View Full Version : Need some advice about upgrading 502 mpi?

01-11-2012, 09:37 PM
I am trying to put together my 502 mpi, I recently picked up so Gill wet offshore exhaust but other than that it is pretty much stock with the exception of a K&N flame arrestor. The engine has recently been rebuilt and is in strong working order. That being said, I really have no reason, nor funds to be able to pull the motor this year, which takes upgrading the cam out of the question. So I am wondering if it is really worth doing anything else with a stock cam? Originally, I was planning on adding an adjustable fuel pressure regulator and have Mark Boos tune my ECM to see if I could possibly go up in prop size and maybe squeeze a few extra mph out of her.
Lately, I have been talking with some guys on OSO who are telling me I should pull and sell my EFI system and use that money to buy an RPM dual plane intake and a Holley 800 or 850 carb and upgrade the ignition. That all sounds simple enough, but I am wondering if any or all of that work is even worth it when running a stock cam and heads? I do plan to upgrade the cam, but probably not until next year and just want to bolt on some HP with the engine in the boat. I would love to buy a whipple, but unless I also want to pay for a divorse lawyer as well, it not happening this year.:bonk:
Anyways, any advice would be greatly appreciated. Thanks

Carl C
01-11-2012, 10:19 PM
There are folks here who know more than I do about this but I think the guy who told you to ditch the EFI is nuts. Pro-charger would be the cheapest and easiest way to make about 600 hp I think. IMO, if you want to stay n/a then you need to wait until you can do the whole engine right and get between 550 to 600 hp the expensive way. With a whipple the sky is the limit. You are looking at $5,000 - $20,000.

01-11-2012, 11:59 PM
I think the thought behind losing he EFI for a carb is easier and cheaper HP.

01-12-2012, 12:09 AM
5,000-20,000? If your going to go boosted I'd figure on the higher figure to do it right and have it dyno tuned. When I did my rebuild last winter on my stock 6.2 we pretty much changed everything but the block, crank and accessories and when all things considered it was in the ballpark of the high figure as stated, however that was a Polished EFI Whipple, alum heads, forged dished pistons, cam, rockers, 7 qt oil pan, added a oil cooler, along with adding a intercooler strainer and seperate thru hull fitting to supply it, your 502 if still healthy shouldn't need as much in the lower end but things add up quickly, very quickly!! If I had to do it all over again I'd do it just the same. The trial run after the re-power was priceless, when I let off the throttle and the GPS speedo was still climbing cuz the speed hadnt cought up with the speed of the boat was impressive to say the least :shocker:. Just be prepared to spend about double what you think your going to and you won't be dissappointed when you do. Steve

01-12-2012, 12:19 AM
might check OSO theres a guy selling a 525 intake and everything to convert your 502 to a 525 for 2,400! might be a good way to go, cant seem to copy and paste link but for 2,400 you could basically have a 525 including the ECM, cam etc. Steve

01-12-2012, 02:09 AM
Here you go. http://www.offshoreonly.com/classifieds/mercruiser_525_efi_system-o43535-en.html

01-12-2012, 02:17 AM
Thanks Bill thats the link maybe it will benefit someone here

The Hedgehog
01-12-2012, 09:23 AM
Question #1. How much horsepower do you want to make and do you think you will ever want the option to have more.

Until you answer that question, all else is simply entertaining discussion. I have taken a base 502 MAG all the way to 942hp. I have made a stop or two in between.

If you are staying EFI, I would not buy a THING without talking to your tuner (Marc). He is the guy that writes the program that makes it all come together. I will give you an example. If you by a 525 intake without doing the heads, you are wasting money. He will have a hell of a time tuning that combo. Buy and exhaust system without enough riser.....wasted dollars. Get too radical? You will need to take it to him for a dyno session and then boat tune it. Follow his recipe and you will be just fine.

Carl C
01-12-2012, 09:35 AM
There is a lot more to a true 525 than the intake system. Custom Edelbrock heads, cam, valve train, CNC block, forged crank and pistons, H-beam rods, distributorless ignition, closed cooling, CMI headers, billet pulleys, mech/elec hybrid fuel system and more I'm sure.

