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This s a post I had on corvetteforum but didn't get much exposure so it'll be a bit less fluid than normal.
Hello, recently my wideband decided to stop working so I had to replace it. Around the same time I noticed a bit of knocking (High load, low RPM is what i was experiencing, not paying attention to WOT). I swapped out my fuel filter (it's got 80K on it and was a bit clogged (Slight resistance when blowing through it). The knocking got better but watching AFR at WOT on the new WB it's in the 14s!! I have it tuned to about .1 of commanded AFR and 12.9-13.0 at WOT In all gears at those areas in my tables. It would ALWAYS be almost on my commanded AFR so I know something is amiss. I threw a mechanical gauge on and I see 58psi and idle and cruise. At WOT it will drop to 55psi and stay steady. I'm fairly sure this is fine especially since It never goes below that regardless of fuel requirement. I think my next step is to get the injectors cleaned/flowed. The injectors are stock and I want to keep it way. The fuel pump is also stock. Car runs/drives perfect aside from this. I have spoke w/ PLX regarding this and they feel if the results of testing came back good (They did) there is no way that the WOT AFR would be that far off from the part throttle and idle (which I am seeing 14.6-14.7). So at this point I either have a bad WB or I'm thinking injectors. I cannot see not knocking like crazy on 92 octane in 90 degree weather at higher loads if I was in the 14:1 zone at WOT. I haven't been watching the knock sensors as I don't have the laptop plugged in but last I checked it certainly isn't what I'd expect for a 14:1 AFR at WOT I do not believe I have any exhaust leaks pre o2. What do you guys think?
Update:
So I went ahead and added 10% to my PE table as a test and it did lean out about 1 full point so that tells me the fuel pump is fine as are the injectors. However as before I am about 1 full point leaner than commanded (12.8 commanded, seeing 13.8). It's almost like my IFR is wrong but NOTHING in this tune has changed from when it was a perfectly functional tune for about 10 years. As a reminder, as a test I did swap in another set of stock 2000 injectors. I've also tried another WBo2 just to ensure it wasn't that so I'm really out of ideas here. Anyone have any tidbits of info? I compared my stock IFR to what it is now and they are the same. Can any other settings cause this? It's not a multiplier as all i'm concerned about right now is my commanded vs. actual AFR. Looking at my tune the only other thing i see is that from the stock tune that may affect this that I had changed my MAF frequency vs. airflow using a spreadsheet and tons of logging years ago and of course my VE table.. It was fine for quite some time after that and I don't even recall if the MAF is used during WOT fueling?
I wonder if for some reason i need to retune the VE and MAF frequency tables? I did clean the MAF also but it was very clean to begin with.
The ONLY thing I can think of is a few years back (and long after I had tuned the MAF) i swapped the vararam intake supplied filter for a K&N and IF it flows better it may be leaning out out as more air could be entering the engine than the MAF is calculating for perhaps? It seems I had added about at 5% increase to my MAF frequency table across the entire range, probably to account for the additional airflow and design of the vararam intake system. Looking at my logs now I'm still 7-10% low charting wb02 vs. MAF frequency. Admittedly I have very few data samples for this log shown below but it is indicative of the issue I'm seeing. I'm debating just massaging the MAF frequency table a bit more but I'm not sure I just want to start ***** nilly playing with things as I'm very rusty w/ tuning as I haven't touched the thing in 10 plus years.
Last update,
Was definitely the MAF table. I'm actually shocked the computer couldn't actually correct for this. It HAD to be the filter change which makes sense as if it flows much better it'll lean it out some IF it was a restriction before. Goes to show w/ a vararam AND a K&N you need to add more (in my case about 15% more) airflow to the table. You'd think that it'd just measure more airflow and compensate w/a higher frequency. However that isn't occurring here, perhaps I don't understand something about this.
That's exactly why you have to tune your MAF. At a set hz, it believes it has X amount of airflow, if you raise or lower that, and do not adjust it in the tune, it's still going to believe x hz= x airflow.
What I don't get it this. Airflow is airflow. So a certain air velocity should always be the same frequency regardless. Why would a reduction and increase in flow change anything here, the flow is still the same, it just may be more or less laminar. A restriction would reduce airflow, and therefore lower the frequency but i don't see why anything else would matter given you are using the stock MAF and it's associated table.
Your MAF doesn't really directly measure airflow though by how it is designed. It's a heated wire that trying to stay at a certain temp via electrical input while the airflow cools it. Remember your trying to calculate airmass not just airflow so temp matters here as well as you're going to cool that wire faster with lower temps aka more dense air.
Your factory MAF is tuned to account for stock incoming airmass via the frequency in Hz it has to increase to get the wire temp to where it prefers to be. If you change intake size, volocities and temp you inherently changed that calibration. Since a 1 Hz change to cool that wire in a 1 inch intake is very different in mass to a 1 Hz change in a 5 inch intake.
Thank you, that makes more sense. I actually knew that but didn't even consider that. Also since the air is now coming in from in front of the bumper with a ram air effect it will be a cooler airmass to boot. But i'd think more air and a cooler mass would mean that it would cause the wire to want to cool down more requiring more current which would generally indicate more air and MORE fuel wouldn't it? I'd think that'd richen it up if nothing else no?
Dave
Originally Posted by 1987firechicken
Your MAF doesn't really directly measure airflow though by how it is designed. It's a heated wire that trying to stay at a certain temp via electrical input while the airflow cools it. Remember your trying to calculate airmass not just airflow so temp matters here as well as you're going to cool that wire faster with lower temps aka more dense air.
