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Nockin the Rods

Many of us have heard the deep knocking sound that strikes fear into the heart of all automotive enthusiasts. We try to tell ourselves its excessive valve lash. We try to tell ourselves it might be any number of problems, but when we look into the oil drain pan and see we struck gold, the cold reality sets in. Often the mind and wallet shudder together in unison.

The purpose of this paper is not to relive bad memories, but rather to shine some light on the often misunderstood subject of why rods fail. The author has caused over 10 catastrophic rod failures in his own cars, and has been responsible for ensuring a proper repair for many more.

Rods fail due to excessive forces, or due to inadequate lubrication to the rod bearing. In this paper, we will first dig into breaking down and understanding the forces at work, then move onto lubrication.

Many forces occur on the connecting rods of your engine. For the scope of this paper, we are going to stick with the two main types of loading the rod experiences. Cylinder pressure force transmitted through the piston, and inertial loading.

Let’s begin with cylinder pressure forces. If we just follow the piston through its cycles, it will provide an order to express the forces in.

First, we have the intake valve open, and the piston begins to make its way down the bore. In a naturally aspirated engine, we know pressure must drop below the pressure in the plenum to cause the cylinder to fill. The pressure on the back side of the piston is determined by crankcase pressure, which on an engine with healthy ring seal and no dry sump vacuum pump stages or exhaust venturi crankcase scavenging should be just a bit above atmospheric pressure. This means that the force on the rod on this stage can either be compressive (pushing) or tensile (pulling) depending on throttle position and/or boost pressure. Fortunately for our engines, this force is going to provide very low stress on our rod, as the pressure on the top and bottom of the piston are quite close to being in equilibrium through this period.

Next, the piston reaches the bottom and begins the journey back up the bore. The intake charge is still rushing in to fill the cylinder as it begins to raise, but soon enough the intake valve will close and the mixture will begin compression. This compression work results in pressure increasing to become roughly 160-260psi above the pressure on the back side of the piston. For an engine with a 2.95” (75mm) bore, we have 6.86 square inches of piston area. When we multiply a pressure by an area, we get a force as the result. In this case the peak compression force on the rod is around 1100-1800lbs. Fortunately, this force is a helper force for the rod. That will be covered later.

So, now we’ve compressed the mixture, and the spark started the flame wave propagating through the mixture. Pressure begins to increase as the piston starts dropping. Somewhere around 18-22deg ATDC, this game of pressure from combustion building matches the rate that the volume of the cylinder increases, and pressure reaches its maxima. This typically would vary between 800-1800psi (peak pressure, note IMEP is much lower), depending largely on aspiration type and compression ratio. Again we multiply piston face area by pressure and we see the force due to combustion pressure ranges between 5,500-12,500 lbs of compressive force on the rod. Fortunately, this high intensity pressure only occurs for a very short amount of time each cycle, otherwise the oil film that the rod bearing floats on would be displaced and the bearing would contact the crank. Fortunately for us, this compressive pressure is well inside the safety margins of all Honda connecting rods, even the little D15B.

While combustion pressure may seem like a lot, during abnormal combustion (like knock), this force on the rod may double or triple in intensity. This causes the oil film to be rapidly displaced, with a much increased chance of bearing surface contacting crank. It also causes excessive heat and pressure related stress on the face of the piston, which can deform the ring package or break the piston. While its possible to have only a very small knock that causes no damage to the engine and/or even helps economy and power production, this sort of tuning should be left only to folks who don’t need to be reading this paper. The knocks that our sensors or ears detect are all large enough to be damaging our engines. It’s rarely ever excessive power production that causes a rod failure due to cylinder pressure. This is true even for the weakest of the Honda blocks. Abnormal combustion is almost always the cause of any pressure related failures. Items like forged pistons and aftermarket stronger rods exist for the purpose of preparing the engine to endure those occasional and inevitable bits of knock that occur for various reasons during the engines life.

So, after combustion pressure, we next deal with the exhaust cycle. The exhaust cycle always begins with more pressure than atmospheric, and always ends with less than atmospheric pressure for naturally aspirated cars, and can vary in turbo cars depending on manifold design and waste gate position among other variables. Most all of the time, exhaust pressure ends higher than atmospheric pressure for turbo cars. This means the cylinder pressure caused force on the rod starts out as compressive and ends as a tensile force. The pressure ranges it experiences are comparatively low. Expect to see anywhere between -10 to +10psi (gage) in the cylinder, which amounts to around 70lbs of force either compressing or pulling on the rod. Turbocharged engines will of course see higher exhaust pressures. However, as you can see, comparatively the rod sees very little stress from the exhaust pressure, so much like the cylinder pressure caused forces from the induction stroke; we will toss this force out of our calculations.

