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WARNING: This is pretty long and technical, so you have been forewarned!
Ok, to start off, I just wanted to start this discussion because this is a problem that has been bothering me for a while now, and I have (what I believe to be) a good hypothesis for why this is a problem (and how to fix it, of course).
Here's the problem: You add a BPV, and next thing you know, you're running way too rich and backfiring on shifts...WTF?
Here's my hypothesis: The BPV spring is too tight is not developing LTFTs where they need to be, which means that you need to adjust it so that it's looser.
Ok, to understand what I mean, let's start off with a little background on how a stock/stock-like BPV works in our car.
BACKGROUND
Our BPV is a differential pressure valve which is arranged roughly like this:
The air in this picture flows from right to left as the engine runs, and the BPV closes by using the spring in the picture to push in the direction that the double arrows point. This pushes the piston against the hole leading to the Cold Pipe and keeps the air in there from coming into the valve and getting recirculated back into the intake. By manipulating the pressures at locations 1 and 2, you can force the spring to expand or compress on demand. For example, if you apply the same pressure to both 1 and 2, the valve will remain shut, because the spring is allowed to push the piston shut. Now, if you were change things such that the pressure at 1 is lower than the pressure at 2, the piston would be sucked upwards, opening up the passage from the Cold Pipe so that air can get recirculated back to the intake as pictured. Here is what happens in practice:
When the throttle is opened, the pressures at locations 1 and 2 (numbered in red) are about the same, which means that the spring is allowed to keep the piston shut...preventing recirculation of air back to the intake. When the throttle shuts, however, a vacuum is formed in the Intake Manifold, while the Cold Pipe section is left at a higher pressure. This would suck the BPV piston upward, which would force the BPV open...allowing air to recirculate back to the intake. Translated into everyday terms: as the throttle shuts every time you shift, the 16 psi sitting in the cold pipe gets released and recirculated back into the intake in a PSSH!
Now that we have covered the fully open and fully closed throttle positions, the important part of this discussion comes into play: what happens at the various partial throttle positions?
At partial throttle, the pressures at locations 1 and 2 are different, but not THAT different. This means that there's a bit of suction upward on the piston. This difference in pressure increases as the throttle closes more and more...eventually pulling upward on the piston enough to overcome the force that the spring is pushing down on the piston with so that air starts recirculating back into the intake. This point is what I'll refer to as the Release Point of the BPV. A higher release point will refer to requiring a larger pressure difference between locations 1 and 2 before the valve pops open...whereas a lower release point will refer to having a lower pressure difference between locations 1 and 2 before the valve starts opening.
The way the Release Point of the BPV works implies that the stronger the spring in the BPV is, the more the throttle can close (higher release point) before the suction at location 1 becomes strong enough to force the valve to open and leak air back to the intake.
Ok...that's enough about BPVs for now...here's some info on fuel trims that you need to know:
Our car uses Short Term Fuel Trims (STFT) and Long Term Fuel Trims (LTFT) to adjust the AFRs to account for variations in the intake system. The reason this feature is in place is because when the ECU goes to decide how much fuel to add, it cannot trust its sensors 100%. When the ECU reads from the MAF sensor how much air has entered the system, it calculates how much fuel it needs to inject in order to achieve a commanded AFR...let's just say this is 14.7:1 at this moment. Well, what happens when it injects the amount it thinks it needs to inject, but then reads the wideband O2 sensor to find out that it came up lean and actually got a measured AFR of 15.5:1? Why was there this disparity? The answer is that sensors aren't perfect, and may be off by a bit. In order to account for this, the ECU uses what is known as a STFT to add/subtract an offset to the current amount of fuel that it's injecting to increase/decrease the measured AFR to match its commanded AFR. If the ECU keeps seeing a STFT of the same value, it saves this value into what's known as a LTFT. This LTFT is then applied to the commanded fuel injection amount to account for the inaccuracy of the sensors. After applying the LTFT, checks the wideband O2 sensor to see how close it got to its commanded AFR and decides if it still needs to add/subtract an additional STFT to achieve its commanded AFR. This allows the ECU to "learn" how this particular car works and apply fueling accurately. In a car that is operating normally, the ECU will take 20-50 miles (roughly...your mileage may vary) to learn the LTFTs that the car requires, and then run with nearly-zero STFTs most of the time. This, incidentally, is what is meant by allowing the ECU to "learn" after a reset.
There are four distinct LTFTs that I have seen our ECU to use. One or more of these may just happen to turn out to be the same, but they are usually different from what I've seen (since flow through the intake system changes under increasing flow conditions/turbo operations). These fuel trims cover:
1. Idle
2. Engine breaking
3. Light Throttle
4. Heavy Throttle (used for WOT operation)
These LTFTs are applied at these particular times in conjunction with STFTs (except at WOT...explained in a sec).
