Question of the Month
496 Sport Tubes
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.

Matthew Burrows
Lake Elsinore, Calif.

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.

You don't need a drug test
I am trying to find some information on a boat that used multiple outboard power heads coupled to outdrives by a common shaft. The engines (four on each side) could be uncoupled in the event of a catastrophic failure. I thought it was campaigned under the name of Jesse James.

The guys at work looked at me like I needed random drug testing when I told them about this. I believe that it might have been in the mid-1980s. Do you have any knowledge of this?

Buzz Brenneman
Cincinnati

The year was 1986. The boat was a 35-foot Cougar catamaran open-class offshore race boat owned by Al Copeland of Popeyes Chicken & Biscuits fame. It raced only once that I know of. Bill Sirois, a prominent outboard tunnel-boat racer at that time, along with input from Jerry Gilbreath and Fred Hauenstein, are probably the people who came up with the idea.

Sirois and Hauenstein, who is still employed by Mercury Racing on the outboard side, designed the installation. They coordinated with Clive Curtis (Steve Curtis' dad) who built the boat overseas. As I recall, Jerry Gilbreath (who later went on to throttle many other offshore boats including Alcone Motorsports and Reliable Carriers) was involved as the crew chief of the "many outboards-inboard" boat, Popeyes. Gilbreath also was involved in rigging it.

To the best of my knowledge, Copeland drove the boat and Sirois was the throttleman. Sirois and Gilbreath also were outboard tunnel and Mod VP racers at the time. Gilbreath developed a Mod VP hull from a modified Spectra 19-foot jet boat that became known as a GMT (Gilbreath Marine Tunnel).

As a side note, Gilbreath also raced in the Grand National Inboard Endurance class in a Rayson Craft called Crazy Horse, which I competed against for many years. I am guessing that Gilbreath's and Sirois' outboard racing experience also were factors of where the idea came from for the project, and it was Copeland's money (and maybe some of Mercury's) that made it come to fruition.

APBA Offshore racing rules in effect at the time dictated that the maximum number of cubic inches allowed in the class was 1,000. So, they had a choice of running either eight 2.0-liter or six 2.4-liter "Bridgeport" outboard race motors mounted inboard. The boat was outfitted to accommodate eight Mercury Racing outboard motor power heads that were mounted laying on their sides. Four were mounted on each side on top of the tunnel adjacent to each sponson. Each group of four was connected by Gilmer belts to a common shaft that was connected to a single No. 2 Speedmaster drive.

The placement of the motors allowed Curtis to streamline the boat quite a bit by cutting the deck height down over the sponsons. There was some sort of a clutch mechanism on each engine that could allow a failed engine to be disconnected and enable the race boat to continue with the remaining running engines.

One of the original purposes of the boat was to set the APBA Offshore kilo record. In order to do that, the boat, driver and throttleman had to first qualify the boat by running it in a sanctioned race. In order to satisfy the requirement, Copeland and Sirois competed in the Bahamas at the first racing event of the 1986 season. For this race, they opted to power the boat with six Mercury Racing 2.4-liter Bridgeport motors mounted in the stringers for simplicity purposes.

Then, for the kilo attempt, the boat was fitted with the eight 2.0-liter hot rod outboard motors. Later, in 1986, the boat actually set an APBA Offshore kilo record somewhere in Louisiana with Copeland and Sirois onboard.

As I recall, the combined two-way speed was in excess of 160 mph. It actually did go faster on the first pass because during the backup run, one of the motors kicked a rod out and severed a fuel line, which spilled gasoline in the bilge and resulted in a fire. When Copeland and Sirois stopped the boat, the water in the bilge carried the burning gasoline forward under the cockpit and spread the fire. The resulting damage was significant.

Eventually, the fire-damaged boat was procured by Bob Idoni, who repaired and outfitted it with conventional inboard V-8 power. Ultimately, the boat hull was probably too light for the inboard offshore wars of that era.

Engine reliability was a significant factor for offshore racing in the 1980s. This was especially true because the average race was 225 to 250 miles. Another advantage of the design was that the Popeyes boat could lose an engine, disconnect it and continue racing without stopping or bringing the boat off plane.

