Naturally aspirated engine tuning is plain simple: (yet so ever challenging!)
A) Think your engine as an air pump
B) Air is always moving from higher pressure to lower
Thus, in order to increase the amount of air your naturally aspirated engine can transfer, you can:
1) Increase the size of an air pump (cc, cid)
2) Increase the effective speed of an air pump (rpm)
3) Optimize the gas flow for higher rpm band (Cylinder heads, valves, cams, headers etc.)
4) Increase the density of air (Cooler air temperature, reduce the pressure losses to minimum)
5) Optimize intake resonances (e.g. by shortening intake runner length, changing plenum size)
Like we learned in the other article, a naturally aspirated engine is as strong as its weakest link. There are no shortcuts.
The Foundation of Naturally Aspirated engine: Displacement
Actually this applies for forced induction engines too. The very base foundation for HP/Torque capability is the engine displacement. Even in turbocharged applications, there is no replacement for displacement. More is better. Always.
RULE: In naturally aspirated engine, an engine displacement will give you torque characteristics for low to mid rpm range. The maximum torque you can achieve is approx. 120nm per litre on NA-engine.
Maximum torque per litre, naturally aspirated engines
80 nm/l: stock 2-valve NA-engines
100nm/l: Best 2-valve NA-engines
100-110 nm/l: Modern, stock 4-valve NA-engines
110-120 nm/l The best stock, and modified 4-valve NA-engines
The character of naturally aspirated engine: Cams
No matter how cool looking “Cool air intake system” your engine has – if you have not cammed your engine – anything else is just a waste of time.
“Why hotter cams 101”: If you seek for more power, you have to stretch the power band towards higher rpm range.
When you rev higher, the following will happen:
A) Less and less time your engine valves stays open (in milliseconds) in relation to rpm.
B) Flowing gasses create more and more inertia, and fluid friction.
Take home points:
Problem A) can be tackled by assembling higher degree cams. This enable valves to stay open longer (in milliseconds) on higher rpm range.
Problem B) Increased overlap paired with good exhaust headers can take advantage of increased gas inertia. Exiting exhaust gasses in the header are creating both positive and negative pressure pulses, which can create low pressure areas, thus creating ‘scavenging effect’. OK, let’s explain this in the other words: While gasses are always looking for a route from higher pressure area to lower, during scavenging effect exhaust gasses have enough kinetic energy to keep moving forward – which in ideal situations – can create draft, thus creating ideal conditions for the exhaust valve to open. This is called scavenging effect. This effect can assist combustion chamber to empty burnt gasses more efficiently, while simultaneously sucking fresh air from induction system. This effect can improve your engine’s Volumetric Efficiency (VE), and produce more torque on upper rpm range.
Scavencing effect – also known as reverse or negative supercharging – is typically combined with inertial, or inertia supercharging, which in short can utilize A) kinetic energy of incoming air, and B) acoustic supercharging (rebouncing resonances ) in between a closing intake valve and intake manifold plenum.
NOTE TO SELF: The best efficiency I have personally reached is 113 nm per litre, measured on highly respectable hub dynanometer with DIN correction. On street, under optimal circumstances with +7 C outside temperature, the log data has shown engine loads that can be transferred as high as 119,4nm per litre. Please note this is NOT torque by DIN standard, only peak calculated torque from maximal engine load, hardly comparable with any other engines than my own.
RULE: In naturally aspirated engine, hotter cams will give you torque characteristics for high rpm range. Cams are the character of your engine. Period.
Reducing pressure losses in the intake manifold
Higher intake manifold pressure than atmospheric pressure on naturally aspirated engine is rare sports. This phenomena has occurred to me only couple of times, although I have a lot of data at my disposal. MAP pressure was shown to be 0,1 kPa higher than atmospheric, and it last only approx. 10 milliseconds. Being skeptical, this may be due to a margin of error, nevertheless indicating minimal pressure losses in the air induction path. Location of MAP sensor has always been in its default location in M6x intake manifold. Although that being said, this data does not prove – nor deny – the existence of inertial supercharging, which can happen closer to a intake valve.