Now, you ask: why bother with LTFTs when you already have STFTs? Well, the reason for this is that LTFTs allow you to get things right the first time without having to react in order to correct a lean condition, fixing the problem before it happens! Equally important, LTFTs allow us to safely run in what's known as "Open Loop".
Our fuel system has two operation modes: "Open Loop" and "Closed Loop". These operation modes are used for different reasons...Closed Loop is basically an emissions/fuel conservation mode whose main goal is to maintain an AFR of 14.7:1, which lowers emissions and increases fuel economy...at the cost of raw power generation. In this mode, the ECU monitors its sensors very carefully and uses its STFTs and LTFTs to maintain 14.7:1 as closely as it can (with slight dips to protect the engine from damage as load varies, of course...but it always settles to 14:7). Closed Loop is usually accompanied with obscenely high spark advance (as high as 50+ degrees) and other assorted efficiency optimizations. This is the mode that we run in most of the time in our car...unless you go heavy on the gas pedal, of course, which causes the car to go into what is known as: Open Loop is a mode whose main goal is to safely achieve high power output while controlling engine cylinder temps and avoiding knock. At this point, the ECU ignores its O2 sensors and simply runs off of preloaded tables to decide what AFRs, timings, etc. that it will command in order to raise power output without regard for fuel consumption and emissions.
This is where LTFTs are absolutely imperative: The sensors are every bit as untrustworthy in Open Loop as they are in Closed Loop. How can the ECU reasonably expect its commanded AFRs to be properly achieved if it's not monitoring the O2 sensors? The answer is that it applies the LTFTs (without using STFTs afterwards) it learned while running at heavy throttle. The assumption is, of course, that heavy throttle flow and sensor characteristics mostly mirror the flow and sensor characteristics of WOT operation. This system usually works decently well and gives the ECU roughly the AFRs that it commanded.
Ok, now that the background is out of the way, let's move on to the point:
THE POINT
During partial throttle operations, the ECU is calculating LTFTs at the same time the BPV is struggling with its Release Point. Since a stronger spring will adjust the Release Point higher, this can definitely affect the LTFTs, since a stronger spring will cause the valve to stay closed at a more and more closed throttle positions.
My hypothesis is that our ECU is programmed to expect a low release point, since the stock BPV has a relatively weak spring holding the valve shut. I think that this stock Release Point is expected to be DURING the operation/learning range of the heavy throttle LTFTs. This is the same LTFT that is used during WOT (Open Loop) operation. When you add an aftermarket BPV which raises the Release Point of the BPV such that it puts it out of the operation/learning range of the heavy throttle LTFTs, which means that the ECU will learn these LTFTs with a fully shut valve (vs the slightly leaking recirc of the stock weak spring), which means that more air would actually get into the engine, making the ECU calculate higher LTFTs, since it sees more air in the system than it's used to (making it add more fuel). This illustration shows what I mean by increasing release points putting themselves out of heavy throttle LTFT operation/learning range:
The important part to notice is that at WOT, the BPV would have been fully shut in both the stock spring and stiffer spring case, so what just happened here is that we artificially introduced a higher LTFT that the ECU would use during Open Loop operations...making the measured WOT AFRs richer. This means that when you romp on your fun pedal, your car's going to run richer than you're used to, and then backfire when you shift because it's running so damn rich. This, of course, can be fixed by simply adjusting your valve to have a lower spring tension, or tuning the WOT AFRs to command a leaner AFR to begin with.
Ok, obviously, there's going to be doubters here, but before you start ripping into me, check out why I think this is true: I've been beta testing BPVs for a few months now, and I've been through a NUMBER of spring settings in these prototypes. This has given me a wide range of observed BPV release points. Toward the beginning of my testing, I went way too heavy on my spring settings on one of the prototypes and got crazy rich AFRs on WOT along with backfires. This was on a fully stock exhaust car, so I was rather weirded out. To troubleshoot this, I went back to the stock BPV (because the BPV was the only thing I had changed), and suddenly, my AFRs were back to normal after 20 miles of learning. I swapped back to the prototype valve, and the problem came back. I swapped the spring tension (lowered it) on the prototype valve, and what do you know: AFRs back to normal again! Ever since then, I've been extra careful in adjusting my spring tension settings on these valves (before, I only played around with them to tune for pewpew sounds). I've also been a constant advocate of adjusting spring tensions to anyone I see on the forums with similar problems.
Ok...sorry for the wall of text, but I figured I would get some discussion and awareness out there about what I think is a pretty interesting problem that a number of people have been running into. Please post constructive criticism about what you think of my thought process!
