I'm looking at buying a boat with raw water cooled 502 Mag MPI's that have 485 hrs. After doing a rolling compression test we found that most cylinders where in the 160 - 170 psi range except for #5 on both engines that are 110. Still had 110 after putting some oil in the cylinders so it seems to be a top end problem. Also most of the spark plugs on the starboard sides of both engines where pretty rusty and it looked like the manifolds might be leaking water down the outside of the head? I had heard some rumors that low rolling compression was a problem on 502's but wasn't sure if it was true and I was a little surprised to see this on engines with only 485 hrs.

So if I get the boat it appears that I will be rebuilding he top end on both engines. I'm thinking that while I have it apart I'll do some moderate upgrades as part of the rebuild. The boat is a Formula 400SS so I'm not looking to make them into fire breathing monsters because the drives on these boats are fragile already. Figuring I'd make a little bit of a power increase while I had the opportunity but I want to keep it reliable. This assumes that I don't pop the heads off and find water damage, cylinder scoring, etc that would require an entire rebuild. Here's what I'm thinking and some questions, let me know your opinions.

-Are the stock 502 heads oval port or rectangle? If oval are they the large oval or small style truck port? Assuming that the castings are OK I would freshen them up with a valve job, guides or liners, severe duty stainless intake and inconnel exhaust valves, springs, retainers, locks, roller rockers, maybe mill them a bit for a slight bump in compression................or..............new Dart Iron Eagle or Merlin iron heads

-Thinking about the possibility of adding closed cooling also but haven't yet researched what's involved. If I decided to do closed cooling and also did new heads is there any down side to using aluminum heads?

-I'd probably do a hydraulic roller upgrade while I'm in there and assume that I'd have to have the ECU's reflashed with a new calibration. Can the stock ECU's be reflashed or do you need to get new ECU's in order to make calibration changes? Any sugestions on who to work with to get cams and ECU recalibration?

-As far as intake manifold it seems that I could use the stock intake system but it's very restrictive? Ugrade to a 500 EFI set up? Or if I'm going that far is it better to get a the Holley set up that I've seen Young's Performance talking about with the single plane carb style intake and Dominator style throttle body? With the Holley set up will the stock wiring harness work or does it need to be cut up? Remembering that this is in a big sport cruiser and I'm not looking for every last HP is it even worth changing intakes or is it pretty much required to take advantage of the roller cam and any exhaust upgrades?

-Speaking of exhaust, any recomendations? The boat has Captains Call that I want to keep so I'd be looking at manifolds and risers that would work with that. Mainly just want to go to a dry joint at the manifold to riser seam but I'm not up to speed on what's the best exhaust option these days

-Last questions, anybody have an approx. idea of what a marina would charge to freshen up the top end back to stock specs? I'm not going to go that route but I need to get an idea so we can reduce the purchase price on the boat accordingly since this was discovered after we had negotiated our deal. I'm going to check with a couple places here locally and the seller is going to have his guy come up with a number, but figure it can't hurt to get as much info as possible

If you made it this far sorry for the long post and thanks in advance for any feedback and suggestions.

Pat

Was the compression test done with the engine warm? The 502 Mags have GM Rec port heads, they can be gasket matched, with a little port work they work well. Closed cooling is not really nessesary unless you are in saltwater. Call RMbuilder for your cam and valvetrain. Do a search and you will find a lot of info on his work. If you want to build big power get rid of the captains call and go with aftermarket exhaust with long risers. If you want it quiet get mufflers or switchable mufflers. Find a good engine builder/Machine shop in your area that is familiar with Marine builds. There are a lot of guys here on the forum that build Marine engines. Good Luck!!

yeah,,,,what he said

Nope, engines were already winterized and cold but it was his guy doing the testing so I wasn't going to argue, I was just there to observe. If I end up geting the boat I will do a full rolling compression test and leak down with engines warmed up before I rip anything apart. Because there is a potential problem, we've agreed to factor in a price reduction equal to what it would cost him to have his guy freshen the heads so I want to get my own idea of what a typical marina would charge so we're on the same page. Either way I don't like the signs of leaky manifolds and don't like the factory manifold to riser joint all all so that needs to be adressed at minimum. Beyond that I figured I'll investigate options and if nothing else it'll get my wife on board with the idea of some performance upgrades ................"hey we have to fix it anyway honey so I might as well make it more "efficient" while we're in there. We should really do this to get better fuel mileage !"

BTW, I just wanted to make the point that I'm not hoping to take advantage of the guy in any way but it is what it is. A couple holes come up low so it's got to come apart to know for sure what's going on. As a side note, he's taking mine in trade towards his and both of my engines came up with 190 in every hole on both engines and they are winterized and were cold when tested by his guy an hour later with the same gauge. Just wanted to make that point, carry on.

Thanks, Pat

i am not a fan of the 502 iron heads,they are known for dropped exhaist seats.if it is in the budget replace them with edelbrock marine heads,and a cam from hp500.this will put you at 550 hp area.

Well.....let's see.
The compression should have been equal across the board, cold or not. Doing it cold may not give you the most accurate number, but it still should be even. If you are seeing rust and signs of water leakage on the outside of the manifolds, then you can bet that it is leaking on the inside. It very well may have caused some rust in the cylinders that is now piled up in the ring lands. When the piston comes up and the rings scrape off the rust on the cylinder walls, the rust ends up going in the ring lands and causes the rings to stick. This will cause a low number during the comp. test. I would do a leak down to be sure of what is causing the low numbers. 60 psi down on a comp. test is a lot. I'll bet a leak down will show 40-50% minimun leakage.
As for a freshen up on the heads, I just did a quote for another OSO member. It included new Manley valves, Isky springs, Comp. 10* retainers and locks, ARP studs, and Comp. guide plates, and seals. Machine work included surfacing the heads, valve job, hone guides, lap valves, etc. and completely assemble them. The entire quote was right around $1500.
The stock rectangle heads are actually pretty good. They can make some decent power. Of course, aluminum heads will be much better for making a more powerful and safer engine. I would not waste my money with purchasing a 500 efi intake setup. I would go straight for the Holley setup. With the Holley intake and the stock steel heads (and some port work), you can make about 575+ hp. With aluminum heads and the Holley intake, you can easily make well over 600 hp. These are safe, pump gas engines at these hp levels. They are by no means on the edge and set on kill. They will do just fine in that big boat.
Your ecm and harness can be reused with the Holley setup. The harness will have to be modified to work, but it can be done. It is no problem to reprogram your existing ecm. I am more than capable of doing so.
A good set of manifolds will work fine.(Stainless Marine, IMCO, Dana, etc.). I agree with the above statement to get rid of the silent choice and use a muffler, swithable or full time. You can then use more camshaft then you can with the silent choice. That will allow you to make more power.
Feel free to give me a call if I can be of assistance. Good luck with the purchase.
Eddie

Eddie,
Thanks for the response and all the info. Looks like we came to an agreement today and should be closing on the deal within a couple weeks. I'll be giving you a call either Tuesday or Wednesday to discuss things further.

Thanks again,
Pat

Sounds good. I look forward to hearing from you.
Eddie

Roughly how much is the Holley throttle body with a decent intake