Air Horn O' Plenty
A Simon air horn throttle body supplies more
02omph to a stock Mercury 3oox big block outboard
Late last year, as we ran off pass after pass with our Stroker shoot-out rigs equipped with Yamaha and Mercury big-block outboards at speeds over 100 mph on John's Lake near Orlando, Florida, we were just getting warmed up. Rigged with a stock Mercury 300X, our one-off Stroker had a unique hull with a special, updated pad design, though it's not perfected yet and won't be available for some time. Yet, we had a chance to run it in this developmen tal stage. In a departure from the norm, Stroker rigged this boat with the steering on the port side, hoping to create a more level ride without having to add too many lead shot bags to the port side of the hull to counterbalance the drivers own weight.
The potential of the new design was readily apparent. Coupled with Strokers unconventional yet effective rigging, as well as loads of setup and seat time, this Stroker posted a best top speed of 105.5 mph, with its box-stock Mercury 300X turning a sedate 6300 rpm with a 14 x 32-inch Mercury Lightning ET three-blade over-hub-exhaust stainless race propeller. With a 10 mph wind and a slight chop, that top speed was easily repeatable, and we reached the same mark over several passes.
TALL AND QUICK
Despite the big prop and tall (1.62:1) gear ratio, acceleration results from this combo were very strong. We hit 30 mph from a dead in-gear idle in just 6.2 seconds, and 40 to 60 mph in 4.7 seconds. Upper midrange punch was also 
strong; we recorded 60-80 mph sprints in just 6.3 seconds.These results are surprisingly quick for a full-size, full-weight hull. Our carried load was a 250-pound driver, a 200-plus pound observer, three batteries, a trolling motor, safety gear, a full tank of fuel and a 75- pound shot bag of ballast. We had the engine set at 3 inches above the bottom of the hull with 14 inches of setback.
However, we were not done in our quest for more speed with the experimental Stroker. To help us achieve greater velocity, we turned to Eric Simon, proprietor of California-based Simon Test Results Mocorsports (760/440-9334; simonmotor sports.com), who specializes in hotrodding 3.0L "big block" Mercury outboards - those produced from 1994 to current. One of Simon's more basic hop-up bolt on kits is his 300X Big Bore Throttle Body Conversion Kit.
Expanding on the theory that these engines need more 02 than the stock 3.5-inch diameter throttle body inlet can provide, Simon tested various throttle body air horn diameters, checking progress on a propshaft dynamometer until he settled on 3.8-inch diameter air horn as the best compromise of top end improvement, while retaining drivability and low-end punch . Simon offers this kit as a $425 conversion for 300X out boards, and a more elaborate $1795 kit for earlier (1996-2001) 3.0L EFI engines. The increased cost of this kit is due co the need to swap out the entire intake, plenum assembly and fuel rail on older engine models.
INSTANT GRATIFICATION
The swap was a simple bolt on for our 300X. In less than an hour, the boat was back in the water and ready for the retest. The installation involved simple hand tools and can be performed by anyone with a good working knowledge of how to remove and install parts on an outboard. However, it always helps to be able co perform basic adjustments, such as those performed on throttle and shift linkage and cables. In our case, we needed to perform adjustments to the engine's fu ll throttle and idle stops, as well as check to see that the throttle would return fully after it was floored.
The results were instantaneous and gratifying; our efforts were rewarded with a 200-rpm gain and a new top speed of 107.2 (GPS and radar-veri fied) at 6500 rpm. Acceleration from a dead stop to 30 mph improved by 2/10 of a second, to a flat 6 seconds. Forty to 60 mph elapsed times dropped by 4/10 of a second, from 4.7 to 4.3 seconds. Upper midrange punch remained about the same. The engine's increased pull was very evident. Even without the gauges to measure actual results, we could feel it pulling harder all the way to the top end.
PERFORMANCE BARGAIN
Overall, the results of this simple swap were very impressive. Anytime you can realize a 2 mph gain on a boat that's already running well over 100 for less than $500, you should jump at the chance. Thar's a performance bargain in anyone's book. Simon has made the 3.0L Mercury powerhead his focus and passion, and kits like this truly showcase his talent and expertise.The kit is a real "sleeper"modification; the new plenum looks stock from the outside, and it would be difficult for anyone who doesn't know any better to tell that anything had been modified. In a world where cost effective performance modifications for outboards are as rare as hen's teeth, Simon's 300X Big Bore Throccle Body may well be considered an air horn o' plenty.
John Tiger Jr,
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