During the 1970s and 1980s, offshore races were long. Absent were helicopters or safety boats all over the place, and sometimes one leg of a course could be as long as 50 miles. GPS or any other effective electronic navigational aids did not exist. Navigation was accomplished with compasses and stopwatches. Boats were equipped with spare parts, tools, props and mechanics to actually fix things during a race. Three-blade props were the norm (actually, state of the art) and a set of two-blade props were usually carried onboard so that one could be installed on the remaining running engine in order to get the boat on plane and get back to port.

All boats were required to carry food and water rations, life rafts, shark repellent and other survival gear (firearms were optional). The bottom line is that if you broke, you were on your own. You would be the person missing from the party and hopefully someone would see you at the next race. Racing then was an adventure rather than a show in a stadium. So, now you can understand the idea of having six or eight motors to share the load, which would theoretically increase the odds of finishing the race with the boat still on plane.

The Popeyes "outboards-inboard" boat did race against Mark Lavin in the Jesse James boat in the Bahamas, which was the first race of the 1986 season. Lavin's accident occurred in Key West, Fla., at the end of that season. As a result of the Jesse James crash, Lavin's family and colleagues proactively developed a set of guidelines for offshore boat safety that are still used today.

George Linder (original designer of the Chris Cat) is currently involved in the updating of the Lavin Foundation Guidelines. The guidelines have evolved over the years and are greatly responsible for the current generation of safer offshore race-boat-canopy systems.

How much do you want?
I own a Formula 292 FAS3Tech with twin 6.2 EFI MerCruiser engines and Bravo One drives with a 1.65:1 gear ratio. I would like to get more top speed (5 to 10 mph) out of the boat without sacrificing reliability and general cruising. I looked into Whipple supercharging the engines, which would also require upgrading the drives and changing the gear ratio.

My question is, are there are other options (intake, headers, drive upgrades, computer) that could provide the desired results without going with the superchargers?

Jim Waldman
Inverness, Ill.

There is nothing that compares to the performance increase that you would realize by adding the supercharger kits. If you did add the Whipple Superchargers, you would definitely need to change the drive gear ratio to the more common 1.5:1. The most cost-effective way to accomplish this, if you also consider reliability, is to sell your current Bravo One drives and replace them with XR versions with the 1.5:1 ratio.

The combination of adding the Whipple Superchargers, XR drives and propellers will turn your boat into a completely different animal. You could expect top-speed performance gains in the neighborhood of 15 to 20 mph.

In addition, because of the extra horsepower, gear ratio and higher-pitch propellers, your midrange cruising rpm at any given speed would be significantly less.

In lieu of going with the "full Monty" described earlier, huge speed increases (10 mph or more) will probably not be realized by just changing external engine components and propellers on your boat. You could expect to gain 5 mph (maybe not 10 mph) by adding aftermarket performance exhaust systems and better propellers.

The small-block engines do respond well to the improved breathing afforded by a better exhaust system. One of the best choices for your motors is the CMI E-Top or Sport Tube headers. Another good choice is the manifold and riser combination available from IMCO Marine.

If you have restrictive mufflers, "Silent Thunder" with all the holes plugged in the swim platform, or some other kind of "quiet and quieter" exhaust system on your boat, the advantages of the enhanced header/exhaust system are diminished slightly. There are a number of muffler systems currently available that are capable of quieting down your small-blocks without hindering performance.

Through Transom
I have a MerCruiser 7.4-liter 454 Magnum engine in a 1996 Formula boat. I want to convert the exhaust to a through-transom setup. I am unable to find anyone around my area who knows much about this.

It looks self-explanatory, but just for comfort I am trying to find out proper installation instructions. Obviously I do not want to end up with water in my engine or with an exhaust system that may be getting too hot. Any information would be very much be appreciated.

Parnell Perry
Bastrop, La.

It should be easy to add through-transom exhaust to your Formula. You will need to purchase a pair of stainless-steel 4-inch exhaust tips with flappers, about 4 feet of 4-inch rubber exhaust hose, some No. 72 all-stainless-steel hose clamps, and a few other items depending on how you are going to do the modification.

Currently, you have a pair of elbows that are clamped into place with exhaust hoses on either side that connect your exhaust manifold risers to the exhaust "Y" (sometimes referred to as a "bullhorn") that directs the exhaust out through your drive and prop hub. It is nearly impossible to remove the bullhorn and cap the drive exhaust passage without removing the engine from the boat. Instead, you can accomplish the conversion a couple of ways.

