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.

Most of the guys I talk to say leakdown is just as important as compression and when they did your compression check did they oil anything or go back with compressed air to see where the leak was? Just curious, I am no expert but I have a lot of toys I have been dealing with for years.

They did not do a leak down test. I am pretty sure the mechanic did not add any oil to the cylinders.I talked to the broker, and it turns out it has been a year since it has been run. They are going to put it in the water tomorrow and run it, then recheck the numbers. I agree, a leak down would be the next step.

Gaffrig or Livorsi made a supercharger kit for a couple years that used a Vortech charger. I don't recall it being marketed as a "twin" setup. I think it was called the extreme supercharger and emphasized the better intercooler, or something like that. It was a lot of brain cells ago!

This boat in SE Michigan?

It is a twin setup, and the current owner refers to them as "cool chargers". This setup is starting to look like a bastard child.

Detroit area.

I highly doubt it's a twin supercharger setup. They were good kits and I'm thinking the owner isn't sure what the kit really is.

I have photos, I wish I could figure out how to post them. It is a twin setup, two air pipes, one from each blower. Absolutely positive. I have never seen anything like it.

Pro Charger?

its a livorsi twin centrifugal style superchargers..they were made by powerdyne..when powerdyne tanked so did the twin supercharger kit that livorsi had..cant get parts for them anymore..

i have seen similar numbers on every 502 i work on..they need to get hot to get good numbers and the butterfly wide open..

Livorsi also had a kit that they used a Procharger. This was back b4 Procharger ever had a marine side.

Interesting. You can post the pics on photobucket and link them here.

X2...especially if the boat has been sitting.

Thank you for the responses, gives me a little more to go on.