01hammer

how wild can you build a 454 mpi before having to go to carb? i know you have to go with different internals but just wondering is it woth messing with the mpi system or go carb?

Noise Pollution

I made my 502 into a 540 and keep the MPI for one year. A lot a work to make it work. Big question how much time and money do you want to spend? IN the end not worth it. I went to carb this year. much easier to tune.

ZXXX Donzi

Mag MPI or just MPI

Pismo10

If it's a mag it has good internals already. If a 7.4L then you need internals. Stroke to a 496. The MPI is always usable just need to remap, carbs are easier tho....esp when you have a problem and you will.

stevesxm

yes. a carb will get you closer , sooner. but having said that, there is no single instance where an equivilent injection as far as flow #s go will fail to produce MUCH better performance , reliability and all around engine life then a carb will. a carb will always be wrong in some part of the fuel curve. a properly set up injection will be absolutely perfect in EVERY part of the fuel curve. those individuals that put forth arguments for carbs that suggest easy tuning and reliability as reasons to go that route are simply unwilling to do the injection route PROPERLY the first time.

if an injection /engine management system is mapped CORRECTLY the first time and the installation is done using correct and proper techiques as required for electronics operating in a hostile environment, then that will be the end of it. it will work perfectly and you won't fill your oil up with fuel or belch black smoke or wonder why in hell the thing runs great one minute and poorly the next as will happen with carbs more often than not.

but it has to be done correctly. no twisted and taped wires. no hot wired sensors. no ecu thats been "reprogrammed" by some hot rod shop that thinks more fuel is always better.

in every racing series on earth where the option exists to use EITHER carb or injection, no one uses a carb.

if you are smart, you will go the injection route and never look back.

01hammer

i've built two lt1 camaro and vette engines bored and srtoked to 396cu. ran the fuel inj with nitrous etc.. i know you could buy aftermarket stuff like intakes bigger throttle bodys etc etc for these cars put the car on the dyno to have them tuned. Around here were i'm at theres no performance boat places and i'm new to the fuel inj. on boats can you buy all the aftermarket stuff for this 454 mpi system too? not mag.

t500hps

aftermarket stuff for boats is not available in the variety it is for cars. You can build whatever you want, but count on building some combination that is "custom" and has to be tuned "custom". Like Noise Pollution I turned a pair of 502 (500EFI's) into 540's. Added compression, flowed heads, upgraded throttle body. As Stevesxm mentioned, I shipped a completely dressed motor to a mercury shop to have it dyno/tuned....even after that I had to fly he guy in to adjust the tune in the boat while on the water....pretty expensive!!!!! but it does run well now. (after all the money spent I still think I maybe I should have switched it to carbs???)

You have the 454mpi and not the forged "MAG" version of that motor. Trying to add any significant amount of HP in modifying the top-end of that motor will only ruin the bottom end. You can get it up over 400hp (you have 310hp now) but count on a re-build in the near future (bottom end)

handfulz28

There's 3 different forms of fuel injection on Mercruisers. The "base" MPI system where the throttle body is on the starboard side around the middle of the intake. Then there's the MAG system where the throttle body faces starboard at the front of the intake, and then there's the "HP EFI" which is Arizona Speed & Marine's Ram Jet type setup.

If you have the base MPI, the real answer I bet is nobody really knows what that system can be modded to since nobody has tried it. The bottom end will support 5000rpm without problems. The issue is airflow through the intake and heads. Head work or new heads would make a big improvement. The carb'd versions of these motors respond well to cam/head work/intake changes to the tune of 400hp or so on stock bottom ends.

The other two intake styles support a higher flow and are bolted on top of rectangle port heads. I know a 454 with the HP EFI/ASM intake has made upwards of 485hp and still runs as mild as a stock motor.

Call these guys: http://pmefi.com/
They're probably one of the few places that might have looked into mods for the base MPI engines.

stevesxm

my boat has the mag mpi system. it is dirt simple and very very effective. if you aren't supercharging, it would seem to me a very straightforward deal to map one of these on a dyno to suit what you have modified if your mods are likely to require it...( and often the injection is smart enough to NOT require it...) and get it so close to spot on that it would be hard to imagine. its not rocket science. you watch the egts and don't just do quick pulls. if the motor isn't something really bizzare all you are really talking about is the ignition curve vs the injector pulse width relative to the temp sensor look up table. i did dozens of the user configureable ones on automotive race motors on the dyno and its not hard. the marine requirements aren't going to be any different especially if the motor is even remotely ordinary in its configuration.

Noise Pollution

The 454 and 502 mag mpi intakes and throttle body doesn't flow enough air if you trying to make 630 hp +. So you are restricted some what.
Carb is so simple. While fuel injection might be the best there is, a good carb man and a dyno can get it just about perfect. Seen it in person.