Speedtoys

88 S4 Magicmatic..




What is the source of these lean dips that occurr just off idle, right down 2400rpm, and lastly at 3450 or so?

If I fatten those out, I dork up the rest of the cell really fat..but these are hurdles you can feel in how the car runs accelerating thru those areas.

Rob Edwards

The ones at 2400 and 3400 are probably the second or so when you've gotten off the gas and the idle switch has shut off fueling during deceleration.

The higher load cells just off idle are lean from tromping on the throttle, maybe some added transient enrichment is in order? I confess I don't understand how to tweak that part of the map. Hopefully someone with a clue will happen along.

ptuomov

It looks something resonance related for sure, since it's almost vertical. Almost but not perfectly vertical fits the theory, because I'd assume that the intake manifold air temperature is higher at low load than at high load. The speed of waves (sound) varies slightly depending on air temperature, traveling faster in the hotter air. That would explain the slight angle in the lean spots.

This can be tested. If you set the flappy to be forced open, then the resonances should change and mostly disappear as the pulses combine. Set the ST2 flappy opening and closing to some very large numbers and see if "the red towers of Pisa" disappear.

It could also be that those are spots at which you've lifted off, the deceleration fuel cut off kicks in and the load declines faster than rpm.

If you look at the actual log data, what are the normal conditions in which the lean spots happen. That is, from which direction are you coming from on the fuel map?

Lizard928

His flappy is vacuum held open at this time.

I think it is likely to be turbulence in the MAF, I would put a piece of honey comb in the top of the MAF body to straighten out the air.....

ptuomov

Isn't the flappy closed until 3660 rpm normally at which point it opens if there's enough load, or do I remember this incorrectly / has the setting been changed?

If the flappy is programmed to be forced open, then I don't see how any turbulence could make it to the MAF as all the pulses combine high up in the intake manifold.

If the data log lean spots happens when moving from southeast to northwest, then it's just engine braking and decel cutoff. If the lean spots happen when moving from northwest to southeast, then it's something else.

Rob Edwards

Theo's diagnostic manual says the flappy is open 3585 to 5410 rpm. For whatever that adds to the discussion.

IcemanG17

I remember Bill Ball had a throttle tip in flat spot when he had his supercharger....I can't remember how he fixed it....still its quite odd

Lizard928

Jeff has no flappy solinoid.
It has straight vacuum from the HVAC/cc/vacuum reservoir.

The air entering the MAF could have turbulence and "dance" at these set rpms..... Hence suggesting a honeycomb

ptuomov

https://rennlist.com/forums/3786717-post29.html





(From Erkka, of course...)

ptuomov

Are you suggesting a resonance in the "pipe" from the air box to the throttle body?

jcorenman

So the flappy is always open?? Why?

ptuomov

Just throwing out ideas here: Do you have a stock fuel system? That is, do you still have all those fuel dampers hooked on? Some cars get resonances in the fuel rails with bigger injectors and with the dampers removed.

You could take the MAF signal and convert the MAF signal into mass air flow in kg. Then look at the injector pulse width and mass air flow on both sides of the lean area and at the lean area. If the mass air flow and the injector pulse width predict a stable AFR in the rpm neighborhood of the lean spot but the O2 sensor says it suddenly dips lean at some one rpm, then it is likely to be the fuel pressure changing with resonance in the fuel rail. Worth a check, maybe.

(That's just one more guess, no actual knowledge, other than we did spend the time hook up a couple of dampers to the new fuel system as well. Benefit not demonstrated, of course.)

Lizard928

Yes.

Rotrex SC location.

60# Siemens inj. I ran them in a stock S4 no issues.
His car has 1 dampener and the FPR along with a chopped 5-8 fuel rail, and both rails are dead headed. This is due to Victors SC setup.

ptuomov

I am running those injectors as well, but with an FPR and three fuel dampers, in a geometry similar to the stock system including a return line system.

I am eyeballing from the graph resonance lines at 800, 1600, 2400, and 3600 rpm. That would be consistent with fuel system resonance, but it would of course be consistent with many other things as well.

Speedtoys

This set of logs was no big tromping, the 24/3400 lean bits are cruising up THRU those ranges, not getting off gas, etc.

I can cruise at 2400 and watch 16-18:1 just roll down the ST logs on and on...