakes only).

Clutch

It is essential that your clutch works correctly - disengaging to allow the engine to idle without stalling, engaging without slipping and coming in neither too early (which makes the engine bog down) nor too late (giving rise to excessive wheel spin and engine overheating).

Make sure that the inside of the clutch bell is free of any trace of lubricant, and that the shoes are shaped according to the car's instructions. Flush the bearing out and relubricate with a minimum of something that won't fly out and coat the inside of the clutch.

If you have a Serpent with a Centax clutch and an engine other than a Serpent Mega, take the clutch, bell and engine to your nearest Serpent agent and have them set it up for you, as the instructions provided work only with their own engines. Once this is done, record the measurements with a vernier calliper accurate to 1/1000& and write them down. If you are not in a position to set it up accurately and repeatably, get the Serpent six-shoe clutch as you can bolt it in and depend on it to be hassle-free. The Centax unit offers excellent performance, but it is less than beginner friendly and you will get more enjoyment from your car with a unit that is bullet proof and requires minimal attention to keep consistent.

Bodyshells

Cooling is critical. Ensure your engine gets enough air. The Australian 1/10th Touring Car rules permit up to 50% of the front windscreen to be taken out, the entire front side windows and the entire rear window. The rear of the shell may be removed up to the bottom of the shell's bumper bar. The same applies to 1/8th scale saloons. It is not really an issue in Can Am/Group C as the cylinder head protrudes well through the top of the shell.

If you have one of the .12 powered cars or trucks get the largest optional heat sink you can fit to it. It will improve your reliability immensely. Of those that we have seen only the newly-arrived Dynamite .12 has an anywhere near adequate heat sink fin stack on the cylinder head as standard. We hope this will be rectified with the new generation of ball-raced .12s finding their way onto the market now.

What the adjustments mean

"Top end"

(also known as the main jet) refers to the screw in the stalk above where the fuel goes into the carburettor and it controls all fuel flow into the engine. It is used to tune mid- to full-throttle engine performance and response, and engine temperature. Because it affects all fuel into the engine, large changes to the top end setting will have a side effect on bottom end mixture.

"Bottom end" affects throttle response down low and idle. If you have an engine manufactured by Novarossi the bottom end mixture needle is in the end of the slide itself. If you have one of the newer Novarossi engines with the second base needle adjustment, just mark its position and leave it alone. It is for experts only. The needle in the slide gives more consistent results on these engines.

Low EndIf you have one of the cheaper .15 or .12 engines with a barrel carbie, the low end mixture is usually in the end of the rotating barrel.

The other important adjustment is the idle air screw (aka idle stop screw or stationary revs adjustment) which is usually in the side of the carbie body. It is used to adjust idle revs once bottom end mixture has been correctly set.

Most reputable engine manufacturers ensure their engines leave the factory with carburettors set correctly for first-time startup. Resist the temptation to fiddle with any of the settings!

Low end mixture access

You may need to rotate the carbie slightly to get access to the low end needle if the tank partly obscures it. Keep it tight against the o-ring in the crankcase - air leaks must be avoided.

Running In

Your first priority with any new engine is to get it properly run in. Bedding in the moving parts with a careful running in period hones their fit and will greatly increase the horsepower output, extend the useful life and vastly improve the tunability of your new powerplant. When they are capable of over 40,000 rpm and around 2.5bhp (.21 size) it is not hard to see why.

The continual improvements in engine metallurgy over the last few years means this now takes a little more time, but the reward is a longer-lived engine. So be patient.

Most racers find it easiest to run in their engines in the car. Which method you choose depends on how experienced you are.

If you have limited experience:


Avoid at all costs getting the engine too hot during the running in procedure. Especially for the first three tanks in your car it is safer to run with the bodyshell off. For the first two tanks set the main needle so that it is only lean enough to keep it from four-stroking (firing only every second cycle) between half and full throttle. The heatsink should be warm to the touch only.

After about a tank of really rich running you can lean it slightly, say 1/8th of a turn. After another three to four tanks like this lean it again by 1/16th and allow the engine to get a little warmer, running evenly without stumbling but still blowing lots of smoke. Watch the temperature like a hawk, testing every few minutes, and if it rises so that you cannot leave your hand comfortably on the top of the heat sink richen it back up.

When the engine begins to free up (and you should persevere at this setting until it does) you will hear the sound change from laboured to more free-running. It invariably catches me by surprise and my first thought is it has suddenly gone lean, but check the engine temperature and if is still very good you can keep it there for another tankful. At most lean it another 1/16th.

If it quits through any cause other than an empty tank and it is more than a little warm you are likely too lean and the engine has overheated.

Avoid the temptation to lean the engine too much. If it gets too hot, richen it back up again and keep it like that for another minute or two before trying again. If the inevitable crowd of onlookers is giving you the hurry up, beat them around the head and shoulders with a blunt object. It's your investment, not theirs.

