This brings us to a very important point which is engine ‘squish’. The use of asymmetcrical piston crown design not only continues the swirl process initiated in the intake system, but more importantly having an asymmetrical piston crown forces the air to rapildy move towards one side of the combustion chamber, especially as the piston approaches top dead center. This squish effect near top dead center can be used for several advantages:

1. Getting a good air and fuel mix for more efficient combustion.
2. Moving the air and fuel mixture closer to the spark plug for easier ignition.
3. Reducing the distance between the tip of the spark plug, and the farthest pocket of air and fuel mixture that needs to be burnt. Reducing the flame front travel distance builds cylinder pressure faster in the chamber which reduces timing advance requirements at lower rpms, but also, maintains more torque delivery to the piston as rpms increase (and as the time that the piston spends between top dead center and 17* ATDC) starts to shrink rapidly compared to flame front travel speed past a certain rpm point, as well as for engines with a shorter rod to stroke ratio with faster piston acceleration away from Top Dead Center.
4. Reducing the probability of detonation and uneven combustion from the better air and fuel mixture and better heat distribution.

Furthermore, on some applications we can take this one step further by milling down the cylinder head (or using a different cylinder head casting with shallower combustion chambers) which brings the spark plug down deeper into the bore, and offsetting the loss of combustion chamber volume with further dish in the piston crown. Bringing the air and fuel pocket closer to the spark plug, and bringing the spark plug closer to the center of the combustion chamber formed between the cylinder head contours and the piston crown contours boost engine efficiency, reduces detonation probability, reduces timing advance requirements, and promotes increased efficiencies at higher rpms as described earlier.

When choosing an aftermarket piston for your motor, look for a reverse dome piston top (rather than a flat top or typical dish type piston) with dimpled flat surfaces for better mixture, and still having at least 0.2” material thickness throughout the entire crown of the piston.

If the piston I just described does not exist, a thick piston (high compression) piston can be machined down to make the piston I’m describing by someone who knows enough about this to do it properly (or by requesting a custom style piston from the piston manufacturer themselves).

3- Piston Coatings

Investing the money in getting your pistons coated has several benefits including:

• Increased power
• Reduced emissions
• Better response
• More resistance to catastrophic failure

The best combination of coatings are as follows:

• A thermal barrier coating on the piston crown
• A friction reduction coating on the piston skirts
• An oil shedding coating on engines equipped with oil squirters in the crank case

The thermal barrier coating gives a more consistent finish to the top of the piston crown. It helps reflect heat into the combustion chamber, rather than dissipating it through the piston materials and thus improves combustion speed and the completeness of the combustion process. However, this increase in combustion temps and trapping the power rather than dissipating it does require reduced timing advance, but will as stated earlier, pay back dividends on higher rpm motors or on engines with short rod/stroke ratios where piston acceleration away from TDC becomes a problem for power transfer into the pistons at higher rpms.

One thing to note here is that since the thermal barrier coating improves torque delivery by accelerating the burn rate inside the engine, it can be used to boost torque output on cars with centrifugal superchargers or large turbos to give better response before the boost builds.

Overall, it may seem like it’s disadvantageous to trap more heat in the chamber and that it possibly reduces the octane, boost, or timing limits of the motor but this is not true. The increased heat can be counteracted with timing reduction without power loss (since the burn rate is maintained) and the added advantages are that the thermal coating helps spread the heat out over the entire crown area of the piston, thus protecting any thin or weak spots from being pummeled to failure. Also in the rare event that you do have some minor detonation in the engine due to high load, a bad fill of gas, or some other factors, the thermal coating prevents piston pitting due to minor occurrences of detonation, and thus it prevents the creation of hot spots on the piston crown which could have become hot-beds for further detonation and a prevented runaway towards total piston failure!

Seems like a fair trade off of some timing advance for increased longevity and increased high rpm efficiency.

The low friction coating on the piston skirt reduces frictional losses between the piston sides and the cylinder walls. This protects the pistons from damage and scuffing on cold starts, if the engine is overheated or overboosted (and the piston expands due to heat), and during oil starvation conditions (high cornering G’s, coild oil, first crank after a rebuild).

Reducing friction in the engine delivers more horsepower to the crank, improves the engine’s operation near redline, and gives the engine crisper response. One study showed that using lower profile skirts, with proper friction coating, as well as lower friction wrist pins can reduce the total engine internal friction by 40%…

So, something as simple and non intrusive as coating your piston skirts is definitly worth the effort, especially on boosted street cars that need to run a full skirted piston (as opposed to naturally aspirated motorcycles that will more likely run a forced side relief piston which features a longer skirt in the primary axis of the piston motion as the connecting rod movement shoves and pulls the piston against the bore on the upwards and downwards strokes… and a short or no-skirt in the axis 90* with plane of the connecting rod’s movement).

Finally, oil shedding coatings on the bottom surface of the piston help evacuate oil off of the piston faster. This helps keep the piston lighter and faster moving in its rotation, however since oil is used to cool the piston bottom and increase its longevity (especially in motors that will experience high cycles as we will explain later), then there is a debate as to weather this coating is beneficial or detrimental on different engines.

Engines with oil squirters get plenty of oil volume delivered to the bottom of the piston at a constant stream of flow. These engines can do with a reduced duration for oil clinging to the bottom of the piston.

On the other hand engines that rely on the crankshaft counterweights sloshing in the oil sump and indirectly sending oil up to the pistons to cool them (and up to the wrist pins to lubricate them on engines without a force-lubricated style connecting rod) could use with the oil clinging to the piston bottom for a longer duration.

If you are doing a full rebuild, my recommendation would be to both coat your pistons AND install oil squirters. Otherwise, choose weather or not to use the oil shedding coating based on weather you have a dry sump, wet sump with squirters, or wet sump without squirters oiling system.

4- Other Piston Design Considerations

As mentioned earlier there are other considerations to choosing your piston design which I have eluded to earlier.

Even though most people judge engine life based on mileage, performance engines are more accurately judged on cycles.

For example, an engine can run for 100 continuous miles at 7500 rpms in 1st gear, or at 1500 rpms in 5th gear…. the same engine could be tracked every weekend (spending a healthy portion of its life in the higher rpm ranges) or cruised on a highyway commute to and from work.

Even though these two engines have the same mileage, they have lived through a different number of engine cycles. Engine cycles is what determines the amount of continuous (or accumulated) stress both on your pistons (for thermal management) as well as on your piston rings (for wear management).

The advice given here is primarily for dual purpose vehicles that are both street cars but will see occasional or repeated track use.

Vehicles that will be used primarily for racing, require a thicker crown for better thermal management, will probably use a lower silicone content piston (with much larger piston to bore clearances to allow for the thermal expansion of the piston after it stops slapping and warms up in the bore, and will definitely have lower ring height for the primary compression ring (which reduces the operating temperature of the ring from around 600*F to around 300*F) as well as using a thicker compression ring that is less likely to distort or fail under continuous sustained high rpm abuse (think about a car in Nascar racing that does the entire race over 5000 rpms…)

These cars that are designed for ‘standing mile races’ as I like to call them or long term endurance racing will also have to be tune differently as I have a complete longevity tuning chapter in The Tuner Mastermind.

However, for most street vehicles running up to around 18psi of boost, and having dual street / track duty… then follow exactly the piston recommendations detailed here and you will have a great combination of torque, efficiency, detonation resistance, and reliability.