Lean tip-in bog with Accel DFI

[DLinson]

I have a lean bog on tip-in coming off of idle. I have an Accel DFI Gen VII SuperRam on my 427 big block Chevy. After reading a tip on another thread I modified my linkage to slow down the throttle, attached is a pic. I made a quadrant so the throttle rolls on linearly and smoothly. It also slowed down the movement relative to the pedal.

The tip-in lean bog is less now and the car drives a lot better but I still have a the bog. I can't figure out how to get rid of it. Accelerating when cruising is fine. Once and a while it will die at idle usually when pulling into the driveway or parking. When it does that it seams to be rich. When you try to start it again you have to give it some throttle.

If anyone has any ideas, I could sure use some.

Thanks,
Dennis

[funcars]

It's hard to know what to address since there isn't much info, but here are a couple of ideas: 1) make sure your IAC counts are at a reasonable point and not opened up too far because your throttle body is closed too much, 2) check the cells close to where you are at when it bogs and see if there are large changes in VE numbers.

It's too bad the Accel manual is so pathetic...

Good luck

[DLinson]

The IAC is about 15 to 20 at idle. I can't get it any lowere unless I replace the idle adjust screw with a longer one.

I changed scales on my VE table so the rpm scale is in 200 rpm incraments up to about 2600 rpm. I also moved the MAP scale so the second MAP value is about 35 and then stepped them evenly up to 101. I have a large solid roller cam 242/248 duration @.050 with .657/.663 lift in the 427 BBC.

I just got done moving the air inlet temp sensor to under the air cleaner. It was in the plenum and the air temps were reading about 125 degrees. The bog seamed worse when hot. It didn't have the bog when I just pulled it in and out of the garage when it was cold.

I am curious if there is a table I should be looking at like the Tau vs MAP/ECT chart or something else.

Thanks,
Dennis

[Dr.Corvette]

You have described the problem I am having to a tee. I also have a Accel gen7. It's on a 460 Pontiac. I'm using a Edelbrock Victor EFI intake with a 4bbl. throttle body. My cam is 230 dergree intake duration at .050 on 112 LSA. I have the same tip in problem except mine goes rich (like 10:1 AFR) when you quickly stab the throttle. (I think my throttle linkage is to fast.) Like you said it's worse when completely warmed up. I have made some improvements by using the TAU table. This table seems like were the computer bases it's fueling strategy for throttle transitions. What have you tried besides slowing your throttle down?

[Hammered]

I have the Gen7 on a 462 Pontiac and a Comp Cams 242/250 duration, .576/.577 lift hydraulic roller cam with 1.6 rockers. My tune is not perfect, but I don't have the problem you're talking about. What AFR are you running at idle? It should be a bit rich.

[Skip Fix]

What AFR are you running at idle? It should be a bit rich.

I had a similar problem with a carb that in neutral idle setting AFR was good, in gear the convertor lugged it down enough to be lean. Fattened idle circuit up in gear and no off idle bog.

[ptest100]

I haven't tried this yet, since I am still trying to get my system going, but I would look at all the tables under the Fuel/Transient/ menu in Calmap, especially the Tau v.s. MAP/ECT table. Here is what the manual says about the Tau table:

"In an average naturally aspirated engine at operating temperature at idle, this table should contain TAU numbers generally around 80-85. When you step on the throttle, the load on the engine will increase, the Manifold Pressure will decrease, and the TAU number should decrease accordingly. A small decrease in TAU numbers equals a small amount of fuel added. A bigger decrease in the TAU numbers will add more acceleration fuel to the engine. For example, when the engine is idling, rapidly accelerating it to 3000 RPM will require a large decrease in TAU numbers to provide the additional fuel necessary to generate a large change in RPM. However, when the engine is at 3000 RPM and you want to accelerate it to 4000 RPM, you will need a proportionately smaller decrease in TAU numbers to provide the required fuel. At wide-open throttle (bottom row in the table) typical TAU numbers may be in the 32-45 ranges."

What I think this means is that when you depress the gas pedal, the MAP will go from one value to another. The ECU will then look at the difference in TAU between those MAP values. So you would look at which MAP values are being used when you get the bog, and change the TAU value at one of those points. If the bog is caused by not enough fuel, you would make the second TAU value smaller (= bigger difference from the first TAU value = more fuel added), and vice-versa.

Let us know if this works...

[Dr.Corvette]

Tonight I finally got rid of my tip in bog. After a month of going through every table with no success it finally dawned on me. My map sensor vacuum line was to long. (was about 2 feet). I shortened it to about 3 inches and it's like driving a new car.

Someone once said tuning is like herding cats. I can sympathize.

[Hammered]

Congrats. What ID line were you using?

[Dr.Corvette]

I'm using 5/32.