Ok, to start off, I just wanted to start this discussion because this is a problem that has been bothering me for a while now, and I have (what I believe to be) a good hypothesis for why this is a problem (and how to fix it, of course).
Here's the problem: You add a BPV, and next thing you know, you're running way too rich and backfiring on shifts...WTF?
Here's my hypothesis: The BPV spring is too tight is not developing LTFTs where they need to be, which means that you need to adjust it so that it's looser.
Ok, to understand what I mean, let's start off with a little background on how a stock/stock-like BPV works in our car.
BACKGROUND
Our BPV is a differential pressure valve which is arranged roughly like this:
The air in this picture flows from right to left as the engine runs, and the BPV closes by using the spring in the picture to push in the direction that the double arrows point. This pushes the piston against the hole leading to the Cold Pipe and keeps the air in there from coming into the valve and getting recirculated back into the intake. By manipulating the pressures at locations 1 and 2, you can force the spring to expand or compress on demand. For example, if you apply the same pressure to both 1 and 2, the valve will remain shut, because the spring is allowed to push the piston shut. Now, if you were change things such that the pressure at 1 is lower than the pressure at 2, the piston would be sucked upwards, opening up the passage from the Cold Pipe so that air can get recirculated back to the intake as pictured. Here is what happens in practice:
When the throttle is opened, the pressures at locations 1 and 2 (numbered in red) are about the same, which means that the spring is allowed to keep the piston shut...preventing recirculation of air back to the intake. When the throttle shuts, however, a vacuum is formed in the Intake Manifold, while the Cold Pipe section is left at a higher pressure. This would suck the BPV piston upward, which would force the BPV open...allowing air to recirculate back to the intake. Translated into everyday terms: as the throttle shuts every time you shift, the 16 psi sitting in the cold pipe gets released and recirculated back into the intake in a PSSH!
Now that we have covered the fully open and fully closed throttle positions, the important part of this discussion comes into play: what happens at the various partial throttle positions?
At partial throttle, the pressures at locations 1 and 2 are different, but not THAT different. This means that there's a bit of suction upward on the piston. This difference in pressure increases as the throttle closes more and more...eventually pulling upward on the piston enough to overcome the force that the spring is pushing down on the piston with so that air starts recirculating back into the intake. This point is what I'll refer to as the Release Point of the BPV. A higher release point will refer to requiring a larger pressure difference between locations 1 and 2 before the valve pops open...whereas a lower release point will refer to having a lower pressure difference between locations 1 and 2 before the valve starts opening.
The way the Release Point of the BPV works implies that the stronger the spring in the BPV is, the more the throttle can close (higher release point) before the suction at location 1 becomes strong enough to force the valve to open and leak air back to the intake.
Ok...that's enough about BPVs for now...here's some info on fuel trims that you need to know:
Our car uses Short Term Fuel Trims (STFT) and Long Term Fuel Trims (LTFT) to adjust the AFRs to account for variations in the intake system. The reason this feature is in place is because when the ECU goes to decide how much fuel to add, it cannot trust its sensors 100%. When the ECU reads from the MAF sensor how much air has entered the system, it calculates how much fuel it needs to inject in order to achieve a commanded AFR...let's just say this is 14.7:1 at this moment. Well, what happens when it injects the amount it thinks it needs to inject, but then reads the wideband O2 sensor to find out that it came up lean and actually got a measured AFR of 15.5:1? Why was there this disparity? The answer is that sensors aren't perfect, and may be off by a bit. In order to account for this, the ECU uses what is known as a STFT to add/subtract an offset to the current amount of fuel that it's injecting to increase/decrease the measured AFR to match its commanded AFR. If the ECU keeps seeing a STFT of the same value, it saves this value into what's known as a LTFT. This LTFT is then applied to the commanded fuel injection amount to account for the inaccuracy of the sensors. After applying the LTFT, checks the wideband O2 sensor to see how close it got to its commanded AFR and decides if it still needs to add/subtract an additional STFT to achieve its commanded AFR. This allows the ECU to "learn" how this particular car works and apply fueling accurately. In a car that is operating normally, the ECU will take 20-50 miles (roughly...your mileage may vary) to learn the LTFTs that the car requires, and then run with nearly-zero STFTs most of the time. This, incidentally, is what is meant by allowing the ECU to "learn" after a reset.
There are four distinct LTFTs that I have seen our ECU to use. One or more of these may just happen to turn out to be the same, but they are usually different from what I've seen (since flow through the intake system changes under increasing flow conditions/turbo operations). These fuel trims cover:
1. Idle
2. Engine breaking
3. Light Throttle
4. Heavy Throttle (used for WOT operation)
These LTFTs are applied at these particular times in conjunction with STFTs (except at WOT...explained in a sec).
