D
DAMN YANKEE
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#2 - Viper Injectors, Pulse, Duty Cycle made easy...
You've got injectors...10 of them, you know that, but most don't know how important working with your injectors can be to making your Viper run well. With some simple foundation information, one can learn a great deal about injectors and how they make your car run well....or not.
Where to start? We start with a stock intake valve!
A stock intake valve opens opens to receive injected fuel and air into the cylinder. Sometimes that valve is open for a relatively long time...idling. Sometimes that valve is open for an incredibly short time....roaring at 5,500 RPM. The higher the RPM, the faster the valves are opening and closing! So, for whatever time we have with that intake valve open is the time we have to inject fuel. We measure that time in "milliseconds" (1,000 milliseconds in a second), not to worry, just know we measure in milliseconds.
For a 2000 GTS, the time the intake valve is open:
At 6,000 RPM the intake value opens for 20.0 ms
At 5,500 RPM the intake value opens for 21.8 ms
At 5,000 RPM the intake valve opens for 24.0 ms
At 4,500 RPM the intake valve opens for 26.6 ms
At 4,000 RPM the intake valve opens for 30.0 ms
At 3,500 RPM the intake valve opens for 34.3 ms
At 3,000 RPM the intake valve opens for 40.0 ms
At 2,500 RPM the intake valve opens for 48.0 ms
At 2,000 RPM the intake valve opens for 60.0 ms
At 1,500 RPM the intake valve opens for 80.0 ms
At 1,000 RPM the intake valve opens for 120 ms
At 500 RPM the intake valve opens for 240 ms
So, now you know. Your Viper gives you about 21.8 milliseconds to load up the cylinder at 5,500 RPM and about 240 milliseconds to load up the cylinder at idle. Big difference!
Now that we know the time we have to squirt the fuel in, we need to know how long and how much our injectors can squirt at any given time. Look at it this way, the intake valve opens for 240 milliseconds at 500 rpm (idle) and that is a relatively long time. We sure are not going to inject fuel in for that whole time! Nope, at idle a typical stock PCM will only send a “pulse” to a stock injector to squirt for about 5 or 6 milliseconds. We also know at 5,500 RPM out valve is open for only 21.8 ms, and we only have that much time to squirt all the fuel we need.
For a GTS, a stock injector squirts fuel for approximately this many milliseconds:
At 5,500 RPM, stock injectors are pulsed for 21 ms
At 4,500 RPM, stock injectors are pulsed for 20 ms
At 3,500 RPM, stock injectors are pulsed for 19 ms
At 2,500 RPM, stock injectors are pulsed for 15 ms
At 1,500 RPM, stock injectors are pulsed for 13 ms
At 500 RPM, stock injectors are pulsed for 5-6 ms
Now we know how long a set of stock injectors squirt fuel at a given RPM. We need to know a couple of more things. First is the term “Duty Cycle”. Duty Cycle is the per cent of time the injector is in use while the intake valve is open. Simply, if the injector is firing for the whole time the intake valve is open, you have a 100% Duty Cycle. If the injector is firing for only half the time the intake valve is open you have a 50% Duty Cycle. So….if one was to fire an injector for 20ms at 3,000 RPM…..well? At 3,000 RPM our intake is open for?.....40.0 ms….so, the answer is 50% Duty Cycle. Great.
So, here is the finished "Duty Cycle" table for stock injectors in a GTS:
At 5,500 RPM a stock injector is at almost 100% Duty Cycle
At 4,500 RPM a stock injector is at 75% Duty Cycle
At 3,500 RPM a stock injector is at 55% Duty Cyce
At 2,500 RPM a stock injector is at 31% Duty Cycle
At 1,500 RPM a stock injector is at 16% Duty Cycle
At 500 RPM a stock injector is at 2% Duty Cycel
Folks, stock injectors in stock motors have enough duty cycle to get the job done. The PCM has its Closed Loop and Open Loop routines to regulate the injectors and squirt fuel into the open intakes as required. As we can see, at the max RPM we are using approx 100% of the stock injector's capacity. Thats important, when we build out an engine and increase the air flow, we will need to increase the fuel flow as well. We would look at any Duty Cycle that exceeds 80% as a "limiter" and something that may need to be addressed.
The PCM knows all! And there was a lot for the engineers to have figured out in a stock Viper.
1. The flow of the heads
2. The camshaft profile
3. The Compression Ratio
4. The outside temperature
5. Coolant temperatures
6. The required octane
and more...
Now you know a lot more about your injectors… We want to keep this relatively simple.
We will use the injectors to mix fuel with the incoming air...we want the resulting mixture to be as close to perfect as we can get it. The perfect mixture of air and fuel is called "Lambda" and for a gasoline engine it is 14.7 pounds of air for every pound of fuel. This value changes with various aspects of the car changing. But its a good place to start.
Now if we know how much air can flow into the heads at any given RPM, we know how much fuel we must add to that air to get close to Lambda. The PCM auto mixes this for us by changing the pulses to the injectors based on RPM and other things like engine load. This will all come together as we move forward on our quest to tune modified engines. But we have to start off with the basics.
Its me so we have to end with questions.
If we change our Roller Rockers (valves open wider/longer), or use larger valves, or use larger throttle bodies, smooth tubes, or do head work that allows for more fuel to flow, how do we change our injector pulse to allow for more fuel?
If we do that work and we know that Open Loop is all preprogrammed and locked down in the PCM for stock values, how do we make changes?
What about when we compress air (forced induction, super charged, turbo charged) and throw it into the cylinders at a much faster rate, how do we get more fuel to mix with it if the PCM is tuned for stock everything and locked down?
What about ignition timing? Barometric pressures? Different octane?
