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QUESTION: I always look forward to every edition of Powerboat magazine, and I especially like the Teague on Tech section. Your May 2009 column provided some good coverage of headers. I can’t afford the $3,000 required to own a set of those sexy CMI headers, especially for installation on my saved-from-the-crusher “rat rod,” which is a 1988 21-foot Glasstream FreeSpirit with a 454 Chevy and a Bravo One drive spinning a 26"-pitch Bravo One propeller.

Considering the cost factor of CMI headers in the pursuit of speed, I instead installed these headers (see picture below). For fire safety, I’ve ensured good airflow around the engine and installed a fuel fume detector and a large-volume automatic Halon fire suppression system. The K&N is a certified flame arrestor. Other than being rather loud, I’ve test driven the boat and examined everything in the engine bay for any signs of heat damage. Everything is damage-free so far.

I’m concerned that I could have missed something in the application and installation. I don’t want to reinvent the wheel nor turn to toast, so I’m taking the opportunity to ask you, the voice of experience, is there anything more I need to do to ensure my boating safety?

Thanks again for a great technical column!

ANSWER: It is only a matter of time before you have a fire or get boarded by the local authorities. I am sure the U.S. Coast Guard will have an opinion about compliance issues with your vessel. The boat is red—and loud. It is likely to get noticed.

If you have ever watched an engine being dyno-tested with headers like yours, you would have noticed that they get so hot that they glow red. The radiant heat is likely to eventually take its toll. Low-profile jet boats and V-drive flat bottoms regularly use water-injected headers to knock down radiant heat. But in those installations, the headers are above the deck line of the boat and in the open air.

The downside of using water-injected headers is the mess they make in the boat from the drain holes in the primary tubes. Wrapped headers have occasionally been used in race applications. The cost of wrapped headers equals or exceeds the cost of water-jacketed headers.

Relative to performance, engines will make more horsepower in a cool environment. So a good flowing exhaust that does not create excessive ambient engine compartment temperatures is beneficial to performance.

Your K&N flame arrestor is designed to suppress a flame caused by a backfire in the carburetor. It provides no protection from the effects of the heat generated by the dry headers. Halon extinguishers are most effective in closed engine compartments. Your modifications that resulted in a more ventilated engine compartment render the Halon system ineffective. Probably the best way to extinguish an eventual fire is with a large dry chemical extinguisher, lots of water or by sinking the boat.

I suggest that you go on eBay or check some performance Web sites and find some good used CMI headers. I think you’ll be able to find some takeoff E-Tops from a boat where someone upgraded to Sport Tubes. Be sure to pressure check used headers before you buy them.

Later-model “Cool Collar” CMI headers are more desirable because they have a short hot section at the flange, which helps eliminate water leaks due to heat and material thickness differences.

496 SPORT TUBES

QUESTION: I have a 2006 Advantage 25-foot Citation with a MerCruiser 496 Mag HO and a Bravo One XR drive with a 1.5:1 gear ratio. I have reached that magical time of one year since purchasing this boat and I'm ready to consider upgrading the horsepower.

I am pretty sure I want to add the CMI 496 Sport Tube headers as a simple, relatively inexpensive bolt-on solution. What advice can you provide regarding a propeller pitch change? I am currently running about 4,900 rpm at 63 mph using a 22"-pitch Bravo One propeller. I want to keep my boat as reliable as possible.

ANSWER: The typical gain in horsepower by upgrading to the CMI 496 Sport Tube header system is significant enough that you will need to go to a larger-pitch propeller. I believe that 4,900 rpm is the magic number for the MerCruiser 496 Mag HO at wide-open throttle.

Accordingly, with the horsepower gained by installing the CMI Sport Tubes, you will likely need to go up to a 24"-pitch Bravo One propeller to match the new power level.

The larger-pitch propeller should enable you to gain about 4 or 5 mph at 4,900 to 5,000 rpm. An additional benefit is that you also will be able to cruise at higher speeds at any given rpm.

CAN YOU HEAR ME NOW?

