yes they are interchangeable,

Two different engine applications, closed cooled vs non, coil on plug vs distributor......the 500 runs a t-stat housing where the 525 has a heat exchanger and the t-stat has a block off plate, but when you take everything off the lowers are the same.

They are interchangeable One has a brass crossover and one dont. I have a 525 lower for sale for $250 shipped

Outlaw - you inbox is full

Going to check out what my buddies mechanic has laying around first. Supposed to have a 500 EFI lower with a rail. Bought a couple 500 efi's that had freeze damage. One of the lowers, a t-stat housing and the two blocks were scrap. Not sure what happened to the other rail, but that is the hardest thing for me to find right now. He's out hunting, so may not know anything until later in the week. Regards,

Eric

huge differences between these 2. A bit of other info is The PCM 555 vs MEFI 3. For one there is power reduction mode in the 555 also the fuel delivery and ignition is by totally different. For one the 555 is sequential fire vs outdated batch fire. and the 555 uses a cam and crank sensor no distributor vs a distributor mefi 3 for a jist of info.

Both use 1988 fuel injector design. Way behind in that area.

sequential injection becomes batch fire at higher rpms,,only advantage ,if any is at low rpm . basically sequential was developed for emission standards.

For high rpms and race engines batch fire if the pulse widths are large then the starting point and ending point of each event gets closer and closer so basically you are correct about batch fire. But for 6500 rpms and down along with marine apps as it seems for the power boater and average boater most of the logged time on their engines is idle times. Batch fire does not provide better idle quality / and fueling vs seq fire. But there is more -- Old vs current thoughts for a jist

With batch fire the timing of the actual injection event on any given injector is NOT precisely aligned to each individual intake valve opening event. With that said there is no need to have precise camshaft to the crankshaft signal detection. Batch fire lacks fuel control of rapid / throttling compared to sequential firing. You are leaving power on the table with batch firing. Batch fire is the simplest form and easiest to build including software and internal components of the ecm and is the a lot easier to tune compared to sequential fire.

in shorts terms Seq. firing breaks down the timing of the fuel / injection to a cyc to cyc chain of events. The ecm provides a given order to fire the injectors per the firing order. Much better fuel control and events cyc to cyc.. This has a lot better fuel control for rich / lean conditions when set up correctly by experience tuners. Seq. fire in laymans terms can be set up for better cyc to cyc fuel and ignition control by far over batch fire.

I would call any top tuners and ask them what they recommend, Very few are using batch fire and a distributor period these days... OEM Marine fuel injection set ups were always at least 10 years behind the times compared to the auto side set ups.

Anyways 1100 / 1350 / 1550 / 1650 Merc racing engines has to meet ZERO emissions especially the race 1650 engine - what injection do you think they are using - it is not batch fire.