Volvo Penta V8 7.4L Rebuild 330hp to 430hp
Taking a pair of tired stock 7.4L (454) motors out to 8.1L (496)
At the end of last season I decided to improve the performance of my 330hp 7.4 litre (454ci) petrol engines in my Sunseeker Thunderhawk 43. I decided to go for a popular modification of taking them out to 8.1 litres (496ci). This involves increasing the cylinder bore as well as the stroke.
 One of the blocks in its reborn 8.1L form. Click picture to Enlarge |
Swaymar Marine a new company who build bespoke high performance petrol engines and improved stock engines were keen to do the work. With a balanced Scat 750hp crankshaft, forged con rods and forged high compression pistons the engine internals are considerably tougher than stock components.
 Twin 750hp Scat Marine cranks, forged conrods and pistons will make these motors perform the way they should. Click picture to enlarge |
The compression ratio was raised to 9.5 to 1, more efficient than the previous ratio of 8 to 1.
More compression means more torque, more power, faster fuel combustion and ultimately more efficiency.
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David Vizard
Swaymar have had a marine camshaft developed by renowned US tuning expert David Vizard and made by Comp Cams to provide the power delivery needed by a marine engine.
 David Martin balances each crank for ultimate efficiency and smooth running. Click picture to enlarge |
The cylinder heads and ports were also reworked to a David Vizard recipe (click here for more details). To match this 'Vizard recipe' the inlet ports and inlet manifolds were also reworked. (click here for more details).
Billet steel timing chain sets and roller rockers were used, essential for accurate valve timing. To keep it all sweet modifications to the oil pump and the lubrication system were made (Click here for more details). All of this tuning was designed to work with both petrol and LPG as my engines are dual fuel and are capable of running on either fuel at the flick of a switch.
 Polished aluminium gen 5/6 rocker covers from Hardin Marine in the States. They do not leak like the tin covers fitted to a gen 4 engine. |
To run effectively on lpg stellite exhaust valves are an essential modification and will benefit any engine from a durability point of view. These are much tougher and considerably more heat resistant than stainless valves and can be supplied by Real Steel in the UK or direct from the States.
Standard stainless exhaust valves will pit around the rim with LPG as will the valve seats in the cylinder head. You will lose compression and hence performance over time and fuel consumption will increase.
My original 7.4l engines had compression down to 80psi from the recommended 150psi as a result of pitted valve seats.
New valves were fitted through out with stelitte exhaust valves and all valve seats were re cut.
 Light and lean, this stainless manifold by Hitek Marine lets the motor perform properly. |
 Old and heavy, the original exhausts are inefficient and prone to leaking sea water after a couple of years. |
The motor now needed to breath to perform properly. This meant some decent stainless steel exhaust manifolds. My old stock iron manifolds had previously leaked sea water and would always subsequently pose a threat to my new engines.
I discovered that Hitek Marine in Australia build one piece stainless exhaust systems for a wide range of engines and weigh only 25lbs each, as apposed to 80lbs for stock iron manifolds. Being a true header system as apposed to the primitive stock exhausts, they provide both more power and should also improve economy.
 A finished long block ready for marinizing. Click picture to enlarge |
 Bolted on to the dyno ready to run Click picture to enlarge |
Finally the day came to run these engines on the Dynometer. Once the dyno had showed that the engine had been run enough to bed the piston rings in, it was safe to load it to get torque and power readings.
 Peak power...430hp and 521ft/lbs torque at 4300rpm. Click picture to enlarge |
Our first torque reading was 510ft/lbs at just 3100 rpm some 40 to 50ft/lbs more than I was expecting. Pushing further up the rev range we hit 431hp with 521ft/lbs at 4350 rpm.
David Martin the technical director had built these engines to produce between 420 to 450 hp at 4300 to 4500rpm with bags of grunt low down and he had certainly achieved this, and all with a measly 650cfm stock carburetor (to be upgraded to 850cfm carburetors shortly).
This power delivery has been tailored exactly to my 7.5 tonne Sunseeker Thunderhawk 43, which will soon be tested in earnest on the water.
 Ebbtide Marine check the engines prior to refitting. |
With Thanks to: