CHECK THE TACHS
QUESTION: My boat has twin 5.7-liter Volvo Pentas with the dual outdrives. My question is: How do I get the engines to run the same rpm at idle, and at top speed? The throttles are nowhere near each other. Is there some computer chip or something that would make them run close to the same rpm?

Troy Phelan
Goodlettsville, Tenn.

ANSWER: I would first verify that your tachometers are reading accurately. Connect a test tachometer to the terminals on each of the tachometers in the boat and compare the reading. The fact that one tachometer reads lower in all ranges, including at idle speed, makes me suspicious of their accuracy.

If the tachometers are found to both be reading accurately, then check the throttle linkage and cables to make sure they are both connected in an identical manner. Most helm-mounted throttle assemblies have at least two different locations in the cable lever where the cable can be attached. If one of the cables was connected at a closer "hole" to the pivot point than the other, the throttle rate through the range would not match. Perform the same check at the carburetor or throttle-body linkage lever to make sure the cables are attached the same.

It is common for propellers to be unmatched to some degree. The difference from side to side is not usually more than a couple hundred rpm, though. If the drives have the same gear ratio, the propellers are the same pitch, both drives are being trimmed equally, and both engines are in reasonably good condition, the rpm from side to side should be very close.

WEAKEST LINK
QUESTION: I own a 1998 232 Baja. It was purchased used in 2002 and came with the standard 310-horsepower 454 setup. I could tell the boat would be a lot more fun with more power. In spring 2004, we removed the engine and reworked it completely. The new engine was producing 470 hp on the dyno using natural carburetion. We reinstalled the engine and ran it for less than 10 hours and burned holes in three of the eight pistons.

We completely redesigned the fuel supply and added a fuel pressure gauge and a knock sensor. We installed new rectangular port heads and spent a lot of time dialing in the fuel mixture. The new power plant produced 515 hp and 509 foot-pounds of torque on the dyno.

Using a standard Bravo One drive and a four-blade propeller with a 24" pitch, the boat was running 72 mph on GPS at 5,300 rpm. This little boat was a blast to drive. It had instant acceleration with great power and was almost as fast full as when it was empty. I have always managed my outdrive knowing the power was on the upper end of its capability.

In the last three years, I have put more than 200 hours on the engine with good success and have performed regular maintenance.

Last week the crankshaft broke just behind the fourth main journal saddle. I lost everything below the heads. The crank shows signs of fatigue. There is evidence of significant wear on the thrust surface—it was grooved as if something was pushing the crank forward. The breaking point seems to support this. The combustion chambers were chocolate brown and the pistons were in great shape with no signs of detonation.

Externally the engine looked super, except for the broken crank. My coupler was worn out (I should have put a new one on when I rebuilt the engine but I never thought about it). I know the engine and outdrive were aligned properly. We used the alignment tool during the installation, and each year we've done maintenance on the outdrive, which slid on and off freely.

The question is: Do you have any ideas why the crankshaft broke? I don't want to repeat my failure. The crankshaft was a forged Eagle 4340 (1/4-inch stroke) and Eagle connecting rods also were used. The compression ratio was 10:1 and I only used 93-octane fuel. I am scratching my head (and wallet!).

Joe Boone
Madison, Ind.

ANSWER: There are many things that can cause a crankshaft to break. Probably, the most common causes are inferior quality or a flaw in the material. Harmonics caused by balance problems, engine geometry, propeller vibration and possibly a worn drive coupler can contribute to crankshaft metal fatigue. A high percentage of crankshafts that break, fail at the point where yours did. It is the location in the crankshaft that has six out of the eight rods driving the crankshaft through the point that has the least amount of mass. Let's call it the weakest link.

The other damage that you are seeing probably occurred after the crankshaft broke. The crankshaft end play is established at the rear main bearing. If the thrust surfaces are worn on the rear main bearings, it is an indication that the crankshaft was bent, twisting, or for some reason the drive was forcing the crankshaft forward. The thrust bearings are designed to endure the forces that push the crankshaft forward.

In a car, oil pressure in an automatic transmission torque converter or the pressure plate in a depressed manual transmission clutch exert forward pressure on the crankshaft. During engine assembly, crankshaft end play must be checked. The desired end-play range is .006" to about .009". On earlier GM blocks, it is necessary to "set" the rear main cap to align the rear thrust surfaces of the main cap. This is done by installing the rear cap with about 10 foot-pounds of torque, and moving the main cap into position by first moving the cap back with the crankshaft, then forward.

This can be done by wedging a large flat-blade screwdriver between the No. 4 main cap and the adjacent crankshaft counterweights. Leave the screwdriver in place while the main cap bolts are tightened to the proper torque. Then remove the screwdriver and check the crankshaft end play with a dial indicator on the end of the crankshaft snout. Later-model blocks and most aftermarket blocks have a dowel pin that properly locates the rear main cap. Even with a pin-located-rear-main cap, the crankshaft end play should still be checked.

