STRESS RELIEVER
Controlling fuel pressure and compression ratios.

QUESTION OF THE MONTH: NO FORWARD GEAR
QUESTION: I am in the Air Force stationed in Hawaii, which you think would be a performance-boat Mecca, but that is another story. I have a 1997 Baja 252 with a carbureted 7.4-liter engine with a Bravo One outdrive. I recently docked the boat for dinner and when I was ready to leave, I shifted it into forward and the engine just revved up with no forward motion.

However, I could put it in reverse and go with no problem. What do you think is wrong? I'm pretty much forced to do everything myself here.

Mike Myers
Honolulu

ANSWER: There are a few things that could be the problem, but the one that is most likely, is also the most serious. It is possible that there is a problem with the intermediate shift cable or where it is connected to the shift mechanism in the drive. Its operation can be checked by removing the back cover from the upper drive housing and ensuring that full motion is achieved when the shifter is operated. Occasionally, the shifting mechanism breaks or becomes loose which also can be inspected while the rear cover is off the drive upper housing.

But it is most likely that you have broken the gear floor out of the forward drive gear in the upper. If your prop rotation is right hand, it will be the lower gear in the upper housing that is damaged. This damage is commonly caused by shifting the drive at an elevated rpm, or flying the boat and not throttling properly.

If a gear floor is broken, the upper will need to be rebuilt or replaced. Mercury offers an upgraded gear set for the older Bravo drives that has a stronger gear floor. Usually when the gear floor breaks, it also damages the thrust bearings, the upper vertical shaft and the cone-clutch assembly.

WHIPPLE FOR THE BAT BOAT
QUESTION: I currently have an Armada B-28 Bat Boat with a Volvo DPX500 engine. I was hoping to get another 100 to 150 hp out of this package. I want to install a Whipple Supercharger, but Whipple does not set up its units to work with Volvo engines.

Is there any way to get the charger to work, or is there another way to get the extra horsepower I want without tearing the engine apart?

Douglas Stevens
Edison, N.J.

ANSWER: I wouldn't give up on the Whipple Supercharger installation. Your engine is the same base engine that was used on the Mercury Racing HP500EFI. The Whipple kit will fit on the engine.

You will have to do some custom work dealing with the accessories and the wiring harnesses. At worst, you will have to install some Mercury parts on your motor to complete the installation. Sounds like a fun challenge to me. If you install the Stage 2 kit with the 3.3 Whipple Supercharger and the "standard" intercooler, you can expect to gain near 250 hp. If you want less, opt for lower boost.

OVER CARBURETED
QUESTION: I currently own a 2004 26-foot American Offshore catamaran. I went from a 468-cubic-inch engine to a 540-cubic-inch Generation 5 engine. The motor is equipped with an 8-71 blower and twin 850 CFM carburetors right out of the box. We started out with standard power valves and No. 87 jets in both the primary and secondary sides.

Then, we changed the jets to No. 93 in the primaries and No. 95 in the secondaries. The boost level is at 9 psi. The motor is running too rich and acceleration is poor. The fuel pressure is about 7 psi. What is your recommendation for fuel pressure and jets on this particular setup?

The engine is equipped with MSD ignition, a MSD High Vibration Blaster coil and a MSD distributor that has the timing locked out with 32 degrees total advance. The electric fuel pump is an Aeromotive 1000. Any advice you can give would be greatly appreciated.

Jamie Secor
Chittenango, N.Y.

ANSWER: First of all, depending on what your compression ratio is, 9 psi of boost with an 8-71 blower is probably too high. With your 540-cubic-inch motor, it is likely that you are overdriving the blower quite a bit. You could probably get away with 8 or 9 psi of boost if you were also running an intercooler to take the heat out of the charge created by the blower running at elevated speeds.

Unless you have very efficient cylinder heads, an aggressive cam profile and a decent exhaust system, your 850 CFM carburetors are probably too large. You may be able to find the correct jetting and power valve combination, but if the combination of all eight venturis is too large, the resulting lower velocity in the carburetors is detrimental to throttle response. Finally, I would not lock out your advance curve on the distributor. It may run OK on the big-end, but this practice results in too much spark advance at idle and may create a hard-starting problem.