If you are a racer with plenty of experience:
This procedure is quicker and extends the engine's life but carries much greater risks - run your engine lean and it could be ruined. It is for experts only and is not recommended if you have any doubts about your engine's setup or your ability to tune an engine.

The following assumes you are running a Novarossi engine, and that you have not altered the settings from where they were set at the factory with the flow meter.

The first tank should be run in the car on the bench, with the engine just above idle. Next, run the car on the track with its shell on and tune the engine about 1/16th richer than you would for normal racing. The only difference is that you accelerate gently, and lift part way out of full throttle once you reach second gear.

After about 3 or 4 tanks your engine is ready to drive normally, but it will continue to improve for the next 15-25 minutes running.

The theory behind this is that you are running in your engine at operating temperature, where the liner has expanded to its normal operating size. Less is worn off the piston in the process, giving longer engine life. But (and it's a big but), this is only for those with plenty of experience.

Engine Race Tuning Procedure

After a few more tankfuls you can consider race tuning your engine, and by this stage around one to one-and-a-half litres of fuel should have been consumed.

The procedure runs as follows:

If you have just finished running in your engine using the program above you should already be close to the right starting point.

Setting the Top End

Top end Starting with the top end rich (plenty of smoke on the main straight), run 4 or 5 laps to get the engine to operating temperature before touching the settings. Lean the top end in no more than 1/8th turn increments, reducing to 1/16th as you get closer, once every two laps or so.

You want the engine to pick up cleanly down the straights and have a just barely visible smoke trail at the end of the main straight. It may take a dozen laps or more.

When you think you are close, quickly bring the car into pit lane, pick it up and place a daub of saliva down low on the side of the heatsink head at about the same level as the glowplug. It should sizzle slowly and evaporate away in about 5-6 seconds depending on your generosity.

If it dances and jumps about you are way lean. If it just sits and steams you are too rich (unless you are running in).

Don't put it on the top of the heat sink as the effectiveness of the tall fin stack on the more modern engines will make the results misleading. Always go for the side on a level with the plug.

Once the top end is right, move on to the bottom end (idle) mixture.

Setting the Bottom End and Idle Air

Bring the car to a halt somewhere close, wait five seconds then accelerate hard.

It should have pulled out cleanly without stumbling, but still have richened up a little off the mark. If it didn't, run through the following checklist.

Recognising where you're at with the bottom end adjustments

Bottom end too lean:

Bottom End * The idle speeds up after a few seconds (and maybe quits).
* The engine hesitates (often intermittently or at a particular point on the track) when pulling out from idle, blowing less than normal smoke at the start.

Bottom end too rich:

Mid range adjustment * Idles OK when cold, but idles fast or runs on when at operating
* Car wants to "creep" forward at idle (assuming clutch worktemperature.
* Loads up and dies before 5 seconds is up

Idle too high:

* Engine won't throttle back to idle cleanly ("runs on").

Idle too low:

Location of idle air adjustment screw* Quits under braking or when coming in for a pit stop.

You should always run a lap or two before testing the idle. It should idle cleanly for 5 seconds, but by 6 or 7 seconds it should "load up" (slight stumble with copious smoke) when you accelerate away.

Leaning your engine further than these points may well gain you extra performance. But it is like overgearing an electric motor: it will go very fast for a while and then stop - expensively. The fact that winning racers go to great lengths to ensure that their engines are tuned just right (and that they do not just keep screwing in the mixtures in the pursuit of speed) should convince you.

Tuning troubleshooting

As your pride and joy is a racing engine it will not idle endlessly like an aircraft engine, and as in full size car racing you will need to "blip" the throttle to keep it alight when not driving around. At 1˝-2 second intervals take it quickly all the way up to full throttle and back off instantly again to idle.

The object of the exercise is merely to keep the engine from loading up with fuel if the idle is rich - otherwise it will blubber and/or flame out when you try to pull quickly off the start line. Don't hold the engine at full throttle for more than an instant as sustained full revs with no load will overrev it and risk damage.

Engine hesitation

If, once on the track, the engine hesitates or the revs die suddenly at some point through the rev range when the engine is going hard (in contrast to being smoky and rich) it is too lean. If this takes place at the bottom end, but comes good when you hold the throttle open, richen the needle in the carbie slide by at least 1/4 turn and start the procedure over again. If it occurs higher up the rev range after being good down low, richen the main needle by the same. You may need to screw the bottom end in a fraction to keep the mixture right if you have had to richen the top a lot.

Engine runs well at first but dies off

If the engine note sounds hard and hollow, and the revs gradually fall off, you are also too lean. The engine will feel hot. If it suddenly goes blubbery after running hard, chances are you've blown the plug through being too lean. Replace it and richen by at least 1/8th turn, or more if the results of the spit test indicate.

When to change the plug

The plug should be replaced when the element loses its bright and shiny finish. If it looks

sandblasted you again have been running too lean at either top or bottom. If it goes dull discard it immediately. Dull plugs make tuning a nightmare and if you are having problems getting a consistent setting look here first. Consider them a consumable item.

