454 Mag MPI with CMI exaust. ECU mod???
 

Hi Guys, do i need to mod the ecu after installing some CMI headers on my 454 MPI, they are running through the scilent choice and are wet with stock cam. Hell of an instulation job but its all done. Not sure if this will lean it out enough to warrent an ECU mod ornot. Jas

Re: 454 Mag MPI with CMI exaust. ECU mod???
 

I would not modify the ECU unless you want to up the rev limiter.
Install a couple of new spark plugs and run it hard for a while. Then pull the plugs and read them. If you feel the engine is leaning out, up the fuel pressure a little.
Good Luck
I bet they look awesome!

Re: 454 Mag MPI with CMI exaust. ECU mod???
 

Do you think i should wind the rpm up a bit, wouldnt mind to do that actually. Jas

Re: 454 Mag MPI with CMI exaust. ECU mod???
 

I think the rev limiter is set at 5200.
I suggest you test with the same prop that you were running before you added the headers.
If you have gained a couple of hundred, more speed, and you're close to 5200 then maybe yes. Do some testing before changing the ecu though.
I have tested my boat with many different pitch props. My boat does not go faster propped for 5000+rpm. It seems like 4800-4900 is the sweet spot for my 502MagMPI.

Re: 454 Mag MPI with CMI exaust. ECU mod???
 

Good advice from Pat. I don't really know how true it is, but I have always heard that Merc's engines run a bit on the rich side anyway, and when adding headers it just helps lean it out a bit.

I know of a local guy who has a twin engine boat with stock 502mpi's who removed the Merc cast iron exhaust manifolds that come stock on the 502mpi, and installed some CMI elbow tops without changing anything in the computer. He has been running it that way for several years now. Besides looking great, they added much nicer sound/tone to the engines. However, I think following the advice Pat McPherson gives is wise.