I would use a pair of Holley 750CFM "Double Pumpers" (Part No. 0-4779C) in place of your 850 CFM carburetors. Install 10.5 power valves in the primary metering blocks only instead of "boost referencing" the carburetors. Start with No. 82 jets in the primaries and No. 88 jets in the secondaries. Seven psi of fuel pressure is adequate as long as this pressure is maintained during extended full-throttle operation.

If the fuel pressure drops while the boat is underway at near or wide-open throttle, the indication is that the fuel system is not adequate to flow the required fuel. In many cases, the entire fuel system from the tank pickups to the carburetors needs to be upgraded with larger hoses and fittings. I would obtain the advance mechanism kit that goes with your MSD distributor. If you don't have it left over, you can purchase the kit separately from your MSD Ignition dealer.

Install the black bushing which limits the curve to 20 degrees at the crankshaft in combination with the light blue springs that result in full advance at 2,500 rpm. Then set the total advance at 34 degrees with the engine running at 3,000 rpm. This advance curve setup will allow the engine to start easier with less load on the starter motor and will idle at about 14 to 16 degrees advance. At the point where the throttle is advanced from idle, the timing will not be so advanced to cause detonation. Finally, I would add an intercooler, which will result in more power and reduce harmful detonation.

COULD IT BE WATER IN YOUR CYLINDERS?
QUESTION: I own a 1994 29-foot PowerQuest powered by twin MerCruiser 502 EFI Magnums with IMCO Marine Powerflow exhaust systems. I am currently having a problem with the starboard engine. When motoring out of the marina (engines in gear but at idle speed), after about 15 minutes of idling the engine dies out. These engines are factory equipped with two fuel pumps with VST tanks.

I have checked fuel pressures on high and low sides and especially on the high side, and the pressure does not change when the engine starts to die out. The VST overflow to the manifold diaphragm is fine and there is no rupture on the fuel pressure regulator diaphragm.

At times, when I have the mufflers on while running the motor at home, I rev up the motor just a bit and the rpm stays up for a few seconds more than normal and then the engine pops a bit before idling down to where it should be. Is it possible that the ECM or injectors may be bad? What do you think about switching the ECMs around to pinpoint the problem?

David Hanson
Highland, Ind.

ANSWER: It is unlikely that the problem is associated with an injector. If the injectors were bad, the engine would not idle for 15 minutes and then quit. There is a remote possibility that the fuel is aerated and a mild vapor-lock condition occurs after the engine heats up, but I doubt it. If the ignition system is operating correctly, there are only a few things that can cause your EFI engine to quit while idling. It could be either too rich or too lean. Or there could be water getting into the cylinders at low speed and causing spark plugs to foul and misfire.

It is unlikely that the ECM is causing the problem. However, it is possible that the ECM is responding to a condition or parameter that is causing the problem. I would invest in a scan tool (or borrow one) so that you can look at some ECM values. Rinda Technologies makes the most common, simple-to-use scan tool that will plug into your harness. The price is about $400.

There are a few things that can affect idle quality that you can detect with a scan tool. Once the scan tool is connected to the harness, turn on the ignition and follow the prompts to read the engine data. The scan tool will automatically check for faults. Many times the fault codes shown will tell the story.

With the engine off, before it is started, the manifold air temperature and the coolant temperature should be very close to the same. If one of the sensors is defective, the reading will be way out of whack or at a default reading that usually involves a minus sign. Defective temperature senders affect the fuel mapping. The manifold air pressure (MAP) reading should be close to the barometric pressure when the engine is not running.

If it is way out of whack, you could have a faulty MAP sensor. There is a vacuum line connected to the MAP sensor that will cause the pressure to lower when the engine is at idle. If it does not, check to make sure the vacuum line is connected and does not have leaks.

The throttle position is displayed in voltage and percentage of throttle. Usually, the throttle position sensor (TPS) voltage on your engine is near .55 to .62 volts. If it is significantly higher or lower than this, it will need to be adjusted. Any time the TPS is adjusted, the ignition must be shut off and turned back on so that the new TPS voltage is recognized by the ECM as zero-percent throttle.

