I am new to this forum and living in Australia.

I have twin MIE 454 magnums 385hp version with EFI and Hurth gearboxes, i think 2001 build. Imported these from USA in 2002 with 20 hours an the clock. They are freshwater cooled but the manifolds are raw water cooled. They are in a 38' cabin cruiser.

Firstly can anyone tell me what the specs are - I gather they have 502 heads & some cam but do not know what?

Next, I have destroyed 4 heads because of water reversion.

I have now installed 3" spacer between manifold and elbow/riser as well as a turbulator and resonator (17" from where exhaust hose pushes against riser).
Each 4" rubber exhaust hose extends approx 9' before joining together into 5" outlet at stern of boat. There is a fall all the way. These are mercruiser manifolds/spacers/risers.
What else can I do to stop the problem which I gather is an idle issue due to cam overlap?
Idle is 600rpm - will it help to increase - if so how?
What problems will this bring when engaging gears?

On shut down after extended idle after backing into marina pen etc, I increase rpm to 3000 for 1 minute then throttle down and turn off. Theory is to get the manifold & turbulator hot so to evaporate any water.

Any ideas or comments as I am at wits end.

anyone?

Welcome to the forum! First, make sure your flappers are in working order, if you haven't already. Your shutdown procedure likely doesn't do anything, as at least my manifolds run cooler when at higher RPM's, most likely due to more water flow.

If it were me, I would look into aftermarket exhaust with extended risers, or a full closed cooling system. Either would eliminate reversion issues. Here is an mfr from your "backyard."

http://www.hitekmarine.com.au/

Aftermarket exhaust is expensive, but is cheap compared to what you have dealt with so far. Another option is to change the cams to something that is known not to cause reversion.

Increasing your idle RPM will help, but I am unfamiliar with your drive/tranny setup so I cannot say if it would cause issues.

Richie, there is a procedure to follow to fix this problem, I have done many for Mercruiser, it includes a "full" closed cooling kit to include the exhaust manifolds in the cooling loop and Merc recalibrates your ecms'. You will need new exhaust manifolds and other info to do it properly. PM me for the info.

Old ride please email me at [email protected]

This is from 9 years ago from a guy who probably moved on with 67 posts total. Just saying