Q&A

Ask Away! with Jeff Smith: Adding Power to a Ski Boat SBC—EFI or Carburetor?

(Image/Our State)

I’ve got a direct-drive ski boat with a worn out 350 c.i.d. small-block Chevy.

I’m thinking about building a new engine with modest goals. It’s a family boat, but I can’t resist the urge for a little more power since it’s due for a rebuild anyway.

Here’s my plan: 4.030 x 3.48, flat piston, with -5cc reliefs, 0.005 deck with a 0.041 thick head gasket which should put the compression at around 10.2:1. I’m looking at the Summit 23-degree 195cc heads. I have the lake for a radiator, so it runs 160 degrees all day long. This is a factory roller cam block and the current cam is 197 / 207 degrees at 0.050 with 0.430/0.450-inch valve lift with 1.5:1 rockers. The intake is a Performer RPM with a 750 cfm carb. I’m considering one of the EFI throttle body injection systems.

The exhaust manifolds are terrible, and probably won’t flow much better than a cast iron ram’s horn type manifold, but I do have a 3-inch exhaust. What I want is all the low end torque I can get with about 350 HP and a horsepower peak at around 5,500 rpm.

Slow idle speed is important for pulling skiers and putting in coves, so that’s a key concern too. It currently idles at about 650 rpm, which is perfect. So, essentially, how small of a cam can I put in to make that much power? — D.B.

Jeff Smith: Your entire package sounds really solid. I like the choice of cylinder heads. One thing I see is that your compression will only be 8.9:1 with 76cc heads. But choosing the TFS 64cc head will put the compression to 10.25:1 compression. Running against a 160 degree water temp should work well — depending upon how much timing the engine wants you might be able to get away with 89 octane fuel.

I think you could go up to a 215 at 0.050 cam. COMP makes a 218/224 Xtreme Marine cam with 0.495/0.503-inch lift and a 112-degree lobe separation angle (LSA). It’s that 112-degree LSA that will work well in the boat because it should idle around 14 inches of manifold vacuum and be smooth. This short intake duration will also lower the peak torque and hp rpm levels. As you mentioned, those exhaust manifolds will also hurt the top-end power.

I use a simple HP formula I learned from my buddy Steve Brulé at Westech Performance. His equation assumes a good cylinder head and 10:1 compression and a good set of headers.

Here’s what it looks like:

Displacement x 1.25 (torque per cubic inch) = peak torque 

Multiply peak torque times 0.9 that assumes a 10 percent loss of torque from peak torque to peak horsepower.

Multiply that times peak rpm and divide by 5,252.

This is the classic horsepower equation: HP = (Torque x RPM) / 5,252.

For your application: 355 c.i.d. x 1.25 = 443 peak torque — but with your poor exhaust manifold — drop that to around 420 ft.-lbs.

420 x .9 = 378 ft-lbs.

378 x 5,500 (again, due to those poor exhaust manifolds) = 2,079,000 / 5,252 = 396 hp.

That’s probably close. With headers the numbers would be closer to 420 hp.

The EFI idea has merit, but my personal opinion is that a well set-up carburetor would make as much power, and a good carb tuner could set it up for a lean cruise setting that would be every bit as good as EFI for much less money.

A 750 carb would be my choice — preferably with screw-in bleeds to help tuning. It could be set up so that it would idle warm with a very lean air-fuel mixture and cruise (depending on load) at between 13.5:1 and 13.8:1 air-fuel ratio.

The nice thing about a ski boat is that it operates almost exclusively in three basic modes:

  1. Light cruise in the no-wake zone
  2. A heavier cruise rpm to get where you’re going, and
  3. Heavy power for towing skiers. That’s really pretty easy for a carburetor since there are usually very few changes in throttle. The one exception is when you go to full power for pulling the skiers. This should be easy to tune.

So overall, your engine plan has plenty of merit.

Here’s why I wouldn’t suggest a self-learning EFI.

With a log manifold, it will be difficult to find a spot where steam or water won’t contact the sensor.

If that happens, this liquid will instantly kill the sensor. I’m not familiar with log boat manifolds but a short investigation indicates that finding a safe position for the oxygen sensor will be very difficult.

The solution would be purchasing dry boat headers — but that’s also an issue because you can’t place the sensor in the collector without at least an 18-inch extension because at idle, the sensor will be fooled by returning reversion air brought into the collector.

Plus, those headers don’t appear to work well for ski boats.

So with all these challenges to running an EFI system with a feedback O2 sensor, the best bet would be to stick with a carburetor.

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One Comment

  1. David W Henninger says:

    Nice choices the carb makes good sense on this project
    Dave

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