Injector Sizing Explained

This was written a while back in response to some stellar forum exchanges. I finished it up because apparently it's still badly needed.


Injector sizing concerns itself with the two ends of fuel consumption: idle, and WOT.

At idle, you are limited by time a given injector takes to open, spray minimal amount of fuel, and then close. Different injectors have different characteristics, but generally speaking, the bigger the injector, the bigger the minimal amount of fuel it wants to spray. If this minimal amount of fuel injected is bigger than the need for the fuel at the given airflow consumption, you will dump more fuel than it needs. This will cause carbon buildup, mess up spark plugs, eventually kill O2 sensors, and it generally going to make the car run like poo, since it's basically over fueling. Over fueling is easily recognizable as surging. Engines with big cams, or any other 'race' modifications have a significantly diminished efficiency of the motor at low pressure and low RPM. This means that it needs a lot less fuel than stock setups at idle. On the flip side, hopped up motors have need for bigger injectors at higher RPM and pressures, to match the increased airflow. These two aspects often clash, as the bigger injector is not capable of injecting smaller amount of fuel at idle. This is when the injector sprays the minimal amount it can do, which is often bigger than what the motor needs and you're back to over fueling and surging. This is why you have to raise idle RPM on such setups--bigger cams get up to higher efficiency very quickly with more RPM. You have to figure out at what RPM the airflow requires more fuel than the minimal pulse width of the injector. There are a lot of variables involved, and very few of them are linear, so this is not a problem with an easy solution.

At WOT, the situation is the opposite. You just have to make sure that at any given airflow, the amount of fuel needed isn't bigger than ~85% duty cycle of the injectors. On naturally aspirated setups this is simple, as the airflow usually just goes up with RPM as MAP is pegged at atmospheric pressure's level. On boosted setups, you can boost a lot more in midrange than up top with fairly small injectors. This is exactly what the 'chipped' 1.8t Audi/VW's do, 17psi at 4000rpm, and back to like 7psi by redline, so the dinky stock injectors have enough time to spray in required amount of fuel at high RPM, while at lower RPM they just take longer to provide adequate amount of fuel. These are two crucial ideas:

• 1.Injectors cannot regulate how much they spray in any way other than how long they are ‘ON’. That time has to be smaller than the time between ignition, and that’s exclusively RPM dependent. So don’t look just at the Pulse Width, you can only judge it against RPM.
• 2.Airflow depends on many factors: VE, boost, RPM and of course, displacement. You must take ALL of them into consideration at ALL possible values. A small engine at midrange RPM can make a lot of airflow with enough boost.

The duty cycle is an often misunderstood. It's a ratio between the pulse width required for proper AFR and the length of time you have to inject that amount of fuel. It is dictated by the RPM and the Otto cycle. 20ms pulse width at 3000rpm is just fine (50% duty cycle), because the 'window of opportunity' is 40ms. But the same 20ms pulse width at 6000rpm is 100% duty cycle because the 'window of opportunity' is also 20ms. Very often on forums you get people making statements like 'my car runs just fine at 130% duty cycle, so the stock injectors must be underrated.' What they really mean is their injectors don’t have the capacity to provide enough fuel in the time allotted. The computer calculates air mass, figures it needs 30ms worth of fuel and starts spraying, not thinking if it has that much time before the next ignition or not. This is where the ridiculous 'acceptable' duty cycle numbers come from. Computer calculates that it wants 30ms fuel with 20ms to do it in, it's going to show 150% duty cycle. This however doesn't mean the fuel gets there. It physically cannot get there in less time than it's allowed, as the intake valve closes shut. So what happens then?

Not only the ignition will happen without all of the fuel necessary at the time, but the fuel is still spraying during the power stroke, puddling on top of a hot valve. Then the injectors turn off and recharge for another spray cycle half way through another cycle already, when they should be spraying fuel. So while one cycle might be fueled fine, another one is going to be lacking proper fueling. Such sequences of events cause severely unpredictable fueling and widely scattered AFR, fluctuating EGT's, and generally making it unpredictable and difficult to tune. In addition to that, you're stressing the injectors beyond their operational range, overheating them, and possibly causing a pre-ignition.

Without proper injectors, at best your car is going to be running like shit; at worse it will cause a meltdown. It's just not worth risking. Injectors are cheap, I will never understand people who are willing to drop 3k in heads that flow few CFM better than others half the price, but won't even think of spending $300 to make sure that the airflow can be matched with fuel and your expensive parts don’t melt away. Don’t be a cheap dumbass, be fast and reliable instead.