Your factory MAF is tuned to account for stock incoming airmass via the frequency in Hz it has to increase to get the wire temp to where it prefers to be. If you change intake size, volocities and temp you inherently changed that calibration. Since a 1 Hz change to cool that wire in a 1 inch intake is very different in mass to a 1 Hz change in a 5 inch intake.
Not necessarily, you changed more that just possibly the temps and flow rate.
You also changed HOW the air flows through your intake, air flows similarly to water in a way so you may have actually reduced the mass of air flowing over that wire now. It may be creating little currents that are avoiding your sensor and making it see "less" mass than a stock airbox. That's why really anytime you change anything before the MAF you should tune it to match, and also why that honeycomb screen is there to try to lessen that effect as much as possible by making the flow more laminar but it can't fix it 100%. Also it's why you should never take that screen out unless you really like trying to tune MAF's.
Think of it this way, you could blow on that wire with a straw and certainly make it think it has more air mass coming through it than your new intake.
I get what you are saying but I cannot see a low pressure area or air vortices of some sort caused around the wire from this somehow tricking it into thinking there is less air coming through the MAF. Two things are for sure here though. The velocity has increased and the incoming air temperature is reduced (maybe only slightly but i've not monitored IATs before/after) with my intake system. Both of these would cool the wire much quicker which I have to assume would indicate the engine is taking in more air thus richening the mixture. Unless SOMEHOW something w/ airflow is that disrupted to not be actually flowing through the center of the MAF where the wire is as much but If that was the case I would assume these intake systems wouldn't be that useful. BTW, this is a C5 w/a vararam intake which uses the foglight holes to feed in air. So it's a cooler ram air effect. I did notice quite a gain at the track with it and if i recall i did see the g/sec increase significantly but this was 10+ years ago so hard to recall.
Your MAF have a honeycomb or not?
Is the MAF right after the vararam?
I also have a C5, 85mm MAF. Been tuned MAF only by me since 2013. always tuned at the dragstrip.
If you didn’t change the MAF switch over point from stock, your car is MAF only past 4000 rpm. That’s the stock setting.
The fueling with a Vararam absolutely needs tuned at speed. The stock filter sucks. You changed that to a K&N. I did that right off the bat with a TR6 filter glued in
Here is what I found.
Fueling in low gear, low speed was far different after getting to second and then 3rd. It took a bunch of iterations to get actual vs commanded to be a reasonably flat line in the 1/4 mile.
Yes, stock screened MAF from my car. I have a manual trans and I did not mess w/ MAF points so yes from my recent research it's referencing MAF about 4K. It was still lean in the lower rpms also however. i also do all my tuning on the street and track. My commanded AFR was always very accurate at the track w/ the stock vararam filter. I basically globally added 5% to the table and it worked well. I probably did this based on some excel spreadsheets but it was so long ago I don't recall. Did some logging and saw it was about 10% low so I increased the MAF another 10% and it's good now. So in essence I'm guessing the switch from the Varanasi to K&N filter increased airflow a good 10%. I still don't exactly understand why the MAF wouldn't detect the added airflow given it's a heated wire and I assume the ECM monitors current to keep the wire at a predetermined temp? I'll have to research this a bit more and I'm very curious.
I guess I haven't explained it to where you fully understand. Your MAF isn't telling you that you are getting less air into the manifold. That wire is what ... like 1 mm in size if that. It is measuring a sliver of airflow in a very specific area, not over the entire intake tube.
For the sake of argument imagine we have 2 different tubes, one is 3 inches in diameter and the other is 4 inches in diameter. To keep it very simple lets say velocity, airflow dynamics, and temp are identical between these 2 pipes they would measure the same on a MAF sensor because you're only reading that small section right in the center. You would have to calibrate it for each tubes volume since you would be getting more mass with the 4 in pipe than the 3 in pipe.
That is in a perfect scenario but the fact is, is that you changed the airflow in your intake and that is a very large difference in calibration for that small area of the sensor. Air could be hugging an outer section of your intake now and not hitting the center as much. This is why you tune the fueling first then the MAF. You know your fueling is correct from the VE without using the MAF, then you enable the MAF and tune it for your explicit car since they are all different. Using anyone else's car as anything other than a very rough guide is not something I would do, there are way too many variables at work to alter airflow through your intake system.
I guess I haven't explained it to where you fully understand. Your MAF isn't telling you that you are getting less air into the manifold. That wire is what ... like 1 mm in size if that. It is measuring a sliver of airflow in a very specific area, not over the entire intake tube.
For the sake of argument imagine we have 2 different tubes, one is 3 inches in diameter and the other is 4 inches in diameter. To keep it very simple lets say velocity, airflow dynamics, and temp are identical between these 2 pipes they would measure the same on a MAF sensor because you're only reading that small section right in the center. You would have to calibrate it for each tubes volume since you would be getting more mass with the 4 in pipe than the 3 in pipe.
That is in a perfect scenario but the fact is, is that you changed the airflow in your intake and that is a very large difference in calibration for that small area of the sensor. Air could be hugging an outer section of your intake now and not hitting the center as much. This is why you tune the fueling first then the MAF. You know your fueling is correct from the VE without using the MAF, then you enable the MAF and tune it for your explicit car since they are all different. Using anyone else's car as anything other than a very rough guide is not something I would do, there are way too many variables at work to alter airflow through your intake system.
Fair enough, thanks for the info. In reality here, the vararam would have made my car 15% leaner had I not tuned it which I find odd. I'd love to see AFR runs w/ this intake on an untuned car. If I recall I gained a good .2 and 2 mph w/ this mod. I wonder if because these cars are tuned a bit rich from the factory some of the HP gain is from it being leaned out by the intake.
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Fuel pressure/AFR question 