The reason that your one engine seems to return to idle speed slowly after revving is because the idle air control (IAC) is staying open for a period of time. The ECM has a designated idle programmed into it. If the engine is idling too low, the program causes the spark timing to advance and open the IAC, which creates a controlled vacuum leak that simulates opening the throttle butterfly slightly. It is possible that your whole problem will be resolved by adjusting the idle screw on the throttle body butterfly.

With the scan tool set to monitor the IAC position, advance the idle slightly until the IAC position value returns to zero or near zero. Any adjustment of the primary butterfly will affect the TPS setting. So, after the idle is adjusted, turn off the ignition, and restart the engine. Check the TPS setting and make sure the IAC reading works its way down to near zero in a reasonable time frame.

It is important to wait until the idle speed normalizes before shifting into gear. Shifting at the higher idle speeds is destructive to drives and transmissions. This is one of the reasons that we want the IAC to close in a reasonable time frame so that the operator does not have to wait excessively before shifting into gear.

If all of the settings above are correct, I suspect that exhaust water is reverting into your cylinders at idle. If the symptoms include the engine idling slower and slower until it dies, I would bet that this is the case.

In order to find out, take a spark plug socket and ratchet with you and as soon as it dies, remove the No. 6 or 8 spark plug and see if there is water on it. I suggest that you inspect the No. 6 or 8 spark plugs because they are at the rear of the engine on the side that may have the shorter riser pipe.

The problem could be caused simply because the idle setting is too low. If it occurs quicker with the mufflers closed, that is another indication that it is water related. It is also possible that there is a slight water leak in the gasket between the IMCO exhaust manifold and riser pipe. If there is evidence of water running down the exhaust manifold on the outside, you can bet that it is also running down the inside.

The problem could be as simple as a leaky gasket or as complex as a crack in the manifold or riser. In rare cases, there could be water leaking into a cylinder through a head gasket or a flaw in a head casting, especially around the exhaust valve guide.

COMPRESSION AND TEMPERATURE
QUESTION: I have questions about compression ratio in conjunction with using the Edelbrock big-block marine oval port heads. I just talked to a representative for Edelbrock at the National Hot Road Association U.S. Nationals. He is trying to tell me that I need higher compression for the water temperature of the 140-degree thermostat that I run in my stock 300-horsepower 454. He said that 140 degrees is not enough temperature for 9:1 aluminum heads to run efficiently. Can you clear this up for me?

Also, can you tell me what the compression ratio would be if I installed the Edelbrock oval port marine heads on my 1994 300-hp 454?

Keenan Buchanan
Peru, Ind.

ANSWER: I will start my answer by saying that boats are different from cars. Cars have pressurized cooling systems with an antifreeze solution in them. Both of these factors raise the boiling point of the coolant and minimize the formulation of steam pockets in critical hot spots in the engine. Mineral and salt deposits and formulations also are not a concern in the well-maintained automotive cooling system. These factors alone are justification to continue running your 140-degree thermostat.

Other factors would include the immense difference in load that is placed on a marine engine as opposed to the typical automobile or light truck engine. Boat engines are always running at a significant load with minimal intake manifold vacuum. Car engines run at light loads most of the time and have time to breathe when coasting. Boats don't coast. That's why they don't need brakes.

The Edelbrock oval port marine head assembly is a very good choice for your stock 300-hp 454. The same applies for other models of the base level of this marine engine also known as the 330-horsepower and the 7.4-liter versions. After checking several of these engines, I have discovered that the stock compression ratio is a little overstated as verified by CNC'ing the chambers, head gaskets and pistons in the bores.

I have found that the compression ratio with the stock iron heads ranges from 8.1:1 to 8.3:1. The stock chamber is about 118 cubic centimeters in volume. The Edelbrock chamber is about 110 cc. This reduced size in the combustion chambers will result in raising the compression ratio of your engine about six-tenths. In other words, if your current compression ratio is 8.3:1, it will be about 8.9:1 with the Edelbrock heads, which is still very safe.

Aluminum heads do dissipate heat better than iron heads, which will help control detonation. I would take this opportunity to also install a performance oval port intake manifold and carburetor to realize the maximum benefits of your cylinder head upgrade.