Terry Burger

iTrader: (4)

I set the cruise @ 75mph (I have 3.73s) and hit a 10 mile straight path. In 4th gear, with the converter locked, I averaged 5.75 ms injector pulsewidth. When I kicked it in to 3rd gear (keeping same speed), the rpms jumped up 1000ish rpm, but the average pulsewidth went down to 4.40. Does lower ms mean better fuel economy, speed being equal?? Both tests were done over similar terain, with cruise on, at 75mph.


Thanks!
Terry

monodax

Pulsewidth is relative to RPM, A lower pulsewidth @ low RPM's = lower duty cycle, whereas the same pulsewidth @ higher rpm = high duty cycle.

20 ms Pulsewidth @ 2000 RPM = (2000 * 20) / 1200 = 20% duty cycle
20 ms Pulsewidth @ 5000 RPM = (5000 * 20) / 1200 = 83% duty cycle

Terry Burger

iTrader: (4)

Hmm. Isn't that just because the injector has less time to get the fuel in as the RPMs rise? 10 ms of fuel is 10 ms of fuel no matter what RPM (as long as it isn't static), right??

EFILive

iTrader: (2)

Yes, 10ms of fuel is 10ms of fuel (assuming fuel rail pressure is constant).

What monodax is saying is, at higher rpm's, although you may get shorter "on times" for the injectors, you get them more often - giving a higher total fuel flow. That measurement is called duty cycle and is calculated as mondax said above. The higher the duty cycle the more fuel you are using, regardless of the injector pulse width.
Paul

Plan B

Terry,

That would depend upon what your tune is doing at that particular load point. Measuring with a wide band at those points without the correction of the o2 sensors would tell you exactly what is going on.

It’s only fuel/air and ignition timing….. The rest is up to you.

Pete.

jimmyblue

iTrader: (11)

10ms of fuel per revolution is 10 ms of fuel per revolution.
But your time averaged fuel flow is seen in the duty cycle,
not the pulse width. Go make the duty cycle calculation
another column in Excel, and see what that tells you.

John_D.

iTrader: (2)

In this case, not necessarily. Because the injectors were pulsing more often, even though they were open for a shorter period on each pulse.

They didn't have to be held open as long, to generate the same hp. (because hp is directly related to fuel consumed) Just the same total fuel was needed. (roughly speaking)

And it takes the same rwhp/torque to move the car at 75mph, regardless of the gear you're in. (without getting into additional frictional losses in the engine or ve differences)

__________________________

Just to rough it in a bit:

I'm turning about 2500 rpms in 4th (auto) at 75mph. So I'll use that for now.

An increase of 1000 rpms is 40%. That's 40% more firing for the same distance traveled. So rpms were multiplied by 1.4

But about the same amount of fuel would be needed to generate the hp to move the car at 75mph.

Now if you multiplied the rpms by 1.4, you should be able to divide the pulsewidth by 1.4 and end up with the same total fuel.

5.75 / 1.4 = 4.10.
So you were actually using more total fuel if the pulsewidth was 4.40.

__________________________

(edit - corrected the percentage to be 40 not 20, and corrected the pulsewidth division....)

Terry Burger

iTrader: (4)

It makes sense now. Thanks!!