The efficiency of an engine can be divided into three main factors: Volumetric efficiency (Ve), Thermal efficiency (Te) and Mechanical efficiency (Me). Te and Me stay more or less constant when basic bolt-on engine modifications are made. (For more info check: http://www.fordscorpio.co.uk/efficiency.htm)
Te tells how well engine makes use of the energy that is produced when fuel is burned in the cylinder. Compression ratio and combustion chamber design affect this factor. Higher compression ratio -> higher Te.
Me tells how well engine transfers energy from burned fuel into rotational force that turns the crankshaft. Higher inner and outer mechanical losses means lower Me. For example lowering the masses of rotating engine parts lowers internal inertia. Instead of using power to accelerate engine internals more power is fed through the crankshaft to the transmission to propel the car. Lower internal inertia and other internal losses -> higher Me.
Volumetric efficiency is what one is improving when basic bolt-ons are installed. Ideally, during the intake stroke, cylinder would “suck” in as much air as the mechanical volume of the cylinder is. In B6ZE’s case this would be 399.25 cc. If the volume of the air “taken in” is under that, the Ve is under 100%.
Naturally aspirating engine works optimally (= Ve is close to or even over 100%) in a narrow rpm range. Variable valve timing, valve lift and variable geometry intake designs are used to widen the optimal rpm range.
Ve tells how well the gases flow through the engine. First air flows through the entire intake tract to make its way into the cylinder. After work stroke exhaust gases flow out of the engine through the entire exhaust system. If there are flow restrictions either in the intake or exhaust side of this “gas pump”, Ve is decreased. It’s that simple! Take out restrictors and Ve is increased. Higher Ve -> higher torque. The trick is to know which restrictor should be eliminated first. Throwing in a race cam to an otherwise unmodified engine does not improve Ve at all because flow is heavily held back by other parts of the engine.
That was basic introduction to combustion engine’s efficiency. Now back to business.
Graphs shown here are from two different sources: Import Tuner Magazine and my own engine build. Both are lacking a baseline graph from a completely stock engine. Import Tuner’s stock engine’s torque curve is an estimation done by me (some discussion about it in my previous post) and “Einspritzer motor” is a copy of some kind of a manufacturer’s graph from some German site. All figures are presented (and converted, if needed) in crank torque and hp. To make comparison easier all curves are presented in similarly scaled graphs.
When I compare Import Tuner’s (IT) findings to my own many similarities and some differences can be found.
First similarities:
- Stock intake and exhaust (together) form a flow restriction worth for 10-20 Nm and 10-15 max hp. That is 10-15 % power loss across the entire rpm range.
- Stock intake makes about half of that. High quality cone filter is a must compared to stock airbox. Snorkel’s diameter is small and in stock airbox there are too many tight bends. Just make sure intake air is as cold as possible, at least not hotter than with the stock airbox.
- Stock engine benefits from a better flowing and better tuned exhaust manifold and gains worth for 5-7 Nm can be found. A little torque is lost below 3500 rpm.
- Both engines start losing torque above 5500-5750 rpm. Reason for that is cam specs (duration and timing).
Then differences:
- IT’s engine made its max hp at 6500 rpm and after that power dropped quickly. Exhaust manifold improved flow but torque curve’s form stayed more or less the same.
- My engine showed no signs of choking above 6500. Reason for that is a bigger AFM that outflows the stock unit. Stock AFM limits air flow to previously mentioned 165CFM, which is about 130-135 crank hp. Exhaust manifold’s true potential can be seen above 6500 rpm because stock AFM isn’t there to limit the flow.
- IT’s engine made about 10 Nm more low- and midrange torque. I have no real explanation for that. Maybe high flowing cat does its job there or drive train losses were less than 20% that was used for calculations. If losses were 18% instead of 20%, max figures would have been 153Nm and 128hp.
Enough with the speculations! My point here is not to stare at the numbers but the shapes of the curves. They will guide you to think of the right things.
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