This leaves us with 2 significant forces on the rod due to cylinder pressure. Compressive pressure and combustion pressure.

Now let’s take a look at force on the rod due to accelerating, stopping, and accelerating of the weight of the piston and the rod itself. These forces increase at the square of the RPM. While this is simple to say, I think the impact of RPM on the stress of the rod is often overlooked. Here is a simplified graph of how the force on an engine connecting rod is changed by RPM. I chose 8,000rpm to represent 100% of the factory design stress for the engine.

Fortunately for the reader, I have removed the elaborate pages of calculations to describe these forces, and prepared some graphs to visually describe the forces. I think you will find the graphs to be much more pleasing. The effect of rod length /stroke ratio plays an effect on the stress on the connecting rod. The graph below shows us two different velocity and acceleration plots. Both plots feature the 90mm stroke of the D16A, yet one features the rod length / stroke ratio of the Honda B16A, which is 1.74:1. This is to show the mostly small difference in stress on the rod that rod length / stroke ratio actually plays. From an engine stress standpoint, I think this minor difference is often overstated. However, you will notice what is certainly not understated is G-force loading on the rod. These numbers are all calculated at 8000rpm, and with 0 degrees representing BDC, not TDC as would commonly be done.

You will note that we have a bit over 2000g’s of acceleration forces on the rod for the lower portion of the stroke. To estimate force on the rod, we must first know piston and rod reciprocating mass. If we use some common Honda piston and rod mass, and multiply this by the g-force, we find we are dealing with around 3,800lbs of force compressing the rod as moves up from BDC accelerating the piston. It holds this force for a long period of crank rotation, about 75deg. The rate of acceleration beings to taper off, and the piston reaches peak velocity around 110deg. Now the crank and rod assembly must work to slow this piston down to a stop before it reaches the top of the cylinder bore. However, it only has about 35deg of crank rotation to accomplish an equal and opposite amount of acceleration that it previously did over a period of around 75deg of crank rotation. This results in the intensity of the acceleration being about double of what it felt when it was being accelerated towards the head from BDC. Also, since we are working to slow the piston down now, all of this work is tensile loading. Upon doing the math, we see around 7600lbs! Most importantly, the engine feels about this force for a very long time, about 60deg. From just looking at the graph you must remember that after the rod has tugged on the piston to slow it to a stop, with out interruption in rod loading it must now accelerate it rapidly back towards BDC again. I didn’t show this on the graph, as it just appears as a mirror reflected version of this same graph continued on. This continuation is why the rod continues to feel the stress for about 30deg BTDC and 30deg ATDC.

Now, hopefully some of you may have noticed that these cylinder pressure forces and inertial loading forces are occurring at the same time. This is very fortunate for our engines, as they just happen to help cancel and reduce stress on our connecting rod. For example, during the compression stroke, we calculated that we have around 1100-1800lbs of compressive force on our piston at the same time we have around 7600lbs of tensile force working to slow the piston (at 8,000rpm). These forces cancel and reduce the force on the rod from 7600lbs down to 6500-5800lbs of tensile force on the rod during the compression stroke. Now, the combustion cylinder pressure comes along to help us change directions of the piston and work to accelerate it towards BDC again. If we look at our earlier calculated figures, we see that we found peak cylinder pressure force to be around 5,500-12,500lbs of compressive loading. We know at 8,000rpm our engine is feeling approximately 7600lbs of force in tensile loading. These cancel to leave our engine feeling somewhere between 2,100lbs tensile clear up to 4900lbs compressive force. Since we cross zero in this range, it does mean that most engines will have some point where RPM based inertial loading and combustion pressure loading equal a zero sum force on the connecting rod during part of the power stroke.

This brings us to the exhaust stroke and intake stroke. We see our available cylinder pressure caused forces are too small to be of much use, yet the piston and rod still having the same mass and are still moving at the same speeds. This means the engine must endure the 7600lbs of tensile force over a painfully long period of about 65deg of crank rotation! Fortunately after this cycle has squeezed and displaced much of the oil film under the rod bearing, it gets the much more gentle compression stroke and power stroke to restore the oil film.

Now, let’s imagine your engine is at 8,000rpm with the throttle closed. Our rod and bearing don’t have the cushion of the compression and combustion cylinder pressure to ease the inertial loading. Our engine feels the full force of 60deg of 7600lbs of force twice as often as it does when the engine is wide open throttle. This gives much less time for the oil film under the bearing to refill before it is hammered again by inertial loading. It also fatigues the rod and bolts at roughly double the rate they are fatigued when under full throttle.

