Internal combustion engines experience the majority of their wear when they are started.  The three main reasons for this wear are:

  • inadequate anti-wear additives in the oil
  • inadequate oil flow
  • gasoline contamination

In an engine, there are 4 regimes of lubrication and moving parts within an engine pass through these regimes starting from stopped to operational speed.  See Lubrication for more information.  Also see Car Engine Oil Lubrication Automotive Appreciation video.

  • Boundary Lubrication
  • Mixed Lubrication
  • Elasto Hydrodynamic Lubrication
  • Fluid Film Lubrication
    • Hydrostatic Lubrication
    • Hydrodynamic Lubrication

Of specific concern for older flat-tappet engines is minimizing wear on the camshaft lobes and lifters as well as on the crankshaft bearings during the initial Boundary Lubrication. Before the engine is started, there is obviously no motion between the component surfaces.  As the camshaft begins to rotate, the tappet starts to slide along the surface of the lobe. At the initial startup, the lack of any oil flow causes these surfaces to be in boundary lubrication regime. In the boundary lubrication regime, there is insufficient oil film to keep the parts separated, and contact between the cam and the tappet will occur.  With oil flow and once rotating quickly enough, the wear surfaces are separated by the oil wedge formed by Hydrodynamic Lubrication.  See Effects of Shearing for more information about weak oil film.


Frozen Car


ZDDP (aka ZDP, Zinc DialkyDithioPhosphate) is a common antiwear additive in engine oil and engines rely on the antiwear additives in the oil to prevent wear in the boundary and mixed lubrication regimes.  ZDDP was originally used as an anti-oxidant and it still has this function today.  In bearings, ZDDP provides sacrificial protection, meaning that once it is worn off, it needs to be replenished. ZDDP is a polar compound, meaning that one end of the molecule is attracted to the iron wear surface and must compete for location with other polar additives (like detergents and friction modifiers).  ZDDP only plates out (adsorbs) onto the wear surfaces when activated by heat and pressure from close contact between moving parts.  If the ZDDP layer is worn away, adhesive wear can occur which could cause surface irregularies larger than the ZDDP layer.  If this happens, there is no way to stop the wear from accelerating and it's only a matter of time before a camshaft lobe is wiped.  See Friction Modifiers.

The rate at which the protective ZDDP layer is worn away depends up on the bearing loads and the length of time the wear surfaces remain in the boundary lubrication regime.  Once the oil is hot, the protective ZDDP layer will rebuild itself from the remaining ZDDP in the oil.  Once the ZDDP layer has repaired itself, any additional ZDDP will not make this layer any thicker.  According to a GM Techlink service bulletin (December 2007), Bob Olree (GM Powertrain Fuels and Lubricants Group), excessive amounts of ZDP is not beneficial:

A higher level of ZDP was good for flat-tappet valve-train scuffing and wear, but it turned out that more was not better. Although break-in scuffing was reduced by using more phosphorus, longer-term wear increased when phosphorus rose above 0.14%. And, at about 0.20% phosphorus, the ZDP started attacking the grain boundaries in the iron, resulting in camshaft spalling.

It's better to use an oil formulated with the right amount of ZDDP and other additives than guessing you've added the right amount.  The main reason for using an engine oil with sufficient ZDDP is to protect the lobes of a flat tappet camshaft. Modern engines with roller lifters don't have this need, which is why Starburst Oils contain 0.06% to 0.08% phosphorus to prevent poisoning of their catalytic converters.  Pre-2007 (ie, before API CJ-4) heavy duty engine oils typically had phosphorus contents less than 0.14% (1400 ppm).  The current CJ-4 and CK-4 categories phosphorus limit is 1200 ppm and this concentration has been found to work well to protect flat tappet camshafts.  ZDDP additive manufacturer ZPlus recommends 1200 ppm of phosphorus:  see ZDDPlus™ Tech Brief #2 - ZDDP and Cam Wear:  Just Another Engine Oil Myth?


