When asking advice on how to break in a new RC nitro engine, you will almost always hear different ways from every RC car driver. Many will say to idle the engine on first tank of fuel; second tank ¼ throttle, third tank ½ throttle, fourth tank ¾ throttle, and fifth tank full throttle. Some say to break it in super rich for a few tanks and then lean out the needles. Others say heat cycle with a race tune. Have also heard just to fuel it up, lean it out, and it’s ready to race!
By trying many of those, except for being scared the last. They all seem to have some benefits, so I believe the best way incorporates a little of each. If you follow proper engine break-in procedures you will find your new engine will last longer and run more reliably.
Heat Cycle Break-in
A heat cycle involves getting the engine to an operating temperature of at least 190 degrees Fahrenheit. This temperature is critical so that the metals that make up the piston and sleeve can expand and conform to the proper size. I feel this is one of the main procedures that should be followed when breaking in a new engine. Different engine brands recommend different running temps but a good rule of thumb is to keep the engine running between 200 to 270 F max.
For proper engine break-in, run your engine for at least three minutes and a total of five times, letting the engine cool between runs. A good judge on how long to run the engine is by a standard size tank of fuel. I like to run a tank then let the engine cool.
Also running the engine rich is a good idea, while making sure the engine still gets up to temp so its heat has cycled. Try not to lean the motor to get the temps up, as valuable lubricants in the fuel are essential during the break-in process.
The richer the better: Just make sure the engine gets up to temp. You can use a handheld temperature gauge to check the temps. These are usually more accurate than onboard temperature gauges (unless they are properly calibrated). Handheld gauges are usually more accurate since by had you can find the hottest spot on the engine (should be center of the head around the glow plug).
The engine temps will vary with fuel level in the tank. Because tank pressure is what acts as the fuel pump of RC cars, the less fuel in the tank, the leaner the motor is running. This means the motor will run the coolest at full tank, and hottest before the tank is empty. Remember this while trying to keep the engine in good temperature ranges (200-270 F).
Warning: Engines that have excessively tight pinch at top dead center (TDC) can wear out a connecting rod quickly, meaning you will have to invest in a new piston and sleeve even sooner. The faster you get the engine up to temperature properly will help prevent this premature wear.
Step 1: By using aluminum foil, heat tape or some sort of wrap to block off cooling fins on the heat sink, you can somewhat manipulate the engine’s temperature so the engine can reach optimum temperature quicker.
Step 2: For the initial start prime the fuel lines to the engine by turning the engine over and having your finger over the exhaust tip to force fuel to the carburetor more quickly. Also a small amount of fuel in from the glow plug opening also helps to get initial lubricants into the motor prior to the first start.
If the motor is very hard to turn over the glow plug might need to be loosened to relieve some compression or the motor my need to be heated (hair dryer or heat gun to the head of the motor). The engine will start with the plug slightly loosened, but needs to be tightened immediately after starting.
Step 3: While the engine is idling and warming up, leave the glow driver attached to the glow plug. This reduces the chance the engine might flame out early.
For the first tank, idling the motor to VERY low throttle is recommended. After more break-in time on the motor / tanks of fuel, higher revs can be made upon more heat cycles through the motor.
Avoid revving the engine high rpm’s to warm it faster: that will actually wear out the piston and sleeve faster. You might see some guys doing this but this does more harm than good. You can give it some short moments of 1/4 throttle to help build engine temp and to keep the engine running while on the starter box or the bench.
Another method is to simply place the RC car on the ground and start driving in circles or figure eight patterns smoothly at low throttle. Ease on and off the throttle. Try not to blip the throttle, this puts extra stress on the rod and crank pin.
Step 4: While the engine cools between break-in runs / tanks of fuel, always make sure the piston is bottom dead center (BDC) when the engine is cooling.
A good way to tell this is by rotating the flywheel, after the compression of TDC (Top Dead Center) is passed the flywheel will immediately become much easier to rotate. Another tip that many racers use is by looking through the exhaust port before the manifold is mounted or the glow plug opening and seeing when the piston is the furthest down (BDC), then marking the flywheel with a permanent marker to easily see this from the flywheel opening under the RC car.
Step 5: After this first tank (should be around a 15-minute break-in period at idle) and the motor has cooled with the piston at BDC (bottom dead center) after each tank.
The RC motor should then be ran at least 4 more tanks while also cooling at BDC between these runs. Run the next tanks few tanks at no more than half throttle. For the last tank or two do the same procedure with some short full-throttle burst. Again for these, make sure the engine gets up to temp quickly.
When the break-in process is finished, you should have a total of 30 minutes to an hour of run time and break-in on the engine.
Piston and Sleeve Dynamics
Here is a brief explanation of why getting the engine up to temp quickly and letting cool at BDC is so important.
RC nitro engines are a lapped piston and sleeve design. This means that the bore becomes gradually smaller as the piston reaches the top of the sleeve.
This gives you that pinch at TDC (Top Dead Center). The pinch is required to keep the piston and sleeve sealed when the engine gets up to temp, since RC nitro engines to not have rings like larger engines.
That is why it’s important to get the engine up to at least 190 when breaking-in. If you don’t, the sleeve will never expand and the piston will wear out faster. The piston will end up wearing down as a result of friction, tapering the head of the piston to match the size of the sleeve. When the engine finally does get up to temp and the sleeve expands, the piston is now too small, allowing for blow by (loss of compression).
That means the cylinder pressure blows by the piston down to the crankcase, and it will cause the engine to overheat and stall. Unfortunately, the only way to remedy this is a complete rebuild with new piston, sleeve and connecting rod.