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Stock 502 Mods for efficiancy

Old 11-15-2008, 09:05 AM
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Good morning Speedreeder,
Thank you for your response. I wanted to share an example; One of my boating friends and I were talking last night about this. His boat is an 07 42 Fountain with Merc 700's and #6's, holds 220 gal of fuel. He can run 4 days (avg time on H2O/day ?) hard and still have fuel. Mine 31 Sunseeker Hawk twin 502 MPI's over B1's, 120 gal of fuel running moderate might make it a day....maybe. Granted Reggie has developed an efficient hull, but something needs attention. These engines had 270 hours of salt before I purchased the boat, only 5 hours in the last 5 years from the previous owner. They now have just under 500hrs the balance in fresh water.
I did commit to purchase 3 set of reworked 502 heads last night that have 10 hours on them (I'm not sure but I think Joey Griffin did the rework) and I will sell one set off.
With what you are talking about roller rockers are in the heads right?(showing my hand a little) Did I jump too quick at these heads for what you are proposing?
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Old 11-15-2008, 09:24 AM
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Default US1 Fountain

Hello my boating friend,
Efficiency is my real goal. I love gen6 502's (something nasty-n-gnarly about them for me, like an old shovelhead) but I prefer stone stock if possible for the reliability side. My hull is not very fast it's a Sportcruiser after all. When I feel the need for speed I ride with friends.
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Old 11-15-2008, 10:16 AM
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I failed to mention that I run with my through hulls closed a lot but try to keep them open above 3K RPM.
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Old 11-15-2008, 10:44 AM
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Moondance, I think it will be very hard for you to get any efficiency with the weight and design of your boat. I'm talking about any that you can feel. Do you have a flowscan or something equivalent to that (fuel flow). That would be a meter that you could look at and see SOME difference. Then I think you should try some 4 or 5 blade props for the way that you want to run your boat (not for top speed). Speed and fuel savings will not ever go hand in hand on a boat of that weight. I do think that you are past do on hours to change or rebuild your heads. You need the casting number from your heads to see if they are right for your setup. What kind of work (porting) has been done to the heads that you bought. I do feel you would get power and economy if you would port match the intakes to the heads with gaskets and also match them to some aluminum header manifolds. The guy on swap shop or ads that is selling the MARINEPOWER manifolds are very good and they are exact castings of IMCO thumper series. Good luck and ask anything else. Thanks
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Old 11-15-2008, 10:49 AM
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I think adding a decent set of the better breathing aftermarket exhaust manifolds is a good place to start without having to make computer mod adjustments---as I have always heard the 502mpi's run on the rich side from Merc anyway. Not sure if a computer mod would be required with adding 1.8 rocker arms though---although I think adding roller rocker arms would be a good upgrade----and the better K&N breather especially if it is true that you can get 400-600rpm increase without having to make cam mods and other internal stuff.
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Old 11-15-2008, 11:45 AM
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Years ago in Hot Boat Magazine, Tyler Crocket did a tech article on upgrading/mods of the Merc 502mpi engine. If I remember correctly he made these changes....

-surface cut the heads .020" (increasing the comp ratio)
-competition valve job (3-angle???)
-added a 2.25" intake valve (vs the stock 2.19" valve)
-upgraded to some CMI elbowed exhaust header system
-replaced the stock breather with a better breathing breather
-added a slightly larger cam profile
-replaced stock steel stamped rocker arms with ROLLER rocker arms (I think they were Crane Golds)
-remap of the computer
In his article I think he said these mods were good for about 100hp from the 502mpi.

I think I remember Crocket mentioning in the article that he had found that porting the heads to any extent did not really bennefit in boosting the power in the upgrading the 502mpi. I think all he did was clean up the bowls and remove any of the sharp edges on the short side radius.
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Old 11-15-2008, 12:13 PM
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Default Many Thanks

Thank you so much for your input! I, like you pointed out am looking for lowering my fuel consumption. My boat is possibly around 10.5 or 11k wet weight. It runs around 57 mph wfo which is fine with me. It has mph robbing bottom paint now but I am changing to white Interlux VC teflon paint this winter (mainly for the look instead of blue bottom paint). I will look at the mods that you have suggested. I assumed that I should look at prop changes but wanted to correct any mechanical issues first. I am lucky to have access to various sizes of props after I solidify the mechanical.
I deduct from your input that I am past due on a refresh which the heads may be a large part of fuel consumption then props the other part. I had a good mechanic evaluate these engines at the beginning of this season. All but one cylinder was within a pound of each other. His conclusion was time to freshen them both but he doesn't want to do the job. He wants me to repower with 496 HO's. Not going to happen, I love 502's. If I was planning to repower, it will be back to Diesels as this boat was built.
I may have three set of heads to sell.
Last, Where is your shop located?
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Old 11-15-2008, 12:19 PM
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Default Any input Tyler

I realize that stockers is not "your Bag" though.
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Old 11-15-2008, 12:33 PM
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Default Kaama

Thank you for your input. I may see if I can pull that piece from the internet. But I don't want more horses just less juice. Of all things to say; I want them to suck less not sux.

But after any mods this winter and no improvement, I will still LOVE my boat, shut the F... up and just feed it fuel.

Kaama by your name am I to assume that you also own a Kaama/Formula? One of my boating buddies has one.

Thanks everyone for your input it helps me a lot.
Moondance
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Old 11-15-2008, 02:43 PM
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Originally Posted by Moondance
Kaama by your name am I to assume that you also own a Kaama/Formula?
Thanks everyone for your input it helps me a lot.
Moondance
No, but I once owned a KAAMA SE 38' Wellcraft w/TRS drives. I now own a 32' Active Thunder w/Bravo One's.

If you're trying to get better performance and efficiency with the gas the engine is already using, then I would install the better breathing exhaust, better breathing air breather, install roller designed rocker arms, and if you take the engines apart I would also clean up the rough edges and short side radius within the bowl/pockets on the heads. A little clean up porting and good breathing exhaust system is usually refered to as "free horsepower"----but then, we all know nothing is really for free---especially these days and in the marine world! Just have fun while you doing it.
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