I restored Sven's airbox thermostat this past fall and had my best winter fuel economy numbers ever this past winter. I averaged about 24 mpg, and that's with shorter trips than usual as I was laid off Nov-Feb. Previous winters were more like 21-22 mpg and that was on long highway runs. I credit the restored airbox thermostat system for a good part of that improvement.

As part of my fuel-economy project/obsession I'd decided to take on the airbox heater. I figured it might be worthwhile as I, like scorron and others, have noticed that fuel economy improves steadily as temperatures rise. My best mpg numbers were during the hottest parts of last summer, about 30-32 mpg highway with auto tranny.

I measure my mpg obsessively/compulsively at each fillup and drive about 120 miles/day so I have plenty data to review. Top summer temps here are between 95-100 deg. F and winters go down to 10-15 deg. F. A pretty wide range of temperatures.

I noticed the airbox has a funnel or horn (forward-facing intake) that receives raw outside air in and blows it on the thermostat. So it's reading outside air temp, not the mixed air temp. Based on the temp of incoming outside air, it sets the damper the best it can to control net air temperature to the throttle body.

I learned from Bentley (section 150-2) that the damper is designed to be at full cold above 59 deg. F and at full hot below 41 deg. F. This has in fact turned out to be the case in my restored system.

I got a digital interior/exterior thermometer ($10) and installed the "exterior" sensor on its long wire to read the temperature of air flowing through the airbox. The "interior" unit reads cabin temperature. Having the thermometer will let me know when the airbox thermostat kicks the bucket so I don't kill the AMM. Plus I get to see the readout of intake air temperature.

I restored the manifold tin shield and the aluminum duct. I wrapped the duct with pipe insulation held on with spiral wound duct tape to maximize the temperature of preheated air going to the airbox. Net result, on the coldest days this past winter the intake air was around 75-80 degrees - it was like summer inside that engine!

In more moderate temperatures between 35-50 degrees, the intake air hovers around 55 deg. F. We're just now starting to see temperatures above 55 deg. but I know what will happen - the damper closes off the preheated air when outside temp reaches about 59 deg. and that's all the preheating you'll get.

Future plans include modifying the stuff inside the airbox so it will pull a higher percentage of preheated air, and hopefully to get intake air temp up around 90-100 deg. without it being summer.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.