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502 MPIs, A Long Story Getting Longer

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Old 09-06-2010, 11:34 PM
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Default 502 MPIs, A Long Story Getting Longer

OK, I have 2 502 MPIs with 500 EFI cams, larger valves in the heads and stainless marine exhaust. The MEFI 1s were reprogramed by Tyler Crockett. The boat also has stellings extension boxes set up in the highest position. Last Labor Day was the first time out with this combination and the boat hit 83 MPH at about 5300 RPMs with 29" mirage plus props. Lake Michigan was a little too rough for an extended speed run so the boat was still picking up speed at that point. That trip only lasted @ 15 minutes as one input shaft twisted off in the bravo one drive. After that trip out the transom was very sooty which I later found out was from running the stock fuel pressure regulators. Jump forward to summer 2010. I discovered the drive failure was probably caused by the stellings drive shafts being out of phase which was corrected. Adjustable fuel pressure regulators installed and set to 36 PSI (recommended by Tyler). And yes another change, 30" pitch bravo one props. The boat would only run 79 mph at @ 4600 RPMs. I connected my laptop with the Diacom software and found there was a lot of spark retard (9.9 degrees) when running 4600 RPMs at WOT. The total advance recorded was @ 18 - 19 degrees at 100 % throttle. As I had read in the past that the stellings drive shafts could set off the knock detectors on the Merc MPI engines I disconnected the lead from the knock sensor into the knock sensor module. The next trip out, the top speed was down to 73 MPH at @ 4000 RPMs. There was no spark retard during this run and the advance recorded was 27 -28 degrees at 100 % throttle. On the 4th data taking pass at 4000 RPM it sounded like an instant exhaust gasket leak from one cylinder. Upon inspection there was one plug wire with just the porcelin part of the spark plug attached hanging from the engine... OUCH. Not sure where the rest of the spark plug is... I don't see it in the bilge. I will have to pull the rear seat this week to get a look but it sounds like the engine will be coming out. So here are the questions: Could the fuel pressure be too low causing a lean condition causing detonation? Does it sound like the knock sensors were detecting actual detonation and not the drive shaft (since the engines had less power with more total advance)? What should the total advance be on this type of engine, I would expect 29 to 32 degrees total but I believe without any retard input from the knock sensor these are at 35 to 36 degrees at WOT( 8 initial plus 27 input from the computer)? Does anyone know if the advance recorded with the Diacom software is total advance or just that which is controlled by the computer (I suspect it is just the advance controlled by the computer because it would not know what the initial setting was)? I have timing set to 8 degrees per the procedure for this engine. I run 87 octane fuel as recommended by Tyler. Any ideas would be appreciated.

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Old 09-07-2010, 01:13 AM
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Sounds like a ECM program. Send a PM to Jason Smith.

I have Stellings Boxes and don't have any issues with the knock sensors, but I think mine are MEFI 3's.
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Old 09-08-2010, 09:30 PM
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Pulled out what was left of the number 3 cyl. spark plug... nothing left but the stainless barrel with the negative electrode covered with what looks like piston droplets... damn. I then called Tyler to see if could give me any insight into why the pcms were retarding spark 9.9 degrees with the engine set up to match the tuning he flashed. He suggested that the props were too big, not allowing the engine to reach the proper WOT RPM, thus causing the knock or detination sensor to retard the timing. I don't understand this logic, the engines were turing 4600 RPM which is only 100 RPM lower than these engines ran as stock 502 MPIs? Second he suggested that 36 PSI fuel pressure may have been too low, but the original pressue of 38 PSI left the transom of the boat covered with soot... His third possibility was that the original 502 MPI fuel pumps could not supply enough fuel for high RPM operation. This sounds like the most probable cause of the spark retard I observed before disconnecting the knock sensors. Tyler said that once my engines are repaired I should bring one over for a day of dyno tuning for @ $1100 to get the programing dialed in to my engines. So much for an off the shelf tune to match my components. Maybe new intakes, carbs and MSD ignitions would be more user friendly for someone without a dyno... Or maybe tuneable software and a pair of O2 sensors to dial in the tune myself on the water. I guess I have a long winter to decide.
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Old 09-08-2010, 09:40 PM
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Man that hurt just reading it.
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Old 09-08-2010, 10:51 PM
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Off the shelf tunes are no good. Dyno it.
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Old 09-09-2010, 12:11 AM
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Quote:
Originally Posted by Inspector View Post
So much for an off the shelf tune to match my components.

... Or maybe tuneable software and a pair of O2 sensors to dial in the tune myself on the water.
First, no such thing as an off the shelf tune. Every engine is different and takes a little different tune.....even if they are 100% identical. That is why I will not do any tuning through the mail. It can not be reliably done IMO. Even when I build 2 identical engines, they always take a little different tune. I will put the first tune in the second engine as a starting point, but then it will get worked from there.
It will still need to be checked in the boat. You can not perfectly simulate a boat on the dyno. I always check and fine tune in the boat after it has been dynoed. For the amount of changes you made, the engine needs to be put on the dyno and tuned.
You should install O2 bungs in the exhaust and check the engine in the boat even if you dyno it. You can not just adjust the fuel pressure without verifying the O2's.
You should also upgrade the fuel system in the boat if it hasn't been done. Most need new pickups in the tank, removal of anti-siphon valve if present, larger lines, larger pump and most importantly, a larger fuel filter head. The stock 502 Mag filter head with it's 1/4 npt ports will not flow enough fuel for that setup. You will also need to return the fuel to the tank, instead of to the filter head.
There is any number of things that could have caused you to smoke a piston......the tune, fuel pressure or delivery, aerated fuel being returned to the filter instead of the tank, etc...
First start with a good tune on the dyno, then take care of the fuel system in the boat. After that, check the O2's and do any needed fine tuning in the boat. Let me know if I can help.
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