Trailmaster Install
Uniform flow has no impact on anything but the MAF readings.
Have you ever seen a wind tunnel? Ever wondered why there is a big screen in front of the 'test area' of the tunnel? The sole purpose of that screen is to straighten the air through the area of interest. Otherwise any readings or observations made in the test area would be skewed and therefore unreliable.
Take that reasoning and apply it to the MAF sensor. In the absence of uniform flow, you can have areas of increased velocity that can influence the MAF readings. If the flow is moving faster over the thermistors than it is throughout the rest of the 'throat', the MAF will report a higher than actual air flow. Flow moving slower over the thermistors than the rest of the cross-section through the 'throat' and it will report less than actual. This is the phenomenon that I witnessed on my Bonneville and was verified by many other people in the 3800s2 community, both s/c & n/a.
In the absence of the screen, one should make ALL attempts possible to make the intake duct as straight as possible from the filter to the MAF. Bends, corrugations, and/or any other disruptions should be removed to keep the flow uniform.
For the record, the intake duct work where a MAF screen does its job isn't really any different between a s/c motor than it is on an n/a motor as it is all before the throttle body. Intake ducts are intake ducts. If it were somewhere after the throttle body, I might agree with you.
As far as my observations on an AFR imbalance, I attribute this to more EGR gases being pulled into the front cylinders (1, 2, and 4). The ports for cylinders 2 & 4 are up front in the plenum on the driver side of the LIM and the port for cylinder 1 is up front on the passenger side (they cross over in the LIM). With a functioning deflector, at normal driving conditions when the EGR valve is open, air from the throttle body is being directed towards the middle of the plenum where it can pull and more evenly distribute the EGR gases. Remove the deflector and now air is being introduced to the front of the plenum. In those cylinders, the EGR gases were displacing combustion supporting oxygen making a fuel rich, oxygen lean condition while the other cylinders were fuel lean, oxygen rich to even things back out at the O2 sensor. This was visible through the LTFT/STFT imbalance side to side. With two cylinders (2 & 4) sucking in more inert exhaust gases than the third (6), the Bank 2 trims were rich. Bank 1 trims were less affected as two cylinders were burning closer to the necessary ratio. I never let the plugs foul as I cleaned them each time I removed them.
I drove my truck of close to 6k miles with the blade cut off because I could not find a replacement blade. I ended up trading the truck a week after I grabbed a throttle body off a '96 Astro. I noticed no increase in mileage with the cut down deflector. After the 'honeymoon' with the increased off idle throttle response that was present below 50% throttle, the increased sensitivity became annoying as all get out in every day driving.
If this is a toy and/or you drive at WOT all the time, you won't notice the AFR imbalance as the EGR valve is not active at WOT. If it was strictly an offroad truck, program out the EGR completely and use a block off plate and that concern goes away. The MAF inconsistencies will still be present though and with such an influential sensor in the operation of the engine... Will it hurt anything, no. Will you really gain anything, IMO, not without many other MAJOR modifications to increase the breathing of the engine (intake manifold, cam, headers, port work, etc) whilst ensuring that the flow through the MAF is as straight and smooth as possible.
Okay then... /diatribe I've stated my peace on this and won't disrupt your thread again. I'm speaking from experience and from knowledge of the flow of fluids/gases. I am just trying to save folks time and, as often is the case, money.
Have you ever seen a wind tunnel? Ever wondered why there is a big screen in front of the 'test area' of the tunnel? The sole purpose of that screen is to straighten the air through the area of interest. Otherwise any readings or observations made in the test area would be skewed and therefore unreliable.
Take that reasoning and apply it to the MAF sensor. In the absence of uniform flow, you can have areas of increased velocity that can influence the MAF readings. If the flow is moving faster over the thermistors than it is throughout the rest of the 'throat', the MAF will report a higher than actual air flow. Flow moving slower over the thermistors than the rest of the cross-section through the 'throat' and it will report less than actual. This is the phenomenon that I witnessed on my Bonneville and was verified by many other people in the 3800s2 community, both s/c & n/a.