Many people install 4 1/4-inch stroke cranks in standard deck blocks. The 9.8-inch (crankshaft centerline to head deck surface) blocks were originally designed for the 396- and 427-cubic-inch Chevrolet Mark IV engines. These engines had a 3.76-inch stroke. The 454- and 502-cubic-inch engines use a 4-inch stroke crankshaft with a rod length of 6.135 inches.

While this combination resulted in a little less desirable rod-length-to-stroke ratio, it is still very acceptable. Going to a 4 1/4-inch stroke with the standard 9.8-inch deck height block requires the use of a longer rod to allow the underside of the pistons to clear the crankshaft counterweights. This combination results in an even less desirable rod-length-to-stroke ratio. The geometry is changed so that the down force exerted by the connecting rod is more sideways to the crankshaft rod journal compared to what it would be with a shorter stroke.

Also, when a longer connecting rod is used, the wrist pin location in the piston must be higher, which crowds the ring pack and shortens the overall height of the piston. Pistons that are short tend to rock more in the cylinder bore, which shortens the life of the engine. Many times the use of a longer connecting rod in combination with a standard deck height block require that the wrist pins be located under the oil ring and must then be installed with a rail support. This combination is not the best for endurance marine performance applications.

My belief is that a tall deck (10.2 inches from crankshaft centerline to head deck surface) is required whenever you intend to run a stroker crankshaft. Using the tall deck block allows the use of combinations that have desirable geometry and design while at the same time allowing you to use a piston design that is up to the rigors of the performance marine environment.

For the most part, the highest-end crankshafts are made for the Mark IV (two-piece rear main seal) blocks. Buy an American-made crankshaft. There are some pretty good cranks in the $1,500 range from companies like Crower and Callies. One of the very best crankshafts money can buy for marine endurance applications is a Sonny Bryant Racing billet crankshaft. They are pricey (about $2,800), but worth every penny.

When you rebuild your motor, go with an internally balanced assembly with a neutral balanced damper and flywheel. Internally balanced rotating assemblies allow forces to be spread more evenly over the crankshaft. Externally balanced rotating assemblies transmit the out-of-balance forces to the ends of the crankshaft, where the counterbalance is achieved through the damper and flywheel.

It is possible that your worn-out drive coupler contributed to crankshaft fatigue. If it was worn to the point that the splined input shaft was loose enough to wobble in the coupler, it would cause the crankshaft assembly to be more out of balance. The resulting harmonics could have contributed to fatigue of the crankshaft metal.

All vibration and harmonics in the drivetrain are transmitted to some degree to the crankshaft and absorbed by the vibration damper. This includes the propeller on your outdrive. If you have a propeller that is out of balance, either statically or due to uneven blade thrust, the resulting harmonics can be transmitted to the crankshaft, which can cause material fatigue. This also is true for boats that have surfacing propellers.

BLUE AND BLACK
QUESTION: I recently purchased a 21-foot Scarab from my neighbor. The stock setup is a 260 MerCruiser with an Alpha One drive. I pulled the motor and had it rebuilt. It's now bored .020" oversize. It was kept as close to stock as possible.

I pulled the stock Thunderbolt IV ignition and will be replacing it with the MSD setup you mentioned in a previous article. I can't find any information on the stock advance curve, only where to set my initial advance. What spring/bushing combination would you recommend using in the MSD marine distributor?

Josh Morrel
Holland, Pa.

ANSWER: I would not be concerned about what the advance curve was with your Thunderbolt IV ignition system. It was ambiguous and maximum advance was achieved at a higher rpm than desirable. I would install the black bushing, which will result in the shortest advance curve. Then install the blue springs on both posts to achieve full advance at about 2,500 rpm.

The black bushing limits the range of advance to about 20 crankshaft degrees. Set your timing at 34 to 36 degrees BTDC at 3,000 rpm. Don't be concerned about setting the timing at idle. It will be about 14 to 16 degrees BTDC. This is higher than it was with your Thunderbolt IV ignition, and will result in a stronger idle with less tendency to revert exhaust water. Having the advance come in a little lower rpm than before will give you more midrange pep.

NIX THE ADDITIVES
QUESTION: I have a 2000 502 MPI with 100 hours on it. I just changed to a Mobil 1 synthetic 10-30W. At the same time, I also added Slick 50, as I have in all my other MerCruiser motors in the past. Just to complicate things even more, I also threw in a can of engine restore in hopes of making the engine last. I run the boat normally and to excess.

Last night my oil temperature rose to 300 degrees and at the same time, my oil pressure hovered around 20 psi. This morning I took the boat out for five minutes. When I started out, the oil pressure gauge indicated about 50 psi, and the oil temperature gauge was not indicating any temperature. After five minutes of running at 3,500 rpm, I brought the boat off plane and my oil pressure dropped to 5 psi at idle (enough to set off the warning system alarm), and the oil temperature had already reached 240 degrees.

