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
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
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
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.
GENTLEMEN, PLACE YOUR BETS
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.
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.
MAKING YOUR DECISION
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
