cableguy1979

Im in a position to rebuild my 454 mag mpi but as I understand it if I go to big...water reversion may be a issue. Has anyone here upgraded a 454 mag mpi successfully with stock exhuast and if so what was the upgrades. I really like my silent choice exhuast and want to keep it. So im only looking for successful upgrades or things people have done without success.

Also if I up the compression will the computer need retuned? If I try and match the compression how close does it need to be? I will be adding a cam and retuneing anyways but I don't want to add a compression problem to this retune if it's that big a deal. As I understand it the engine stock is 385hp 118cc heads and 8.6:1 compression stock with domes. Any help understanding this MPI engine is greatly appreciated.

Trash

Yes there is a limit to cam choice when utilizing stock manifolds. I'm not going to say there is a hard cutoff for duration and overlap as every combination is a little different. With a big block and stock exhaust your reversion limits are roughly, and I mean roughly limited to 238-240 duration at .050" assuming a 112-114 LSA. Please have others chime in for sage advice on this.


You are overthinking tuning regarding compression and adding a cam. They all work together. The tuning PROCESS isn't any different whether you have Cam "A" with 8.5:1 or Cam "A" with 9.8:1 compression. Your tune may be slightly different between the two, but not the process.


Cam, heads and cubic inches will yield the best results. They all work together. Don't get heads so big they outflow the limits of the MPI system. Don't forget about injector sizing and headroom.


Keep it simple. Get a good set of heads, perhaps a stroker kit, ample injector sizing and a moderate cam and start tuning. The tuning process is not difficult and very few tables need to be altered. Primarily the fuel and spark tables. Don't mess with the majority of other tables, scalars, constants etc.


My last word of advice. Don't touch the stock tune UNTIL you get the motor started under no load, preferably on a test stand. This will offer several advantages. 1) You can leak check the motor and verify basic rebuild integrity. 2)If the motor doesn't run you can eliminate the tune issue because you haven't altered it yet (yes it will run on the stock tune and run safely at idle on a stand). 3)You can set and verify base timing easily.

Once that is done THEN get into the fuel tables and adjust for appropriate AFRs with the motor in the boat under various operating conditions. Wideband is a must.

TomZ

A good read to start... this is member Articfriends thread on his 502 refresh, but a ton of good info:

modified 502mpi intake on 502 dyno session at my shop

And member Haxbyspeed and his upgrade for a customer’s 502 MPI:

502 MPI upgrade results

Exhaust is probably the single-most important upgrade to do before considering anything else. Without doing this your upgrade result will be extremely limited and probably not worth trying. If you’re not able to upgrade the exhaust and the engine runs well, the best bet it to leave it alone. The 454 Mag will go 1000+ hours so long as it’s treated well and proper maintenance is performed. The cylinder heads may need a refresh, and it’s important to keep after the injectors, but the bottom end will stay together for a long time.

TomZ

Responding by phone so I didn’t put two and two together regarding issues you had last year with your 454 and the need to do a rebuild.

I still stand behind the exhaust being a most important first for upgrading. The factory exhaust is that poor. if you can’t change the exhaust, give it a stock refresh.

mggdoors

What’s your budget

cableguy1979

I will not be changing the exhuast. I'd rather put the engine back to stock for budget reasons but since I will be replacing the cam and lifters I do plan on getting a few ponies out of it but since i am buying all new internals I'd like proven results from someone that has successfully done this with stock exhuast. It seems to me that a 502 mag has stock exhaust so it should be able to yield 425hp with out reversion. Exhaust is to expensive. I will be using the stock heads unless I come across some better ones on the cheap which is highly unlikely. Since im building it maybe it's best to stroke it but with the stroke and cam im afraid to find a cam builder that has a tune for a specific 496 stroker on stock heads and exhaust. Im trying to use as much stock components as possible. Ideally I'd like to have the performance of the 502 mag mpi maybe a little more.

TomZ

My opinion...

Rebuild it to stock specs and enjoy it for what it is. The expense of replacing the cam and associated parts and reprogramming the ECM will not be worth the small gains. You’ll be lucky to get a mile an hour or two out of it.

mggdoors

The likelihood of someone having a tune for whatever cam you put in there along with whatever else you do is highly unlikely. I would either rebuild it to stock specs instead of stroke it, and use that money for exhaust, which can be had for a grand used all day. Do a mild cam, tune it and enjoy it. You are going to be limited as your intake is a bottleneck. Be moderate and cam it for the limitations that exist. Even if it doesn’t gain mega mph it will accelerate a hell of a lot better, which I tend to prefer a quick boat. Just funner to drive.

Griff

The 502 mag uses the exact same cam as the 454 mag and gets its extra 30hp from the cubic inches and that extra 30hp is worth about 2 mph on the top end speed. It is a complete waste of time to upgrade the cam without upgrading the exhaust. You might gain 10hp which is worth about a 0.5-1 mph on top speed.

cableguy1979

This is helpful. I have to replace my cam due to the old one is too worn to reuse. Thinking Tyler Crockett has a cam and ecu program. I am not looking for top end but rather the pull and better sound. Sounds like stroking it and replacing the cam with the Tyler Crockett cam that is barely over stock should yield me the same power as a 502 and maybe a better lope.