vit's post on the 8th gen forum regarding injectors -
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Injector matching and what it really means to YOU
Several injector manufacturers have a process they follow to test and flow injectors and sell them as "matched" sets. Why do they do this? Injector Dynamics has a good article on why this matters:
Does Dynamic Matching Really Matter? - Injector Dynamics
But what does this actually mean to YOU and YOUR car? Everyone and their mom is trying to sell a "Bosch" injector to you. From my experience all their marketing hype isn't what it's cracked out to be. Most simply do not do this kind of dynamic matching and it shows in the logs and on the cars I work on.
Basically, in simple terms, the idea behind dynamic matching is to package a set of injectors that flow the same (or damn near close) across the workable "pulsewidth" range of the injector, 2ms to 10ms (we actually run them to 15-16ms, but that's such a large pulsewidth that if you're already matched at 10ms you're fine beyond that). The better matched the set of injectors, the smoother your car will run. This is notable as the injectors get larger and larger in size (a 10% deviation on a 400cc injector isn't nearly as much as 10% on a 1000cc or larger injector).
Why is this the case? Well, let's take your typical 1000cc injector and apply it to your standard stock motor K20 or K24 Civic. At idle you will be around 1.8ms pulsewidth. At normal "light" driving and cruising you will be around 2.0-4.0ms. It is very important for idle quality and light throttle/cruise drivability that the injectors be perfectly matched as a large enough deviation and you start feeling light "jerks" or "stutters" at cruise or light throttle acceleration, all the way to full blow misfires (on a single cylinder usually). Your idle also sounds very "poppy", and it is, because the motor is basically misfiring every few cycles.
But why does this happen? Well first of all at light throttle and idle the motor runs at "stoic" fuel targets, 1.0 lambda (14.7 afr or so), this is essentially your "economy" mode under normal driving conditions. If you have a 12-15% deviation between injectors, at idle you could essentially be seeing something like this on the 4 cylinders:
- 14.5 afr, 15.2 afr, 13.8 afr, 16.3 afr
Not very well balanced, and in all likelihood the 16 afr will cause a slight miss, or at the very least a "stumble" in the engine (basically a hesitation).
What if we only have a 5% deviation? We might see something like
- 14.5 afr, 14.8 afr, 13.9 afr, 15.3 afr
Better right? But still not very smooth and we might get the occasional stumble.
Now how does this translate to part throttle in the 2.0-4.0 range? Well when you're in the 2.0-3.0 range the motor will be very touchy to having a deviation between injectors, and you also have to take into consideration a few other variables like how the injectors are flowed, their spray pattern, etc. I've seen injectors that have been matched pretty well still have some slight issues in the 2.0-3.0 range, but if that deviation is high in that area, you essentially can run into a situation where the motor runs something like
- 14.7 afr, 14.6 afr, 14.3 afr, 15.5 afr
Now you won't get a full blown misfire, but the motor will sort of "stumble" (and doesn't necessarily happen every cycle) and you'll feel a very very light hesitation at cruise or as you try to lightly accelerate. In this situation you'll add throttle and once you're in the 3.5ms+ pulsewidth area things smooth out.
Keep in mind this is just a very short explanation for something that I have seen a lot of over the years. I'm in no way an "injector expert", so my explanation may have holes, but it's a good idea of what's going on when you experience this drivability nuance. And no, a "tune" technically can't fix this behavior, as it is a mechanical issue related to the combination of parts on the car.
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