I just had my boat at the shop and had them pull the spark plugs. Unfortuntately we had oil on a few of them. The shop then checked the compression and did a leakdown and I am low across the board in almost all of them! I had noticed a RPM drop of 300 and a top speed drop of 4 mph this year and now I know why....

Here is my question: The engine is a 1994 Mercuiser 454 Magnum EFI (385 HP) with 250 hours on it. Im being told that I have leaking valve seals and the heads need to be replaced. Im thinking if I need to do this I should go with some better heads and was wondering what everyone would recommend? Im hoping to get up around 475 hp--is this realistic? (PS--Cost is a consideration. I dont want to get so out of line that I should just do a long block).
What would everyone recommend?

Why would you replace the heads because of leaking valve seals. Even if you're talking valve seats a valve job is much cheaper than a set of decent heads.

I was told my heads would need to be replaced or rebuilt to fix. Would leaking valve seals cause the low compression?

Leaking valve seals shouldn't have anything to do with compression. The valve seats will cause compression to be lost if they are in bad shape. But a valve job and new seals should only be around 200 bucks at most. Throw some new springs and seals in and maybe you'll have 500 bucks at most.

The only reason to replace the heads would be if they are cracked or corroded badly. Sounds like the shop is trying to get you to spend more money than needed.... IMO.

if your description is accurate , nothing about their reccomendations makes sense. also, nothing about your wish to get another 100 hp on a budget makes any sense either.

you really need to do the tests again and get both some compression and leak down numbers and while you are doing the leakdown, check and see where the air is coming out , if the numbers are bad. that tells you what you have to fix.

rebuilding those heads is a piece of cake... IF you actually can convince yourself that they are the problem... which based on your description so far is a long shot i think.

You need them to be more specific as to the reason for the compression loss. And as stated valve SEALS have no affect on actual compression figures.

Just like a medical issue, sounds like you need a second opinion to me.

+1

I 2nd the second opinion!

Just got an email from my mechanic and this is what he said:

"Figured you might want these numbers for your compression, cylinder #, followed by compression psi
1: 150, 2: 130, 3: 60, 4: 70, 5: 75, 6: 75, 7: 75, 8: 145.

I normally see compression numbers in the range of 150-160 for your engine. The oil on the plugs is cause by the seals leaking and compression loss is caused by the valve seats."



So, it sounds like I can just do a valve job and fix my problem.

I did find these heads for sale locally. Not sure if Im going to go this route but was told these are 465 hp heads. Does this sound right?

big block chev heads, 454/502/540, square port, casting number 14097088, all new stainless steel valves, 2.25" intakes, 1.88" exhausts, new valve springs and retainers.

They are for sale for $1000.

well... if those number are correct then those valve are bent or rusted open and i would want to know why THAT happened before i went any furthur.

but either way, assuming the numbers are correct, if it were mine, i would have the heads off and make a determination from there. and i would leak the bottom end while the heads were off.

i still don't see any reason to buy new heads yet. it is entirely possible that you rusted those valves open and bent them on start up one day or something like that and for the sake of a set of valves and a valve job you could be out of this...

as for the oil, we have this discussion all the time. i for one haven't seen meaningful cylinder oiling resulting from a seal issue unless the guides are really scrap. you say this motor is a 96 w/ 250 hours . thats 20 hours a year . i wonder if this thing has sat around so often and so long that it has rusted some rings up or something.