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Old 01-28-2008, 04:55 PM   #1
williaty
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Default Car Bucking, Belching Black Smoke

OK, so I went out to log some stuff today and discovered to my horror that somewhere along the line I've pissed off my car. I'm not sure if this is a tuning problem or a hardware problem (only possibility for that is the new intake).

The problem comes when I floor it from idle. Between idle and about 2k RPM, the car bucks like it's being hit by mortar fire. As soon as it starts getting its act together, it belches out a bunch of thick black smoke, then proceeds to be perfectly fine. Because the problem only lasts for such a short time, I logged it 3 times, then combined the logs, then sorted the result by RPM so there's more samples to look at:
Code:
RPM	g/rev	MRP	MAFv	AFL1	AFR1	FIPW	TotTim
961	1.22	-0.55	2.28	-3.12	12.52	17.255	-8
994	1.24	-0.49	2.14	-5.47	11.02	16.974	-7.5
1025	1.29	-0.5	2.26	-5.47	11.02	17.161	-7
1040	1.35	-0.5	2.04	-5.47	11.02	18.521	-7
1115	1.94	-0.52	2.56	-5.47	11.02	27.354	-7.5
1118	1.8	-0.53	2.46	-5.47	11.02	24.239	-4
1124	1.75	-0.52	2.28	-5.47	11.02	24.086	-4
1131	1.75	-0.53	2.28	-5.47	11.02	24.14	-4
1160	1.97	-0.55	2.84	-5.47	11.02	27.478	-1.5
1167	1.97	-0.54	2.68	-5.47	11.02	27.287	0.5
1205	1.97	-0.55	2.52	-5.47	11.02	27.565	-0.5
1253	1.89	-0.58	3	-5.47	11.02	25.874	7.5
1298	1.9	-0.56	2.84	-5.47	11.02	26.718	6
1354	1.91	-0.55	2.96	-5.47	11.02	25.36	6
1386	1.96	-0.5	3.1	-5.47	11.02	26.554	2.5
1405	1.92	-0.52	2.64	-5.47	11.02	25.564	6.5
1466	1.81	-0.51	2.54	-5.47	11.02	21.198	6
1470	1.89	-0.5	2.76	-5.47	11.02	25.322	5.5
1517	1.65	-0.5	2.36	-5.47	11.02	21.787	7.5
1560	1.65	-0.51	2.68	-5.47	11.02	21.728	8.5
1571	1.6	-0.51	2.62	-5.47	11.02	20.432	8.5
1588	1.76	-0.53	2.7	-5.47	11.02	21.873	4.5
1616	1.82	-0.52	2.34	-5.47	11.02	25.74	8.5
1685	1.22	-0.51	2.32	-5.47	11.02	15.102	8
1713	1.19	-0.51	2.42	-5.47	11.02	14.858	8
1864	1.1	-0.51	2.4	-5.47	11.83	13.618	11.5
1882	1.1	-0.5	2.48	-5.47	12.17	13.57	12
1948	1.07	-0.48	2.5	-5.47	12.52	13.253	14.5
2012	1.07	-0.5	2.68	-5.47	12.86	13.362	15.5
2058	1.08	-0.5	2.66	-5.47	12.86	13.715	15.5
2181	1.11	-0.5	2.62	-5.47	13.21	14.318	16.5
2373	1.14	-0.49	2.64	-5.47	12.98	14.914	18.5
2396	1.13	-0.5	2.72	-5.47	12.98	14.945	19.5
Here's the only thing that caught my attention:


It looks like right after I open the throttle, MAFv spikes (believe me, I would be going wild if I thought the car was actually hitting a 1.9x load, but it's never broken 1.4) which sends the AFRs down to bat**** insane town (11.02 is the reporting limit for the ECU, actual AFRs could be horrifyingly richer). As RPM rises, MAFv comes back to sanity and the ECU sorts everything out.

WTF is going on and do I fix it in hardware or software?
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Old 01-28-2008, 05:04 PM   #2
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You have done a bit of adjusting to the MAF on your car in that engine speed/ load right? Have you done anything on tip in?
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Old 01-28-2008, 05:10 PM   #3
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The MAFv range involved in the problem has been scaled properly for where it occurrs higher in the rev range. That's the same MAFv I get for WOT and about ~3500 RPM. So if I screw with the scaling to fix this, I'd screw things up at higher RPM. Haven't touched tip in, but this doesn't happen if I floor it at 2kRPM or higher.
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Old 01-28-2008, 05:15 PM   #4
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What does the fuel map lool like in this RPM and load areas?
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Old 01-28-2008, 05:22 PM   #5
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12.5 through all of that. The maps in the ECU only go up to 1.3, so all the problem area is above the top of the map, meaning no change based on load. Through those couple of RPM rows, there's no change either.
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Old 01-28-2008, 06:02 PM   #6
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I seem to be having the same issue especially bucking between 2k and 3k rpm.
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Old 01-28-2008, 06:08 PM   #7
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had you had a turbo car I call compressor surge but that definitely not the case here.

