morrowcarl72

i currently have a 502mag that im going to rebuild (was going to buy a zz502 but that fell through)....

long story short what can i do with the efi to make it compensate for more cam

basically how can i pull more hp and torque out of a 502mag?

options for more power :
1.) bigger cam
2.) stroker crank

im on a budget so no crazy ideas but thoughts would be helpful

baywatch

You can"t do much without updating the computers. I just had mine totally rebuilt and went up one size on the cam which should take me from 415hp to 450ish a piece. Much more than than that you have to start re-calibrating stuff. I'll see If I can find my Cam cards to see what they were.

Boat1

The 502 MPI is tough to drastically improve. The cam is about as large as you can go with stock exhaust. The intake and throttle body are restrictive to additional air flow. To upgrade you need performance exhaust, bigger camshaft, tie-bar lifters, pushrods, roller rockers, ECM programming, bigger oil cooler, as a start, expect 500-510 HP. Second choice is remove all the upper end and install a single plane manifold and carburetor, then upgrade the rest, may get to 550 HP. Good luck.

Kidnova

Would help to know your budget. Not being nosey....

Before I bored/stroked my ZZ to a 557 I was planning on staying 502, and adding AFR 305's better intake and cam, more carb. I pow-wowed with rmbuilder aka Bob Madara, and cfm. I figured with parts and labor, it would cost in the $5,000.00 range.... oh, add an exhaust to that 5K. Figured about 600-625ish hp would have been attainable, while still staying dependable.

Like Boat1 mentioned, the mpi 502 intake is restrictive. I know that only because a friend (rattlesnake jake) ran into a similar problem on his 509 build.

If you choose to go stroker, I can tell you 1st hand, the cost will increase further, and further yet (and again 1st hand....I went with Callies Racemaster) with a good quality rotating assembly.

HP is not cheap... but damn it's fun (o:

morrowcarl72

see the ****ty thing is that i know this but i still dream there is a way around it....


ps side note... found a 502 with freshwater cooling complete efi-pan with exhaust that came out running for a grand

jeffswav

Like was said, what is your budget? Also what is usable or rebuildable on your current engine?

morrowcarl72

budget isnt too crazy i have to rebuild the motor and was thinking about tossing a few bucks into it during the rebuild process

all of my enternals look to be good on my motor and i came across a running throttle body efi 502 im going to be buying

plan is as follows....

1.)have block looked at and machined as necessary
2.)reuse crank and rods with necessary polishing, balancing and or turning
3.) potentially if i have no choice go with stock cam and roller hydraulic lifters
4.) have heads gone through and fixed as necessary ie guides seats new springs and so on
5.) slap efi set up back on and run like new


i would like to go with a bigger cam if i could with efi set up to gain a little bit

if possible with factory efi i would like to possibly go with a 4.25 stroke crank


if i cant make any changes or hp gains with efi set up it will go back to stock until i can afford to make the expensive changes and or build a supercharged motor down the line.... any ideas any one know anything about the throttle body efi setups... i know as for gm factory tbi set ups were garbage but im not sure what this set up even is


either way this motor goes together im going to have alot of left over parts and did i mention the motor im buying has a close loop cooling :-)

Gimme Fuel

I built a 540 out of my 97 502 MAG MPI........There are a lot of things in that motor to be upgraded if you are going to stroke it. The factory intake and throttle body are the main bottlenecks and it is expensive to modify those. Better off going with a whole new intake and EFI system. The fuel system is very limiting on these motors too. Either rebuild it to stock or be prepared to spend some coin. Once you start, there is no going back and no half azzing anything. You will be amazed at how many little things you need will start to add up.

What is your budget? That would help for reccomendations.

Kidnova

There ya go, what Gimme Fuel said.

Regardless of which path you follow, I would (and did) get a qualified marine engine machinist/mechanic who you can trust to do the build. Poke around here on OSO and on other hi-perf boat sites and you find way too many horror stories from simple rebuilds to wicked mods that went south, and caused guys a ton of grief, and allot of $$. If it were me I'd do a good solid rebuild for now then save up for a hp build down the road.

morrowcarl72

so....factory cam is 227/211 at .050 and 342 lift on both

there is no way i can go one small step up? maybe a bit bigger on exhaust size or possible use the stock cam and go from 1.7ratio rockers to some nice billet 1.8:1's

just get a few ponies out without touching the ecu