This is a summary of our latest testing of the Allison Grand Sport, with our Project 150 Mercury four stroke. Let’s call it a backyard 150R. We received our gearcase back from Jason Wolf, who did the modifications, and got a chance to do some runs on the Caloosahatchee River in Florida. I wanted to get some fuel mileage numbers, and obviously some top speed runs. Here is a rundown.

First, the gearcase now has a long torpedo shape. In general, the cone is probably a bit too pointy, a more rounded tip is actually faster, and produces less drag than a pointy tip. I may modify this in the future, but for now it works, allowing us to raise the engine without worry. In fact, I am running it maxed out on the plate, which is about ¼ below the pad because it was setup conservatively at the start. I am going to mount the engine higher on the plate, to have a little more adjustability.

Although the ProXS mounts are an improvement, they are still too mushy for precise steering input at higher speeds. This thing just needs solid mounts. I will look into doing at least the lowers, this should take considerable slop out of the steering.

We also didn’t get our Trophy prop yet, and just ran our stock 28” ProMax. All around performance is quite good with the ProMax, once the RPM gets over 3000, the acceleration improves drastically. The prop, and the fact it’s a 150 HP 4 cylinder don’t make it a dragster, but the hole shot is decent, and once it gets up to about 40 MPH, the Mercury fourstroke has good midrange power.

Top speed was 76 MPH at 5876 RPM. At 4800 RPM, the Grand Sport was doing 65 MPH, achieving 6.2 MPG. (10.4 GPH). That might be the most impressive thing about the Mercury Marine 150 four stroke, it is really efficient. Because it is a single overhead cam engine, with big displacement, there is plenty of torque and seems to make good power throughout the RPM range. The drawback is that the 3.0L Merc is not very rev happy, with a max recommended RPM of 5,800.

Data

4500 RPM, 60 MPH, (8.9 GPH) 6.75 MPG

4800 RPM, 65 MPH, (10.4 GPH) 6.2 MPG

Top Speed, 76 MPH at 5876 RPM

With some adjusting, I am confident we can get to 77 or 78 MPH. If only Mercury offered the new 150 ProXS with a sportmaster and solid mounts, it would be a great little engine.