We have known since the introduction of the Mercruiser "cross ram" style intake manifold, that it is very restrictive for performance applications. They make good torque at lower RPM's, but fall short in supplying enough air to feed the engine in the higher RPM range (4500 and up). When upgrading a 502 and larger cubic inch engine, that style intake struggles to keep up, especially above 5000, where you need it most. Ideally, we want the horsepower to continue to rise until it reaches the optimum power for the camshaft and cylinder head design. In this case, we designed a camshaft, and set up the cylinder heads with valve size and porting for a maximum RPM of 6000 to 6100. Piston design, rings, and many other factors were all taken into account for this build.

The goal was 600 + horsepower with 9.0:1 compression, without breaking the bank. The test bed would be my 28' Daytona. The base engine we started with was a 2001 Mercruiser 502 Mag. These engines are a good base with quality parts. Forged crankshaft, 7/16? rods, rectangle port heads, 4 bolt block, etc. We were confident we could reach this goal with the cast iron heads which would greatly reduce the cost of this upgrade and allow us to shift some of the expense to improving the intake air flow. While aluminum heads would give us the ability to run 87 instead of 91 octane fuel, maintaining the same power level, it would take approximately 10,000 gallons of fuel to break even on the added expense and our goal was to make this upgrade as inexpensive as possible.

Just a few of the basic details of the engine are:

Bore and torque plate hone the block to get a round bore with a specific hone finish that will give good ring seal for hundreds of hours. Straighten the line bore by line honing the block. Surface and square the block to a specified deck height.
509 Cubic inches .030 larger bore than the 502
Higher quality and re-sized valves
Mild cylinder head porting with a specific chamber volume
B&D special design, tight lash, solid roller camshaft

I know the solid roller lifters immediately raises eyebrows to a history of high maintenance. A camshaft lobe profile designed to work in this RPM range, 6100 and below, along with being a "tight lash" lobe, almost eliminates that concern. Over the course of 6 years, the valves have been checked twice, and no adjustments were needed. Anyone that has done any boating with me knows, I am not easy on these engines.

In an effort to keep this interesting and not bore you with too many details, I will describe the build
process and hopefully hit on the parts I feel will be of interest and most important. I think the intake manifold modifications and results are very interesting.

One modification that is mandatory to the cylinder heads are the rocker arm threads. They are 3/8-16 from GM and with the increased load from the more aggressive cam lobe and higher pressure valve springs, they must be enlarged to a 7/16-14 thread.

Heads 1.jpg

Heads 2.jpg

I'm going to skip to the intake modifications here. Engine assembly is very tedious and all the details, in my opinion, would make this boring to most. One of the most important things in building an engine is making sure the fuel delivery is more than adequate. I wanted COMPLETE control of what was happening in relation to the engine control. For the EFI, we used Accell's DFI system. A sophisticated, laptop controlled engine management. It gives an almost infinite amount of control over ignition and fuel. We also installed a wide band O2 sensors in the tail pipes to allow O2 sensor control to meet our target air/fuel table.

We knew the stock fuel injectors would not be able to keep up with the fuel requirements of these engines. On top of that, we don?t like disk type injector design for performance applications. We also like a higher fuel pressure than that type of injector can take. Our choice was a Siemens triple cone 48# injector. These injectors produce a much better spray pattern and better atomization.

I had (2) 500 EFI intake manifolds left over from installing Whipple kits on 500 EFI's. I wanted to see if we could modify and produce a bolt on top for the 500's that would allow almost any horsepower upgrades without air restriction. We took the plenum (top), and machined the whole top away. We then took a piece of aluminum plate and made a dual throttle body top and welded it to the plenum. We topped it off with (2) billet 1250 cfm throttle bodies. (with port fuel injection, the engine will only pull the air it needs and the fuel delivery is not dependent upon a vacuum signal so "too much air" really doesn't apply)

Feb 6 2006 016.jpg

Daytona Upgrade 009.jpg

The end result, was 630 horsepower at 6000 RPM?s and 647 lb ft torque at 4000 RPm's. The torque curve was very linear and exactly what we were hoping for. It made 612 at 3000 and over 600 all the way to 5300 with a strong 560 at 6000.

Now the comparison between the dual and single top:
As suspected, the single produced its maximum torque of 630 at 3600 RPM?s. The dual top produced the same torque numbers at 3600, but continued to climb until it?s max numbers at 4000 RPM's. This is where the dual top really showed the improvement. The single top at 5800 was a full 30 lb ft less than the dual. And at 6000 the dual made a full 35 more horsepower.

Now remember, the engine was controlled by a wide band O2 in closed loop for all the dyno pulls, so the tune was exactly the same for all the tests.

written by Brian Fetherolf of B&D Marine