Engines require oil FLOW and oil PRESSURE is related to flow and viscosity. Oil pumps are positive displacement, which means their flow is proportional to speed. The pressure measured by an oil pressure gauge is typically at the oil pump or in the main oil gallery.  Once the oil reaches its destination (tappets, bearings, etc), its pressure is essentially atmospheric (ie 0 psi) or close to it so the measured oil pressure is what the oil pump develops in response to the flow it is generating.  Excessive pressure can cause oil filters to rupture or oil pump gears to shear so hydraulic pumping systems always have a safety relief valve to limit the pressure developed at the pump.  Excess pressure is bled off by the relief valve back to the sump so you're better off keeping the oil's viscosity to the lowest that protects the bearings.

As you can see by the following photo, engine oils become more viscous (thicker) as they get colder but the 0W-30 oils flow far better than 15W-40.


Engine Oil Viscosity Comparison


Since oil pumps are positive displacement, the pressure developed by the pump should almost instantaneously increase if all of oil passages are filled with oil.  When the ambient temperature starts to get low, you've probably noticed that it takes longer for your oil pressure to come up or for your oil pressure warning lamp to go out. The reason for this delay is that excessively thick oil is causing one of two things: vortexing or cavitation.

Vortexing results from the oil becoming so viscous that it "gels" and becomes a semi-solid material that cannot flow past the oil pickup screen and into the oil pump.  Instead, the vacuum created by the oil pump allows air to pulled into the oil pickup, thereby causing air-binding.  An air-bound pump does not flow oil and therefore is unable to build oil pressure.  See Thermal History of the Engine Oil and Its Effects on Low-Temperature Pumpability and Gelation Formation.

Cavitation is similar in that that oil is too viscous to flow through the pickup screen as fast as the oil pump requires. The vacuum in the oil pickup causes the oil to "flash" into vapour bubbles, which then collapse in the oil pump.  A cavitating pump is inefficient and is unable to generate as much flow as it should.  Severe cases of flow restriction can lead to flow-limited failure.

Richard Widman makes this observation on page 2 about hydrodynamic lubrication:  Selection of the Right Motor Oil for the Corvair and other Engines:

A cushion of liquid oil surrounds the lubricated item and holds it away from the rest of  the parts.  When the proper oil viscosity is used in a properly built engine at operating velocities, the crankshaft is in hydrodynamic lubrication.  It has no contact with the bearings.  The only physical contact is during startup before velocity is attained or under lugging from improper gear range.  If the oil is too thin, it can be displaced and allow contact. If it is too thick it takes longer to get to the bearings and valve train as well as build pressure (the cushion) in the bearings creating additional wear.

To ensure that engine oil flows well to the oil pump at low temperatures, the oil must have adequate pumpability.  The "W" (w means "winter") viscosity rating (0W, 5W, 10W, 15W, 20W, 25W) of the SAE Viscosity Grade is what describes how pumpable the oil is at low temperatures.  Pour Point Depressant additives are sometimes added to prevent gelation by reducing low temperature wax crystal formation.

Depending upon the design of the engine and the orientation of the oil filter, another thing to consider is leakage from the anti-drainback valve (ADBV, check valve, non-return valve) in the filter.  If this valve does not seal properly, oil will drain out of the filter and, no matter the ambient temperature, there will always be a delay for the oil pressure to come up.  If this is the case, you need to find another filter with a better ADBV.

If you're concerned about oil flow to your bearings, you should install oil pressure and and oil temperature gauges to measure how much pressure is being developed by your oil pump and how hot your oil runs.  If your engine is in good condition and its oil pressure at fully warm idle is within the manufacturer's specifications, your oil pump should have enough capacity at higher engine speeds.  At higher engine speeds, you should see your oil pressure top-out at the rating of the safety relief valve.

Hydrodynamic oil friction from higher engine speeds and/or loads generates more heat in the engine oil.  Heat is rejected from the engine oil mainly via the oil pan and the oil filter.  Higher oil temperatures will cause the engine oil to become less viscous and therefore its oil pressure will decrease with increasing temperature. Unless your oil runs really hot, you probably don't need much more than a 30-grade oil (0W-30, 5W-30, 10W-30).  Due to fluid friction, with the same engine load and speed, less viscous (thinner) oils tend to run cooler.

