Direct Injection vs Sequential Fuel Injection

More Torque and Less Fuel - PCM Direct Injection Engines

Direct Injection vs. Sequential Fuel Injection

Ever since PCM came out with the new direct injection H5 and H6 engines, we have been asked what the difference is between direct and sequential injection and if PCM is the only company using direct injection. I hope to answer both questions below.  Here is some basic information about injection systems.

Basically, a SFI (sequential fuel injection) system has a fuel injector for each cylinder.  Each of these injectors puts fuel into the system at 30-40 PSI before the intake valve.  This air/fuel mixture is pulled into the cylinder past the intake valve on the intake stroke of the piston and is then compressed and ignited.  This system was way more fuel efficient and created way more power than the carburation systems it replaced.  Sequential fuel injection has been used in marine engines for several years and is the system that is currently used in most new towboats and the system PCM used on engines until 2016.

DI (Direct Injection) is the next evolution of fuel injection where the fuel is injected after the intake valve directly into each cylinder at over 3000 psi.  What are the advantages DI of over the old SFI system?  Antuan Goodwin (engine writer for Cnet) said that readers may have noticed that during the jumps from carburetion to SPFI (single port) to MPFI (multi-port), the point at which fuel is added to the intake charge, has moved from before the throttle to the intake manifold and onward to the individual intake runners — closer and closer to the combustion chamber. Direct injection takes this evolution to the next level by placing the injector inside the combustion chamber. By moving the injector into the combustion chamber, GDI (gasoline direct injection) gains a few advantages over the previously discussed systems.

By putting the injector inside the cylinder, the engine’s computer gains even more precision control over the amount of fuel injected during the intake stroke, further optimizing the air/fuel mixture to create a clean burning explosion with very little wasted fuel and increased power delivery.

A GDI system also has more flexibility regarding when in the combustion cycle the fuel is added. MPFI systems can only add fuel during the intake stroke of the piston, when the intake valve is open. GDI can add fuel whenever it needs to. For example, some GDI engines can adjust the timing so that a lesser amount of fuel is injected during the compression stroke, creating a much smaller, controlled explosion in the cylinder. This so-called ultra-lean burn mode sacrifices a bit of outright power but greatly reduces the amount of fuel used during times when the (boat) requires very little grunt (idling, decelerating, etc.).

Well, enough of all that tech stuff – So what does direct injection mean for you?  It means you can have an engine that has more torque (low end power) than a sequential fuel injection system has, and you get that extra power while burning less fuel.  I can tell you from personal experience that the new H5 just plain stomps the last year’s PCM ZR409 out of the hole on a Centurion FS33 with max ballast.  But the really big surprise happened when we tested the new PCM H6 engine here at altitude in Utah. We were able to get a fully ballasted Ri237 (10,500 lbs. total weight) up to wakeboard speed at Deer Creek.  We were really surprised because that is the max amount of weight that we could get to wakeboard speed with my 2015 supercharged 550 hp PCM XR7.  We tested them side by side, and the difference was negligible.

Needless to say that after our personal experience, we weren’t terribly surprised when we received the engine comparison specs from PCM.  The New PCM Direct Injection H5 and H6 are 33% faster 0 to 23 mph than last year’s PCM ZR409 & ZR450.  The best part though was that these new engines are up to 27% more fuel efficient than last year’s comparable engines.  More power, less fuel.  That’s a combination I can live with.  It is no wonder a good friend of mine, Trey Thurman from PCM, told me a year ago that PCM had something special coming for 2016 and that I would love it.  Trey was right, these motors are very, very impressive.

Side note, just in case you were wondering, PCM is the only towboat engine manufacture using direct injection engines for 2016.    That means if the towboat you are looking at is not a Centurion, Nautique, or Supreme, it most likely does not have Direct Injection.

“Life is Short, Let’s Ride”


  • Paul Edwards| October 21, 2016 at 9:45 pm

    I absolutely love my H5 in my SV233. I boat at an elevation of 6,070′.

  •| October 7, 2016 at 9:05 pm

    What is gasoline direct injection and how is it different from traditional methods of getting fuel into your car’s engine?

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