I get this question all the time. You're rebuilding a small-block Chevy or a Ford 351 for your boat, the car cam is cheaper and sitting right there on the shelf, and the guy at the parts store says "a 350 is a 350." It's not that simple once that engine is sitting in a hull. Here's why.
A Boat Engine Is Always Climbing a Hill
A car cruising down the highway barely works. Once it's up to speed it coasts, and the engine loafs along using a small slice of its power.
A boat never coasts. Water drags on the hull every second you're moving. Let off the throttle and the boat slows down almost right away. So a boat engine runs under heavy, steady load the whole time, like a truck pulling a trailer up a long grade that never ends.
That changes what you want out of a cam. You don't want peak horsepower way up at the top of the rev range. You want strong, usable torque from just off idle through cruise, so the engine can push the hull up on plane and hold it there without straining.
Water in the Cylinders (Reversion)
This is the reason that matters most. Most inboards and stern drives have a wet exhaust. Cooling water gets dumped into the exhaust at the risers and rides out with the exhaust gas.
Now think about what a cam does. There's a moment at the top of the exhaust stroke when the intake and exhaust valves are both cracked open at the same time. That's called overlap. Performance car cams use a lot of overlap because it helps at high RPM.
At low RPM and idle, though, all that overlap lets exhaust flow backwards into the cylinder. Engine builders call that reversion. In a car it just makes a lumpy idle. In a boat, what's sitting in that exhaust is water.
- Exhaust (and water) can flow backwards at idle
- Rust on valves, seats and cylinder walls
- Stuck valves after the boat sits
- Hydrolock risk in the worst case
- Less overlap
- Wider lobe separation angle (around 112° or more)
- Keeps exhaust and water moving the right way
- Built for a wet-exhaust engine
Torque Down Low, a Clean Idle, and Good Vacuum
A marine cam is built to make its power where a boat actually runs. Look at the specs on Comp Cams' Xtreme Marine 212/218 roller cam for the small-block Chevy: 112° lobe separation and a working range of about 1,200 to 5,200 RPM. That's right where a typical stern drive lives.
You get three things out of that:
- Hole shot. More low-end torque gets a heavy hull up on plane faster.
- Smooth idle. You spend a lot of time idling through no-wake zones and around the dock. A choppy car cam makes that miserable, and it's hard on shifting in and out of gear.
- Engine vacuum. Good vacuum keeps the carburetor metering right at low speed and keeps vacuum-operated parts happy.
A big car cam that pulls hard at 6,000 RPM does you no good if your prop keeps the engine at 4,600 wide open.
Twin Engines: Watch Out for Reverse Rotation
On a lot of twin-engine inboard boats, one engine spins the opposite direction so the props don't pull the boat to one side. That opposite-rotation engine needs a reverse-rotation camshaft. The lobes are ground to work turning the other way. A standard car cam in that engine won't run right, period.
Who Makes Marine Cams
You've got good choices whether you want stock-style or a mild upgrade.
| Maker | Line | What to Know |
|---|---|---|
| Comp Cams | Xtreme Marine | Hydraulic roller and flat-tappet grinds. The 212/218 SBC roller runs 1,200–5,200 RPM on a 112° LSA. |
| Crower | Marine hydraulic flat tappet | Four SBC grinds on 112–113° LSA, from a mild 1,700–5,100 RPM cam up to 2,400–6,000 RPM. |
| OEM (MerCruiser, Volvo Penta, Indmar, PCM, Crusader) | Factory replacement | Matches the original spec. The safest pick for a stock rebuild, and the way to go for reverse-rotation engines. |
Whatever you buy, check that it says marine and matches your engine's rotation. Also match it to your lifter type: flat tappet or roller.
Common Mistakes I See
- Buying the "RV" or "towing" car cam and calling it close enough. It might have decent low-end torque, but it isn't ground with marine overlap in mind.
- Going too big. A lumpy cam sounds great at the ramp and runs terrible everywhere else. It also won't reach its power band behind a stock prop.
- Forgetting rotation on a twin. One wrong cam in a counter-rotating engine and you're pulling it back apart.
- Skipping the break-in on a flat-tappet cam. Use zinc-rich break-in oil and follow the maker's break-in steps, or you can wipe a lobe in minutes.
- Not matching the rest of the combo. The carb, intake, exhaust and prop all need to work with the cam you pick.
The Bottom Line
A marine camshaft costs about the same as a good car cam. What you get for that money is an engine that idles clean, pulls hard where your boat actually runs, and doesn't suck water back into the cylinders. When you're rebuilding anyway, that's the cheapest insurance you'll ever buy.