I find it odd at this time the MRP doesn't change at all. Is your car DBW or DBC?
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Old 01-28-2008, 07:45 PM   #8
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I have seen MAFv oscillate when manifold absolute pressure was smooth. It can be caused by compressor surge or it can be caused by the intake resonating.

I have also seen it on aftermarket intakes with no Helmholz resonator. It only happens at high load and low rpm. I have never datalogged such high load below 2000 rpm so I can't comment on your particular case. I've seen it at 17 - 18 psi at 2600 rpm on VF40 LGTs. For the LGT, I know it wasn't compressor surge because we have the surgeline for the VF40 and the 2.5L LGT engine clears it easily. Also, when my friend replaced his SPT intake with his stock air box with resonator, the surging almost disappeared.
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Old 01-28-2008, 08:04 PM   #9
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As a test, I swapped back to the stock intake piping with the Helmholtz resonator. Problem solved. Looks like you can't run an intake on a MAF-based 2.5RS
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Old 02-02-2008, 08:03 PM   #10
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do the WRX's have this Helmholtz resonator? What does it look like?
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Old 02-02-2008, 08:43 PM   #11
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Actually, after extensive logging and testing of several cars, it's the "snorkus" in the fender that prevents this from happening. It looks like you shouldn't EVER take that off a RS.

I also tested a WRX, which didn't have any problem at all.
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Old 02-03-2008, 08:22 AM   #12
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well, glad you found the problem!
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Old 02-04-2008, 02:42 PM   #13
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Quote:
Originally Posted by williaty View Post
I also tested a WRX, which didn't have any problem at all.
the fender piece was removed as well?
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Old 02-04-2008, 02:53 PM   #14
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Yes, that's what I'm saying. A snorkus delete (wither just by itself, or as a consequence of installing a CAI or SRI) will horridly upset the MAF sensor (MAFv's literally more than 2V higher than make sense) on a MAF-based NA whereas the on a Turbo car it doesn't affect the MAF at all.
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Old 02-16-2008, 06:58 PM   #15
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You might be looking at this from the wrong end. The box at the throttlebody (torquebox) of the stock intake may be your issue here. I noticed this peg-rich effect when I installed my intake last year. The torquebox provides a mass of air right at the throttlebody (so goes the theory). Once we got some reliable n/a defs I pulled out some tip-in enrichment and with some playing with the numbers, cleaned this right up. When you tip in, the ECU dumps in fuel, and in the case of a torquebox-eliminating intake, FAR too much. Chokes the motor, you see some flutter at the MAF, then it works out as regularly scheduled programming is resumed. The extra voltage you're seeing at the MAF with the snorkus removed is just the MAF seeing more air, and colder air due to that stupid maze not being on the other side of the filter anymore.

Give it a shot.
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Old 02-16-2008, 07:02 PM   #16
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Quote:
Originally Posted by FalconRS View Post
You might be looking at this from the wrong end. The box at the throttlebody (torquebox) of the stock intake may be your issue here. I noticed this peg-rich effect when I installed my intake last year. The torquebox provides a mass of air right at the throttlebody (so goes the theory). Once we got some reliable n/a defs I pulled out some tip-in enrichment and with some playing with the numbers, cleaned this right up. When you tip in, the ECU dumps in fuel, and in the case of a torquebox-eliminating intake, FAR too much. Chokes the motor, you see some flutter at the MAF, then it works out as regularly scheduled programming is resumed. The extra voltage you're seeing at the MAF with the snorkus removed is just the MAF seeing more air, and colder air due to that stupid maze not being on the other side of the filter anymore.

Give it a shot.
I've already done this. I was able to isolate it specifically to the snorkus.


Haven't played with tip-in yet as I just got my LC-1 this week. Gotta clean up the big problems before I can tackle tip-in.
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Old 02-16-2008, 07:17 PM   #17
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Keep in mind you're also trying this from 900-ish RPM, which, if you're 3rd gear or higher, is heavy in the bog range. These engines don't like heavy loads down there at the best of times. But I really do think tip-in is the answer to at least cleaning it up down there.

I realize you don't much care for anything I have to say, but hopefully you'll get past that and at least give it a shot. Just trying to help you out dude. Not many of us N/A-tuners around, as evidenced by every thread posted about n/a is immediately filled with a lineup of answers referencing turbo cars. Don't think we have anything to gain with any kind of feud.
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Old 02-16-2008, 08:36 PM   #18
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Don't get me wrong, I definitely intend to look at tip-in once I get overall fueling dialed in.

However, I know that it's not the problem in this case because I was able to compare stock vs. snorkus delete via logging and see exactly what's being skewed and the results thereof.
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Old 02-16-2008, 11:21 PM   #19
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Quote:
Originally Posted by williaty View Post
Don't get me wrong, I definitely intend to look at tip-in once I get overall fueling dialed in.

