GM/Merc 496HO knock sensor
01-02-2006, 03:58 PM
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Re: GM/Merc 496HO knock sensor
Rage
I think you need to back up a little bit and think about what really causes most knock (detonation) in a marine high performance engine. Knock is not usually the result of upgrading performance in a hi/po marine engine. Almost every motor under continious load can expierence knock under the right condtions. The condtions in a motor that tend to contribute to knock seem to be overly advanced ignition timing, very high combustion chamber temperatures usually from lean fuel mixture or very high compression, poor combustion chamber design and mixture burn, poor fuel quality or to low of octane number,etc. The PCM555 is an ECU that still uses a speed density calculation to select spark timing and fuel delivery based on a MAP input, throttle position, intake air temp and coolant temperature. The real knock control job of the ECM is to keep timing and fuel delivery within a range that has been determined by testing to let a particular engine run and make power without developing knock. The programmed values in a PCM555 have literally hundreds of combinations of fuel and spark delivery based on volumetric efficiency that the ECM uses to calculate how much of each it will deliver at any given rpm and throttle position. The knock sensor Mercury uses with the program that Mercury has programmed into the PCM is preset to recognize distinctive knock signals in the 496. When the ECM sees a definite knock event over a predetermined time and intensity it will add fuel on a Bank A or Bank B basis, not on a cylinder by cylinder basis. This is why there are two knock sensors on this motor. The program will also, at the same time start subtracting ignition timing to preset levels for a period of time until the knock stops. If the knocking is large and over a longer period of time the PCM555 program will actualy set a code and go into a "Guardian" reduced power mode to hopefully reduce damage to the engine. the PCM555 is programmed to record certain knock events and record their frequency. These can be pulled up from history in the ECM with the correct scan tool. What I am attempting to say here is that increasing the performance of a motor does not necessesarily cuase knock in a motor. If the performance upgrades increase the volumetric efficiency of a normally aspirated motor within ranges that the ECM is programmed to see and calculate for, the ECM can deliver added fuel and spark to a point where the motor can make more power and still not produce knock. This is why our 525 hp kit works on a stock 496HO without reprogramming. The components of the kit like heads, cam and intake were developed to produce a serious increase in volumetric efficiency which the program in the PCM555 is capable of recalculating and delivering the correct amount of fuel at a corrected fuel pressure to keep the air fuel ratios at all RPMs at almost identical values of what Mercury programmed and measures on a stock 496HO. Motors with our kits actually see no increased knock events in actual use and yet they are producing 100 more hp on the stock Mercury PCM555 program. How did we do that, well thats a Raylar secret, but your concerns about increased or uncontrolled knock should only be a concern for any individual, engine builder, or aftermarket company who makes major mechanical changes to a 496 engine without
good testing and design that insures the so called upgrades will produce a good end result without increased reliability problems or engine damage.
Its good you are asking these questions, because others on the forum get some great feedback and information they might not have known or understood. I would just be careful that you don't overcompensate for your concerns by trying to over engineer your planned changes and end up making your performance boating expierence less enjoyable. I doubt if you had chosen a good brain surgeon for your own brain surgery you would be getting this detailed on his surgical methods. If you just love to know how everything works and the science of motors is one of your joys in boating, then you should be having fun and learning a lot!
Best Regards,
Ray @ Raylar
I think you need to back up a little bit and think about what really causes most knock (detonation) in a marine high performance engine. Knock is not usually the result of upgrading performance in a hi/po marine engine. Almost every motor under continious load can expierence knock under the right condtions. The condtions in a motor that tend to contribute to knock seem to be overly advanced ignition timing, very high combustion chamber temperatures usually from lean fuel mixture or very high compression, poor combustion chamber design and mixture burn, poor fuel quality or to low of octane number,etc. The PCM555 is an ECU that still uses a speed density calculation to select spark timing and fuel delivery based on a MAP input, throttle position, intake air temp and coolant temperature. The real knock control job of the ECM is to keep timing and fuel delivery within a range that has been determined by testing to let a particular engine run and make power without developing knock. The programmed values in a PCM555 have literally hundreds of combinations of fuel and spark delivery based on volumetric efficiency that the ECM uses to calculate how much of each it will deliver at any given rpm and throttle position. The knock sensor Mercury uses with the program that Mercury has programmed into the PCM is preset to recognize distinctive knock signals in the 496. When the ECM sees a definite knock event over a predetermined time and intensity it will add fuel on a Bank A or Bank B basis, not on a cylinder by cylinder basis. This is why there are two knock sensors on this motor. The program will also, at the same time start subtracting ignition timing to preset levels for a period of time until the knock stops. If the knocking is large and over a longer period of time the PCM555 program will actualy set a code and go into a "Guardian" reduced power mode to hopefully reduce damage to the engine. the PCM555 is programmed to record certain knock events and record their frequency. These can be pulled up from history in the ECM with the correct scan tool. What I am attempting to say here is that increasing the performance of a motor does not necessesarily cuase knock in a motor. If the performance upgrades increase the volumetric efficiency of a normally aspirated motor within ranges that the ECM is programmed to see and calculate for, the ECM can deliver added fuel and spark to a point where the motor can make more power and still not produce knock. This is why our 525 hp kit works on a stock 496HO without reprogramming. The components of the kit like heads, cam and intake were developed to produce a serious increase in volumetric efficiency which the program in the PCM555 is capable of recalculating and delivering the correct amount of fuel at a corrected fuel pressure to keep the air fuel ratios at all RPMs at almost identical values of what Mercury programmed and measures on a stock 496HO. Motors with our kits actually see no increased knock events in actual use and yet they are producing 100 more hp on the stock Mercury PCM555 program. How did we do that, well thats a Raylar secret, but your concerns about increased or uncontrolled knock should only be a concern for any individual, engine builder, or aftermarket company who makes major mechanical changes to a 496 engine without
good testing and design that insures the so called upgrades will produce a good end result without increased reliability problems or engine damage.
Its good you are asking these questions, because others on the forum get some great feedback and information they might not have known or understood. I would just be careful that you don't overcompensate for your concerns by trying to over engineer your planned changes and end up making your performance boating expierence less enjoyable. I doubt if you had chosen a good brain surgeon for your own brain surgery you would be getting this detailed on his surgical methods. If you just love to know how everything works and the science of motors is one of your joys in boating, then you should be having fun and learning a lot!
Best Regards,
Ray @ Raylar
Re: GM/Merc 496HO knock sensor
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