ITB air flow

Question about the Borla EightStack system and how it is tuned and balanced.

Background: The system is modeled after a Weber 48IDA carburetor with the injectors buried in the fuel bowls. The injectors are ABOVE the throttle plates and the spray discharge is onto the throttle plate and then into the airstream as a carburetor would.

The system has a "balance" port that will leak air around the throttle plate when closed. The concept is to balance the two bores, the port is opened on the lower flowing unit so they both flow the same air.

The installation has been balanced and flows equally on all cylinders at idle and also at 2000 and 3000 rpm. Each pair of throttle bodies then has one cylinder with the bleed port closed and the second cylinder with the bleed port cracked open to get the air flows the same.

The questions

1) If the injector is told to flow "X" milliseconds to make the car idle at lambda 1.0, for the cylinder with the balance port screw closed, is it correct to assume if the fuel open time is accurate, the lambda would be as targeted, say 1.0

2) on the side on which the bleed port is opened to get the airflow in balance, at that same setting, would that side be leaner by some amount since some of the airflow is going "around" the blade and not pulling fuel with it?

3) at some point of the throttle opening, do not these bleed ports shut down and air flow is strictly what it going past the blades. At that point and beyond, with the airflow even on both throttles, other than for flow/discharge tolerance, the two cylinders would then have similar lambda's?

I am chasing a side-to-side imbalance on my Short Term Fuel settings and trying to understand all the various factors that could come into play. If I am correct with my assumptions above, on each side, I could have two cylinders running leaner than the other two and the O2 sensor the each bank being confused as the lambda fluctuates.

Or am I overthinking all this?

Paul

Paul,

I'm guessing you are balancing the bleed screws using a synchrometer, but please correct me if I'm wrong in that.

1,2) I would expect fuel mass delivered to each cylinder to be similar. Every injector has variance built in, but hopefully you have a good set. Keep in mind they don't necessarily have the same variance at all on times, pressures. With your particular application, how much you deliver is just one aspect of fuel delivery. If the fuel puddles and "drips" between the plate and bore wall, the quality of fuel delivery can be poor enough to cause poor combustion and inconsistent result lambda. It may appear more stable than it is if you're only monitoring lambda per bank. As the throttle plate opens further, then this isn't so much of a concern beyond typical wall wetting. I tell people take a go pro video at the highest frame rate, film idle shooting down the ITB, then open throttle a little and see how much the fuel mist behavior changes. It helps put things in perspective.

3) Depending on the mechanical design, some ports for idle balancing are blocked off at higher throttle opening, but many remain open. If yours is an external bleed screw it's likely just a fixed orifice effectively, so always open. The air coming through a tiny bleed like that becomes a very small percentage of the whole air mass ingested once the throttle is significantly open, so it has little to negligible impact in terms of balancing once throttle is significantly open.

At higher airflow you start to run into factors like adjacent cylinders potentially "stealing" air from each other, variances in exhaust manifold runner bends and port shapes causing differences in per cylinder airflow.

Are you using a common airbox, per bank airbox, open trumpets?

Are all trumpet lengths uniform and how close are the openings to each other?

Mike

Yes, I am using a Synchrometer. It is a multi-step process to set idle, since as the flow balances, the idle typically increases. I am using the bleed screws on the side of the body to balance the barrels of each unit. The fore-aft balance on each side is done using the interlink between the two units. The bank balance is done two ways:

1) while on idle, the main throttle body (1&2 and 7&8) idle screws are used to balance the banks.

2) once the throttles begin to open, the bellcrank/cross links are adjusted to obtain balance.

I spoke to Borla and a couple of the higher level customers they put me in touch with. Borla purchases Ford injectors however they are the aftermarket units (individual) rather than the matched sets used in production vehicles. Both of the tuners I spoke to said they had never run into an issue of the injectors being so far out of balance they required replacement.

You are right about the fuel quantity being delivered, it is metered by banks so all four on one bank are getting the same shot. The cylinders with the air bleeds slightly opened are probably running leaner but I don't have a way of determining how much. I have my transport delay calculated and set to 6 revolutions which I calculated the air mass running thru the headers to the O2 sensor. However I don't have a lot of confidence in this. I have not been able to verify my settings with a fuel change due to the fluctuation of my Lambda readings.

I've attached a picture of the throttle body, which looks exactly like a Weber 48IDA. I am running open trumpets, each of which is a couple inches away from the others. I have installed a baffle into the hood scoop on the Cobra to preclude the air flow shearing off the front units air flow. The trumpets are the same length on all units

You'll notice the four air bleed screws on an angle near the base of the throttle bodies.

My next step is to open up the fuel supply lines, looking for maybe a flapper from a mis-assembled AN fitting. I doubt this will show up since the lines are PTFE tube but there may be something floating in there - I don't know and need to verify so I can rule it out.

Paul

Paul,

For what it's worth, your balance procedure sounds good. I also use minimal external bleed, max throttle opening with shower injection to reduce puddling.

