Quote:
Originally Posted by California98Civic
I don't see the major fuel economy gains, though, unless it is just by getting into closed loop sooner.
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It's not just about getting into closed loop faster. Modern cars are tuned to intentionally waste a bunch of extra fuel in order to heat the cat up faster.
Here are the things the stock tune on my Subaru does to (presumably) heat the cat faster:
- Idles in stratified fueling mode up to coolant temps of 60C (increasing fuel use by at least 50% at idle)
- Retards ignition timing for the first 3 minutes after engine start.
- Retards exhaust cam timing by 20 to 50 degrees until the coolant temperature is above 50C.
- 1800rpm idle speed up to coolant temps of 40C, and even for a while after hot starts.
I've pretty much tuned all these things out, with great benefits to short-trip fuel economy. They don't affect driveability. I can only assume the engine is tuned like this to heat up the cat quickly for regulatory reasons. I'd be surprised if overall NOx emissions were even reduced much.