How To Break In Your Engine

How To Break In Your Engine For Maximum Power

No matter how well an engine is assembled, one of the most critical parts of the engine building process is the break in. These principles apply to all 4 stroke engines: Street or Race Motorcycles, Cars, Snowmobiles, Airplanes & yes even Mullenix LS Racing Engines. So what’s the best way to break in a new engine? The Short Answer: Run it Hard!

Why Run it Hard?

Nowadays, the piston ring seal is really what the break in process is all about. Contrary to popular belief, piston rings don’t seal the combustion pressure by spring tension. Ring tension is necessary only to “scrape” the oil to prevent it from entering the combustion chamber.

If you think about it, the ring exerts maybe 5-10 lbs of spring tension against the cylinder wall. How can such a small amount of spring tension seal against thousands of PSI (Pounds Per Square Inch) of combustion pressure? Of course it can’t. So then how Do Rings Seal Against Tremendous Combustion Pressure? From the actual gas pressure itself! It passes over the top of the ring, and gets behind it to force it outward against the cylinder wall. The problem is that new rings are far from perfect and they must be worn in quite a bit in order to completely seal all the way around the bore. With strong gas pressure during the engine’s first miles of operation, then the entire ring will wear into the cylinder surface to seal the combustion pressure as well as possible.

The Problem With “Easy Break In”

The honed crosshatch pattern in the cylinder bore acts like a file to allow the rings to wear. The rings quickly wear down the “peaks” of this roughness, regardless of how hard the engine is run. There’s a very small window (about 20 minutes) of opportunity to get the rings to seal really well. If the rings aren’t forced against the walls soon enough, they’ll use up the roughness before they fully seat. Once that happens there is no solution but to re-hone the cylinders, install new rings and start over again. Fortunately, most new hot rod owners can’t resist the urge to “open it up” once or twice, which is why more engines don’t have this problem.

Break-In on Dyno

Warm the engine up completely!

Then, using 4th gear:

Do Three 1/2 Throttle dyno runs from
40% – 60% of your engine’s max rpm
Let it Cool Down For About 15 Minutes

Do Three 3/4 Throttle dyno runs from
40% – 80% of your engine’s max rpm
Let it Cool Down For About 15 Minutes

Do Three Full Throttle dyno runs from
30% – 100% of your engine’s max rpm
Let it Cool Down For About 15 Minutes
Go For It!

NOTE: If you use a dyno with a brake, it’s critical during break-in that you allow the engine to decelerate fully on it’s own (don’t use the dyno brake). The engine vacuum created during closed throttle deceleration sucks the excess oil and metal off the cylinder walls.

The point of this is to remove the very small (micro) particles of ring and cylinder material which are part of the normal wear during this process. During deceleration, the particles suspended in the oil blow out the exhaust, rather than accumulating in the ring grooves between the piston and rings. This keeps the rings from wearing too much. You’ll notice that at first the engine “smokes” on decel, this is normal, as the rings haven’t sealed yet. When you’re doing it right, you’ll notice that the smoke goes away after about 7-8 runs.

The above “cool down” instructions only apply if you are using a dyno machine to break in your engine. The reason for cool down on a dyno has nothing to do with “Heat Cycles”! Cool Down on a dyno is important since the cooling fans used at most dyno facilities are too small to equal the amount of air coming into the radiator at actual riding speeds. On a dyno, the water temperature will become high enough to cause it to boil out of the radiator after about 4 dyno runs. This will happen to a brand new engine just as it will happen to a very old engine.

If you’re breaking your engine in on the street or racetrack, the high speed incoming air will keep the engine temperature in the normal range.

Break-In on the Street

Warm the engine up completely: Because of the wind resistance, you don’t need to use higher gears like you would on a dyno machine. The main thing is to load the engine by opening the throttle hard in 2nd, 3rd and 4th gear.

Realistically, you won’t be able to do full throttle runs even in 2nd gear on most bikes without exceeding 65 mph / 104 kph. The best method is to alternate between short bursts of hard acceleration and deceleration. You don’t have to go over 65 mph / 104 kph to properly load the rings. Also, make sure that you’re not being followed by another bike or car when you decelerate, most drivers won’t expect that you’ll suddenly slow down, and we don’t want anyone to get hit from behind!

The biggest problem with breaking your engine in on the street (besides police) is if you ride the bike on the freeway (too little throttle = not enough pressure on the rings) or if you get stuck in slow city traffic. For the first 200 miles or so, get out into the country where you can vary the speed more and run it through the gears.

Be Safe On The Street!

When you’re not used to the handling of a new vehicle, you should accelerate only on the straightaways, then slow down extra early for the turns. Remember that both hard acceleration and hard engine braking (deceleration) are equally important during the break in process.