First it would be simplier to block off the upper grill area and stop the massive amount of air flow through the engine compartment. Second is not to heat up the intake air too much because . . . you have a plastic intake manifold and heat is not going to make it last longer - it is one of the reasons that the intake is in front of the engine for additional cooling. You may also want to block off the air gap between the hood and the top of the radiator since that gap is letting a lot of air in also.
- The throttle bodies are warmed up with coolant to avoid condense/icing. It do is possible to have this, even without a venturi because the airflow is increasing there drastically when the throttle valve is open for e.g. 15%.
- You will not damage anything by warming up the airtemp after the MAFsensor. The MAFsensor measures temperature to calculate the exact mass of air passing by at that point. What happens afterwards doesn't change a thing to the airmass entering the engine (as long as all is airtight).
I've got a '06 Toyota Corolla with an AT and have some experiences/results that all may ponder.
My engine setup is very similar to the OP's Scion (well, it is a Toyota). I too did the 3" aluminum duct from the engine exhaust manifold and similarly had poor temp rise. Generally about 10 degrees over ambient. I even tried to make a sheet metal shroud over the existing exhaust manifold to help capture the heat, didn't help. One problem with this approach is the difficulty working on the engine in that area, it's a long reach and generally a PITA.
For my next experiment I placed a "air box" (for lack of a better name) directly behind the radiator. This was made out of a plastic mud box (for drywalling) I bought at Home Depot. dimensions are appox 5"W x 14"L x 4"D. The open end rested against the radiator, with a 3" hole on one side coupled to the 3" duct. Generally air temps are about 35 degrees over ambient. (all temps from my ScanGauge). I installed this on August 8, 2009 and have been running like this since then.
Now on to the results. I track every gas purchase and mileage in a spreadsheet. While one may fault my reasoning. I thought a year-to-year comparison may be the best way to see if there is any improvement as any variations due to weather winds, driving style, tire pressure, etc. should cancel out. So here's the numbers:
From 8/8/2009 to 3/09/2010 - 18,836 miles using 472.19 gallons = 39.89 MPG
From 8/8/2008 to 3/12/2009 - 19,297 miles using 497.85 gallons = 38.76 MPG
So based on this, my average mileage increased 1.13 MPG or just under 3%
Mitigating factors. I believe my driving style has improved over this same period as I have been more conscious in my attempt to get better mileage. That could account for the change by itself. The mix of driving really hasn't changed at all, I've got a one way commute of 35 miles with 90% of it on highways.
My conclusion. WAI may work, but I think a 3% increase could still be in the "noise" and is not definite proof. I may try and get a bit more aggressive and work on getting a higher temp, but I already block the front grill and have a partial belly pan to cover up most of the area under the front bumper. Plus in the winter I do a partial block on the radiator to help with warm-up times (it gets cold here).
Sorry for the long post, but thought some may find this interesting Good luck to all others in their personal quest for better mileage!
I bought the R-30 insulation. The thinner stuff was R-13, and wasn't narrower at all. Both were 15" wide, to fit between 16" and 18" stud centers. There was only a $0.40 difference in price between rolls, so I went with the thicker insulation. The humongous roll (~36" wide) is simply 40' of ~3" thick insulation, while the thinner roll (~16" wide) is 40' of ~1-1/2" thick insulation.
So, I'm almost ready for phase 3 of the testing. I'll wrap the WAI with the insulation this week, and try to get some testing in. Temperatures have been in the 70s this week. It's so warm that my cooling fan is going on with the engine off at stoplights. Apparently my lower grill blocks really do have an effect, as my fan never went off last summer, sans grill blocks. I monitored IAT, but not engine water temperature. IATs reached a high of 112?F while P&Ging yesterday, uninsulated WAI connected.
That R-30 is really thick when it uncompresses. I had a difficult time wrapping my WAI with it. I ended up cutting three pieces of it, each about 2' long, and wrapping them around my WAI before I went out to run some errands today. It didn't go well. The insulation is too thick, and I left gaps. I'll have to thin the insulation, and tape it closed around my WAI when I try again. Under 70 degree ambient temperature, my weakly insulated WAI only gave me 103? F IAT while P&Ging. That's about a 10? F improvement over the uninsulated WAI, and about 35? over ambient temperature.
An r-value only states how well it insulates, not how it burns. Fiberglass insulation is made from glass fibers. Glass does not burn. Maybe r-13 insulation made from another material will burn, but not fiberglass.