Now, you ask: why bother with LTFTs when you already have STFTs? Well, the reason for this is that LTFTs allow you to get things right the first time without having to react in order to correct a lean condition, fixing the problem before it happens! Equally important, LTFTs allow us to safely run in what's known as "Open Loop".
Our fuel system has two operation modes: "Open Loop" and "Closed Loop". These operation modes are used for different reasons...Closed Loop is basically an emissions/fuel conservation mode whose main goal is to maintain an AFR of 14.7:1, which lowers emissions and increases fuel economy...at the cost of raw power generation. In this mode, the ECU monitors its sensors very carefully and uses its STFTs and LTFTs to maintain 14.7:1 as closely as it can (with slight dips to protect the engine from damage as load varies, of course...but it always settles to 14:7). Closed Loop is usually accompanied with obscenely high spark advance (as high as 50+ degrees) and other assorted efficiency optimizations. This is the mode that we run in most of the time in our car...unless you go heavy on the gas pedal, of course, which causes the car to go into what is known as: Open Loop is a mode whose main goal is to safely achieve high power output while controlling engine cylinder temps and avoiding knock. At this point, the ECU ignores its O2 sensors and simply runs off of preloaded tables to decide what AFRs, timings, etc. that it will command in order to raise power output without regard for fuel consumption and emissions.
This is where LTFTs are absolutely imperative: The sensors are every bit as untrustworthy in Open Loop as they are in Closed Loop. How can the ECU reasonably expect its commanded AFRs to be properly achieved if it's not monitoring the O2 sensors? The answer is that it applies the LTFTs (without using STFTs afterwards) it learned while running at heavy throttle. The assumption is, of course, that heavy throttle flow and sensor characteristics mostly mirror the flow and sensor characteristics of WOT operation. This system usually works decently well and gives the ECU roughly the AFRs that it commanded.
Ok, now that the background is out of the way, let's move on to the point:
THE POINT
During partial throttle operations, the ECU is calculating LTFTs at the same time the BPV is struggling with its Release Point. Since a stronger spring will adjust the Release Point higher, this can definitely affect the LTFTs, since a stronger spring will cause the valve to stay closed at a more and more closed throttle positions.
My hypothesis is that our ECU is programmed to expect a low release point, since the stock BPV has a relatively weak spring holding the valve shut. I think that this stock Release Point is expected to be DURING the operation/learning range of the heavy throttle LTFTs. This is the same LTFT that is used during WOT (Open Loop) operation. When you add an aftermarket BPV which raises the Release Point of the BPV such that it puts it out of the operation/learning range of the heavy throttle LTFTs, which means that the ECU will learn these LTFTs with a fully shut valve (vs the slightly leaking recirc of the stock weak spring), which means that more air would actually get into the engine, making the ECU calculate higher LTFTs, since it sees more air in the system than it's used to (making it add more fuel). This illustration shows what I mean by increasing release points putting themselves out of heavy throttle LTFT operation/learning range:
The important part to notice is that at WOT, the BPV would have been fully shut in both the stock spring and stiffer spring case, so what just happened here is that we artificially introduced a higher LTFT that the ECU would use during Open Loop operations...making the measured WOT AFRs richer. This means that when you romp on your fun pedal, your car's going to run richer than you're used to, and then backfire when you shift because it's running so damn rich. This, of course, can be fixed by simply adjusting your valve to have a lower spring tension, or tuning the WOT AFRs to command a leaner AFR to begin with.
Ok, obviously, there's going to be doubters here, but before you start ripping into me, check out why I think this is true: I've been beta testing BPVs for a few months now, and I've been through a NUMBER of spring settings in these prototypes. This has given me a wide range of observed BPV release points. Toward the beginning of my testing, I went way too heavy on my spring settings on one of the prototypes and got crazy rich AFRs on WOT along with backfires. This was on a fully stock exhaust car, so I was rather weirded out. To troubleshoot this, I went back to the stock BPV (because the BPV was the only thing I had changed), and suddenly, my AFRs were back to normal after 20 miles of learning. I swapped back to the prototype valve, and the problem came back. I swapped the spring tension (lowered it) on the prototype valve, and what do you know: AFRs back to normal again! Ever since then, I've been extra careful in adjusting my spring tension settings on these valves (before, I only played around with them to tune for pewpew sounds). I've also been a constant advocate of adjusting spring tensions to anyone I see on the forums with similar problems.
Ok...sorry for the wall of text, but I figured I would get some discussion and awareness out there about what I think is a pretty interesting problem that a number of people have been running into. Please post constructive criticism about what you think of my thought process!
