Need help understanding scan data

[QB93Z]

The last time I was at the drag strip I thought my 1993 ZR-1's performance was lower than usual. I had slower ET's and lower top speed.

A scan I ran during one pass showed 1 instance of knock spark retard which I have seen before.

But the scan also showed Injector Duty Cycle of way over 100%.

Thinking that I may have a fuel supply problem, I changed the fuel filter and tested the fuel pumps using the FSM. Both fuel pumps appear to be working properly. I get 49 psi, steady at WOT.

I recorded another scan yesterday after the fuel filter change. Here is a portion of the scan during a hard acceleration to 100 mph:
http://i407.photobucket.com/albums/pp152/QB93Z/Misc/ZR1SCAN2.jpg


Am I right in my understanding that INJ DC being high means the computer is making the injector pulse width longer in order to get enough fuel to the cylinder?

Any ideas on where to look next?

Jim

[tpepmeie]

Datamaster does not correctly apply the pulsewidth scalar when the 2ndary's are active. Your real DC is approx half of what is shown.

[Paul Workman]

I compared your data to one of my WOT scans with the 90 Z, and with one from my former LT1 Vette.

What jumps out at me is your injector dwell time on one (right) side, and the O2 readings on that same side:

The PW is nuts on the one (right?) side. However, the O2 reading likewise indicates very rich, so the sensor appears to be working. (The pros here that look at this stuff almost daily...please step in.) So, I would be wanting to eliminate a connection problem between the O2 and the ECM first. I'd like to hear what Jeffvette or some other turners think. (I'm subscribed on this one!:thumbsup:)

P.

[tpepmeie]

The PW is nuts on the one (right?) side.

P.

No, the PW are fine, 23.59 ms each side. The numbers you are citing (16.9/49.6ms) are also normal.

Todd

[Aurora40]

From what I understand (from Todd), you basically divide the duty cycle/puslewidths in half when the secondaries are operating. Datamaster isn't doing that.

Edit: The only thing sorta off is the MAP pressure? Seems sorta low, especially if the ambient pressure was over 100kPa that day. If you scroll back to when you were at WOT but at a low RPM, what is the pressure there? How much is it dropping as the RPMs rise? I usually see about a 2-3kPa drop or so. More than that, and perhaps your air filter is dirty or your duct is collapsing or something like that?

[QB93Z]

Thanks for the responses.

I understand now about the Injector duty cycle reading, divide by two. I went back to some older scans and the readings were similar to yesterdays, but I never noticed the X2 value before. I guess I was concentrating on other readings.

Bob, I looked at some other WOT data on this scan (long pull from 40 to 70 mph in fifth gear, 1700 to 2800 rpm. MAP flucuates from 99.3 to 100.1 for the 5 seconds duration (avg 99.7). During the 0 to 100 mph run pictured above, the MAP flucuates from about 97.5 to 98.2 KPa ( avg 97.85). Is the difference you are pointing out?

I will check out the duct and the air filter. Thanks.

Jim

[Aurora40]

Hey Jim,

What I meant is, look at the MAP at low RPM at WOT. What is the highest the MAP reading goes up to? That is potentially the atmospheric pressure at that time. It's better if the RPM is really low, but at the dragstrip you probably aren't driving that way.

If the max pressure is that 100.1kPa, and in the snapshot it is 96.7kPa, then you are getting about a 3.5kPa pressure drop. That seems a little bit high, though not a lot. I think I usually have closer to a 2kPa drop as the revs climb. Possibly it gets lower as the RPMs go up from there? That snapshot is at 6,300.

Intake pressure directly affects power. Though here we're talking maybe 2% at the max, which on a 400hp motor would be like 8hp. Certainly that's not nothing but hardly huge. How "off" were your runs?

[XfireZ51]

Benchmark the MAP sensor first before attempting to compare WOT. When datalogging, I first record all sensors with ignition ON but motor OFF. That will give you MAP reading at atmosphere. Its not always 100kPa. MAP will fluctuate at WOT and be at its highest point at torque peak, for instance, 5300-5500rpm for my motor.

