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Old 12-24-2009, 01:03 AM   #7 (permalink)
Almost fuel free!
Join Date: Nov 2008
Location: Georgia
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Everyone is talking about modifying the plug gaps across all cylinders in the engine, but has anyone tried using different plug gaps for different cylinders? It sounds crazy, but I was reading an article on Autospeed about cylinder-specific tuning, and the variance in AF ratios and other factors for different cylinders in the engine was reasonably large.

The Autospeed article
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For instance, one of the situations in which cylinder imbalance of Air/Fuel ratios would occur happens when an assymetrical intake is utilized:
In this situation, the air is taken in through one side of the manifold, before being distributed through the various, equal length runners to the engine. From the flow diagram, it seems that the runner farthest from the intake port would be receiving the most air, while the runner closest to the port would be receiving the least.

Now, because the engine is running in closed loop (feedback from the O2 sensor and various other engine points determines fuel delivery), the amount of air reaching each cylinder plays a major role in determining whether the car runs as close to 14.7 to 1 as possible. Due to the fact that many cars run a single O2 sensor, that sensor is only seeing the average of AF ratios from the engine. Also, each cylinder recieves the same amount of fuel (because there is only one O2 sensor, fuel isn't automatically adjusted for each cylinder). This would mean that some cylinders are receiving more fuel than necessary, and others less than they need.

For instance, the cylinder farthest from the intake (I'll call it cylinder 1) is receiving much more air than the other cylinders, while also getting the same amount of fuel. This would cause it to run leaner than the other cylinders. The opposite cylinder, closest to the intake is receiving less air with the same amount of fuel, causing a rich condition in that cylinder. To protect the engine, most cars will err on the side of delivering too much fuel, rather than too little. This means, if the O2 sensor finds that the average cylinder AF ratio is too lean, it will immediately enrich the mixture, thus impacting fuel economy. While this will average out to approx. 14.7-to-1, the cylinder that is running lean is also influencing the knock sensor of the engine. Engine knock, or detonation is primarily seen when an engine is running too lean for the operating conditions. When knock is detected in fuel injected cars, the vehicle's ECU immediately responds by retarding the ignition timing (less power) and dumping more fuel into the engine to eliminate the lean condition.

So basically, cylinder imbalance can result in the car using more fuel than necessary to compensate for the differences across the cylinders.

What I'm wondering is whether it would make sense to counter this with variable plug gaps across a given engine. My 96 Accord uses an intake setup similar to the flow diagram pictured, so I figured that by using a narrower plug gap at the same position as cylinder 1, and a wider gap at cylinder 4 (in my case, the cylinder closest to the intake), the cylinder that is running rich would burn more fuel, while the cylinder running lean would burn less.
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