01-12-2012, 10:11 AM
Yep, add to what Carl said;
FI system- $2,500.00
Aluminum heads- $3000.00
Headers- $3,00.00-$6,000.00
Now, you're at $10,00 and it's not assembled or tuned.

I think there's nothing wrong w/Merc blue motors, but I also think there're ways to get the same output for less investment simply due to the availability of components and wide spread talent.

roadtrip se
01-12-2012, 10:28 AM
I have a 525 intake on top of my motor. It works great and really made a difference in the air flow, but as the guys have already said here, there is a bit more sitting under it than a 502MPI. No such thing as cheap horsepower.

One other angle to consider while we are dreaming, a 500EFI take-out. Probably can be had for less than $10K and put you in the low-80's. But again, budget horsepower doesn't really exist.

01-12-2012, 11:38 AM
Thanks for the advice guys. I guess I already new the answer to that question, but was hoping I might be wrong. After speaking more with John at Eastside Offshore I am going to leave it how it is and enjoy it for now. Because the engine has recently been rebuilt and is still in very strong working order, I think I would rather save my money for a Procharger or a used Whipple setup. I have a BX1 swap happening later this month so I might start looking around for a used conservative boost set up that I can have rebuilt after that. A buddy has an M-1SC Procharger I can buy for $2K and then have the ecm re-mapped by Eastside Offshore or Mark Boos. Would that be a better option? To be honest, this is a starter boat for me that I plan to enjoy for a few years, but I am really not interested in doing radical engine work. I would rather wait for the next boat thats a little newer.

The Hedgehog
01-12-2012, 11:51 AM
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Procharger works great. Once again, talk to the tuner first. It is not a big deal, Marc has done many of these. He can tell you what will and won't work. Basically if you remap the ECU, install a thermostatically controlled cooler and a few other goodies you are fine. Cut corners and BOOM. It is not brain surgery but you have to do it right. You won't pull it off for $2K though. You WILL also want a fuel pressure gauge and boost gauge. You WILL want to dial it in with a mechanical gauge.

My biggest mistake was not using a thermostatically controlled oil cooler. Everything else I did worked great.

01-12-2012, 12:11 PM
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Thanks. Another quick question, would the thermostatically controlled cooler be included in a package from Procharger? The unit I am looking at has the self-contained option, or is that something totally different? I just go off the phone with Procharger and they told me that the serial number of the one my buddy has will work great on my boat and that I would just need new mounting brackets. I was also told by Procharger that this unit likes a stock engine and that no other engine mods are needed? Is that correct?

01-12-2012, 04:02 PM
Read this..


Cheap, reliable power.

01-12-2012, 04:22 PM
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That was the post that I was reading when I fist got the itch. That is definatly an option I am considering.

The Hedgehog
01-13-2012, 10:18 AM
First of all, don't believe most of what you hear from Procharger. They probably know the bracket info but that is about it. Ask anyone that has done a bunch of installs and they will tell you the same. With my M-5 SC kit for the 502 MPI, they sent instructions for a carb engine and the boost reference disc for a 7lbs application (I was running a 5 lb pulley). If they had gotten the disc backwards and I had a 7 lbs pulley, instead of running rich and sputtering when the boost comes on, I would have had pistons in my oil pan. Their tech support could not even come close to identifying that problem. I just had them send me a boost reference kit and found it right away. This was after lots of sputtering and moving the fuel pressure around.

Another example, when you remove the thermostat per their instructions, the boat runs in cold start mode. That is like running around with the choke on in lower RPMs. You get a nice black transom and wash the cylinder walls. I had about a half gallon of gas in my oil after 10 hours. The engine was otherwise fine. That is RICH. They don't tell you this, but you HAVE to reprogram to fix this. It will work fine at full power though!

Their kit does NOT have a proper oil cooler. You will need to buy that. I recommend Hardin Marine. I can send you pictures of what will happen if you do not buy one.

I would just talk to Marc Boos. He will tell you what you need and don't. Eddie Young will do the same.

If you want a deal, I have a procharger M-3SC laying around. It is polished with a polished bracket. You will need some other parts but it is not brain surgery. The blower has about 25 hours since rebuild.