Getting even more out of a non-stepped 35-foot Fountain.

As Seen In Speed On The Water

For this issue’s column, I opted to dig into my Q&A archives and pull out a question that not only was a fun one to answer, but also might provide some current value since we know there are many 35-foot V-bottoms from Fountain Powerboats floating around out there.

NEED FOR SPEED

Q: I have a 35-foot Fountain with two 502 engines that are “bow-tie” blocks according to the block identification. They are the 380-horsepower engines, not the 415-hp versions. The boat ran 74 mph at the factory, and 17 years later it still runs 74 mph on GPS. That’s not very fast now, and the engines have 674 hours on them. I am considering rebuilding them to 555 cubic inches with Airflow Research (AFR) heads.

Basically, I want to take them to around 600 to 650 hp, but my question is simple: I would like to get the boat to run 95 mph but I do not have a stepped hull. So, will 600- to 650-hp engines like that push this boat to my desired speed, or should I consider a newer boat with the twin-stepped hull?

The problem I face is that I’d love to drop the boat off at a reputable shop, and then just pick it up with a pair of quality motors. But with the economy the way it is, I can’t do that. I also wonder about the dependability of my motors with lots of hours on them. Only the port engine uses oil, the starboard one doesn’t even use a drop all summer long, and they’ve only had Mobil 1 in them. The engines have never been apart and have no issues at all. That’s the kind of durability I’m looking for.

A: Well, you’ve gotten your money’s worth out of your boat. I’m sure part of your
success is that you take good care of it and don’t abuse the engines.

There is no doubt that a later-model twin-stepped 35 Fountain will run faster with any given power combination. That being said, it is hard to beat the great handling attributes of the Fountain non-stepped 35- and 38-foot boats. I think it is possible to get your boat into the 90-mph range with about 650-hp engines.

Part of the success of the newer boats is that Fountain has continually raised the “X” dimension on its boats. It has been able to do this in part because of the array of propellers available now that weren’t available when your boat was built. At that time, Mirage three-blade propellers were common. Now five-blade Mercury Maximus and six-blade Hering propellers provide maximum performance with prop shafts that are a few inches higher than yours.

If your prop shafts were as high as the current boats, the boat would have never come on plane with the Mirage, Mach or four-blade Hydromotive propellers that were available at that time. I’d install some shorter lower units in combination with the better propellers that are available today. That in itself will improve your top-speed performance.

I am not a big fan of building stroker marine motors using a short-deck block. The result is a piston design that is not quite as reliable. I’m assuming that you are considering a combination that includes a 4.560-inch bore with a 4.25-inch stroke. Your current bore is 4.467 inches. I’d be reluctant to increase the bore nearly .100 of an inch on your old blocks. Instead, I would bore them to 4.500 inches. At this size, there is still plenty of space between the cylinders to protect the head gaskets and dissipate heat, and standard (non-metric) rings are available. The most I would bore the blocks would be to 4.530 inches.

Using a 4.25-inch stroke crankshaft in a 9.8-inch deck block will require you to use a rod that is 6.385 inches long and a piston that has the wrist pin located under the oil ring. This requires the use of an oil ring support in combination with the oil ring. The combination is fine for street machines, but isn’t the best for long-term endurance when used in the rigorous marine environment. I recommend staying with a 4-inch-stroke crankshaft for your rebuild. You will gain performance in your boat with an engine capable of higher rpm. In general, larger cubic inch “stroker” engines are not as happy running higher rpm. The 4-inch-stroke motor can run 5,800 to 5,900 rpm without any strain if it is properly built.

I would keep your stroke stock and bore the blocks to 4.500 or 4.530 inches. This will
result in 509 or 516 cubic inches, respectively. If your crankshafts are in good shape, you’ll still be able to use them. You will definitely want to upgrade your connecting rods from the stock GM pieces. They are notorious for spinning rod bearings and probably have some cracks in the beams along the forging laps. I suggest Manley Part No. 14062, which
is a 6.385-inch-long H-beam rod that’s not too heavy. Using the .250-inch-longer rod will
improve the rod-to-stroke ratio compared to the stock 6.135-inch rod length.

Use a JE or CP piston with a dome that will result in about a 9.5:1 compression ratio in combination with the volume of your cylinder head chambers and head gasket. With this compression ratio, you’ll need premium pump gas. With the stock stroke in combination with the 6.385-inch rod, the piston’s compression distance will be 1.395 inches. Use their
top-line pistons for marine applications instead of the shelf street pistons such as the JE SRP line. The top-line pistons are beefier.

If you want to use the AFR head, I suggest the 315 cc runner design that is CNC ported. Higher-volume runners are not desirable for naturally aspirated engines at around the 509-cubic-inch level.

You will need to upgrade to a hydraulic roller camshaft and valvetrain. I suggest a lift of .610 to .632 inches, with a duration of 236 degrees on the intake and 244 degrees on the exhaust at .050-inch lift. The lobe centerline should be either 112 degrees or 114 degrees a split pattern. This cam profile will provide good power without creating a water reversion problem if you have a good exhaust system.

You’ll need to change the intake manifold to one that makes more power. Try the World Products Merlin X to accommodate a Holley 4500 series Dominator carburetor. The manifold works well out of the box and has brass inserts in the water passages to protect from corrosion. Select a two-circuit 1050 CFM carburetor. Three-circuit carburetors run rich in the midrange. Under the carburetor, install two 1-inch spacers. The top spacer should be a four-hole design, which lengthens the carburetor venturi and improves the booster signal.

This improved influence on the carburetor boosters will improve throttle response with the big carburetor. The spacer next to the intake manifold should be a cloverleaf design that improves the flow transition from the fourhole spacer to the intake manifold plenum. The flame arrestor has to be large enough to not cause a flow restriction. The combination of the intake manifold, spacers and flame arrestor will be higher than your current stock setup. You may have to install SuperChiller fiberglass inserts in the underside of your hatch to accommodate the extra height.

Your stock exhaust system will have to go in the trash bin. Using it will result in loss of power and probable exhaust water reversion. At a minimum, you’ll need something like the IMCO Marine PowerFlow exhaust system. It be better to use headers such as the Custom Marine Inc. (CMI) E-Tops or Sport Tubes. Headers like the CMI Sport Tubes with longer primary tubes are better for performance on naturally aspirated motors. The Sport Tubes are similar to the “sweeper” headers used on the Mercury Racing HP525EFI, HP600SCi and HP700SCi engines. If your boat is currently equipped with Silent Choice, you may have to consider going to switchable muffler exhaust tips to accommodate a header system that will complement your new engine program.

Finally, you’ll need to upgrade the ignition system. Your current ignition system has a rev-limiter well below your new engine’s capabilities. You also have to set your advance curve and limit. I suggest the MSD marine ignition system with a Soft Touch revlimiter. With the MSD billet marine distributor, you can limit the advance curve to 20 crankshaft degrees by using the provided black bushing and have the advance all in at 2,500 rpm by using the provided blue springs. If you set the total advance at about 36 degrees BTC, the engines will have a strong idle advance of 16 degrees, which will make them more manageable around the docks.

–Technical editor and offshore racing world champion Bob Teague is the owner of Teague Custom Marine in Valencia, Calif.

This article appears in:
Speed on the Water, Issue 16; November/December 2015