The inlets to the bullhorn are elliptical. A pair of stainless-steel tubes that are flattened slightly to match the shape of the inlets with a cap welded on one end can be clamped into place using the original connection hoses to serve as block-offs. If the existing bullhorn is not blocked off, or incorporated into the proposed through-transom exhaust system, water will pour into the boat through the openings.

In lieu of blocking off the existing through-hub system, you can install a pair of stainless Y-pieces that allow exhaust to exit through the hub and transom simultaneously. You may also take this opportunity to install a switchable system allowing you to choose from through-transom to through-hub exhaust with a switch at the helm. Mercury Silent Choice, CORSA Captain's Call and CMI Internal Sound Choice are a few of the systems that are common.

The transom tips are installed by drilling 4-inch holes in the transom in-line side-to-side and slightly lower than the exhaust riser outlets on your engine. The exhaust outlet hose should angle down at 10 to 12 degrees to ensure that water flows out and not in. The holes cut in the transom should be sealed with resin and the tips need to be sealed with a good grade of RTV silicone when installed. A combination of internal and external flappers are best to prevent water surging up the exhaust system from waves and wakes from the stern. External flappers are desirable since they are easy to inspect and service.

Mark IV vs. Generation V
I have a 1989 Commander 21-foot jet boat with a 454-cubic-inch engine. I'm going to rebuild it. I have a set of heads from a Mercury Racing HP500 motor with rectangular inlet ports. Will these fit on my 454? Will it run any better with these heads? What compression ratio is desirable? What cam should I use? The boat is an open bow that we use for skiing and cruising.

John Crawford
Torrance, Calif.

So, have you ever wondered why we call the early versions of the big-block Chevrolet "Mark IV" while the later versions are referred to as "Generations?" I was once given an answer that I believe is correct. By the time GM came out with the next version of their big-block Chevy offering, Ford had a Lincoln model called the "Mark V."

So, Ford Motor Co. prevented GM from using the name "Mark V" for the engine. GM then had to change the designation of their next versions of the original "Turbo Jet" big-block Chevys to "Generations." The first of the one-piece rear main seal big-block Chevy engines is therefore known as a "Generation V," and the subsequent roller cam version of the motor is known as a "Generation VI."

This trivia is interesting, but it also has an impact on your proposed engine project. Mark IV blocks and Generation V and VI heads are really not compatible. The head bolt pattern is identical, however, the design of the block to head surfaces are different enough that head gasket water leaks can occur when mismatching parts from different engine generations. The area of most concern is at the top of the deck surface (nearest to the lifter valley).

The Mark IV block surface does not extend enough to match the cooling holes in the Generation V head along the contour of the sealing surface. Even modifications to the decks, combined with careful selection of the head gaskets, leads to marginal results. The most common problem is that coolant leaks into the engine valley between the head and the block. Virtually all of the aftermarket iron and aluminum heads available today will work with all of the block designs as long as the correct head gasket (determined by water passage design, shape and flow) is used.

The Mercury HP450 (454 cubic inch) and HP500 engines were both based from the GM Generation V big-block Chevy. The heads used on both engines had the same size combustion chambers and used the same valves. Your 1989 454 is a Mark IV unless the engine was actually produced in 1991 or later.

The bottom line is that your HP500 heads are a mismatch for your older 454 Chevy. The best solution is to get a pair of aftermarket iron or aluminum head assemblies from Air Flow Research, Brodix, Dart, Edelbrock or World Products, to name a few. Make sure that you are purchasing head assemblies intended for marine use. Many of the aftermarket providers have a marine line of heads. Don't be fooled by the car guys and discount houses.

I would upgrade the engine with the use of a hydraulic roller cam. Crane Cams Part No. 139731 (13HR00146 on a billet core) would be about right. This cam has a .587" lift on the intakes and .610" lift on the exhaust valves. The duration at .050" lift is 226 and 234 degrees, respectively. When you buy the head assemblies, indicate that you are using a hydraulic roller cam to ensure that the proper valve springs are provided. The on-the-seat valve spring pressure should be about 150 pounds with a tension of approximately 450 pounds "over the nose."

A compression ratio of about 9:1 will still allow you to run pump gas and make decent power. Add a later-style intake manifold such as World Products' Merlin X marine manifold and a Holley 800 CFM marine carburetor and you will be on your way to making 450 to 500 manageable horsepower for your jet boat.