If the element is bent, but remains shiny, the engine is overcompressed for the amount of nitro in the fuel. Add a 0.1mm shim under the cylinder head, or go to a lower nitro fuel.

If you have blown the plug element and the engine temperature on the track has been fine, try a colder plug. If engine revs fall off after the power is disconnected but the plug is new, the plug is too cold. Try the next number down (hotter).

Is it the clutch?

If your clutch is too heavy the engine will be slow out of corners with little exhaust smoke and is easy to confuse with a lean bottom end. If your clutch is too light and slips, it will overheat the motor, sag out of corners and can be confused for a rich bottom end, especially as it may take a couple of minutes to exhibit.

When to have your engine overhauled.

Preventing Engine Failure

The connecting rod in your engine is subjected to an enormous load, and is an item that you should replace on a regular basis. For .21 size engines, every 12-15 hours is rough guide, but certain events may make it prudent to do it sooner.

For example, overrevving the engine with no load such as in a runaway caused by radio failure, or missed gear shifts in 2-speed cars can quickly tire out and even stretch the rod. Overheating the engine by a large margin can destroy the rod's tempering.

Rod failure can easily wreck the entire engine as the forces involved fling metal debris about, often with great violence. Replacing the rod before it fails is cheap insurance and will save you money.

After run engine maintenance

A little maintenance at the end of the day before you put the car away very important, and is an easy way to extend the life of your engine.

Simply run the car out of fuel at just slightly above idle, disconnect the fuel tube and continue to restart it until it won't light up at all. Put half a dozen drops of Model Technics After-run Oil down each of the carbie and cylinder and spin the engine over for a couple of seconds on your starter. This ensures your engine bearings remain rust free.

Some fuels with poorer quality nitro will break down into acidic byproducts if left in your engine longer than a few hours, so don't skip this procedure.

If by some chance you forget to run your engine completely dry, or feel that the bearings are less than pristine, it is better to replace at least the main bearing. Rust and flat spots are likely to make the bearing balls skate and bounce, which at best means poorer performance and can lead to the balls shedding shards of metal. Get it replaced if you have any doubts.

"Dink!"

Where you have been unfortunate enough to have done the unmentionable to your engine, in most cases it is possible to salvage something workable from it. Avoid turning your engine over and if possible remove the backplate and flush out the crankcase with auto transmission fluid or a mixture of petrol and machine oil. Return it for inspection to the point of purchase.

Extending Your Engine's Life

Piston/liner fit

Where the rest of the engine is still in good condition but the piston/liner fit is starting to exhibit leakdown, evidenced by performance loss and poor idling in spite of following this tuning guide, and you are fortunate enough to own a Novarossi engine it is possible to have a new piston fitted by the Australian importer at the same time as the new rod. This gives about a 65% saving on a new piston/liner/rod assembly and will bring the engine back as good as new. For other makes of engine enquire at the point of purchase.

We do not recommend that any but the most experienced racers disassemble engines. Most nitro car specialists will perform these overhauls free of labour charges (ie just the price of the parts) on reasonably current engines.

Don't change your fuel's nitro content

Nor do we recommend changing the nitromethane content of your fuel unless absolutely necessary. You should run your engine in using the nitro content you’re going to race with.

To quote model engine authority C. David Gierke from his book Two Stroke Glow engines for R/C Aircraft, on running in ABC engines:

"For break-in use the same nitro percentage as you plan to fly (race) with. Why? Generally, the more nitromethane a fuel contains, the higher the cylinder head temperature will be. Higher cylinder head temperatures mean greater expansion of the upper cylinder and, to some degree, the piston. If you break in an engine with 5-percent-nitro fuel, it will actually be too loose when 15-percent nitro is used because the cylinder expands faster as temperatures increase."

A liner that is too loose means lost power.

Conclusion

Nitro powered model cars are not "Plug'n'Play" toys - they are the closest thing in radio control to the real thing. As with any leading edge equipment they are perhaps a little more prone to accidental death by mistreatment, but all that is required is some common sense and informed observation.

Hopefully, as a result of this article, the "informed" part of your observation skill is no longer a problem.

Respect it and treat it well and your engine will last a very long time, giving you many hours of high performance enjoyment.

Sources:

Ron Paris, Paris Racing Engines Operating Instructions, 1990. http://www.parisracing.com

Carroll Smith, Tune to Win, Aero Publishers Inc 1978.

C David Kierke, 2-Stroke Glow Engines for Model Aircraft, Model Airplane News Publications 1994.

Cesare Rossi, Novarossi Miniature Engines Operating Instructions, 1994.

Photos:

Carb overview: The Serpent Network - http://www.serpent.nl/. Reproduced with permission.
All others: John Hawkins / Adrenalin Model Motorsport.


 You are visitor number [ ] since 1 December 1998.

Copyright © 1998-99 Adrenalin Model Motorsport.
Product trademarks and copyright thereto are the property of their respective holders.
Last modified: March 12, 2000.