Moondance

Thanks that's exactly the information I was looking for. I assume the SUCK/BANG/BLOW theory would apply. "SUCK" in more air, "BANG" fire the gas right, and "BLOW" out more air.
Next question; Who's intake and who's exaust would work best for me?
Thanks Kaama

KAAMA

iTrader: (1)

Moondance---

Well usually, when you suck a little more air it requires a little more fuel so you don't burn the engines down. But as I have said, Merc usually runs their engines more on the rich side vs lean. So, those mods MAY not require adding more fuel....or a fuel map change, but perhaps you can have it checked after the mods are completed just to make sure---you dont need to be burning the engines down.

As far as an exhaust system goes I would consider Stainless Marine, but someone else (Speedreeder) mentioned Marinepower manifolds that are supposedly the same exact casting as the IMCO "Thumper Series", but I do not know how accurate that is so I would continue to dig a little deeper. If speedreeder has done several of these mods then perhaps then he would know much more than I would. You can call him and ask LOTS of questions about his claims and experience with the 502mpi and the mods he has done to them. Also, I my friend "Articfriends" is a good guy to ask---he knows quite a bit about the 502mpi and he has a good reputation here on OSO and will tell you straight!

You mentioned the word "intake"....I do not believe you have mentioned that before. I would just leave the intake alone. Just put the better breathing aftermarket breather on it.

Moondance

So you think the 500 intake and plenum are wasted $, for little improvement. I will look for the best breather to satisfy that part, say the K&N units I believe they bolt right on.
You mentioned port matching the heads to the engine, I did commit to purchase three set of (I believe they are Joey Griffin) heads. If they don't match I can sell them for what I got in them. And I'll purchase the best exaust units I can find for my application. Then I'll re-prop next spring.
As I mentioned before these are salties and they just need some TLC.
Thanks for your input to my forum it help me stay on the right track instead of chasing ideas.

KAAMA

iTrader: (1)

I did NOT mention anything about port matching the heads to the intake---although that would be a good thing to do. What I DID say was to CLEAN UP THE BOWLS within the heads and remove any sharp edges and casting marks/flaws especially on the short side radius----nothing major at all.

Also, you are asking the wrong guy about the 500 intakes. I do not know what if any gains you will make with them. Some things I know well, but if I don't know I am not going to pretend that I do. That's why I said that you might try calling "speedreeder" as he claims to know much about the 502mpi engine and mods---BUT I would ASK LOTS OF QUESTIONS about his claims and do your diligence and homework before you make any decisions.

Perhaps others who really know the 502mpi well and can chime in on the type of mods you are seeking and interested in. Speedreeder has already made some interesting suggestions, but I do not know him well. I hope I have helped you to some degree so far.

KAAMA

iTrader: (1)

Also, I don't know where you're located, but if you're looking for a good shop with an awesome reputation to rebuild your engines without any "hype" you may want to consider WESCO Racing Engines in Jenison, Michigan. Dave Wesseldyke is the owner and 98% of the engines he builds are marine hi-perf offshore engines. His number is 616-662-2250.

Moondance

I have gotten a lot out of your input.
I am located in S.C. just an hour out of Atlanta. We have a few shops close but all the engine builders that I have been exposed to are all "Hot Power" guys. Scotty Cannon is a friend and a local, but he's doing 700's to 2200's and up with big $. Jerry Grimes is just down the road in Alt. but same...big horsepower. Joey Griffin in Washington, N.C. does mainly race boats for Fountain, and poker runners in the off season, he may entertain my stockers if he has a hole in his schedule. I have been to Joey's shop several times with friends and their Hot Boats and he absolutely positively knows what's going on.
Thank you for you "engine Guy" reference. I have drugg boats up there before for the "Silver Cup Series", so it's not out of the question.

KAAMA

iTrader: (1)

Well, if you have someone who is local that has an excellent reputation with marine hi-perf offshore engines then I would use them.

My builder told me he treats each engine with the same respect as his high dollar stuff. It does not matter whether it is a bone stocker or an all out high pressure thumper race engine. There is a given number of point checks he does on each engine that he believes is paramount in how an engine survives.

Anyway, whatever you decide to do I hope it all goes well for you.

SB

iTrader: (1)

If your talking vehicle fuel efficiency vs BSFC (amt of fuel per horsepower) then:

If the engines are in perfect working order including tune, then your fuel mileage is almost solely dependant on the boat, it's set up, and it's driver.

If your boat takes 500hp to cruise at 50mph and your friends boat takes 300hp to cruise at 50mph, well, he's using a ton less fuel then you are at that speed.

HP=fuel !

Weight and hull design are 'way numero uno' in regards to fuel mileage. Propping/drive placement follows.

BTW: Idling = 0 mpg.

Moondance

Thanks SB,
I think that through this blog I have determined that my engines are due a top end job. That may be the only real issue that I have other than some reversion on my starboard engine which I think replacing exhaust manifolds will correct that.