Allison Grand Sport Test Update: The Mercury 150 Hits 80 MPH

Although the Mercury 150 four stroke was never designed to be a high performance engine, in the end, the reasonable weight (455 Lbs.), adequate gear ratio and reliability, make the popular 150 HP outboard a really good repower engine for many boats. The idea of putting it on an Allison Grand Sport seems a little odd at first, but those same attributes make the 3.0L 4 cylinder possible to turn into good performer, even on the ultra efficient Allison boat. We simply added an aftermarket nose cone and the Mercury ProXS 150 upper mounts to stiffen up the handling.

After running the Grand Sport boat with a 28” Mercury Racing ProMax prop for a while, I decided to try the 30” ProMax again. For everyday use, the 28” ProMax is excellent overall, but does hit the rev limiter (5940 RPM) at 77 MPH. The Mercury 150 has a recommended operating RPM up to 5,800 RPM, and the engine makes 158 HP in that RPM range. It feels like if the rev limiter would allow it to go a couple hundred more RPM, the boat would it 80 MPH no problem with the 28”.

What seems like a strange combo works really well. Darris Allison has always been concerned with efficiency. The most efficient boat is the fastest boat. The 30” ProMax is an incredible prop on this setup

When I originally tried the 30” ProMax, the air temperature was over 100 degrees and I had about a ¾ tank of fuel. I could get 77 MPH at 5600 RPM but it just couldn’t break through. The 30” feels pretty good overall, you notice the higher pitch at lower RPM, the boat is a little less punchy. The Mercury four stroke is fairly torquee, and with a 26” Trophy the Allison is punchy, the 28” is smooth, the 30” is a little bit noticeable. I should’ve done an acceleration test, but I didn’t. I ran the 28” the most because you can have 4 people in the boat and there is no problem, I don’t think the 30” would do well loaded up. However, the temperature has dropped, the engine has 20 more hours on it and the tank was around a ¼ full.

The plain white and black striped Allison looks lean and mean.

On a perfect Texas evening, I gave the 30” another shot. The first pass I knew right away the favorable conditions made a difference. 79 MPH at 5740 RPM. On another straight away, with a little ripple on the water 80 MPH, just shy of 5800 RPM, and I was hoping I could get past 5800, but as long as I held it I couldn’t. Considering we are at about a 7% theoretical slip, I’m really happy. With the 30”, the Grand Sport cruises at around 71 MPH at 5000 RPM.

Is there anything else we can do? I have about 45 lbs. of counterweight for balance, we ran with less than a quarter tank and there isn’t much else in the boat, so I can’t reduce weight. The engine is mounted so that I’m ¾ above the pad, I could go higher but for overall drivability and safety, it runs perfect. That said, we could run a couple different props, and make a ¼ increase in engine height and probably get that 81 or 82 MPH. When I looked at the dial, it certainly looks like that’s in the cards with just a little more RPM left. One other thing I’ve talked about is going with solid mounts, this would improve overall handling and probably my next move. (I’ve been lazy)

Mercury makes a ProXS version of this engine, with a few differences, the mounts, which we have on ours, the gearcase which is different and uses lower 2.08 gears, and it is tuned to run higher RPM to 6,000. But, the ProXS won’t be faster, because the gear ratio actually makes it slower technically. It would have to spin over 6300 RPM to equal the regular 150 Merc four stroke. If Mercury had the option of a 1.85 or 1.75 on the ProXS, it would be great.

Seating for 5, side storage and cup holders, a glove box, rear storage compartments and under the bow storage makes for a very practical 20’ sport boat.

For the Allison Grand Sport, it continues to amaze me. Being able to cruise around 40 MPH and get 10 MPG is incredible. The usability of the boat is amazing for the size. Not many 20 foot boats can have this much storage, seating for 5 and perform like this. With the 150 Mercury, or other 150 engines, you get affordability and usability. Ours was a certified pre-owned with a full warranty and cost far less than a new one. The money saved on purchase with a 150 HP, at the pump and running cost is real. Beyond that, the boat itself is just a blast to drive. When it comes to agility, speed and the fun factor, I don’t think there is anything out there like it. Driving it is rewarding and the boat is just so thoroughly well designed. In the cockpit, you are sitting deep inside, sheltered and securely “in” the boat. The Grand Sport’s low profile belies the amount of storage and practicality the 20 foot hull has. You can see why they get so much money on the used market. Many people buy them new and keep them for life.

If I was repowering a Grand Sport and wanted to go with new technology economically, I think the modified 150 Mercury is a great option. If you could change the gears on a ProXS, even better. Yamaha has the 4 cyl 150 Vmax SHO which is a great engine and the gears, 2.00:1, would put it on par with the Merc because the Yamaha can rev a little higher to 6000+ RPM. The 175 Vmax SHO is a sleeper because it has the 1.86 gear ratio. The 175 SHO could push the Allison to the mid 80s most likely. The Yamaha’s are double overhead cam engines too, but they do push the weight limit for the boat. Those are probably the best engine options if you want new tech. The Evinrude G2 V6 150 SHO pushes the weight limit and the gearing is too low, although the gearcase can be swapped for the 1.85 version. Evinrude’s new 3 cylinder 150 is very light but again is geared a bit too low.