QUESTION: Thank you for all the great articles and advice. My question is about my single-engine catamaran powered by a 572-cubic-inch, 800-horsepower EFI engine. I am considering changing my tailpipes from wet to dry.

My engine builder suggested running dry tailpipes to free up some power that is being lost due to back pressure caused by the water in my exhaust. Will the dry pipes cause a heat problem where the pipes go through the hull?

The headers are similar to the ones from Custom Marine Inc. (CMI) with wet tailpipes. I understand the fuel mixture will need to be checked and possibly adjusted, which I am capable of doing. Will the decibel level be higher?

ANSWER: If your existing headers are similar to CMI's, they also should be equipped with tailpipes that are water jacketed, which means that there is no water in the exhaust stream until the aft end of the tailpipe. With the CMI-type system, the water entering the exhaust stream at the end of the tail should have very little affect on the performance of your engine.

You could opt to extend water-jacketed tailpipes through the transom instead of using transom tips that are connected with rubber exhaust hoses. With through-transom tips, you will have to use transom rings and seals to keep the water out of the boat. A through-transom tailpipe can either have the water exit at the end of the pipe or a little further forward.

If you want to plumb the exhaust water overboard via a through-hull, you will still have a slight amount of water exiting where the inner pipe is swedged out to the outer pipe. It is not advisable to weld the inner and outer pipe together at the tip because of the temperature differential between the two tubes. The inner pipe gets hotter and expands at a greater rate than the outer pipe. The result will likely be a crack in the inner pipe, which could result in water getting into your engine. You may gain only a slight amount of performance by using a CMI-style dry tailpipe. I do not believe that having water exiting in your exhaust is robbing your boat of horsepower.

If your headers are the type that a rubber exhaust hose clamps to, it will be next to impossible to add a dry pipe to them. A water-jacketed tailpipe should not cause a problem with heat at the transom as long as there is enough water flowing through the tailpipe jacket.

Here's the bottom line: Dry tailpipes are loud. Marine-style mufflers are pretty much not designed to work without the presence of water. When it comes to pleasure boating, you will get tired of the noise and everyone else will probably get tired of it as well. The attention you get may not be from the folks you want it from such as local law enforcement.

CHEVY HEADS

QUESTION: Can you tell me what cylinder heads MerCruiser used on the 1987 330-hp 454 Magnum engine package?

ANSWER: The 330-hp version of the 454-cubic-inch MerCruiser engine was not referred to as a Magnum. The Magnum version of the engine was rated at 360, 365 or 385 horsepower depending on the model and induction system. The 330-hp engine was a marine version of the GM 454 truck engine. It was a Mark IV big-block Chevrolet with small intake runner oval port iron cylinder heads. Typically, the exhaust valves also had rotator assemblies under the valve springs.

The Magnum version of the 454 engine is a marine version of the Chevrolet high-performance 454-cubic-inch engine. This engine uses the larger rectangular port intake runner Mark IV cast iron heads that are similar to the heads first found on the LS-7 hot rod motor that was first available around 1970. Both the oval port and rectangular port heads I am referring to are "open chamber" design.

The very early models of these heads are not suitable for use with today's unleaded gasoline because they were not fitted with hardened exhaust seats. These heads cannot be retrofitted because there is not enough material in the casting around the exhaust seats to properly install a hardened seat insert.

Generation V and VI GM cylinder heads should not be used on your Mark IV engine because of water sealing problems with the head gaskets. If you are thinking of reworking or upgrading your heads, don't. The best solution is to select one of the many aftermarket head assemblies that are offered specifically for marine use.

I have had good success with head assemblies available from Edelbrock and World Products (Merlin). I also would consider going with aluminum heads, which better dissipate the heat that is generated by today's gasoline formulations. Edelbrock has a marine head assembly that is anodized and has a smaller 110 cc chamber. Using this head will result in raising your compression a little, which will improve the engine's performance.

—Need some guidance from Bob Teague, Powerboat's lead test driver and owner of Teague Custom Marine? To send him an email click here.

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