Intro To Tuning
An improperly tuned engine can get bad gas mileage, and fail emissions tests. If your engine is modified, these are the least of your worries. An engine that is running too rich won't perform to its potential, and an engine running lean can detonate and destroy itself. Regardless the error, an improperly tuned engine can cost you a race, a championship, or... just money!
When tuning modified engines, accurate, wideband oxygen sensing equipment is critical. The keys to this process are:
A heated, 4-wire oxygen sensor like the Bosch LSM-11, or preferably a 5-wire NTK/NGK UEGO (or the Bosch LSM-4 UEGO). This is not the $28 sensor from the parts store. Good sensors start at $500 and go to over $1000. It is important that they be relatively new (less than 500 hours) and kept at EGT's (exhaust gas temperatures) less than 900c.
The sensor must be correctly located. A sensor hanging in the tailpipe will react too slowly to changes in the mixture. A sensor mounted too close to the exhaust manifold (particularly in boosted or rotary engines) will be destroyed by the excessive temperatures. So, somewhere close (but not too close) to the manifold is preferable. It is not difficult to weld the proper bung (a nut) to a down pipe (turbo) or collector.
In addition to the sensor, you must have accurate hardware (a "controller unit") to make corrections for exhaust gas temperature and convert the 0-5v readings from the sensor into corrected data. The correction factors are critical when tuning for maximum performance.
Last, and this sounds obvious, but its easy to overlook, you need to be able display the O2 readings in a meaningful format - for a given O2 sensor reading, you must know "where you are" in terms of RPM's, boost, etc... so you can interpret your data. This tells you and your tuner where to correct your fuel injection maps, make carb adjustments, or other changes to the fuel/air/ignition combination.
At part throttle and low loads (cruise), very lean mixtures are possible with things such as high energy ignitions, charge stratification or special combustion chamber shapes. Knock sensors are not normally needed under these conditions. When increased power is needed, the mixture is richened closer to the stoichiometric ratio, and the potential for detonation increases as cylinder filling increases.
When the peak pressure inside the combustion chamber occurs too early (as the piston is still rising or at TDC in the bore), knocking occurs. Knocking is the collision of two flame fronts in the combustion chamber (also known as pre-ignition). Pre-ignition can be caused by:
1. Carbon deposits or a sharp edge (a nick in the head or piston) causing combustion to start away from the plug.
2. A lean mixture pre-igniting due to combustion chamber pressure (the lean mixture requires less pressure to spontaneously ignite) prior to the spark plug firing.
3. Too early spark advance.
Fuel with too-low resistance to combustion (low octane rating).
This hammer blow is the actual knocking, which in conjunction with higher temps can destroy the piston head (or an apex seal in a rotary engine). It also places a heavy load on the rod bearings and the hydrodynamic oil cushion, which is why lugging the engine (low revs, high load); can wipe out bearings and journals.
If the peak pressure wave occurs early, but the cylinder head is lightly filled, the piston can still compress the fireball without problem (sometimes light pinging).
The results of heavy detonation, especially in turbocharged/supercharged applications can be found on the desktops, and in garbage cans of racers everywhere.
How an Oxygen Sensor Works
A Lambda, or O2 sensor might be described as a chemical generator. When it is heated to a minimum of 600F (about 300o) it will begin to produce voltage ranging from 100 to 600 milli volts. Once this temperature is reached, the sensor will begin to respond to changes in the content of the exhaust oxygen (O2). When the oxygen content of the exhausts is high, the engine control computer assumes that the engine is running lean.
The theory behind the O2 sensor is that an engine running at correct air-fuel ration of 14.7:1 will consume all of the oxygen and all of the fuel simultaneously. If the engine is running rich, then the oxygen will be consumed before the fuel. The average oxygen sensor has difficulty distinguishing between a rich-running and perfect-running engine. However, a lean running engine will leave large amounts of residual oxygen in the exhaust.
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