When talking about the response time difference between a direct injected engine and a port injected engine, it got me thinking so I did some quick math.
At 4K rpm, a GM LS engine would have an intake port air velocity of roughly 150 feet per second. In the average V8 port injected engine, the injector nozzle is roughly 6" from the intake valve. So the fuel has to travel 6" before it can be combusted.
At 4K rpm the injector will fire 33 times per second. The math suggests 66 times per second, but keep in mind that in a 4 stroke engine, combustion only takes place every two revolutions of the crankshaft.
At 150 feet per second air velocity, the fuel from the port injector takes 1/300 of a second (or .00333 if you prefer) to get from the injector to the combustion chamber.
A direct port engine fires the fuel directly into the chamber.
Doesn't sound like much difference, does it?
If it takes 16.95 seconds to get through the slalom course on a pass, a single injector fires 559 times during the pass. That equates to a 1.86 second total time delay vs direct injection over the course of a pass through the course.
That sounds like a huge number when you are only talking about 16.95 seconds to complete the run, doesn't it?
Well that would depend upon how much of the 16.95 seconds it's making fuel delivery corrections. It's making them all the way through the pass, then I can definitely see why response would be better with direct injection. I've never ridden in a boat with ZO, so I've never been able to hear what the engine is doing first hand, but I'd bet it is correcting quite a bit of the time.
Before I did the math, I was questioning if Direct Injection was really making that much difference in response. After doing the math, assuming it corrects a LOT, I would think it makes a pretty noticeable difference to a short line skier over port injection.
This is when it would really be great to hear from some port injected engine ZO boat owners, vs the newer DI engine ZO boat owners. No amount of data will override real world user experience.