502 compression specs
 

I am looking at a 2000 Donzi 26 ZX with a 502. I had a compression test done and #1 and #2 came in at 120 psi, #3 100, #4 & #8 at 110, #6 at 115, and #5&7 at 90. i cannot seem to locate numbers on this motor. I would think those numbers should be closer to 140-150.

I realize camshaft and other factors are in play. It should be noted someone in the past has added a Gaffrig twin blower setup. I have not seen the boat in person, and am not familiar with this company. I am wondering if the engine has been redone internally to accommodate the blower system. It has 350 hours on the motor and is very clean otherwise. Any thoughts?

It's over 20% difference from highest to lowest. Regardless of spec that shows a problem

Its very likely that the engine had lower compression pistons put in it, and possibly a different cam. Both of those things, will reflect low cranking psi numbers. A stock 502 mag, would be much higher. Either way, those numbers are all over the place, and the engine is probably tired. A supercharged engine with 350 hours, yea, its probably overdue for a rebuild, and probably why he selling the boat.

Whats a gaffrig twin blower setup? Never heard of it.

plan on rebuilding it,so offer accordingly.

Mercruiser specs for all engine states all compression should be an min of 100 psi.

here is Mercruiser compression testing and leak down specs. You have to open the throttle blade(s) wide open when doing compression tests as well.

anyways a good read for all about Mercruiser engines. Volvo has a good worksheet on compression as well. It is the same that the auto world is speced at basically.

Read this link all of it.

http://www.marinemechanic.com/merc/d...iontesting.pdf

I must have had a set on last year ... Every time I ran hard the gaffrig boost and pyro would start moving and there were two of them....:lolhit::party-smiley-004:

could be talking about his flame arrestors.:D

I have never heard go Gaffrig either. I see they make gauges and other accessories, but could not find blowers on their website. It looks to be nicely done, and I would assume the pistons were changed to compensate for them, and possible the cam? I just haven't been around supercharged engines.

The overall condition of the boat is very nice. No bottom paint, stored on a lift, freshwater only, stored in a heated facility during the winter as it is located in Michigan. It is a beautiful boat.

I agree that the percentage of variation on the pressures is what disturbs me the most.

can you post a picture of the top of your motor so someone could see what exactly you have

I don't seem to have the smarts to post a picture. Any tips? However, it is a dual system, one on each side of he block, belt driven. Hangs where the alternator and A/C would on an automotive application.