Hilton

Hmm.. sounds like a job for a 3D printer? Paging Mike Fry

Tony

Makes a great apple slicer, I wonder when my wife will miss it?

http://www.walmart.com/ip/Farberware...ioner/14964900...red is faster

ptuomov

Trying to solve this lean spot problem without a MAF honeycomb is like going hunting without an accordion.

jcorenman

Good point. It's not the MAF, or an airflow issue, however.

Jeff logged a bunch of mid-range data yesterday afternoon, here's what it looks like in Sharkplotter:



Nice stripes, very consistent. AFR goes from mid-14's to mid-16's in the lean stripes.
The question is whether those stripes are lean because (a) the MAF is under-estimating the amount of air, and injecting less fuel as a result; or (b) The MAF is reading accurately and the lack of fuel is caused by something else, e.g. a drop in fuel pressure at those RPM's.

So I dug around in the log files that Jeff kindly sent, and found a good example where the throttle was held constant through the 2400 and 2800 RPM stripes. I selected that range of points in SP, expecting to see the "path" wander up and down (i.e. changing MAF load) if the MAF signal was inconsistent with real airflow.
Here's what it looks like (single run file, expanded scale):



That looks pretty straight to me. I don't think the MAF output is doing anything inconsistent with simple, constant acceleration.

I then copied that 23 seconds worth of data to Excel and plotted AFR against RPM, and also the injector pulsewidth. AFR varied as RPM's crossed the "stripes", injector pulses did not.



So the LH, listening to the MAF, was calling for constant fueling but the AFR's say otherwise.
That can only be, I think, a reduction in fuel pressure as the RPM's cross those critical points.
A resonance in the fuel rail, in other words.

The next step would be to get a scope on the fuel pressure. A conventional gauge probably won't show it, if it is a resonance effect.
But a scope and a fast electronic transducer might.
Or just reconfigure the fuel rails and see if that helps.

Speedtoys

Im buying into the fuel resonance here...

INvestigating options.

ptuomov

...therefore, trying to solve it without a MAF honeycomb would be like going hunting without an accordion.

Lizard928

Very interesting.

If it is a fuel resonance within the pipe, maybe you need to mount an accumulator onto the end of both rails....

SeanR

I've seen maps/plots exactly like this on mine. Doesn't happen every time but has shown up frequently. Hans saw 4 of them at 3rd Coast and it worried him. The next day i did 8 logs during the "drive" and only had one that looked like that.

Will be keeping a close eye on this thread.

jcorenman

Fixed it for you

ptuomov

Since fuel doesn't compress much, accumulator doesn't work -- not even as a joke... ;-b

More seriously, I think the solution is to refit the stock dampers, get some simpler 944/951 dampers as I did, or get some of those aftermarket dampers from Marren.

Standard flow path for fuel would also help, with fuel from the front separately to each rail and a FPR and return line in the back. Don't know if this is possible in this case, though.

Speedtoys

The stock flow could be possible, yes.

Know offhand the type of fitting that the fuel system uses up front?

dprantl

Is the wideband O2 sensor mounted before the exhaust joins so it is monitoring only one bank of cylinders? It would be an interesting test to swap it to the other side and do a similar logging run.

Dan
'91 928GT S/C 475hp/460lb.ft

Speedtoys

It is mounted that way..but would be a huge pain to recable it for the other side.

Tony

fyi...Installed two of those "apple slicers" in my intake yesterday. One at the exit of my 90 turn from the filter which is appx 10" from the MAF sensor...the other installed where the mesh screen woulb be.

While i dont recall seeing anything like what has been shown on those sharkplots, i have had a consistant slight surge above 2300rpm or so while accelerating. Lived with it for ages, but this thread and a few others i searched lead me to try what i did.

The results actually amazed me. The surging is gone and the acceleration is so much crisper and linear in feel. Definitely is someting to be said about the quality of air delivered to the MAF.

I will probably look for a "honeycomb" solution at some point but right now what i have seems to work great.

Speedtoys

Pics?