I finally have a running motor after cobbling together parts I have been accumulating for a few years now. The car is a 1974 Corvette with the following setup: 350 (std bore), Trick Flow 23 deg aluminum heads, headers with true duals, unported TPI plenum and base w/ stock runners and throttle body.

I've read and re-read the stickies and a thousand other posts here, so I hope my newbie question doesn't offend anyone too much.

I started with an ARAP bin, figuring the aluminum heads can take the extra timing, and I'm going to make sure I don't load the engine up too much until I'm comfortable that detonation won't kill it. The only mods to the ARAP are VATS (turned off), and ALDL spark advance set to 0 since I don't have a cable that will let me log at 8192 baud, so I need to keep the resistor in place while I drive around.

So here are my questions: (1) If I'm seeing BLMs >128 across the board (and I only drove around the neighborhood, so there aren't many data points in the log), should I go ahead and adjust the MAF scalars to add fuel, or just stick to the MAF flow tables for now. I know my engine flows more than a stock L98 at pretty much any load and RPM, so I was figuring I'd adjust scalars to get the BLMs closer to 128 initially. (2) The high INTs make sense, but I'm not entirely sure I understand the two data points that are <128-- does this mean the ECU has learned that it needs to add fuel, so the short term fuel trim is getting back to "normal"?

I'm sure I'll have more questions as I log more. Thanks in advance.

 

I would personally adjust the injector constant if its an across the board lean condition OR scale all the MAF tables by a uniform ammount, depending on what has been done. The MAF scalars/tables are mainly there to translate the MAFs output into g/sec airflow rate, and are dependant only on the MAF in use, and not on the engine itself. It looks like the actual fuel being delivered by the injectors may not correspond with the injector flowrate. This is espeically true if you have aftermarket injectors. The actual and advertised flowrates may not truely match. The other possibility is that the MAFs output no longer properly correlates with the MAF tables. Is the MAF stock, or do you have an aftermarket unit? What kind of injectors are in use?

 

Thanks Dimented. Stock MAF and 24lb injectors.

I've read a few of the theories on changing injector constant and fuel pressure instead of the MAF tables. Here's what I don't understand about the MAF tables: it makes sense that the MAF just reports air flow, independent of mods done to the engine. But, it looks like most folks running a MAF are changing the MAF tables anyway, to get BLMs in line. So is it true that the MAF reports flow regardless of the motor it's attached to, or do the MAF tables represent flow for a given voltage in just the stock configuration. In other words, did GM change the MAF tables for different intakes and cams even though they used exactly the same MAF?

 

My experience with MAF is that the tables just calibrate the MAFs output to airflow. They should normally not be changed. I've done head and cam swaps and there was no need to change the MAF tables. The only time you would need to change the tables is when the intake ducting around the MAF or the MAF itself is changed.

Now, there are some things that can happen that may cause the calibration to fall out of tune. One possibility is that the MAFs output may drift as it ages. IOW, the MAF may no longer have the same voltage vs. flowrate curve that it had new. A dirty sensing element or aging circuitry in both the ECM or the MAF can cause this. Another possibility is that there are ground biases present. With a voltage based MAF, this will in essance change its output as the ground voltage reference for the A/D within the ECM and the MAF may not be at the same potential. This can cause major problems with the voltage based systems. The fact that everything is off globally points to some issue like this.

You could simply just scale the injector constant or all the MAF tables by a fixed ammount to fix it. But, it sounds like there may be a more simple and fundamental problem with the MAF, injectors, fuel pressure, etc. Checking the fuel pressure, electrical connections, and maybe trying another MAF sensor would be worthwile before potentially trying to tune around the problem.

 

I think I understand what you're saying. When you changed cams, didn't your BLMs move away from 128? If you had the same injectors and the same MAF, did you tune anyting to get them closer to 128, or did you leave the tune like it was? I'm fine leaving my BLMs higher than 128, so long as I don't peg them at the high end. From what I understand, as long as I don't max out the BLMs, then the ECU is still correcting to a 14.7 AFR. I just thought from reading all the stickies and posts on this board that those people tuning MAF cars were adjusting constants to get BLMs close to 128.

 

If you adjust the Maf tables, you are calibrating the tables to your actual Maf sensor. If it is older it will read less flow (volts out).

Changing your Maf table to "fix" an injector fuel flow problem will recalibrate your Maf and add error to your air flow. This will then effect all your other air/fuel calculations.

If you change your Injector constant, you are calibrating the constant to your actual fuel flow.

Changing the injector constant to "fix" a Maf problem will recalibrate your cold start, PE, and AE amounts. These will then need "Fixed" retuned.

If you get it right there should be very little to change, a miss will send you down the "I have to tune everything" road.