Quote:
Originally Posted by Power Hungry
One other thing to pay close attention to is the timing. At lower RPMs with even a modest load, timing can drop to almost zero which can seriously affect fuel economy. This is done in order to help prevent detonation under those types of load conditions. Larger tires compound the problem by dropping the RPM at certain speeds.
For example, my Expy with 3.31 gears gets better fuel economy at 80 than it does at 75 because of the load and RPM range. Oddly enough though, it also gets good economy at 65 which, despite the lower RPM and load range, I assume is due to the lower wind resistance at that speed.
One other thing to remember is that in order to help reduce droning from the CAI kits, we remove the modified Cam Timing curves and revert back to the stock ones. This has a slight economy hit as well.
Take care.
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Yipes! This also means my interpretation of the effect of combining a custom tune with a commercial CAI is somewhat wrong! The custom tune actually DOES
reduce the effectiveness of the CAI a bit due to the reduced timing. (I guess we could call it an unfortunate interaction). Reduced timing means reduced power, and, a corresponding reduction in fuel economy.
I'm sure glad I'm using my homemade CAI!
And, larger tires will mean lower rpms at any cruise speed, so, the reduction in timing can be an additional hit on top of the increased moment of inertia of the larger rotating mass as well as larger frontal drag due to more surface area and the increased rolling resistance of the bigger tires.
- Jack