Mercury 300hp V8 Review

It took six years to get to the simple outcome before us, a 64 degree V8 of 4.6 litre capacity with double overhead camshafts and 24 valves. During the six years of development, department heads in Australia played a major role in the core requirements which mirror what we have seen in other successful launches like the 2.1L 75-115hp outboard and the impressive 135-150hp 3.0L engine.

The new V8 is an extension of the recently launched V6; it is effectively the V6 with two more cylinders. It carries over the cam-chain and low-maintenance solid lifters of the V6 meaning low service costs. At 4.6 litres it has a bigger swept capacity than the Yamaha V6 rated to 300hp and is lighter yet its narrow angle keeps it compact ideal for close twins and triples.

The use of a fly-by-wire throttle body and closed loop fuel control should help efficiency through constant adjustment based on load, revs and throttle position. The same sensors also allow the engines to have adaptive speed control where the engine raises revs to maintain speed in turns, the fantastic technology only seen in ski and wake boats before.


One thing I really like about the V8 is the way Merc has planned out the variants for you. The existing FourStroke is the entry model with a racey ProXS above it, a Verado that has extra goodies in it and two commercially oriented options in the SeaPro CMS and SeaPro AMS.

The FourStroke comes standard to suit hydraulic steering and cable controlled throttle, the excellent Digital Throttle & Shift (DTS) is an option. The 300hp V8 Verado comes with a bigger alternator (115amp up from 85) a switchable exhaust that raises the volume through a bypass valve above the water line, Integrated Electro-Hydraulic Power-Steering and DTS as standard. The ProXS takes the exhaust bypass as standard (which loses it some weight) but leaves the steering hydraulic and throttles cable, again upgrades are optional. The two SeaPro’s are targeted at commercial operators with the SeaPro AMS coming with the fantastic Advanced Midsection (AMS), bigger alternator and DTS as standard. The Conventional Midsection (CMS) SeaPro drops the fancy steering, DTS and bigger alternator, effectively mirroring the spec of the FourStroke but with a commercially rated warranty.

On the warranty, we reported in News a while back that Mercury was moving to a 3+3 year warranty on all 2018 and newer engines, this stands on the V8 and importantly it is non-declining meaning full coverage for the complete engine for six years should you maintain scheduled services.

At the press conference, Mercury touched on a number of key selling points based on research done both locally and overseas. The key take homes were that users wanted simplicity, reliability, fuel efficiency, excitement and low weight. Simplicity and reliability should come from the simplified powerhead touched on above and I’ll cover off the aural feel later but first the weight.


Weight savings are important but the saving of 4.5kg over ‘nearest competitor’ (that's the Yamaha by the way) is negligible. One less six-pack of beer, a summer of morning jogs or leaving a couple of rod and reel combos at home and you have that difference covered.

Head up the range and the story is really not all rosey. Take the Verado, with its integrated power steering at 272 kg (20in leg again) and it's actually heavier than the G2 ETEC which also has power steering built in. The only 300hp V8’s lighter than the rest are the ProXS and FourStroke and not by enough to have me excited.

On fuel efficiency, the company claim up to eight percent better than its main rival but till we test ourselves, we will not comment on it. On noise and feel, the US based designers and engineers have done an outstanding job. The bimodal exhaust on the Verado gives you the best of both worlds offering super quiet mode or eyebrow raising rumble while the ProXS keeps the rumble on at all times. The use of the AMS on the Verado is also a master stroke as it has made these new engines the smoothest in the market, by a long way.


We got to sample the FourStroke, ProXS and Verado on a few boats in Queensland and were frankly blown away. Forget the insignificant weight savings, what matters is how smooth the Advanced Midsection (AMS) equipped Verado was and how raucous the ProXS was. eWe had two Verado’s hanging off the back of a new Southern 24 that were so smooth and quiet we could hold a conversation at 4300rpm without really raising our voice. The mantle of quietest engine, once held by Honda, is now Mercury’s. At WOT we saw a frankly insane 52 kts although it did take a lot of fine trimming to see it, comfortable was 49-50 kts. The ProXS saves a bit of weight through a different exhaust that gives it a distinctive noise at idle and bark at startup. Underway it was louder than the standard FourStroke but not intrusive. 

Check out the full review in issue #504 of Trade-a-Boat magazine. Subscribe today for all the latest boat news, reviews and travel inspiration.