One of the most popular engines Mercury Racing ever built was the 500 EFI. This engine has been used in a wide variety of vessels from family sportboats to multi-engine poker run boats—and even offshore racers. As these engines reach rebuild time, we are often asked about upgrade options. With newer engines making more horsepower and torque, one of the first questions usually is, “What can we do to make more power?”

Budget and application are two key questions Potter Performance Engines’ customers need to answer before they upgrade their 500 EFI engines.

Case in point: We recently received a call from a gentleman up north looking to rebuild and upgrade his 500 EFI from his 27-foot Fountain Powerboats V-bottom. I gave him several options starting with the average cost for a complete stock rebuild. This is important information as it gives everyone a starting point. The most popular option has been to upgrade this engine to a 540-cubic-inch, 625-hp naturally aspirated package.

Of course, there are other options including supercharging, which can make as much as 800 hp, but you need to be aware that the Bravo drive is always the weak link in the equation. Other factors also play a role in your options. Hatch clearance on many boats is an issue. This can limit the use of supercharger installations especially if an intercooler is used. You also need to consider the boat. How much more power is reasonable for your catamaran or V-bottom?

My customer from up north elected to go with the 540/625 option as this best fits his budget and gave him the extra power he was looking for. We like to have our customers send us their engine complete—meaning with its coolers, hoses, mounts, flywheel housings and exhaust. The idea here is not only to be able to properly inspect these items, but to return the engine to the customer in a complete drop-in-ready package.

We also dyno the engine once the upgrade/rebuild is complete. This way, we know that when the engine leaves our shop everything was OK. Our customer doesn’t have to worry about putting parts back on their engine prior to installation as well.

Here’s a closer, more detailed look at the process.

After an engine is delivered to our facility, we start by disassembling the entire engine and cleaning all parts to be inspected and reused. Many times we find brackets and mounts in poor and rusted condition. In these cases, we send all items needing attention out for stripping, blasting and powder coating.

After inspection, the engine block goes into our machine shop for boring, honing, square-and-deck, as well as deburring and cleaning. Cylinder heads are disassembled, checked for cracks and inspected prior to porting work and competition valve job. Heads are reassembled with some of the best valve-train components available from Ferrea, Isky, Manley, Comp and Crane.

Hands-on in every engine rebuild—the author hard at work (click image to enlarge).

Once the crankshaft, rods and pistons have arrived, we balance the rotating assembly. We internally balance all of our engines, which receive new rotating assemblies, and replace the factory-cast flywheel and balancer with steel components. This is a good idea whenever you’re increasing the power level of a stock engine. We have seen more than a few balancers and flywheels fail!

As soon as we receive the new high-volume oil pump, we like to disassemble the pumps for inspection and deburring. We also modify the regulator valve for better pressure control. The oil pump pick-up screen is installed on the pump, aligned and welded. The intake manifold receives some mild porting work and fuel pressure regulator is modified for adjustability. A new, larger mono-blade throttle body is installed for increased airflow.

The short-block is assembled keeping all bearing clearances to between 22 and 27 ten-thousands-of-an-inch. This is very important to maintain proper oil pressure and bearing life. We use coated bearings in all of our upgrades.

The new camshaft is installed and degreed for proper valve timing. The cylinder heads are installed with new ARP head bolts. New Crower stainless rockers and custom .135 wall push rods are installed and adjusted. The long block is completed and prepped for paint.

All items that need repainting are prepped before receiving an application of acid-etching primer and three coats of DuPont urethane paint. The engine is put on a special final assembly cart and dressed. Any items that did not check out OK such as starters, alternators, couplers, fuel pumps, etc. are replaced. The ECM is reprogramed to accommodate the larger displacement and fuel requirements.

Once the engine has been thoroughly inspected it’s off to the dynamometer. All of our engines are run in for at least one hour on the dyno while timing is set and temperatures and pressures are maintained. We send our customers detailed dyno sheets, as well as photos of the entire assembly process. With top-quality components, we find these engines will last for many years and have great reliability.

Editor’s note: This is engine builder Ron Potter’s third bi-weekly column for speedonthewater.com. His next installment will go live on Monday, Feb. 29.

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