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hi Im new here, searching this subject brought me to the forum so Im starting here. First off, I have an RX8, incredible little potent engine with a very modern emissions setup. But for those that never knew... city driving yields about 12-14 mpg out of that 1.3L engine. I have over 80k miles, and it runs very clean. It's internal seals are also incredible. Ive pulled vacuum lines to find it still holding high vacuum over an hour after shutting down.
On to the subject. Heating an intake charge is not what we look for to increase performance and thats proven on the drag strip by boosted cars using ice water in air/water intercoolers. Also proven by dyno testing when heat soak consistently lowers horsepower. Now the RX8 uses coolant to heat the throttle body. I suspect this is one of the late additions that lowered its horsepower rating upon release. Attempts have been made to bypass the throttle body in order increase power. Ive never seen dyno results of this but I think it would give a very small gain but show resistance to power fade due to heat soak in repeated testing. The side effect of the bypass is that MPG suffered greatly (some reported 1/3 total mpg losses). Here's my reference for that, note user comments at the bottom. Never mind I need 5 posts to post a link here.
now racerc2000 brings up a good point about turbo cars. Ive personally seen several turbocharged cars pulling mid 30s for mileage and several of these are 80's cars that bleed oil like its cool. There are 2 reasons a turbo car gets awesome mpg, first the turbocharger heats the intake charge a LOT. Intercoolers are widely utilized to cool this down to increase power and reduce detonation risk. Also note that a critical element to creating boost is load and the heat generated on the turbine side of the turbo increases load. Methods are employed here to coat and wrap the metal so that the heat gets to the impeller and is not soaked into the metal and lost into the engine bay.
These 2 together are about all the proof I need that a heated air charge does indeed raise your mpg. Yes this comes at the risk over power output but probably not greatly so and ultimately im talking highway driving very city/spirited driving for when you want this heat.
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