Wanted to see how intake air would affect FE with increased airflow and then with a restricted airflow. Removed the original 18 airduct that went from the air box to the forward corner of the engine compartment and replaced it with a 4 long piece of 2 diameter PVC that is directed towards the exhaust manifold. Reset the ECU and did runs with car configured with normal, 2 hose, and restrictor plate on a 41 mile test loop. The restrictor plate was made buy covering 2 hose with cardboard with a 1x1hole cut in the center. Each mod showed an improvement. With the restrictor there was a slight decrease in acceleration but good drivability. Max RPM was 3575 and Max speed was 60 MPH.
The restricted airflow shows promise but I forgot to reset the ECU for that run. During the run with the restricted airflow when I would shift into neutral to coast the idle RPM would only drop to around 1300 until the car would come to a stop then drop down to its normal 650 RPM. Towards the end of the run the idle was dropping to 1000 RPM so I think that it will eventually get back to its 700-800 RPM range which will save on fuel. Scan gauge results below.
Conditions: Beginning of runs calm winds 76
End of runs calm winds 70
Im just guessing but there is probably a sweet spot on the amount of air restricted to get the best fuel flow and still have performance acceleration without getting squashed getting on the highway. I basically reduced the airflow by 2/3. The original area of the 2 opening was 3.14 reduced to 1. I might try Ύ x Ύ opening in the restrictor and see if it chokes off to much air. The way it is set up now the acceleration starts to drop off once the RPM reach about 3200. My normal shift point on the automatic is 2500 RPM. I think that most of the fuel saving comes from the acceleration factor so if you were doing a lot of highway miles it might not show any improvement.
Perhaps this is part of the reason behind the smaller diameter throttle body on the D15Z1, 40mm vs 56mm I think?
I am in agreement, but the manifold is also vastly different to suit that so it's not just the smaller TB. I'd say a D15Z1 IM/TB would be quite a good FE upgrade, considering the way it is designed with the resonance and all that that Bunger explained.
Another thing I would be concerned about is the TPS. On OBD2 at least, the TPS plays a part in fuel injection rate. If you had to open the throttle more it seems like the ECU would want to dump in more fuel.
Horsepower is how hard you hit the wall, torque is how much of the wall you take with you.