[Aurora40]

Why would the MAP be at it's highest point at peak torque?

[flyin ryan]

Peak TQ. is always the most volumetric efficent point.

[Aurora40]

Yes, but MAP is the pressure in the manifold. Why would it increase with VE? I'd think if anything, it would be the worst where the VE is highest, since that's where the intake system would be most taxed.


I do have sort of a side question though... Is total airflow highest at the torque peak? I realize each cylinder is filling the most completely, but that event is happening less frequently than at higher RPMs. If I were to guess, I'd think the total airflow would be highest at the power peak, but I'm just guessing.


Edit: Also, Dominic, that's a good idea about looking at the MAP with the engine off. It's funny, but that never occurred to me. I always tried to look at it at WOT at a low RPM, figuring that the intake would be pretty close to atmospheric there. But now that you say that, it seems so obvious. :)

[XfireZ51]

Yes, but MAP is the pressure in the manifold. Why would it increase with VE? I'd think if anything, it would be the worst where the VE is highest, since that's where the intake system would be most taxed.


I do have sort of a side question though... Is total airflow highest at the torque peak? I realize each cylinder is filling the most completely, but that event is happening less frequently than at higher RPMs. If I were to guess, I'd think the total airflow would be highest at the power peak, but I'm just guessing.


Edit: Also, Dominic, that's a good idea about looking at the MAP with the engine off. It's funny, but that never occurred to me. I always tried to look at it at WOT at a low RPM, figuring that the intake would be pretty close to atmospheric there. But now that you say that, it seems so obvious. :)

Bob,

Remember what's being described by MAP. Its the differential pressure in the manifold v atmosphere. So higher MAP means you are getting closer to being equal to no difference between the manifold pressure and atmosphere. Anything less than that means there is something inhibiting that airflow. VE dropping after torque peak is easier to see on OHV motors. In tuning my 84, the VE number would drop fairly rapidly from torque peak(3800rpm) to power peak (4800rpm). I just did a datalog session yesterday at the track and MAP was 99.3kPa from 4900 to 5800rpm then dropped to 97.9 kPa up to 7200rpm where 100kPa was 14.6psi at Key ON. As for VE, don't forget that at higher rpms, things are happening very rapidly. Dropping VE says you're not filling those cylinders simply because air has inertia and only so much gets in within a rapidly shrinking window of time. Gotta love those DOHC motors.

[Aurora40]

Right, doesn't the MAP just indicate how well the air filter and TB are working to keep air going into the motor? Maybe we're saying the same thing and I'm misunderstanding you, or maybe I've got it wrong (again)?

If you are seeing the MAP pressure drop above 5800 rpm, doesn't that mean your intake system (TB, air filter, duct, etc) is not able to keep up with the consumption rate of the motor? Ideally wouldn't it be the case that the MAP would not drop as RPMs increase?

It sounds like what you describe is what I see. As RPM increases, the MAP reading drops fairly linearly. But VE doesn't drop in the same way. At 1,500 rpm, the engine has worse VE than at 4,500, correct? At 1,500 rpm at WOT, the MAP reading will definitely be higher or as high as at 4,500.

Also, the MAP is not differential, it is an absolute pressure measurement, isn't it? For example, the sensor can differentiate between a day with high ambient pressure vs a day with low ambient pressure, can't it?

Sorry for all the questions, I'm just trying to understand.

[tpepmeie]

Bob,
you understand this better than you give yourself credit for. Ideally, there would be no pressure loss through the throttle and intake at WOT. That is rarely the case, but usually the pressure drop is only slight (say, 1 kpa). If the Key on Engine off MAP reading is 99.6, then you might see 98.6 or so at WOT, and it will get worse with increasing RPM. Whoever suggested the OP to check for collapsing air duct may just be on to something.