Running in a nitro engine

The first few tanks through a new nitro engine decide how well it runs for the rest of its life. Running in lets the piston and sleeve, and the bearings and con rod, bed in gently before you ask for full power. Rush it, run it lean or let it overheat, and you can lose compression before you've had any fun.

There isn't one correct method. Engine makers publish their own, and racers argue about the rest. Always follow your engine's manual first. This guide explains the principles, walks through a published manufacturer procedure, and covers the mistakes that ruin engines.

Before you start

• Read your engine and car manuals. They give the factory needle settings and the maker's running-in procedure.
• Use the fuel you'll use every day. Traxxas advises against special break-in fuels; if you change fuel or nitro percentage later, you'll need to re-tune (Traxxas engine manual). See nitro fuel.
• Check the air filter is clean, oiled and sealed onto the carburettor with no gaps. A leak lets dirt straight into a brand-new engine.
• Check the factory needle settings haven't been disturbed. Traxxas, for example, sets the high-speed needle 4 turns out from closed for break-in.
• Pick the right day and place: a large, flat, paved area outdoors, avoiding very hot or very cold days. Traxxas recommends the engine itself is at room temperature (about 21°C) or above for the first start, so keep the car indoors until you're ready. Don't run below about 2°C, when fuel can start to gel.
• Have the kit ready: igniter, fuel, spare glow plugs, and ideally an infrared temperature gun. The full list is in nitro starter kits.

A manufacturer's method: Traxxas, tank by tank

Traxxas publishes a clear procedure for its TRX engines. It's a good illustration of the gentle, gradual approach most makers take. It takes about 1 to 1.5 hours and five tanks (Traxxas engine manual):

Traxxas says no sustained high-speed running until the sixth tank.

Target temperature

Traxxas gives 200 to 215°F (about 93 to 102°C) at the cooling head during break-in (Traxxas). An infrared temperature gun pointed at the head makes this easy. Without one, Traxxas describes a water-drop test: a drop of water on the head near the glow plug should sizzle for about 6 to 8 seconds; if it vanishes in a couple of seconds, it's over about 104°C (220°F) and too hot; if it doesn't evaporate, it's too cool.

If the engine runs too cool (Traxxas notes this below about 7°C, when engines can sit at 71 to 82°C), don't lean the mixture to heat it up. Leaning cuts lubrication and wears the piston and sleeve. Instead, partly wrap the cooling head with a rag or sock to keep the heat in.

The heat-cycle method

Many racers use a heat-cycling approach instead of long gentle running. One widely shared version runs the engine on the ground from the first start in 2 to 3-minute runs at around 93°C (200°F), tuned slightly rich, letting it cool completely between runs, then moves to 3 to 4-minute runs, for about 20 to 25 minutes in total before tuning (RCTech discussion).

Both methods share the same principles:

• Get the engine up to proper working temperature. Don't run it cold and very rich for long.
• Let it cool between early runs.
• Build up throttle and run time gradually.
• Never run it lean while it's new.

If your engine's manual gives a method, use it. If it doesn't, either approach above is a sensible starting point.

Mistakes that ruin new engines

1. Idling it on a stand for tanks on end. Traxxas says plainly not to break in its engines by idling them on a stand, because it gives poor results. Racers are split on stand running: some idle on a stand with the head wrapped to keep the temperature up, while others prefer running on the ground from the start. Unwrapped idling for tank after tank is the version to avoid.
2. Leaning the mixture to make it hotter. This is the commonest way to damage a new engine. Traxxas lists the lean warning signs: cutting out, sudden loss of power, overheating (over about 132°C/270°F at the plug) and little or no blue smoke. A high temperature alone doesn't prove it's lean, so look for the other signs too. Richen the high-speed needle ¼ turn if you see them.
3. Running the tank dry. The last of the fuel runs lean and can burn the glow plug or overheat the engine.
4. Over-priming. One to two seconds is enough. More floods the engine.
5. Hammering it straight away. Full-throttle runs come at the end, not the start.
6. Running on very hot or very cold days. Temperature extremes make it much harder to hold the right engine temperature.
7. Skipping the air filter. A new engine is especially vulnerable to dirt. Traxxas recommends cleaning the filter after every hour of running, including running in.

After running in

• Fit a new glow plug. Deposits from running in can make the original plug fail, so Traxxas recommends replacing it once running in is finished. See glow plugs explained.
• Clean and re-oil the air filter.
• Tune for normal running, always from the rich side, keeping a light trail of blue smoke.
• Do the after-run routine at the end of the day (not mid-run): drain the tank, then idle the engine with no load until it stops, then put after-run oil into the carburettor and plug hole and turn it over. Methanol fuel attracts moisture, which can rust the bearings and crank. Our RC car maintenance guide has the rest.

Not sure the engine is running right? Work through RC car troubleshooting, or check nitro fuel if you've changed fuel. Back to the RC cars hub. Considering going electric instead? See NiMH vs LiPo for the batteries side.