Adding RamJet to 350 MAG

[Zackman]

I have a 1999 350 MAG in my current boat and was wondering if retrofitting the intake with the Ramjet (GM P/N 12489371) was practical? I would be hoping to gain about 20 Hp and 20 Ft*lbf of torque. Also, would this produce a flatter torque curve with higher torque at lower rpms? What other modifications would I need to make?

Zack

[stonebreaker]

Are you currently fuel injected or carb'd?

[Zackman]

MPI. Also, I believe Mercruiser uses a similar arrangement for their Black Scorpion Ski Boat Motors in both the 350 and 377 (stroked 350) versions.

[stonebreaker]

If you're already MPI, I think you might be a little optimistic as far as gaining 20 ft-lbs switching manifolds. I think a more cost-effective mod would be swapping the air filter for a K&N piece, and changing the stock valve springs for a higher performance set, such as the Comp Cams 915 beehives. The stiffer valve springs will allow you to rev the engine higher, increasing your top speed. Swapping out the paper filter for a K&N piece is worth about 10 hp on the dyno. Another mod that's worth about 15 hp is swapping the stock rockers for 1.6 ratio roller rockers.

[Zackman]

Stonebreaker, the air intake is a flame arrestor. Hp and torque drop off on this engine around 5100 rpm and WOT is now around 5000 rpm. Are the current rockers 1.5?

[stonebreaker]

The stock rockers are 1.5's, but more importantly they are the old ball type and not roller trunnions. The roller trunions reduce friction, freeing up horsepower. Going with 1.5 rollers will give you about 10 more peak hp at the prop, changing to 1.6 rockers will give you an additional 5 or so. Adding the stiffer springs will allow you to rev higher, assuming you don't have a rev limit programmed into your computer.

Do you exhaust through the prop or through the transom? That's another way of freeing up horsepower without putting any more stress on the engine - exhausting through the transom.

You can do all of these mods without retuning the computer, because they are mods that tend to free up existing horsepower instead of creating new horsepower that would require a computer retune. If you do run a little lean, you can buy an adjustable fuel pressure regulator and turn the pressure up a bit without having to pay for a full computer tune, although if you add anything substantial like a cam you will need to tune the computer to compensate.

[Zackman]

I have the captains call exhaust thru the transom. It's difficult for me to tell for sure, since I don't run on static water (i.e., rivers), but I believe I get about 1 to 2 mph gps more with the exhaust open thru the transom. I do have a rev limiter, but I'm not sure if I am running up against it or the torque limitation due to the impeller size. With the exhaust thru the prop I do about 55 mph gps. The boat is about 4600 lbs with the B3.

[stonebreaker]

2 more mph sounds about right for a 30 hp or so increase. I know it doesn't sound like much, but the rule of thumb formula for increase in speed is percent change in horsepower is equivalent to percent change in velocity cubed:

(57/55)^3 = X/300 hp

(57/55)^3 * 300 hp = 333

X = 333 hp

Thus opening the captain's choice is good for about 30 hp - which is a reasonable number. We see similar increases on the dyno when we swap out the stock exhaust systems on our cars with free flowing performance exhausts.

The GM Powertrain site says that the standard rev limiter is a fuel shutoff type, which means that when you hit the rev limiter it will shut the gas off to the engine; so unless Mercruiser changed it you have a fuel shutoff at what looks like 5500 rpm or so on the power graph.

The next place I would look at to find horsepower is the valve springs. Any vortec - compatible spring kit would work. A set of GM performance springs and retainers will run you around $50 or so (I'm pretty sure the LT4 springs will fit your heads), or you can go all out and pick up a set of Comp Cams p/n 915 beehive springs and retainers for around $230. I'd go for the beehives because their design is inherently resistant to float and thus they don't need dampers - they will run cooler and, because of their small retainers, will actually put less stress on the valve train than a conventionally designed spring.