Do you have any idea what is causing my problems?

Dick McKean
Troutman, N.C.

ANSWER: Mercury Marine recommends oils with a higher viscosity for your engine. This is especially true for the summer climate in North Carolina. I believe that the minimum viscosity oil for your boat should be 25W-40, 25W-50 or straight SAE 40W. Only part of your problem is that your oil is too thin and not providing enough "cushion" in your engine. The thin oil is exaggerating the effects that your concoction has created.

Slick 50 contains Teflon and your engine-restore additive is more than likely cleaning it out. The debris from this cleaning process is being transported through the oil lines and trapped in the oil filter. It is likely that your oil filter is now clogged. If the filter is clogged, it prevents the oil from also flowing to the oil cooler, which is plumbed in series with the oil filter. Instead, the oil is being routed through the block-mounted oil bypass valve. The result is that your engine is running on unfiltered, non-cooled oil that is loaded with garbage. Eventually, the debris in your oil could clog the smaller oil passages such as those in the hydraulic lifters and push rods.

So, what do you do now? Immediately stop running the engine. Drain the oil, flush the oil lines, change the oil filter and replace the oil with a high-grade SAE 40 oil such as Kendall GT. Run the engine for about an hour and repeat the process until you are sure that all of your concoction and its effects are gone. Hopefully, permanent damage to the engine has not occurred.

Then you can switch to a high-quality synthetic oil that is the proper viscosity. AMSOIL Series 2000 25W-50 Racing Oil is a good choice for hydraulic roller lifter marine engines. High-quality engine oils are formulated to provide complete protection without any additional additives. Sometimes, certain additives can compromise a portion of the protection the oil itself provides. So, use high-quality oil with the proper viscosity and forget the additives.

DO THE WHIPPLE
QUESTION: I have a MerCruiser 496 Mag (not the HO) in a 2005 Glastron GX255. The boat is extremely stable, and runs 60 mph at 4,500 feet above sea level. I always run at altitudes between 4,500 and 8,000 feet, and I am interested in how I can get 75 mph out of the boat (which is apparently what it runs with the 496 Mag HO at sea level).

ProCharger is hesitant to even install a 3.5-psi (low boost) system on my engine because I was told that MerCruiser had apparently changed to a "softer piston" in late 2004. Are there other options to get 500 to 550 hp out of my engine?

Clay Baker
Pocatello, Idaho

ANSWER: The MerCruiser 496 Mag engines use a Hypereutectic piston, and always have. I would not describe the piston as "softer." Instead, I would refer to the piston as weaker and possibly more brittle than a forged piston. Any way you look at it, the piston is less tolerant to the effects of detonation.

What has changed are the crankshafts. The early 496 Mag HO engine models had a forged steel crankshaft while the non-HO Mags had a cast-iron crankshaft.

Now, for at least a couple of years, both models are equipped with the cast-iron crankshaft and the forged offering was discontinued. New crankshafts can't be purchased separately. You'll need to purchase a complete short-block assembly to get a crankshaft. So, the only differences between the two motors are the camshaft, ECU program and the throttle body.

Whipple Superchargers produces a kit for your motor that has good success. Included with the purchase price is the service of reprogramming the ECU for the new setup. This kit is especially good if you are using it to replace the power lost by running at higher altitudes. The 496 Mag HO is a better candidate for the Whipple Supercharger conversion for only one reason—the camshaft.

The "smaller" cam profile used in the non-HO Mag results in a less desirable higher cylinder pressure condition when used in combination with a forced-induction system. The good news is that all you have to do is replace the camshaft with the 496 Mag HO "bump stick" and you will end up with the exact same motor as a 496 Mag HO once the Whipple Supercharger kit is installed.

This is because the other two differences in the two motors (the ECU and throttle body) are changed when the Whipple Supercharger kit is installed. The ECU is reprogrammed and the throttle body is replaced by a Whipple throttle body. Everything from the intake manifold up (including the intake manifold) is replaced when the Whipple kit is installed.

PROBABLY NOT
QUESTION: I have a 1990 Advantage 21SR with a stock 454 Magnum, a Bravo One drive, and a 23"-pitch, three-blade Mirage propeller. I believe the horsepower is 365. I can get the boat to a top speed of 65 mph. Would a higher-pitch, four-blade prop give me more top speed without taking too much performance from my bottom-end?

Carlos Lopez
Bullhead City, Ariz.

ANSWER: It is likely that the X dimension on your Advantage is set at a pretty conservative level, which means that the prop is running fairly deep. If the boat goes on plane well with the three-blade Mirage propeller, chances are that a higher-pitch, four-blade propeller won't provide any benefits, especially for top-end speed.

With your smaller V-bottom, the more hooked up four-blade propeller may cause the boat to lean in the opposite direction of the propeller rotation during acceleration, and the other way during deceleration. A four-blade will probably be more efficient in the midrange, but that is about it.