I'm in the process of fuel tuning my '87 manual vette (383, MAF, $6E).

In most of the BLM cells where there is throttle activity and vehicle speed(light load, medium load, low rpms, medium rpms, etc), the BLMs are ranging from 124 to 118, indicating it is a little rich.

In the BLM cells where there is no throttle activity (but there is vehicle speed), mainly deceleration, the BLMs are averaging much higher. They are in the 138 - 144 range.

Is this common for a manual car? To get the "deceleration" BLMs in line, I should concentrate on the DFCO and DF enlean elements, correct?

I am also getting some "popping" out of the exhaust when decelerating. What causes this? Do I tune it out through timing or decel fueling?

Thank you!!

 

https://www.thirdgen.org/forums/diy-...t-low-map.html

Experinced this last summer as well m8.

cheers

 

Thanks again!!

 

You should turn off fuel (means use DFCO) at low MAP and closed throttle. (Adjust MAP settings to get DFCO). It's almost pointless to try to fuel that area correctly, and there's really no need to fuel it when the engine is warm. The popping is unburnt fuel, because it's hard to get stable combustion at low MAP - it's just too weak and diluted with exhaust gas, and the PWs are terribly small and inconsistant. The popping also means INT and BLM will go up falsely due to misfire.

At lower RPMs, when fuel returns, calibrate it better, but also disable closed loop if it's an option. You may find it better to have that area leaner, so that closed loop INT goes up a little more than necessary just as it returns to idle.

If you're driving and have slight throttle but very low MAP, adjust your IAC throttle following to get higher MAP with very slight throttle. Don't overdo it, or it'll jerk too much when fuel returns.

Decel enlean has little to do with this problem, except in transient state.

 

Thanks RednGold86Z!

I have a MAF set up, so I would make the DFCO adjustments based on load and rpm, correct? I am seeing in the data log where DFCO comes in after about 2 seconds of very small inj PW (.57 - .85). This is probably where the popping is occuring. Right now, I have the "DFCO enable if conditions met for X" set to 1.75 seconds. Can I reduce that to something like .25 seconds?

Thank you for confirming the false high INT / BLM numbers as well. I'll work on getting the acceleration / cruising cells fueled properly, as they are averaging around 120-124.

I really appreciate the guidance -

 

Yeah, LV8 is basically the same as MAP. The small PWs are not accurate with multi-point saturation style injectors. That's one reason sequential is better, because the PWs are ~doubled.

Reducing the enable time should be no problem. It might make a small drivability difference for normal driving (amount and timing of engine braking and how comfortably the torque changes transfer to your butt), but probably will feel pretty good still.

 

Cool. I'll give .25 a try. If the transfer feels too rough, I'll start bringing it back up until it feels ok again.

Thank you so much!