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Wait, What Performance?...
At first blush it might sound like an oxymoron but in fact economy and performance aren't such strange bedfellows. After all performance is really all about efficiency in all its iterations. The nature of good tuning should support an increase in daily MPG's, primarily at light throttle cruise, by virtue of more precise calibrations for the particular driving environment, fuel quality, and performance mods that increase engine efficiency.
While higher than stock levels of horsepower certainly do increase tailpipe emissions when in higher throttle/higher boost driving, the increase in efficiency should translate to better light throttle cruise MPG's where most drivers spend the majority of the time. This typically nets an overall reduction in tailpipe emissions when compared across the entire range of driving conditions.
Air Fuel Ratio Matters
An interesting side bar that I've noted is that not all fuel mapping is the same. What I mean is that given two identical cars in stock form the fuel mapping may not be exactly the same. In some cases it's quite different. As for MPG in these cars: XC's are the worst because they have so much against them. Less optimal aerodynamic profile, highest parasitic loss from AWD, Auto trans, and poor stock mapping IMO. My 99XC is a prime example. I can't get much over 18 in city on stock map and around 26 on highway. However all stock fuel maps are not the same, typically an air fuel ratio (AFR) of 14.7:1 is ideal for both reduced emissions and maximum efficiency but factory mapping is not consistent in every car. Most Volvo models that I've seen are programmed for this 'stoichiometric' value of 14.7:1 but I've seen press cars with maps (I affectionately call them 'Press Maps') that have the fuel trims at 16:1. Obviously this leaner fuel target is only at light throttle cruise and has typical AFR values when under load and in boost.
AFR 16:1 Is Too Lean IMHO
Point being there's some room in the middle to take advantage of this. Now 16:1 in my experience is too lean and could end up reducing vehicle longevity even if only by ~10%... it's too lean for my comfort level in what I would provide out the door to a customer. In the particular case of a vehicle with a 'press map' that I most recently read out that car was a 2000 S70 GLT with 120K on the odometer, so take that for what it's worth.
Three MPG Strategies
There are three basic strategies that tuners employ when trying to increase MPG's:
- Reduced load transient sensitivity. Basically filter out the smaller movements in the throttle that would normally cause tip in enrichment when the movement is less than 5%, essentially this helps keep the ECU from enriching the mixture for throttle tip in when it's just small variations in driver foot positioning. Again this is done primarily at light throttle cruise.
- Advanced timing strategy based on fuel and mods. Adjusting the timing maps based on typical fuel octane the driver uses and with respect to modifications on the car (exhaust, intake, etc..) these maps can help to increase engine efficiency and lead to increased MPG's.
- Active AFR. By adjusting AFR by not just the front wideband 02 sensor based on fuel map target but also offset by the exhaust gas temperature model. The EGT model is a feedback value that normally enriches mixture to lower EGT's if they get too high, but with a bit of rework you can also use that map double duty to lean the light throttle cruise AFR's based on EGT modeling. So if EGT will allow it, the ECU will lean the mixture to a point. If EGT's get too high it will bring AFR's back down to keep the model limits satisfied.
Be advised you'll really only see increases like this at highway speeds since the inertia and energy required to move the car stop light to stop light is fixed and can't really be changed. So in town gains are pretty small but on the highway bigger gains can be had.
More Ways To Use Less Fuel
As time goes on and we continue to push forward with what we can safely gain in MPG's over stock, we'll be looking for even more ways to use less fuel. I've recently started work with students at a local university to introduce the use of water/meth injection for the purpose of reducing fuel usage by substituting some of the fuel injection quantity with water/meth. The idea behind this is that by using water/meth we can take advantage of the considerable increase in timing from the cooling effect it provides while running the engine AFR leaner that we otherwise might. This additional cooling prevents EGT's from rising higher than would otherwise be safe at these leaner AFR targets.
More Ways to Think About Efficiency
By the time you are reading this I will be just finishing our last day at the SEMA show and by next Friday who knows what we'll be talking about then. New products and fresh ideas abound at the show, and we'll be bringing back the best and brightest we find.
Robert Lucky Arnold
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