However, I know that it's not the problem in this case because I was able to compare stock vs. snorkus delete via logging and see exactly what's being skewed and the results thereof.
Alright, well then take into consideration that I started that process 6-8 months ago. All I'm suggesting is to give it a try. The torquebox effect has been well documented over the years, it's only just now that the n/a community has the tools to do anything about it so correcting it is a new process. The effect you're seeing at the MAF with just a snorkus removal is simply the MAF reacting to better air. Try it, you'll see.

EDIT: Lean out the tip-in slightly with a snorkus delete to account for warm air coming from the torquebox vs. the much cooler than normal IAT seen at the MAF. For the intake, you'll just have to lean it out even more. The IAT from MAF to TB will be closer, but you'll have much less air immediately available right on tip-in. You have to experiment a little to get it where you want it, I still go through adjustments here and there as I'm playing around with other things. But it's been effective.
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Old 02-16-2008, 11:41 PM   #20
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Old 02-17-2008, 12:28 AM   #21
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Yep, looks right. More voltage needed to maintain the MAF's heated element temperature, which is how it measures airflow. More/colder air will cause that spike. Once the air is moving with air flow increasing steadily through it comes back down. You're seeing the effect of pulling cold fender air directly.

Look dude, you want me to leave you alone, just say so. Never particularly been a fan of this site anyway. I'm just trying to help you out, based on what I've seen on my car.
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Old 02-17-2008, 12:55 AM   #22
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Quote:
Originally Posted by FalconRS View Post
Yep, looks right. More voltage needed to maintain the MAF's heated element temperature, which is how it measures airflow. More/colder air will cause that spike. Once the air is moving with air flow increasing steadily through it comes back down. You're seeing the effect of pulling cold fender air directly.

Look dude, you want me to leave you alone, just say so. Never particularly been a fan of this site anyway. I'm just trying to help you out, based on what I've seen on my car.
Yes, I understand how the MAF works.

However, what you're saying doesn't make sense. If the effect were solely one of pulling in cooler air, then the MAFv line for Snorkus Delete would have a roughly constant vertical offset from Stock over the entire plot. The graph blows up solely below 2kRPM. Also, the MAFv's are ~50% higher in the problem area. If we're assuming that the change in MAFv is SOLELY temperature-related, then the temperature in the fender would need to be half the temperature of the air picked up by the stock intake. Another temperature-based explanation could be that colder air is denser, also raising MAFv. To account for this ~50% increase in air density, the air sucked up by the stock intake would have to be nearly 300 degrees Celsius hotter than the air in the fender. Due to the lack of my car catching on fire, it's a relatively safe assumption that thermal issues aren't causing the errant MAFv readings.

A very likely explanation is that the snorkus is designed to damp a resonant mode of the intake system. Without this damper in place, the resonant mode pushes a given unit of air past the MAF sensor is opposing directions multiple times causing the air entering the engine to be over-estimated. This exact problem is covered in several textbooks.
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Old 02-17-2008, 10:04 AM   #23
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There is also the solution of not heavily loading your car at 900 rpm, people improve their intakes for the areas of the rev range they drive in most often, and/or for performance ranges, and I certainly don't go around torturing my car with 900 rpm 3rd gear throttle stabs. There's a stick on the center console that fixes the problem very efficiently.

Just eyeballing that graph, the snorkus-less intake does appear to have a fairly consistently higher (though not much) MAFv once it straightens out, which indicates more airflow. In the end, that's what we're after in modding our cars, more airflow. If you're correct and it's all in the snorkus that's causing your "problem", then I'd consider that the trade-off for an intake modification. All performance mods have trade-offs. Change a part, drivability changes.
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Old 02-17-2008, 10:41 AM   #24
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Lol, yeah, I know.

It actually took me a month of driving to find the problem. The only two situations in which I can see it occurring in real driving are rallycross (pitch it too sideways into a turn and you can end up below 2k if you accidentally get the tires to grip again) and if you're idling along in gear in stop-and-go traffic and suddenly see a hole to dart into. The very fact that it's possible to have the problem happen makes me want to prevent it from being possible. Also, the cure (leaving the snorkus in) has less than a 3% effect on airflow. So the problem can be avoided without any meaningful hit to performance.
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Old 02-17-2008, 11:34 PM   #25
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Falcon, radio chatter during the Subie run today caused me to have an interesting thought. A potential reason why this isn't complained about is the Closed Loop Delay. My tune has eliminated the CLD, going from CL to OL as soon as the throttle goes past 40%. If a person were to do the snorkus delete on a car with a stock tune, I wouldn't be surprised if the CLD would outlast the problem. I'd really love to log a stock-tuned 05 with a snorkus delete. I bet if I ran the test, I'd see the MAFv blow up like the graph above shows but AF Correction #1 peg at -20% and keep the AFRs somewhat reasonable.

Also, regarding your tradeoff argument, I somewhat agree. The hybrid-style intake I'm running now shows 97% (that's a real number, not one made up to make my point) of the gain that a full CAI/SRI does. To me, I'm willing to take the undetectable decrease in power from a full intake (and it's still considerably more than stock) in return for not having the problem down low.
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