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This handbook is currently under construction.
With modern engines it has become extremely important to nail the air/fuel ratios at which your engine should be running. Severe problems can arise from incorrect air/fuel ratios, and these problems occur quickly and are often devastating. Broken rings, burnt pistons, cracked heads, burnt valves, and overtaxed bearings are just a few of failures that can and will occur when tuning is off.
In the past there was no real way for the average car guy to determine what their air/fuel ratios were, but with the advent of accurate air/fuel measuring equipment that is affordable, everyone can afford to have their own. This handbook will be very useful with this equipment to know what the readings mean, where they should be, and how to get them where you want them. We go over the reasons why air/fuel ratios need to be correct, and when they aren’t how to make the necessary changes. When using this new equipment you can see immediate results for each change made . No more inaccurate readings from spark plugs, and no more guessing by listening for engine detonation or pre ignition. For the cost of a few hours on a dyno you can now own your own equipment and take readings under actual driving conditions, not just at whatever driving schedule the dyno guys run your car through.
Check the tech info page for air/fuel ratio monitoring equipment that we have used.
Here’s a sample:
Air to Fuel Ratio
Today we have the ability of checking on our carburetor adjustment progress very simply. For a relatively small investment you can now own an air fuel ratio tool that can help you get your carburetor, or fuel injection, tuned perfectly. I am using the Innovate Motorsports LM1 (they also have several other air/fuel products), although there are other choices in the performance market.. It is a very versatile and easy to use tool that can help you get every bit of good from your engine. It will help you to improve performance, mileage, and engine life. But before I begin with just what you need to know you must first understand this one rule, and when things don’t seem to be going right always come back to it, it may be the answer to your problem. This tool, and others like it, cannot provide you with information that is usable unless you first;
MAKE SURE THAT THE ENGINE IS HEALTHY AND RUNNING ON ALL OF ITS CYLINDERS.
Otherwise the information provided by the test equipment will be of no value. Air/fuel ratio equipment functions by sampling the amount of oxygen in the exhaust. If an engine is misfiring then the oxygen not burned during what should be the combustion process will end up in the exhaust. Where oxygen sensors are concerned, high oxygen content in the exhaust indicates a lean condition; low content indicates a rich situation. But this information is only useful if all of the cylinders are firing properly, and that the exhaust is tight, not allowing any outside air to enter the exhaust. And its only accurate when sampled in front of a catalytic converter. When using an air/fuel ratio device you have to always remember “oxygen” is what it’s sampling, not fuel.
If you’re in doubt of the engine’s health it is wise to do a compression test, a cylinder leak test, a running cylinder contribution test, and an ignition analysis. Also make sure that the engine fuel supply is constant throughout the entire rpm range.
If you have an older application utilizing an AIR pump make sure that the switching and check valves are operating properly.
On all applications rule out any possibilities of vacuum leaks before you begin. You also want to make sure that the big camshaft you installed has a sufficiently modified carburetor in order for idle and part throttle to work properly.
Gas Analyzer
CO was the original bogey for identifying air fuel ratio but O2 is what lets you know if an engine is running rich or lean. Oxygen content in the exhaust is the real indicator for air to fuel ratio, so it is not necessary to look at the other gasses to set up an engine for best performance.
AIR/FUEL RATIO RECOMMENDATIONS
You need to check with an expert on just what your engine will require for a safe or best air to fuel ratio, but most naturally aspirated engines will run 10 to 15 percent richer than the stoichiometric air/fuel ratio for a modern closed loop engine, which is 14.7 to 1. That means that most of us will want our engines to be running, under load, between 12.5 - 13.3 to one, depending on compression, etc. This can be attained by changing jets, power valves, metering rods, air bleeds, idle screw adjustment or injector pulse width, if you have the ability to modify your fuel injection system through a laptop. Nitrous cars run much richer, usually about 11.9 to 12.0 to one, but these are high power cars. At idle and part throttle you may want to run leaner, for economy, but staying a little richer is always safer than running too lean. With most mild street cars I tend to run around 14 to 1 at idle and part throttle. Engine load is very light at idle and part throttle so detonation is not a factor, but economy is. And running too rich at idle is one of those things that really soots the spark plugs and makes for poor mileage, smelly exhaust, fouled spark plugs, and reduced engine life. When stuck in traffic I don’t want to be worrying about whether the plugs are going to foul and cause the engine to start to misfire. But if you set the idle too lean it might be a real bear to warm that engine up in the morning, so there’s going to be a trade off in there somewhere.
But when I say that you want to run around 14 to 1, you need to be aware that with a system that is an open loop system (no feed back from an oxygen sensor to a computer to control air/fuel) then the adjustments you make are going to be approximate and will bounce around, especially at low speeds. It is impossible to sustain a consistent air/fuel ratio without the aid of a computer. Even computer controlled fuel systems only average a specific air/fuel ratio, they cannot maintain one exactly. Engines are subject to too many varying factors. On a dyno with complete control over all aspects of the engine and its environment things are different, but in the real world we have to settle for “close enough.”
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