Can I use larger injectors? If so how do I tune?
Get the picture?
You've got injectors...10 of them, you know that, but most don't know how important working with your injectors can be to making your Viper run well. With some simple foundation information, one can learn a great deal about injectors and how they make your car run well....or not.
Where to start? We start with a stock intake valve!
A stock intake valve opens opens to receive injected fuel and air into the cylinder. Sometimes that valve is open for a relatively long time...idling. Sometimes that valve is open for an incredibly short time....roaring at 5,500 RPM. The higher the RPM, the faster the valves are opening and closing! So, for whatever time we have with that intake valve open is the time we have to inject fuel. We measure that time in "milliseconds" (1,000 milliseconds in a second), not to worry, just know we measure in milliseconds.
For a 2000 GTS, the time the intake valve is open:
At 6,000 RPM the intake value opens for 20.0 ms
At 5,500 RPM the intake value opens for 21.8 ms
At 5,000 RPM the intake valve opens for 24.0 ms
At 4,500 RPM the intake valve opens for 26.6 ms
At 4,000 RPM the intake valve opens for 30.0 ms
At 3,500 RPM the intake valve opens for 34.3 ms
At 3,000 RPM the intake valve opens for 40.0 ms
At 2,500 RPM the intake valve opens for 48.0 ms
At 2,000 RPM the intake valve opens for 60.0 ms
At 1,500 RPM the intake valve opens for 80.0 ms
At 1,000 RPM the intake valve opens for 120 ms
At 500 RPM the intake valve opens for 240 ms
So, now you know. Your Viper gives you about 21.8 milliseconds to load up the cylinder at 5,500 RPM and about 240 milliseconds to load up the cylinder at idle. Big difference!
Now that we know the time we have to squirt the fuel in, we need to know how long and how much our injectors can squirt at any given time. Look at it this way, the intake valve opens for 240 milliseconds at 500 rpm (idle) and that is a relatively long time. We sure are not going to inject fuel in for that whole time! Nope, at idle a typical stock PCM will only send a “pulse” to a stock injector to squirt for about 5 or 6 milliseconds. We also know at 5,500 RPM out valve is open for only 21.8 ms, and we only have that much time to squirt all the fuel we need.
For a GTS, a stock injector squirts fuel for approximately this many milliseconds:
At 5,500 RPM, stock injectors are pulsed for 21 ms
At 4,500 RPM, stock injectors are pulsed for 20 ms
At 3,500 RPM, stock injectors are pulsed for 19 ms
At 2,500 RPM, stock injectors are pulsed for 15 ms
At 1,500 RPM, stock injectors are pulsed for 13 ms
At 500 RPM, stock injectors are pulsed for 5-6 ms
Now we know how long a set of stock injectors squirt fuel at a given RPM. We need to know a couple of more things. First is the term “Duty Cycle”. Duty Cycle is the per cent of time the injector is in use while the intake valve is open. Simply, if the injector is firing for the whole time the intake valve is open, you have a 100% Duty Cycle. If the injector is firing for only half the time the intake valve is open you have a 50% Duty Cycle. So….if one was to fire an injector for 20ms at 3,000 RPM…..well? At 3,000 RPM our intake is open for?.....40.0 ms….so, the answer is 50% Duty Cycle. Great.
So, here is the finished "Duty Cycle" table for stock injectors in a GTS:
At 5,500 RPM a stock injector is at almost 100% Duty Cycle
At 4,500 RPM a stock injector is at 75% Duty Cycle
At 3,500 RPM a stock injector is at 55% Duty Cyce
At 2,500 RPM a stock injector is at 31% Duty Cycle
At 1,500 RPM a stock injector is at 16% Duty Cycle
At 500 RPM a stock injector is at 2% Duty Cycel
Folks, stock injectors in stock motors have enough duty cycle to get the job done. The PCM has its Closed Loop and Open Loop routines to regulate the injectors and squirt fuel into the open intakes as required. As we can see, at the max RPM we are using approx 100% of the stock injector's capacity. Thats important, when we build out an engine and increase the air flow, we will need to increase the fuel flow as well. We would look at any Duty Cycle that exceeds 80% as a "limiter" and something that may need to be addressed.
The PCM knows all! And there was a lot for the engineers to have figured out in a stock Viper.
1. The flow of the heads
2. The camshaft profile
3. The Compression Ratio
4. The outside temperature
5. Coolant temperatures
6. The required octane
and more...
Now you know a lot more about your injectors… We want to keep this relatively simple.
We will use the injectors to mix fuel with the incoming air...we want the resulting mixture to be as close to perfect as we can get it. The perfect mixture of air and fuel is called "Lambda" and for a gasoline engine it is 14.7 pounds of air for every pound of fuel. This value changes with various aspects of the car changing. But its a good place to start.
Now if we know how much air can flow into the heads at any given RPM, we know how much fuel we must add to that air to get close to Lambda. The PCM auto mixes this for us by changing the pulses to the injectors based on RPM and other things like engine load. This will all come together as we move forward on our quest to tune modified engines. But we have to start off with the basics.
Its me so we have to end with questions.
If we change our Roller Rockers (valves open wider/longer), or use larger valves, or use larger throttle bodies, smooth tubes, or do head work that allows for more fuel to flow, how do we change our injector pulse to allow for more fuel?
If we do that work and we know that Open Loop is all preprogrammed and locked down in the PCM for stock values, how do we make changes?
What about when we compress air (forced induction, super charged, turbo charged) and throw it into the cylinders at a much faster rate, how do we get more fuel to mix with it if the PCM is tuned for stock everything and locked down?
What about ignition timing? Barometric pressures? Different octane?
Can I use larger injectors? If so how do I tune?
Get the picture?
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