An injector is just a vavle. Injectors are controlled by the ECU with an electrical signal that lasts a specific peroid of time called a "pulse width" usually measured in milliseconds. In terms of fuel injectors they're either open or closed. However, since you cannot simply open an injector all the way instantaneously due to simple physics it takes a small amount of time to go from 0% open to 100% open. It's this lag in the injector that causes problems at extreme ends of duty cycle.

Duty cycle (DC) is a measurement of how long the injector is open relative to how long it is closed. 50% DC means that the injector is open half the time. At 100% DC it's just open all the time and delivers the maximum amount of fuel as the injector is rated for. You cannot have a valve open more than 100% of the time. There's no such thing as 110% duty cycle.

If you exceed 80% duty cycle then you get problems with the timing of the injector opening and closing in relation to the intake stroke of the engine when you're telling an injector to open and close 18 times a second or more.

2%-80% duty cycle is fine for normal injectors. The problem of the injector opening and closing quickly enough at elevated DCs is realized as the time it takes the vavle to close and reopen gets shorter and shorter.

Think of 90%-100% DC as "valve float" for fuel injectors. The time between the ECU's electrical signal turning the injector off and back on again is shorter than the time it takes for the physical injector solenoid to open and close. The effect of this is inconsistant fuel delivery. The injector is neither closed nor open and as such it's not delivering the proper amount of fuel. Considering that high duty cycle normally only occurs at elevated manifold pressures and means that your engine is running at maximum cylinder pressure. Essentially you run your engine lean at wide open throttle and you end up with detonation.

The opposite is true with very large injectors at low duty cycle but without the damage from detonation (low cylinder pressure). If you have a small 100cc/min injector at idle/part throttle operation it's easier to control than a 1000cc/min injector in the same engine. If you have the 100cc/min injector running a 10 millisecond pulse width (.010 seconds) you get ~.017ccs of fuel.

Example:
100cc/min / 60 seconds = 1.667ccs per second
1.667ccs/.010 seconds = .017ccs

1000cc/min / 60 seconds = 16.667ccs per second
16.667ccs/.010 seconds = .167ccs

If you ran the 1000cc injector at the same pulse width you'd get ~.17ccs. That's 10 times the amount of fuel you need. So you'd have to crank the injector pulse back to 1/10th of 10 milliseconds or .001 seconds. It's very hard to control a large injector in such a short period of time. The ECU quickly commands the injector to open and close, but by the time the electrical signal ends the 1000cc injector has barely even opened. The effect of large injectors on low rpm operation is that you have inconsistant fueling, poor atomization, harsh driveability, and poor emissions.

So, running larger injectors is generally a good idea if you have extensive modifications to your engine that might push the limits of the factory injectors. Honda likes to run their injectors pretty hard from the factory so there's not a lot of room for extra power. A short term fix for stock injectors is an adjustable fuel pressure regulator. With the adj. regulator you can artificially increase the flow rating of the stock injectors by raising the fuel pressure, but it should only be used until properly sized injectors can be sourced and retuned.

Consider yourself informed.

Pirate

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The pulse width is the raw number of milliseconds that the injector is open, and the duty cycle is just a calculation from that, when you add in rpm.

For instance:

Say you're at 8000rpm, in a 4 stroke engine, you fire the injector once every 2rpm, so take 8000/2 = 4000; 4000 / 60 = 66.66 rev/s; 1/66.66 = .015s or 15ms per revolution.

Now say that you need the injector open for 10ms to get enough fuel in.

10ms/15ms = 66.67% DC

Good info, I just noticed the guy on that G23 thread talking about something along these lines.

In that light, any duty cycle number you get from an online injector size calculator that is over 100% is just telling you that you need a bigger injector or more pressure. That can exist on paper, but not in the real world. Being over 100% duty cycle means that your injector needs longer than the length of one engine cycle at a given rpm to get enough fuel in.

Modified by mgags7 at 2:09 PM 11/21/2007

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Its good to see some good tech here, and good too see someone taking the time to help us noobs out

That's my 2c. I wish I knew it back in the day, maybe it will help out someone else

The same goes for Hondata. The datalogger consistently reads out erratic numbers and sometime goes over 100% as well.

Definite FAQ material.

92 Si, 4th motor

H23/F22B2 build in progress. fresh rebuild on bottom end. nothing major, just something different. and cheap

*shrug*

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I'm not talking about the actual fuel table, though Blue might be, I'm just talking about the datalogger's numbers. It spits out DC % for you, and is often very wrong.

I was referring to Crome's duty cycle table, which is claiming to tell you the actual duty cycle in %.

I found Hondata S200's DC log ok the one time I used it. It did spike to a 100% for no reason though

Anyhow, thanks for clearing this up for me. Looks like I'll be sticking with the 450's after all.

EDIT: Also, how exactly do you know when it's "time" for bigger injectors as far as upgrading things go? You run so much boost from one setup and are fine on one set of injectors, and then get something bigger and all of a sudden need bigger injectors? Duh, the answer is because you need more fuel at any given point in time, but what exactly determines that? And please don't call me a noob, atleast I'm asking legitimate questions here.

Quote, originally posted by mgags7 »

Cheaping out on parts and ruining more poor preludes Get a geo or something.

good stuff pirate and matt

Matt's got it covered in his post where he states 15ms as the time between cycles at 8000 rpm, and 10ms being his pulsewidth = 66.67% duty cycle

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Yeah. You *could* rig something up on the wires and keep a stock setup, but you'd have to buy an oscilloscope or something comparable in order to measure the pulse width, and those things are not cheap by any means.

Another thought I just had was, you could most likely use a high-end AC voltmeter to read duty cycle, some of them will actually measure it directly. But, you'd have to multiply the # you get from it by two because it would be measuring duty cycle for all 2 rotations of the engine between injector fires, when the duty cycle we calc is measured over just 1 rotation.

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Ahem.

Quote, originally posted by mgags7 »

Cheaping out on parts and ruining more poor preludes Get a geo or something.

the injector calculator on RC's site works really well on determining how big of an injector you will need based off of h.p. goals and #of cylinders and fuel type used.

HUGE H22 partout sale and other h-series parts
http://www.hondamarketplace.co...44636