DirectScan (ECM) A/F ratio ?

[Fred 86 GN]

I have noticed that when the A/F ratio gets below 10:1, that knock retard seems to occur. Is this a case of being so rich that detonation occurs??

My O2 numbers are 840+ when this happens.

Also seems to correlate with the activation of my Volt Booster -- which jumps the voltage up from 13.9 to 14.8 volts.

Is the A/F ratio in DS accurate?

Thanks,

 

[bruce]

DS is just reporting what the ecm is calculating.
If all the calabration data is correct.

10:1?
It would appear that your *playing* a game here. Unless your using huge injectors you very well maybe to the stage of trying to get the injectors to pass more fuel then designed.

If your running more then 90% duty cycle, then the injectors are on so much of the time that they can't cycle off.

This is why *TOO* large of injector is safer then too small.

Oh, and as an injector approaches going static (always ON), it may hang half open and actually be lean.

On a calculated 12.2 I was at a 12.6 but I'd lied about the injector size, so in actuality it was correct.

 

[JDEstill]

The a/f ratio in DS is not what you actually have, it is what is being commanded.

In a non Extender situation, the air flow measurement is limited to 255 gm/sec. If you have more air flow than that (which you probably do if you are running 16+ psi) then there are some fudges that help make sure you get the right amount of fuel injected.

Suppose you have 320 gm/sec of air flow. You want an a/f ratio of 12.5:1. You need to inject 320/12.5= 25.6 gm/sec of fuel to get that a/f ratio.

But the ecm is limited to 255 gm/sec air flow reading. So the ecm has to think that it needs an a/f ratio of 255/25.6 = 10:1 if it is going to inject the right amount of fuel.

The chip maker juggles the various PE settings and whatnot so that under those conditions the ecm thinks it needs an a/f ratio of 10:1.

Your problem sounds like a mismatched chip. The chip maker has probably dialed it in so it provides plenty of fuel for a car running boost in the low to mid 20's, but you are only running in the high teens. The chip is dialed in expecting to provide fuel for 320 gm/sec air flow (for example) but you are giving it only 280 gm/sec (again for example). The ecm doesn't know that you aren't doing that, it just sees the MAF signal pegged at 255. So it injects a lot more fuel than what you actually need.

Solution: get a custom chip set up for how you run your car, or turn up the boost, or get an Extender setup. Any of those should get the fueling back in line where it needs to be.

John Estill

 

[Fred 86 GN]

The fuel pressure is at 46psi (vacuum off) and the boost level at 15psi. I was attempting to fix a lean tip-in condition from a JL93 chip with those settings and decided to switch over to another chip I had for the car. Obviously this other chip is 20-30% richer than the JL93, from the mid-800mV signals the O2 sensor was sending and seems to be confirmed by the injector PW vs. the MAF curves (PW % increase per frame occuring at a greater rate than the MAF % increase per frame).

I'll turn the pressure down 2psi and see what happens. If the 8.5:1 ratios "go away" then maybe I will turn the boost up a psi or two and see what the A/F and O2's look like.

John,

I am confused by your statement that the A/F ratio displayed by DS is being "commanded" by the ECM. Why would the ECM be calling for more fuel when the O2 signal is already really rich?

Does it have to do with the BLM being locked in at 147?

Thanks,

 

[bruce]

The ECM commands that the injector be opened. Weither they obey the command is something different.

At WOT the ecm isn't using the O2 sensor in any of the calculatons (the Speedpro with the WB, can)

BTW, the stock O2 readings aren't very meaningful or accurate.

 

[JDEstill]

I'll turn the pressure down 2psi and see what happens. If the 8.5:1 ratios "go away" then maybe I will turn the boost up a psi or two and see what the A/F and O2's look like.

I don't think those ratios will go away with fuel pressure adjustments. It may change the actual amount of fuel being delivered (assuming your BLM stays the same), but the ecm doesn't know that. For the same set of conditions, it should calculate the same injector pw, no matter what the fuel pressure is. That's the reasoning behind using a BLM lock, to make sure that the injector pw comes up the same each time, so that changing the fuel pressure does in fact change the amount of fuel delivered.

I am confused by your statement that the A/F ratio displayed by DS is being "commanded" by the ECM. Why would the ECM be calling for more fuel when the O2 signal is already really rich?

At WOT the ecm is not looking at the O2 sensor any more, it doesn't know that it is rich. It knows the rpm, the MAF, coolant and air temperatures, TPS position, the BLM, stuff like that. From those inputs it calculates the amount of fuel to be injected and tries to inject that. It doesn't look at any feed back to see if it is rich or lean. The chip programmer can change some of the tables around in the chip to get it to inject more or less fuel for a given set of conditions. It just sounded to me like yours has been programmed for more...

John

 

[Fred 86 GN]

Thanks for all the input!

The explanations are very helpful

I think (hope!) I have found the primary problem that is causing the high KR numbers. After removing my Volt Booster (since the high KR seemed to always occur a few frames after the VB activated), I did some more testing. The KR numbers were very low (less than 1 degree) during two of my full throttle tests. These happened to be second gear WOT pulls from 40mph to 70mph with the fuel pressure at 44psi (no vacuum) and at 16psi boost.

The next test was a WOT stab that caused the car to downshift from third to second. Big KR occurred (6 degrees). Another test before the minute-and-a-half of record time was up caused another big down shift from second to first and KR numbers shot up to 15 degrees I immediately let out of the throttle when my audible alarm went off.

After reviewing the DS data, it appears that the combination of low TPS voltage (less than 4.5v), going from low load to high load (LV8 jumped from 150 to 230 in a couple frames) and a RAPID gain in rpm (1000rpm in 0.16 seconds the first time; 1500rpm in 0.50 seconds the second time) caused the high KR numbers to appear.

The transmission is not right in the car, so hopefully valve body and governor will be fixed by Bruce as soon as I rip them out and ship them to him!

Has anyone else had a "free revving" engine cause knock retard problems? Or is the TPS not being at 4.5+volts a real concern?

Thanks,