What are the differences and the pros & cons of a Mercruiser 500EFI and a Mercruiser 525? I assume that the latter makes 25 more horse power but is it a better engine? What about durability, maintenance, cost to operate etc.

In other words, all things be equal, how should it influence a decision in purchasing a used boat?

Thanks,

Padraig

525 Pros - Closer to 50+hp over 500efi, closed cooling system, aluminum cylinder heads.
525 Cons - OEM CMI headers are prone to premature failure.

Everything Lee said plus 500EFI is no longer a "current" engine whereas, the 525EFI is. The 500EFI is a good reliable engine but given the choice of the two, I would opt for the 525EFI.

More on 525 headers... I CANNOT STRESS THIS ENOUGH. If you are looking at a boat with 525's in it, you MUST examine the headers very carefully and if possible have them pressure tested for leaks. Also, do compression and leak down tests but these will not find leaking headers unless the damage has already been done. The OEM CMI headers come in two varieties. Those with leaks and those that will leak. There are some lucky guys who've had no problems but there's a LOT of others including myself, some who've experienced catastrophic engine loss as a result of water ingestion. Having said all that, I would absolutely buy another boat with 525's in it, but they require diligent maintenance and inspection of the headers at least once a season.

Thanks. So far it sounds like the 525 is better unless there are header problems. Hour wise, when will header problems show up? Or do you just figure on the expense of purchasing new CMI headers?

What about the internals? I "think" the 500 is based on a 502 block and the 525 on a 496 block. Are the pistons, crank, etc. Equal?

They're both a 502 block, both good forged internals

Both based on 502 block. As far as the header problem, there's no real predicting when they will fail. Some guys have gone 100's of hours before seeing a problem, others have seen it much sooner. The problem is more a function of the heating & cooling of the headers versus the number of hours, ie: if the motor has overheated at some point or the cooling system been deprived of water flow, the likelihood of developing leaks is higher. Unfortunately, no real way to predict. Some basic preventative maintenance helps a lot. For example, there was a service bulletin that calls for the removal of the bypass hoses that run from the distribution tubes to the tailpipes. That should definitely be done to allow more water flow in the jackets. If you're in salt water, flushing the headers / heat exchanger is a MUST after you run the boat. Checking the spark plugs every so often will let you know if you're getting water in the cylinders and pressure testing the headers at the beginning of the season might save you a lot of hassles. None of this should stop you from buying a 525 boat, just be more educated on what to look for. Believe me, I know firsthand..

Thank you everyone for the excellent information.

Padraig

The 525 intake manifold and throttle body have been improved also and the 525 has hybrid elec/mechanical fuel pumps to eliminate vapor lock.

The 500 EFI's are cheaper and eat less Bravo drives.

525's also have that oddball exhaust pattern on the heads.