If your engine is in good condition but your engine oil pressure tends to be low due to high oil temperature, you can either increase the pressure by switching to a more viscous (heavier) oil (eg, change from 30-grade to 40-grade), upgrading to a higher volume pump, or adding an engine oil cooler. Using a heavier oil gives the added benefit of getting an oil with a higher HTHS (High Temperature, High Shear) rating.  Compared with mineral oils, synthetic oils should also have higher HTHS ratings due to their reduced reliance on Viscosity Index Improver additives.  Higher volume pumps and/or higher viscosity oils place a higher load on the oil pump's drive gear, which can lead to gear failure so be sure that this is a recommended option for your engine.  See SL6 Oil Pump Gear Failure and Oil System Information.

If your oil pressure is within the normal range but you want more flow to your bearings, you would be better off switching the safety relief valve spring to a higher rating than switching to a high volume oil pump. A higher oil pressure relief setting will reduce the amount of oil recirculated back to the sump.  However, this change places additional load on the oil pump so be sure that this is a recommended option for your engine.

The Mopar rule of thumb is 50 psi up to 5000 RPM and then 10 psi for every 1000 RPM increment (ie, 6000 RPM requires 60 psi).  For Chrysler products, a simple oil pressure upgrade is to replace the OEM relief valve spring (45-55 psi) with the 75 psi Hemi Oil spring (PN 2406677) in slant six and B-RB big block engines.  Small block LA engines use the 75 psi (PN P3690944).  With my slant six running 10W-30, I get 70-75 psi at ~2500 RPM on the highway during the hottest summer days here in Ontario.


Engines always run best when they are fully warm.  However, it is the intake manifold that is the specific part of the engine that needs to be warm for the engine to run smoothly.

Gasoline enters the air stream to the engine via the venturi(s) in the carburetor.  Liquid gasoline is atomized in the venturi(s), meaning that it becomes extremely fine droplets.  Some (depending up manifold temperature and vacuum) of this gasoline mist will vaporize, meaning that it will draw heat from the air and evaporate into gasoline fumes.  This vaporization causes the air stream temperature to become colder and it's possible to the intake manifold to be covered in condensation or even frost.

Intake Manifold Frost


Heat must be added to the intake manifold to replace that absorbed by the vaporizing gasoline and this is the reason that the manifold heat control system (aka, heat riser) provides a hot spot under the carburetor.

The following 7:57 minute video explains this process at 4:41:


Some of the gasoline droplets will not make the 90° turn from the carburetor into the intake manifold and will drop out of the air stream to form a puddle on the intake manifold floor.  The hot spot below the carburetor vaporizes the puddled gasoline back into the air stream, which helps to ensure the engine gets the correct fuel mixture.

When the engine is first started, the intake manifold is cold and a greater proportion of the atomized fuel does not vaporize. To compensate, a carburetor has a choke to enrich the fuel mixture until the intake manifold warms up.  Until it does, gasoline will continue to puddle and will flow as liquid film to the cylinders.  Some of this liquid gasoline will flow into the combustion chambers and will then wash the oil off the cylinder walls as it creeps into the crankcase.  Over time, the lack of oil at the cylinders will cause them to gradually lose compression as they wear.  The cylinders getting the largest proportion of start-up liquid gasoline flow will show the most wear.

Some of the gasoline that reaches the crankcase will boil off when the oil becomes hot enough.  However, not all is boiled-off and the amount of residual gasoline in the oil will increase over time.  It generally takes longer for the oil to reach its operating temperature that the coolant.  Gasoline is a poor lubricant and will reduce the oil's viscosity and dilute the additives.  To reduce the oil's gasoline contamination, you should:

  • make sure that your manifold heat control system is working properly
  • make sure that your choke is working properly
  • make sure that heated air system (if so equipped) is working properly
  • plan your trips so that your engine remains at operating temperature for extended periods (at least 30 minutes)