Moondance

Hello my gear head friends!
First I am not a teckie just and engineer (not mechanical) to let you know where I am comming from.
I would like to make my stock 502 more efficient but remain stock as possible, my first thought is to make them breath better. Would I accomplish anything by bolting a 500EFI plenum (and if nessary manifold) to my stockers? Or would I be better off just changing to a K&N Breather?
Also would a change in exaust manifolds help to say CMI casties or somthing that will work with my captian call?

Pat McPherson

I assume you are taking about a 502MagMPI.
Yes, a better flowing exhaust will help. I put Stainless Marine manifolds on my engine.
The AZ Speed/500EFI intake does flow better than the Stock Merc but not worth the $$ and effort on this engine without other mods.
I don't know if a K&N arrestor flows better than a stock unit but they sure look better...

Moondance

Thanks for your reply.
You are correct in assuming that these are MPI's. I need to change my exaust anyway because they are salted and making my starboard engine steam. I have Corsica captians call and will have to bolt up to that. I will look at Stainless Marine to see what they have.
Thanks again!

Griff

The Corsa diverters won't work with headers.

Moondance

Thanks for your input.
I was not planning on going to headers, just a better flowing aftermarket system like mabe cast lowers with a close elbow upper to mate with the Corsica diverter.
I think a lot of companies make them, EMI, Stainless Marine, ect. I don't have a clue who's is the best choice for improving efficiency.

Michael Garibay

Moondance, If you are interested, I have a Rex Marine K&N adapter and filter for the 502 Mag that has very little use and will sell for $75.00 + shipping (new from Rex is $180.00) . call me Mike (909)576-4337

Moondance

Thanks Michael,
I'll keep that in mind. If you look at the adaptor it closes down to a 3 or 4 inch round hole which would limit the CFM. This may not matter much but that is the question I am asking everyone. If you look at a 500EFI plenum, the K&N mounts to a larger opening (unless I am mistaken).
Thank you for your input.

speedreeder

Moondance, I have done quite a few of mods like you want. I would put on IMCO Thumper series manifold headers, that will work with your captians call. They can also be polished for not much more money-around $1200. I would then install some 1.8 roller rockers that would make your cam a little larger and a sharper ramp on opening and closing(more efficient). These two should take you up about 400-600 rpm, Which is at least one prop size up, If not two sizes. Please PM me for a more extensive list and prices. Good Luck, Rob

Griff

Do you have 2 of them???????

US1 Fountain

iTrader: (1)

How many hrs would you have to put on the boat in order to recoupe the cost of installing exhaust from any gains in efficiency? Providing efficiency is the only goal here.