What HP Upgrades can be done on a mid 90's 502 MPI Engine?

Depending on what exhaust manifolds it has, a set of performance minded manifolds would be the best place to start. After that cylinder heads. With those done get the MEFI computer burned with a new tune on it to maximize the upgrades

If you're ok with not having fuel injection, bolting on a Edelbrock air gap, ( or comparable dual plane intake), and 850 Holley will usual get you at least 2 - 4 mph on almost any 502mpi application. Doesn't seem like much ,but most other bolt ons net zero gains (pro charger excluded)...

Literally anything can be done to a 502. Where do you draw the line?

Most everything you need to know in these two threads. Using this info I took my old 32' Fountain from 75 to 92mph and still prop testing when I sold it. I'd do it again in a heartbeat. Cheap and easy.

502 MPI upgrade results - Offshoreonly.com

modified 502mpi intake on 502 dyno session at my shop - Offshoreonly.com

You can keep the existing distributor and run it without the computer - works really well.

What is the best way to go about the wiring when going from an MPI to a carb. Im doing that as we speak as my plan was to switch over to Holley HP but its on backorder since february so to get on the water for this summer I am tossing a carb on the holley 4150 flange injection intake that i was putting injector and a TB on. I planned to keep the stock pin connector and just use the ignition power and gauges pins then eliminate the rest from the harness.
setup:
New GM performance 502 block bored .030 (509)
Stock crank & rods with new wrist pins & .030 SRP pistons
Stock iron heads all freshened up with new Inconel valves, valve springs, roller rockers
ARP head studs, main studs and rob bolts

Isky cam:

• Valve Lift Int/Ext. 0.553/0.578
• Valve Lash Hot Int/Ext. 0.000
• ADV Duration Int/Ext. 275/284
• 0.050 Duration Int/Ext. 228/238
• Lobe Center 112

Eddie marine thunder mainfolds and dry tail risers

I removed the injection harness and redid a little of the Merc harness to power the distributor and coil and extend wiring for the temp senders and warning horn. It was pretty easy to do. I had all of the correct color coded wiring so it came out looking factory.

The two-prong connector at the distributor is power (red wire) and coil negative (brown wire) and is all you need to get it running. Your start and charge harness should be able to accommodate this.




Pinouts for the EST module. You only need the coil connector (if you do not want to remove from your injection harness, the pigtail can be had on Amazon for less than $20).




This is the factory coil connection for reference. The boat tach receives the signal from the ecm so you’ll need do a little surgery on that connector to get your tach signal connected properly. I cannot quite remember what I did on this side when I converted from the MPI. It’s not hard though.

I just did this swap last week on our Bullet using a coil similar to yours (from a TB-IV/MSD). I tied power from the harness (purple) to the positive on the coil. Red wire from the distributor to positive on the coil. Brown wire to the negative on the coil, and then the gray wire from the tach to the negative on the coil.

When it’s done, total timing is all you’ll need to worry about. 32 degrees with iron GM heads should be good. Check and adjust at 3k rpm. If your balancer isn’t marked, you’ll need a dial back light. Your MSD box may not work with some lights (honestly, I’d drop the MSD box - the EST is more than capable without it - just add a rev limiter).

Hope this helps. If you have any other questions, just ask.

**Images belong to MicroSquirt: http://www.useasydocs.com/details/GM_7pinHEI.htm

I forgot to mention I also purchased the following:

MSD 6M-3L marine ignition box
MSD Marine pro-billet distributer
MSD ignition coil
https://www.cpperformance.com/instru...structions.pdf

All I should need is batt power, key power & ground

Not sure if I’d trust the MSD stuff. And the Delco stuff can be fixed with parts from the local parts store pretty easily (order a D1965A module from Sierra to keep as a spare).

I was a fan of MSD it not so much anymore. Too many problems with corrosion and faulty electronics.

Hope the above is useful for those reading.