A new generation of fuel injectors has significantly changed engine tuning. These injectors are more consistent & predictable than ever, and they provide big improvements in idle quality, driveability and power. Still, a lot of people continue to buy old injector designs, mostly because they’re not aware of just how far the state of the art has evolved.
In this article, we’ll discuss the advances in injector technology and their effect on tuning. We’ll also look at why some aftermarket injectors perform much better than others, even when they’re built from the same basic parts. Finally, we’ll touch on how MOTEC & Injector Dynamics have taken advantage of these technological improvements.
First, some definitions:
- Pulsewidth is how long an injector is open.
- Linearity refers to the relationship of injector flow to pulsewidth. The closer to a straight line this is, the better.
- Duty Cycle is the percentage of an engine cycle an injector is on.
- Offset (sometimes referred to as dead time, battery comp, latency, lag, etc) is the time it takes for the injector to transition from closed to open, and then from open to closed.
In the above graph, the “linearity” of the injector is excellent — the blue line is completely straight over the majority of its operating range. Older injectors are aren’t nearly as linear, and this limits what you can do with them.
Older injectors have heavier parts inside that take more time to move from a stop, which happens every time the injector opens and closes. This results in highly nonlinear behavior at the ends of the injector’s range (at min and max flow regions). Since big power requirements force injectors to operate closer to those ends, serious problems can occur.
Here’s why. Big power means big injectors, which means that the pulsewidths during startup, idle and tip-in are small. Fuel corrections (such as for water or air temp) operate on the assumption that your injection is linear. But because the older injectors are more non-linear, a bit of correction can have a bigger-than-intended effect – or none at all!
At really high duty cycles, this nonlinear behavior can put your engine at risk. Have you heard of valve float? Injectors can do the same thing – essentially, the injector never gets to close all the way, so the flow jumps drastically. What you think is just a little tweak can result in a huge fueling change – and you’d better hope it’s not to the lean side. What does all this mean to you? The more non-linear your injectors are, the harder they can be to make idle, and the more dangerous they are to run when you have high RPM and big fuel demands.
One direct benefit to modern injectors’ linearity is the ability to safely run larger injectors at shorter duty cycles – this opens up tuning possibilities. Whereas in the past we were limited to about 85% duty cycle at full power, today I have similar cars running at just 30% duty cycle. This additional duty cycle headroom is a big benefit when running flex-fuel applications like gasoline/E85 since E85 has much higher flow requirements than gasoline.
When I started in this industry, injector timing (when the injector fires with respect to crankshaft position) was often ignored. This was because flow requirements dictated that the injector be open much longer than the intake valve for most of the engine’s operation – any duty cycle greater than 25% in a 4-stroke engine means that the injector is open longer than the intake valve. In recent years, as variable camshafts & intake manifolds have improved low-speed performance by raising intake port velocities, we’ve seen opportunities for better fuel control by timing our injection more accurately.
Properly matched and characterized injectors really help an engine’s tune. One reason we sell Injector Dynamics injectors is the unique way they ensure consistency in their products. Every injector first goes through a break-in process and is then characterized. This makes sure a set is as similar to each other as possible. They also match injectors up by offset – something no one else does. At low pulse widths, this offset is a significant percentage of the amount of time its open. So if your injectors aren’t all the same – not just in flow but also in offset, tuning issues are going to be magnified.
The final item I’d like to point out is how some ECU manufacturers are taking advantage of these improvements. A good example is MOTEC, whose software allows for complex injector dead-time compensations. Most ECUs treat this simply as a function of battery voltage – the lower it is, the more offset you need. However, Injector Dynamics has proven that offset is also related to pulse width. Thanks to MOTEC’s 3D injection comp tables & Injector Dynamics’ data, you can apply corrections in a way that ensures predictable flow. OEMs have been doing this for many years but until now there hasn’t been accurate injector characterization in the aftermarket. Tony Palo from Injector Dynamics put up a great video of a stock Acura idling on 2000cc/min injectors – stock was 240cc/min. In the old days a motor like this would never idle.
The bottom line is this: when looking for aftermarket fuel injectors, consider the advantages of modern technology and proper characterization. More than ever before, people expect their high-performance engines to start, idle and drive as easily as the most mellow street cars, yet make no sacrifices when it comes to power. Linearity is key to this. Properly matched and characterized injectors are important. When dealing with aftermarket injectors, these qualities aren’t necessarily a given. Modern hardware from the right supplier makes for the best result possible.
Special thanks to Paul Yaw from Injector Dyamics and Simon Wagner from MOTEC Systems USA for their help with this post. Also thanks to all the Apex friends & family who are pouring over these and helping refine my prose.