CMI’s Fusion Performance Headers Take the Work Out of Stock Cast-Iron MerCruiser Exhaust Swaps

Who hasn’t admired the stunning engine compartment of a genuine high performance boat. And one of the most eye-catching bling-features is always the gleaming polished stainless steel tubular-style water-jacketed exhaust headers and tailpipes which adorn those engines.
120801-engine compartment CMI Headers.jpg

In all but rare instances, those headers are the work of Custom Marine, Inc. (CMI) located in Neenah, Wisconsin, suppliers of all things exhaust for Mercury Racing and many other leading high performance marine engine manufacturers for the past several decades. But what about the rest of us, proud owners of stock stern drives who also crave a little more “wow factor” under our motor hatches not to mention a nice kick up the horsepower ladder?

CMI obviously heard the cry and have figured out a solution….the “Fusion” performance header system which makes swapping out an original set of cast iron manifolds and risers for “real deal” headers a no-brainer.

The Fusion headers look similar to other CMI exhaust models, but there is a very important difference. Instead of being a custom-fit multi-piece system which is common to other CMI header models, the Fusion is integrated into one piece, header and tailpipe attached, which eliminates extra clamps, gaskets and hoses. The Fusion design is intended for all those “stock” big-block V-8 MerCruiser Bravo packages (454, 496 and 502 c.i. blocks, except for the newer catalyzed models) which come with the familiar black cast-iron manifolds and risers exiting via the standard Mercury tailpipes with thru-transom exhaust outlets.

In order to avoid all the trouble of plugging and re-drilling transom holes, the standard Fusion tailpipe is angled and fit to line-up with conventional MerCruiser thru-transom outlets. All that’s needed to connect the end of the Fusion tailpipe with the inside exhaust tip flange is a standard rubber exhaust hose and clamps. It’s just that easy.

If by chance the existing transom exhaust outlets deviate slightly from the standard MerCruiser pattern, CMI can adjust the Fusion tailpipes at the factory (up to 2 inches horizontally, or no more than 2 inches lower) to accommodate that difference. There is, however, a modest (about $250 to $400) additional charge for custom fitting when ordering.

One other factor you need to be aware of is over the many years of manufacturing big-block V-8 Bravo stern drive packages, MerCruiser models have evolved in a multitude of configurations. Because essential engine accessory components (i.e throttle/shift brackets and cables, top mounted oil filters, etc.) rely on attachment to cast iron manifolds and risers for support, it is important that the proper adapter bracket kits are ordered with the Fusion headers. Various models and year manufactured do make a difference. The way to ensure that you’re getting the right parts (which are sold separately) is accurately identify your specific engine by the correct year manufactured and model designation.

What CMI is banking on are the thousands of MerCruiser packages now in service that are getting older and need replacement exhaust systems because of the inevitable forces of corrosion. Regardless of how meticulous a boater might be about engine flushing after use, cast iron exposed to water and repeated heating and cooling will eventually deteriorate the metal to a point where it is no longer usable. When the time comes, it’s either replace with another set of stock manifolds and risers, or spend a little more for a CMI Fusion stainless steel header system which looks a lot cooler and should perk-up the performance of even the most stock big-block. Suggested retail price for the standard CMI Fusion performance header set (they come in “Platinum” finish which looks like brushed satin) is around $4,000, noticeably less when compared to the more pricey custom high performance/race versions.

Here’s what a typical conversion project looks like and you can do-it-yourself.


Right out of the box. This is the starboard side of the stainless steel CMI Fusion performance header with basic kit accessories including clamps, bolts, gaskets and water distribution system.


This is what can result if proper flushing is not practiced on raw water-cooled engines. Corrosion not only eats up the manifolds, but may also damage the engine cylinder heads.


The only thing this tired and badly rusted cast iron exhaust manifold is good for is an anchor.


This is your typical stock MerCruiser 454 MPI stern drive, 1995 vintage with cast iron manifolds, risers, tailpipes and thru-transom outlets all from Mercury.


Begin the disassembly process by first loosening the double clamps around the rubber exhaust hose at the transom. Always double clamp for safety.


Next, loosen the clamps from the rubber exhaust hose connecting the tailpipe to the riser elbow.


Now unbolt all accessory bracket(s) (i.e. throttle/shift plate, top mounted oil filter, etc.) that may be attached directly to the cast iron manifold or riser.


Don't forget to loosen the bottom clamp from the water inlet hose and disconnect.


After you loosen and remove all eight of the exhaust manifold nuts from the cylinder head studs, you're ready to remove the entire manifold, riser and tailpipe assembly.


This is what it looks like when the manifold, riser and tailpipe have been removed.


Be sure to remove the old cylinder head/manifold gasket and replace it with the new gasket provided in the CMI kit. If you have trouble keeping the new gasket in place on the engine cylinder head while you are lining up the header, use a couple of dabs of grease on the inside of the gasket to keep it where it needs to be.


Now you're ready to install the new CMI Fusion header. Initially, just secure it with a couple of nuts front and back. Don't tighten quite yet.


Next, install the water distribution tube system to the underside fittings on the header with the hose clamps provided. Then, reconnect the water intake hose to the water distribution tube system. If necessary, shorten (cut) the hose if too long.


This fitting at the header elbow will be plugged because the engine does not require a water return line to the engine. Some engine models may require this line to be connected.


Assuming this is a stock thru-transom Mercury set-up, the standard CMI Fusion header and tailpipe will use the same exhaust outlets without modification. Connect the rubber exhaust hose with four clamps (2 on each end).


The starboard side conversion is now complete. The only step remaining is to reinstall needed accessories with CMI bracket kits. Remember, when purchasing the Fusion headers, be sure and specify year and model of your engine. That information will determine which accessory kits will be needed.