MerCruiser puts supercharged lighting in Wellcraft's Thunder. 

WELL, HERE WE GO again. My horsepower nerve is twitching, shooting synapse-popping bursts of energy from my brain to my throttle hand. Sometimes it's the smell of Cam 2 exhaust fumes. Sometimes it's the view through my helmet visor. Today it's the unmuffled roar of 1,200 horsepower blasting from the transom. I have to jam those Zero-Effort levers to the wall and buzz awhile.

Finally out of the no-wake zone, I nurse the boat onto plane, then slam the throttles home. Thunder rolls off the steel hull of a Norwegian cruise liner, a white flash in my peripheral vision as we howl out Government Cut at 75 mph.

MerCruiser has too much to lose to do something poorly. Starting with the supercharger itself, each aluminum rotor, case and manifold is hard-anodized inside and out to protect it from salt corrosion. The 525SC is fitted with a Weiand blower displacing 177 cubic inches, turning at a 2:1 ratio (two blower revolutions to each engine revolution). Both the 600SC and 800SC use a 420-cid B&M Mega-Blower, the 600SC spinning it at a 0.93:1 ratio, the 800SC at a slightly faster 1.18:1 ratio. 

George Craig, MerCruiser Hi-Performance R&D engineer, explained that blower speed was determined after factoring octane characteristics of the target fuel, the heating of intake air as it is compressed, the noise level of the blower and other mechanical factors. One reason the large Mega-Blower was used on the 600SC and 800SC is that it can produce the required compression at a low speed, thus avoiding the stress and noisy "blower whine" typical of high-speed automotive applications.

The blowers themselves were modified by the vendors for this marine project, mostly to help them survive the "light switch" duty cycle of a marine engine (it's either on full throttle or off). All have upgraded oil seals and the previously mentioned anodized coating. 

Weiand cast a special intake manifold for the 525SC that accommodates MerCruiser's larger thermostat and keeps it in the same location used on the HP425. B&M also cast special intake manifolds for the 600SC and 800SC that have more material at the manifold's base for a better seal to the heads. The B&M manifold also accommodates a MerCruiser thermostat housing, which is perched under the snout of the blower drive.

Instead of the cogged drive belt often associated with drag cars, the MerCruiser blowers are driven by a grooved belt, similar to the Kevlar-reinforced serpentine belt used on today's passenger cars, but wider to distribute the load over a larger area. The grooved belt is safer in this application, according to Craig, because it will slip if the engine backfires through the blower.

With a cogged belt, the blower might explode in that situation. The belt has a self-adjusting, spring-loaded tensioner. If the belt should ever break, the engine will simply suck air through the free-wheeling vanes of the supercharger, says Craig. The 525SC, for instance, still produces more than 300 hp without its blower.


Turning the blower faster, of course, will yield more compression and hence, more horsepower. Recognizing that the enterprising, power-crazed customer might be tempted to increase blower rpm by installing a smaller top pulley, MerCruiser acted to foil such irresponsible activity.

"On the Weiand blower we've specified an exclusive spline shape on the drive shaft. Pulleys to fit that spline will only be available from MerCruiser," says Craig. "The Mega-Blower on the 600SC and 800SC uses an exclusive pulley and belt with 10 grooves. You'd have to be pretty determined to change the blower ratio."

Internally, each SC engine was also revised to accept the added pressure of the supercharger. In each case, the compression ratio was changed, lowered in the 525SC (from 8.8:1 to 7.5:1) and the 600SC (from 8.75:1 to 7.5:1) and raised in the 800SC (from 8.6:1 to 9.0:1). The 525SC and 600SC are fitted with reverse-dome pistons, while the 800SC has flat-top slugs. Why a higher ratio on the 800SC?

According to Craig, MerCruiser's target of 800 hp couldn't be met on pump gas without either raising engine rpm (and shortening engine life), increasing displacement (an expensive proposition), or increasing the compression ratio (and running high-octane fuel). The last option was deemed the best way to go in terms of cost and engine durability.

All three engines have special graphite head gaskets. Because these engines make power without running at high rpm, major changes to valve train componentry were not needed. MerCruiser did attend to many details, however, such as adding hardened valve seat inserts, cleaning up the airflow in the heads and combustion chamber, and re-balancing each crankshaft to match it to the new pistons.

The standard exhaust system on each engine is a cast-aluminum Gil/ MerCruiser manifold with a stainless-steel pipe. Stainless-steel headers are available but result in minimal power increase, as the supercharged engines do not rely on exhaust pulses to control flow, according to Craig. Remember, we're packing power in on the front end, but those stainless headers do look nice.

Each engine also runs with a digital electronic ignition system developed by MerCruiser Hi-Performance with Firepower Ignitions that integrates a 5400-rpm rev limiter to save the engines of those who are slow on throttle response. The 525SC and 600SC are offered with MerCruiser Three, Four or Five drives, while the 800SC is sold only with the MerCruiser Six drive (what used to be called the Kiekhaefer Drive).

MerCruiser Hi-Performance is essentially a custom engine shop for boat guys. It's comforting to know, for instance, that your engines were hand-built from the block up by a single technician in MerCruiser's shop in Oshkosh, Wisconsin. Factor in MerCruiser's extensive development capabilities, the rev limiter, and some octane flexibility, and you'll understand MerCruiser's confidence in selling the 525SC and 600SC with a 90-day warranty (try getting that from Big Daddy's Hot Rod Shop). Buy the 800SC, which operates a little closer to the edge, and you're on your own, warranty-wise.

Change the oil and run good gas and these motors, especially the 525SC and the 600SC, will reward you with faithful service, not continuing visits to the engine shop.

Simple high performance is what it's all about.

This review/article originally appeared in Boating Magazine, July 1991 and is written by Charles Plueddeman. For more great powerboat reviews, visit their website and subscribe at: