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?

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.

MORE TOP-END FOR THE TOP CAT

QUESTION: I have a 2005 Conquest Top Cat I with a Mercury Racing HP525EFI and a Bravo XR drive. Currently the boat runs in the mid-70s in ideal conditions. I am curious about any suggestions you have on getting more horsepower and realistically more speed.

I am open to rebuilding the HP525EFI, adding a Whipple Charger or just buying a complete new engine. Of course I would like to maintain reliability and possibly pump gas capability.

I also would like to keep the costs down to a minimum but I am aware that this process doesn't come cheap. I would really like the boat to run in the mid-80-mph range, so I am guessing about 200 additional horsepower is what I will need. Any suggestions would be much appreciated.

ANSWER: Your HP525EFI is a good candidate for adding a Whipple Charger kit. Accomplishing your goal by rebuilding your HP525EFI and leaving it naturally aspirated is unrealistic. The Mercury Racing HP525EFI is pretty good the way it is. You might be able to get another 50 to 75 horsepower out of the package, which won't make a huge difference for your Top Cat.

If your HP525EFI is in good condition, you can simply install the Whipple Stage 1 HP525EFI kit and end up just shy of 800 horsepower. If your engine is the first design HP525EFI, I would suggest changing the head gaskets to either the Cometic style used on the later-model HP525EFIs, or the Felpro Part No. 17048 marine head gasket designed for the Generation 5 GM blocks.

The current design Whipple Charger HP525EFI kit includes everything needed to convert the engine to a MEFI 4 system. The Whipple kit includes a new intake manifold with the intercooler and complete induction system. Everything is changed from the intake manifold gaskets up. You will need to add a separate water pickup and sea strainer for the intercooler.

Being that the center pod on your Conquest Top Cat I is relatively deep, adding a hull pickup in the center pod will work fine. Properly installed, the overall package will be reliable and can run on 91-octane fuel, plus provide decent power throughout the rpm range. The final horsepower of at least 775 should push your Top Cat into the 90-mph range, which means that you will be able to cruise in the 80s.

You will definitely need to go up in prop pitch. You may want to select a Mercury lab-finished Maximus, which will increase the overall efficiency of your deck boat, especially when loaded with people. With the supercharged engine, you will have great out-of-the-hole performance, eliminating the need for a prop that has a lot of slip while coming on plane.

—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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