Your trumpets are rather far apart from each other compared to many setups I've tuned, so I don't expect significant stealing from one another.

At this point my suspicion is you may be up against a fuel pressure oscillation issue, due to resonance in the system not being absorbed since you have PTFE lines. This can be localized and extreme at certain engine speeds and injector flow rates. As a test, I would add a 6 foot (not shorter) coil of soft rubber (NOT PTFE) hose to your fuel rail feed to act as an untuned damper. I'm not suggesting trying a damper unit because they often make things worse when not tuned to match the frequency present in your system.

I would log fuel pressure at the highest rate available to you, before and after this physical change. The issue is often in the rail, so you may not see much if you're logging at the FPR, but it's worth a shot.

Beyond that we get into things that are further afield, like localized changes in air pressure under hood at certain vehicle speeds which can impact ITB setups oddly when they don't share a common airbox.

I've been doing some swapping of parts and testing to narrow this down, hope you guys are still monitoring the thread. Bottom line, it seems to be a difference in Bosch 4.9 O2 sensors.

After changing/swapping positions, etc. I've found the fuel requirements on the right and left bank move rich or lean depending on which side a particular O2 sensor is on. And all the time, the air flow thru the ITB's is equal and balanced. With sensor A on the passenger side, the short term fuel trim adds 10% on that side and removes 8% on the drivers side. When I move sensor A to the drivers side and put the drivers side on the other, these fuel trims swap. Move the sensors back and the trims swap back again. The problem/solution is repeatable and appears to be the O2 sensors.

The sensors are provided by the ECU maker, branded in their name. The manufacturer of the sensor is Walker Products. They are not cheap knock-offs. Is it possible that these sensors are the actual culprit? Or are there additional things I should be looking for? This is way beyond my paygrade but I am learning.

Paul

If everything else is exactly the same and it follows the sensors, then it's 99.9% likely to be down to them, as you surmise.

I'd be getting onto your supplier and/or the manufacturer to see what they say, and if possible have them checked. Been a while, but I seem to recall something about a corrective resistor being built into the sensors, if so, that may be faulty?

Paul,

It sure sounds like you've done some smart work and found your answer.

You mentioned it's a Bosch LSU 4.9 sensor, but then said it's made by Walker Products which I've never heard of so that MIGHT be a factor.

Even if that wasn't the case, your testing indicates at least 1 sensor isn't trustworthy.

I would purchase 2 new Bosch produced Bosch sensors and see how things go. This is a trusted US based vendor:

https://www.bmotorsports.com/shop/index.php/cPath/103_105

Potentially related, wideband o2 sensors are only accurate for so long and their life span varies dramatically with their environment. Any idea how many operating hours they have on these?

Mike

I'd be surprised if they have forty hours on them. Essentially new. But they have been exposed to some abnormal rich and lean environments, if that would cause degradation

I know the labs can test an O2 sensor in a controlled gas environment. Is there a way for a typical home shop to test one? Maybe put it in propane gas and measure voltage?

Paul

Paul

The Saga continues, but to a better note

I replaced the two O2 sensors. returning them the PE to put them on the test bench and verify their accuracy. Took a short ride tonight and it appears the two new sensors are considerably better. The car runs noticeably better. One side Short Term fluctuates around the 100% point by about 1/2% depending on the needs. The other side is fluctuating around 102%.

I am going to talk to the guy who wrote the algorithm for the ECU to understand how the inputs from two O2 sensors are factored into one long term table with only one setting. I wonder if inside the calculations, one side is fighting with the other.

We are a lot closer now. Happy with the results and realize it will never be exactly 100% all the time. Now just curious as to why it acted as it did

Thanks to everyone for looking over my shoulder and offering support and ideas. It means a lot

I'll report back the findings on the two sensors and also the comments on how the ecu calculates its fuel opening times.

Paul

That's great news Paul, well done sticking with it!

Sensors are cheap enough that if I suspect a fault, I replace with new and see if behavior changes.

For tuning, 80 operating hours is my max replacement time, but high heat environments, certain fuels, misfiring, or a turbo or engine fault putting oil into the exhaust will cause me to replace sooner. After enough tuning experience, you can tell when the sensor has started lying to you. Sometimes response gets slow, or ambient air readings are odd, but sometimes they seem OK until you put the engine under heavy load.

If a sensor is being left in a vehicle, then I let leave it up to the car owner, but can tell when data isn't trustworthy. I start seeing skewing at high load despite all other engine factors remaining consistent, and don't make adjustment based on it. This happens a lot on turbocharged road race and rally cars. Then when they replace the sensor(s) I confirm things are still on target, no tuning adjustment needed.

On the other hand I've seen shops torch sensors to clean them up, and swear they get years out of LSU sensors. I cannot confirm or deny whether this practice works as well as replacement of sensors. I've not seen them verify by any scientific means.

You could create a chamber you flush with a control gas to test a sensor, but I'd want to test multiple ranges to feel confident. Given the supplies and time involved it's cheaper for me to replace sensors, but it might be a fun experiment.