Thanks for adding that Nathan. Right down at the "molecular level" as they say, the blower rotors should not make pressure unless you absolutely know the air will flow THROUGH the rotors and not get dragged around and around inside them. Thats why an engine that can make a lot of power (for its displacement) needs a bigger bypass valve that closes later. Any time its not pumping pretty fast with throttle open, the blower needs to coast a bit.
At idle and very low throttle, low revs, the blower may be turning too slow to be effective (and the pulses get big enough to make your air flow sensing devices go crazy without bypass opening). Another critical area when dealing with these blowers is the airflow through the air filter to the Airflow Meter, especially on the common "hot-wire" AFM.
Production air filter housings are carefully designed to keep the air flow through the hot-wire clean and even. Most aftermarket air filter kits use straight through tubing between an air filter and throttle body, with the AFM inserted somewhere in the tube (pretty much wherever the stock wire reaches). On various occasions, I mapped the airflow measurements on some of these. I found big errors and "holes" in the measurement at various parts of the powerband except top end, and idle.
The best "correction", besides putting the good parts back on, is a section of intake tube about 3-4" before the hot-wire that has a pair of flat blades arranged in a T pattern. The upper quadrant of the T can be about 1/3rd down from top of the tube circle. The blades, about 2" chord length, straighten the airflow for two different scenarios: rapid airflow increase on the low end, when throttle is dumped open, and intake tract air spiral length that changes rapidly as the "tuning" gets right passing through the mid-range. The section of tube with the T installed is clamped to rubber tube at each end. This allows you to test air measurement at different orientation of the T. Once you hit the right angle you will know, because all the "bog" and "lean surge" suddenly disappears.
Hot-wire AFMs are most sensitive (give the biggest rate of change per pound of air) at the lower airflows. Near the top end of the AFMs designed capacity the measurement is much less accurate and responsive. On a 300HP engine, that peak flow rate of about 30 lbs/minute may measure anywhere from 28-31 lbs/minute. The ECM doesnt care...its running pretty much on the "open loop power map" and says "whatever....heres a chunk of fuel, so you deal with it". This is where some folks get in trouble with too-large capacity AFM and throttle body. I was told that too-big carb can be trouble also, because it can get into too much airflow for mid-range circuit at those throttle settings. I never messed with those, however, but it stands to reason that full power enrichment areas of throttle plate opening should match the real airflow the blower adds....its probably complicated.
The reason I mention that last AFM bit, is because you can have really strange things happen in the mid-range airflow measurement if something is odd with your air filter or intact tract. One of the celebrity Celicas (I remember John Elway had that car, that year) was definitely weak on mid range acceleration. The problem turned out to be one damaged clip (out of four) on the cover of the air box. When the intake tuning started working in the midrange, the cover would pulse and the AFM measured less airflow. After we replaced that damaged clip, the car ran perfectly. Some of the aftermarket air filters (big, cone shaped) can get soft, over time, and the AFM will undermeasure every time you start getting into the strongest torque range of your engine design.
Intake tuning is no different on a blower engine, than on NA. You still need to hit the tuned lengths for the powerband you are using. If you get it wrong, you can get some heavy pulsing in the intake tract that gives a "shudder" feel at large throttle. When I was working on the Turbo Highlander Hybrid we had some struggles with that. Because the Atkinson Cycle on a Hybrid only runs with large throttle opening we had to run the turbo off the front three cylinders, only (otherwise, it stays spooled all the time). The higher airflow, when we got on boost, was really showing some problems on the air flow measurement side. The intercooler was scrambling the tuned length between the AFM and the cooler inlet, because the other side (the runner side) was seeing the intake "pump back" effect of Atkinson Cycle operation. I moved the length of that inlet tract area out to 14" and it ran fine.
Getting the EVAP system to work, purge the Charcoal Canister, and not throw codes...was a whole other animal. Nature never intended your Charcoal Canister and gas tank to see boost pressure so high and so quick, as what a Hybrid does! In the end, we made a lot of power for street use but we could not overcome two problems. Every time you lifted throttle on a long downgrade, the Hybrid system shuts down the engine (normal). The turbo started cooling and that delayed relight of the Catalytic Convertor (kiss of death for that project!). The other problem is that the tires wouldnt squeal when you started boiling them if you stabbed full throttle at 30mph. Between the big electric motor, and the added push of the turbo V6, the tires just hissed (not even as loud as the intake noise). Your first clue was vague steering and smoke coming out of the dash vents (the Heater-A/C plenum has drain flow open into the front wheel-well area on most cars). I guess thats really operator error.
Now I am getting off track because this is about a fellows Whipple blower. Thanks for your patience with another trip down memory lane!
Good night,
JimL