Anyone know what the guys in the States refer to as pre-fuel cut? It sure looks alot like MAF overrun to me. Has anyone with a datalogger ever seen MAF overrun on a mildly modified 1G? Let me clear up what I mean. Before you hit fuel cut at like 15 psi at rpms over 5,500 or so you'll get the 'excessive knock defense'. So you get my MAF mod done and it lowers the volume of air going through the metered section and the problem goes away. So it is based on the airflow. Now I always thought this was MAF overrun, but I don't have a datalogger to tell me if it is or not. But a couple people in the States have told me this is what they call pre-fuel cut and that you don't hit MAF overrun until well after fuel cut. But they don't know what pre-fuel cut is really... just that it exists. Well I'm unconvinced. I think they are the same thing. Can anyone give inputs that would prove me wonr or support me? Can MAF overrun be hit at boost levels around 15~16 psi? If not do you know what other limit there is out there that would cause the knock LED to go nuts and timing to be retarded based on air volume? I know what I've seen on a lot of 1Gs and I know it exists I just want to call it by the pollitically correct term.

p.s. I don't mean fuel cut or problem with spark.

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Mike Jackson
92 Laser RS-T, FWD
-Quadcylla-
Vortex Racing
http://www3.sympatico.ca/robin.jackson2/HomePage.htm

MAF over run is what happens when the MAF flows too much air. After the maf mods it is still possible to get too much air flowing through the metered section. I think the MAF can only count upto 2000Hz-2200Hz anything higher and the maf loses count and you get a stumble. The data logger only reads up to 1850 I think, So even though it shows as being maxed out there is still a little room left.

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'91 Laser RS Turbo-"Sound Advice" guy.
www.geocities.com/motorcity/speedway/5051
Turbo + Intercooler = The replacement for Displacement

Thanks Chris... I understand all that The datalogger maxes out at 1606 yet the MAF maxes out around where you said. Watching other things though you can tell when the limit is hit. What I really want to know is if that "pre-fuel cut" is really MAF overrun. I know what MAF overrun is and what causes it but I want to know if anyone has hit it (with a datalogger as proof) with maybe just a cat-back, K&N (no airbox) and 16 lbs of boost. Or is the phenomina of 'pre-fuel cut' something that acts exactly the same for a specifically different reason. Pre-fuel cut is also bassed on the air volume so I really am thinking they are the same thing... just hoping someone had proof.

Later....

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Mike Jackson
92 Laser RS-T, FWD
-Quadcylla-
Vortex Racing
http://www3.sympatico.ca/robin.jackson2/HomePage.htm

I have seen mildly modded 1g cars have maf over run. 16 psi, cat back, 14b. No idea why though. But it can happen.

Now fuel cut is always after, never before (in my experience)

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PurplePlymouthGuy
1992 Laser AWD
DSMentia support group leader...
so many mods, I don't remember if its 20G or 20g's

I've seen what I call a flat spot around 4000 rpm were the car kicks back the timing. Is that what your talking about? the timing kicks back to like 8 degrees around 4g and the thing feels like a stone.

Mark... thanks for the input man. So did the knock sum go up, timing get retarded AND the injector pulse width go nuts? I want to make sure the injector pulse width went nuts to confirm that it was the MAF signal going erratic that is causing the knock in this case.

On the other hand... how do people get around this with just an AFC??? The AFC would not solve this problem cuz it only manipulates the MAF signal... but won't correct it's erratic behaviour. The only way I can see to solve it is hacking the unmetered section of the MAF to lower the volume and thus velocity of the air going through the metered section. This would bring the velocity down below where the MAF gets overrun. But if an AFC does solve this particular problem then it can't be directly related to MAF overrun Anyone seen this go away after installing and tuning an AFC? In particular did it solve the erratic injector pulse width at 5k rpm or higher? I can see that just richening it up would get rid of the knock but it still wouldn't get rid of the erratic MAF signal.

I put a similar question into the digest... I'll let you know what comes out of it.

Marco... that could be what you witnessed... did you see the knock sum shoot up and the injector pulse width go nuts? Did the timing stay retarded till higher rpms or just freak out at about 4K?

I'm leaning more and more towards MAF overrun here boys as the problem. My mod will get rid of this (what ever the name).... ask anyone I've done it to if they've seen their knock led come on since then

Later....

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Mike Jackson
92 Laser RS-T, FWD
-Quadcylla-
Vortex Racing
http://www3.sympatico.ca/robin.jackson2/HomePage.htm

Last time Andy B. was in Montreal, I mentioned I was thinking of going to a 2G MAF. Andy, having a 2G, cautioned me that the 1G MAF has some "holes" (probably what Marco is calling "flat spots") where somehow the signal cuts out or something at specific rpms. The 1G ECU knows about this strange behaviour, and compensates for it at the appropriate rpm ranges. Apparently the 2G MAF sends a steady (unbroken) signal to the ECU, so when people do the 1G/2G MAF swap, they end up with some erratic results at specific rpms, precisely because the 1G ECU is automatically compensating for these holes/flat spots even when they are not
there.

Not to say that I fully understand what I just typed here, and I may have expressed it incorrectly (please, Andy, jump in and correct me if I did!). What I think I'm trying to say is that if you've modified the car so that the timing and/or rpms are not within the parameters that the ECU seems to be hard-coded to expect, this could cause those symptoms you are describing.

This is a bit over my head, so if what I said doesn't make any sense, please disregard... if it does make sense, then please be so kind as to explain it to me so I understand it too

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Marta
dsmgrrrl@hotmail.com
92 Talon TSi AWD

I have a hacked mas and the S-AFC and I'm getting a lot of timing retard at high rpm. The injector pulse width did go kind of screwy. The knock sum went through the roof. I think the injectors are maxing out or not firng "properly" which causes the high knock and retarded timing...I'll know more in the spring when I do more tuning and datalog a dyno run.

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'91 Laser RS Turbo-"Sound Advice" guy.
www.geocities.com/motorcity/speedway/5051
Turbo + Intercooler = The replacement for Displacement

I really think it has nothing to do with the mass, causer Ive experienced it mostly on the 2g cars I think It has more to do with engine load at that rpm, cause when I go 3/4 throttle I charges, which leads me to suspect the fuel and timing maps.

BTW chris, I think your problem is with the factory intercooler

Well I got a reply from Todd Day and the limit I'm talking about is a limit set up in the ECU against airflow (not air mass). Since it's a limit against airflow Todd figures it was put in there to prevent us from hitting MAF overrun. When pre-fuel cut is hit the ECU closes the boost control solenoid thus trying to shut down your boost levels. I guess it has no effect on timing or knock sum just the BCS. So if you remove the BCS from the system then it'll get rid of this problem because the ECU will no longer have any control over boost. You'd then be free to hit MAF overrun and fuel cut

So the answer to my original question was 1) a limit in the ECU. Funny how my MAF mods theory post to the digest early this year is proving to be more and more accurate

This is only for 1Gs. Apparently this effect does not exist for 2G guys.

I'll let you all know once I've updated the page... it'll be a very interesting read

Later....

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Mike J

-Quadcylla-
92 Laser RS-T, FWD

Vortex Racing
http://www3.sympatico.ca/robin.jackson2/HomePage.htm