As seen in Speedboat Magazine’s November 2019 Issue.
Headers for 454
Magnum MPI
Dear Bob:
I have a 27′ Checkmate with a 454 Magnum MPI. Currently, the exhaust is stock MerCruiser cast iron manifolds and risers. The boat is also equipped with the Corsa Captain’s Call switchable system that diverts the exhaust through the propeller hub when activated. I need to keep the ability to silence the engine. I was planning on doing some upgrades to the engine soon with a more aggressive camshaft profile that is similar to what is used in the Mercury Racing 525EFI.
I have been told that my stock exhaust system will hinder performance and their use may result in exhaust water reversion with the increased cam duration.
I was considering the CMI part number 13186, but l need to know if they can be installed in conjunction with my current Captain’s Call system. If not, what system will work?
Thanks,
Damian Zapata Odessa, TX
The CMI 13186 is the satin finish 454/502 one-piece Elbow Top header. This header is also available in a polished finish for an additional charge. These headers are the same as the 496 Elbow Tops but do not have the required sensor fittings that are used with the 496 engines. Some single engine boats similar to yours, such as those built by Baja, used a spacer between the exhaust manifold and riser. If your current stock exhaust has that 2″ spacer, then the 13186 CMI headers would not work. If it does not have the spacer between the manifolds and risers, then the 13186 CMI headers will work. If you have the 2-inch spacer blocks between the stock manifolds and risers, the header is also available in a 2-inch taller version that should work the same in that application depending on the location of the exhaust ports in the transom.
If your engine is completely stock, you should opt for the mounting brackets that are added to the headers for the shift bracket, ECU mounting box, and the oil filter mount. These brackets are welded onto the headers, so the stock accessories end up in the same place on the motor. The oil filter mounting bracket varies from engines equipped with vee-belts compared to the engines using a serpentine belt.
The 525EFI cam has a 236 degrees intake duration and 244 degrees exhaust duration at .050″ of camshaft lift. Make sure your version has a 114-degree lobe separation which will help control exhaust water reversion. The cam you selected is normally used with a 502 to 509 cubic inch motor. When used with a smaller cubic inch motor by comparison, the cam will be a little more radical.
CMI 454/502 E-Top (polished version).
You will not be able to use the stock valve springs with the more aggressive higher lift camshaft. You should elect a dual dampened spring that will install at about 150 or 160 pounds on the seat and is close to about 480 pounds “over the nose” with at least .080″ coil bind clearance. If your heads are stock, with stock length valves, Competition Cams part number 929, or PAC TVS 1632 are decent choices at reasonable prices. They install at 1.880″ to achieve the desired set up. I would go with IO-degree Jocks and steel retainers that fit the spring properly. If the valves are too short to accommodate the needed installed height, .100″ longer valves are available. Remember, the exhaust valves must be Inconel or Super Alloy. Isky Tool Room part number 9925 is a premier spring that will install correctly at about 1.950 inches with the longer valves. I also recommend upgrading to higher performance hydraulic roller lifters. If your motor is a Gen VI, with the lifter guide blocks, do not use the guide blocks and “spider” that holds them in place and opt for high quality short travel lifters with tie bars. We use a lifter that is made to our specs by Johnson Lifters tl1at require push rods that are about .175″ longer. The late model Crane lifters seem good lately and use the same length push rod as the stock lifter, with all things being equal. This would also be a good time to upgrade your rocker studs. The Gen V and VJ heads use a stud that is 3/ 8-16 on the underside and 7/16-20 on top. The best choice is the ARP RRS-6H stud available in a kit (part number 135-7102) which is made of their best material.
Most of the good aftermarket lifters have taller bodies. In some cases, the button that holds the tie bar can hit the valley tray that is under the intake manifold. It may be necessary to put 8 dents in the tray to provide clearance. Do not omit the valley tray because it provides oil control to protect the underside of the intake manifold from excessive heat and more importantly serves as a baffle to prevent oil from being sucked up by the PCV valve on engines so equipped.
Your 454 Magnum uses a MEFI 3 ECM which is no longer supported. If you change the cam profile, and add headers, the ECM will have to be reprogrammed, Ultimately, you might 
want to have 02 bungs installed in the headers so that you can verify the new program with a wide band air-fuel ratio monitor. Also, it would be a good idea to modify the fuel system adding an adjustable fuel pressure regulator. Having the extra power might require higher operating fuel pressure.
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