In the absence of the screen, one should make ALL attempts possible to make the intake duct as straight as possible from the filter to the MAF. Bends, corrugations, and/or any other disruptions should be removed to keep the flow uniform.
For the record, the intake duct work where a MAF screen does its job isn't really any different between a s/c motor than it is on an n/a motor as it is all before the throttle body. Intake ducts are intake ducts. If it were somewhere after the throttle body, I might agree with you.
As far as my observations on an AFR imbalance, I attribute this to more EGR gases being pulled into the front cylinders (1, 2, and 4). The ports for cylinders 2 & 4 are up front in the plenum on the driver side of the LIM and the port for cylinder 1 is up front on the passenger side (they cross over in the LIM). With a functioning deflector, at normal driving conditions when the EGR valve is open, air from the throttle body is being directed towards the middle of the plenum where it can pull and more evenly distribute the EGR gases. Remove the deflector and now air is being introduced to the front of the plenum. In those cylinders, the EGR gases were displacing combustion supporting oxygen making a fuel rich, oxygen lean condition while the other cylinders were fuel lean, oxygen rich to even things back out at the O2 sensor. This was visible through the LTFT/STFT imbalance side to side. With two cylinders (2 & 4) sucking in more inert exhaust gases than the third (6), the Bank 2 trims were rich. Bank 1 trims were less affected as two cylinders were burning closer to the necessary ratio. I never let the plugs foul as I cleaned them each time I removed them.
I drove my truck of close to 6k miles with the blade cut off because I could not find a replacement blade. I ended up trading the truck a week after I grabbed a throttle body off a '96 Astro. I noticed no increase in mileage with the cut down deflector. After the 'honeymoon' with the increased off idle throttle response that was present below 50% throttle, the increased sensitivity became annoying as all get out in every day driving.
If this is a toy and/or you drive at WOT all the time, you won't notice the AFR imbalance as the EGR valve is not active at WOT. If it was strictly an offroad truck, program out the EGR completely and use a block off plate and that concern goes away. The MAF inconsistencies will still be present though and with such an influential sensor in the operation of the engine... Will it hurt anything, no. Will you really gain anything, IMO, not without many other MAJOR modifications to increase the breathing of the engine (intake manifold, cam, headers, port work, etc) whilst ensuring that the flow through the MAF is as straight and smooth as possible.
Okay then... /diatribe I've stated my peace on this and won't disrupt your thread again. I'm speaking from experience and from knowledge of the flow of fluids/gases. I am just trying to save folks time and, as often is the case, money.
Indeed, I have been to a wind tunnel, but alas and again sir, unlike in the wind tunnel, the screen in our subject duct is downstream of both the thermistors which mark the sampling region, and also the venturi, which was designed as you are aware to create a localized pressure drop thereby increasing the velocity of the charge over the thermistor elements. Because of the delta in Reynolds number due to the scale of this system as compared to the windtunnel, the blade that was placed immediately upstream of the venturi should serve to steady the flow profile immediately prior to it's introduction to the test area.
Which is exactly where it should be to do the job I am describing. If pressed, I will maintain that perhaps indeed you have it backward sir, and that the function of the screen is to level the velocity profile of the charge just prior to its introduction to the throttle body, and that it was the removal of the screen that was more likely the root cause of your varied fuel mixture and not the increased throttling differential wrt pedal angle from varying the restrictor.
hehe - ya, so basically having your MAF installed backwards will give you issues, and trimming the throttle plate will give you the illusion of pep because you don't have press the pedal as hard.
I would also say that the prudent Blazer owner should rely on the extensive lab and field testing performed by the Product Development and Product Engineering Teams under controlled conditions.
oh, and humble pie ain't so sweet....
I would also say that the prudent Blazer owner should rely on the extensive lab and field testing performed by the Product Development and Product Engineering Teams under controlled conditions.
oh, and humble pie ain't so sweet....
