if you took a box stock late 502 mag mpi and put a 540 kit in it and changed nothing else at all, would the injection as calibrated for the 502 be dangerously lean ? that is to say... i'm sure a custom flash might buy some power with the new combination but i am specifically curious if you know how the stock injection would measure up.

please no carb suggestions.

Years ago Lingenfelter had a 575HP/540 kit for these engines that did utilize the stock Merc intake. I believe the cam was also changed though.

i understand that there are a million 540 variants . i have a client who has the same motors i do. box stock 502 mags. he can buy rotating assemblies for about 1800. so... if all he did was plug in the rotating assys, could he run the box stock injection safely.

no guesswork , please. i never ran a mefi 3 on the dyno so i don't know what these fuel curves look like or how safe they are in stock trim. intuition says that its probably ok up through 4500 rpm but MIGHT get lean after that... but i don't know and im curious if anyone here has done this and does know.

My guess would be that the program would need to be changed. The air/fuel demands in stock to stock configuration would be roughly 10% more due to the added 10% volume. The air will flow, but the fuel will not unless the computer is re-mapped. You might be able to gain a little by bumping up the fuel pressure, but then it will be rich everywhere but WOT, and it still may run lean there. The efi stuff is pretty twitchy. I don't think I'd chance it without having the computer done.

its not that simple. or linear. the manifold on these by all accounts is restrictive. and the annular opening is small for a 540. im not that convinced that the cylinder head flow would be that much greater but then again , as i said, i never ran one of these on my dyno nor have i seen the egts on the stock 502 to make a reasonable guess how safe the fuel curve would be...
that's why im asking.

What Jay said.

Why would anyone go through the trouble of doing the bottom end without reprogram? It is only a few extra pennies to set it up right after all that. My EFI is a little finicky but set up right is great.

the answer to that is a general lack of faith in the aftermarket "reprogram" industry in general and without taking the time to do it on the dyno in a controlled setting, ( i have done that on user programmagle systems before often) the chances of getting it right or even better than stock are slim and the dyno is not an option in this case.

it is just raw guesswork to do it by mail.

but thats anther discussion for another time. my original question still stands.

I would not say it is raw guess work. No more than getting a stock cal from merc.

Build a combo to specs that have been run on a dyno and get someone to put in a good cal. Plumb the exhaust for a wide band O2 sensor and a good tuner should be able to hit it in a couple of sessions.

Or you could start with the stock cal, put in the O2 sensor, run it and have a good tuner make the adjustments. Stay away from the tuners that are giving everyone the problems and you should be fine.

I have lived the pain you are describing. I tried to build up a 310HP/7.4MPI (L-29) with a larger cam and head work. After 3 attempts at programming the ECU with the engine in the boat, I gave up. This is the reason I stayed stock when I rebuilt my 502MagMPI. Bolting together a custom combo that has not ever been tested on a dyno is a crap shoot.
However, if you work with a builder/programmer that has a proven combo, I believe it can be done "by mail".

It would make more power with a better intake and a carb.