Stingray Powerboats
Juice Squeezer
Trailer Boats - February 1999

There's no substitute for cubic inches, right? As our tech editor found in this small-block vs. big-block shoot-out, that old adage may no longer ring true.

When it comes to power in marine applications, I'm fond of the adage: "There is no substitute for cubic inches."

Yet, as technology races forward and engineers squeeze more and more juice out of every cubic inch, many have come to question the old assumptions concerning power vs. cubic inches. In other words, with advances such as multi-point fuel injection (MPI), electronic control modules, digital ignition 350 Mag profile systems, high-flow heads and tuned exhaust systems in the modern marine engine, do you still need to add cubic inches to achieve better acceleration and speed?

To find out how cubic inches and weight affect performance, we pitted two MerCruiser sterndrive engines against each other: the 350 Mag MPI (5.7L/350cid) small-block V-8 and the 7.4L MPI (7.4L/454cid) big-block V-8.

From a pure horsepower standpoint, the two are fairly close. The 7.4L MPI has a slight edge with 310 hp at the prop vs. 300 hp at the prop for the 350 Mag MPI. Both engines will operate on regular fuel.

Before I get any further, I should explain what sets these engines apart from others of the same displacement. The 350 Mag MPI is the only 5.7L sterndrive engine from MerCruiser with multi-point fuel injection. MerCruiser has squeezed more power out of this model than any other engine in this displacement category.

There are two other 5.7s: one with a two-barrel carburetor generating 250 hp and another with throttle-body-style electronic fuel injection producing 260 hp. The compression ratio for all three engines is 9.4:1.

The 7.4L is the least powerful in MerCruiser's two-engine 454-cid sterndrive class. The other is the 385hp 454 Mag MPI. Both engines are equipped with multi-point electronic fuel injection. A major difference between the two engines is the compression ratio, which is 8.0:1 for the 7.4L MPI and 8.6:1 for the 454 Mag MPI.

Our test boat for this shoot-out is a Stingray 230LX Bowrider. This proven hull tips the scales at approximately 2285 pounds. It is not a lightweight, but we were not carrying the heft of an aft cabin cruiser, either. Standard power is a 5.7L EFI with an Alpha 1 drive rated at 260 hp. Top-of-the-line power is the 7.4L MPI.

For the best comparison possible, we used the very same hull and sterndrive for both engine tests, swapping one engine for the other. Pulling out the 7.4L Merc 7.4L profile MPI (the first engine tested) and dropping in the 350 Mag MPI took about four hours.

We mated both engines to a MerCruiser Bravo 1 outdrive. With this drivetrain, the 350 Magnum weighs 133 pounds less than the 7.4L MPI. Yet the weight-per-horsepower ratio for each engine is fairly close. For the small-block V-8, the ratio computes to 11.1 pounds per horsepower, giving the small-block a slight advantage.

Both engines were tested with a 21-inch-pitch Mirage Plus prop, the latest generation in the Mirage line. We tried a 21-inch-pitch Laser II on the small-block package, but it didn't work as well. However, with the same drive and prop on both, we had excellent repeatability of our performance figures with both engines.

The cost difference between the two engines is a bit over $1000, with the big-block costing more.

Before testing began, I took a poll around the shop asking: Which engine would be the fastest? It was a mixed response, but most believed that the big-block would outrun the small-block. A few even quoted the cubic-inch adage.

Radar Gun

Based on these predictions, the test results surprised just about everyone.

When dealing with boat weights in the 3500-pound range, we don't think of 133 pounds as being very significant. Yet that extra weight in the stern of the 7.4L MPI equipped boat was enough to prevent the Stingray from planing at 2000 rpm. It struggled at 9.9 mph while the 350 Mag MPI was up and running at over 18 mph. As expected, the small-block engine consumed less fuel. It averaged 10 percent better fuel economy in the slow and midrange speeds, and nearly 20 percent at top speed. Even at the slower speed, the 350 Mag MPI still delivered approximately 5 percent more miles per gallon.

Optimum cruising speeds were about the same for both boats in terms of rpm and mph. The 350 Mag, however, carried its optimum cruise over a broader rpm range and offered better fuel economy at these speeds.

The big-block was the clear-cut winner in the acceleration department, with 0-to-30 mph times averaging 6.2 seconds. The small-block averaged 6.9 seconds to reach 30 mph. Still, the difference can barely be felt in the seat of the pants, and a skier is unlikely to notice at all.

Which engine took the lead when it came to all-out speed? The 350 Mag MPI posted 62.1 mph, besting its larger cousin by 3.7 mph. The small-block hit its top-end at 4900 rpm, while the big-block topped out at 4700 rpm.

I would consider the small-block 350 Mag MPI to be the engine of choice in this particular application, providing there are no expectations of modifications for more power in the future.

With the exception of acceleration, the small-block offers better overall performance per dollar. This engine is also available with the lighter, faster Alpha 1 drive at an approximate savings of another $700 retail. With the improvements we have seen in the Alpha drive over the years, reliability should still be most satisfactory. Still, the Bravo 1 unit we tested is about as bulletproof as they come.

At 300 hp, however, the 350 Mag MPI doesn't have much room for horsepower increases without serious engine work. At 310 hp, the 7.4L MPI has the potential to add considerable horsepower if you like to tinker and modify engines to boost the output.

If you plan to keep the engine stock, which one should you choose? The acceleration figures tell the tale. If we want to pop a heavily loaded cruiser on top with a minimum of fuss and strain, the big-block is the answer. In such applications, there truly is no substitute for cubic inches. If, on the other hand, you are looking to squeeze a lot of juice out of a lighter, less expensive sterndrive, the 350 Mag MPI might be the engine for you.

Trailer Boats
February 1999



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