Now when we are wondering why our engine is making the dreaded knocking sounds from down, we have some questions to ask ourselves. Did I allow my car to slow while in gear without disengaging the clutch? Did I use downshifting and letting the clutch out with the throttle closed to slow the car? (Known as engine braking, which I call “engine breaking”.) Did I ever miss shift into a gear which caused the engine to over rev? Was the throttle closed when it happened? Did I have extended periods of severe engine knock?

If you answer “no” to all of those questions, you likely lost your bearing due to oiling problems. The connecting rod bearing is an interesting type of bearing called a fluid bearing. With the soft types of fluid bearings we use in engines, the metal surface is never supposed to have any contact with the rod journal. It is designed to keep a steady feed of oil to maintain a film in place to always let the bearing shell be continuously suspended by hydrodynamic forces. Think of this suspending effect like your car hitting standing water at speed and “hydroplaning” along the surface of the water without contacting the road. When you begin to think more about a fluid bearing, you quickly see how critical it becomes to maintain that oil film.

The oil functions as a mechanical part of the bearing. This mechanical part is used and displaced constantly during engine operation. Every time it is used, it drains back to the oil pan with a few more complex oil molecules broken down into more simple parts. Excessive fuel being washed off the cylinder walls also works as a solvent to break down oil molecules into more simple parts. These simple parts unfortunately do not have the same qualities that the bearing requires to endure the forces on the oil wedge. When a large enough portion of the oil is converted into these more simple molecules, it will have increased chances of the oil film getting displaced to an extent that enables metal to contact metal in the engine. In the case of the very soft metal of the connecting rod bearing, this quickly results in excessive clearances developing. This excessive clearance results in too large of a gap for the oil wedge to escape through. This causes it to become more easily displaced, resulting in an increased frequency of metal contacting metal. When this reaches the point where our ears notice a knocking sound, the gap is so large that the oil is easily flung away from the bearing and the rod clicks against the journal.

Oiling problems also happen when this fluid bearing is fed a compressible fluid or gas. Gas can be sucked into the oil pickup from a variety of causes. Running the engine low on oil results in the oil pump pickup being exposed to air. Feeding air into a fluid bearing results in the air simply compressing under the force and allowing the metal surfaces to contact. This causes immediate damage every time the metal contacts metal in the engine. The oil pickup can also suck air during a hard cornering in a pan without proper baffles. The oil is all slung somewhere that the pickup is not, and as a result the bearings are fed air and the engine is damaged.

Another way the engine bearings can be fed gas is caused by suction leaks inside the oil pump, but we rarely see this happen with Honda oil pumps. However, when we spin a stock oil pump much higher than the speeds it was designed to turn; it sometimes isn’t able to draw oil into the pump fast enough. This result is very low pressures coupled with high velocities. These are the ingredients needed to induce bubbles to form inside fluid called cavitations. These bubbles can cause harm to the pump and anything they come in contact with in your engine. Fortunately, the Honda oil pumps can be modified by a professional to give plenty of increased RPM safety by improving the intake passage to have an improved shape.

If the oiling system on the engine is in good condition and the engine is still having issues with rod bearings, first ensure the crank you are using is perfectly round. The crank must always be perfectly round or the bearing can’t function.

Next check the rod big ends. If they are also perfectly round, this may be the cause of the problem for high RPM engine builds. The problem is not that the rod bore is round, it’s that once 7600lbs of force is applied trying to pull the cap off the rod it flexes a bit. If your machinist does a clever process to create a big end on the rod with a slight oval shape that is widest at the split, the rod bearing will elongate under high tensile loading at high RPM to become round during the times the bearing needs to be round most. It can also prevent the bearing from getting crushed in against the sides of the journal and grabbed and spun when the rod is under high tensile stress and deforms into an oval.

When choosing a bearing, if you use a soft bearing like a factory Honda bearing, when particles inevitably get passed the filter and make there way to the bearing, the particles are able to mush flat into the soft surface of the bearing. This prevents damage to the crank that would have occurred with a harder bearing surface. It also causes less damage to the crankshaft during those unfortunate times when metal to metal contact occasionally occurs. Factory Honda bearings are also the only bearing offered in such small sizing steps that one can set the oil clearance to exactly the Honda recommended 0.0015”.

Hopefully next time you hear that terrible knocking sound, you will have a bit better understanding of what sorts of factors were at work that caused damage to that bearing. However, more hopefully, some bit of info provided above will cause you to enjoy only the blissful silence of a healthy rod bearings.