Engine swaps, mileage, emissions and power

I’ve always chuckled quietly to myself when Vanagon people justify their plan to convert their van’s engine by stating, confidently and a priori, that their fuel efficiency will naturally improve by swapping the engine to something “modern”. All that tells me is the person hasn’t viewed the question thru the lens of thermodynamics, in particular the 2nd Law, the simplest and most honest way you can address such questions.

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Work is the measure of the application of force over time, and whether it’s a greater force exerted fewer times, or a lesser force exerted more times, if the net resulting work is the same, the same amount of energy was used. Basically no matter how you set it up, it takes a certain a mount of energy to do a certain amount of work. Conversion efficiency between different reciprocating gasoline engines of similar displacement differs very little, so to push the same mass and aerodynamic shape down the road at the same speed on the same tires requires about the same amount of fuel. This work amount, by the way, is about 40hp to go 65mph on level ground in calm air for a 4000lb. vehicle with the frontal area and aerodynamic drag of a Vanagon.

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“Ahh,” the stock engine haters say, “but the wbx engines are dirty and inefficient.” This demonstrates a lack of basic knowledge of not-even-very-recent engine technology, especially as it pertains to gasoline automotive engines sold in the North American market since around 1981, and later on throughout the world. Since that period, these engines’control systems use lambda-sond, i.e. closed-loop oxygen sensor/fuel trim feedback. Gasoline engines using lambda burn stoichiometric mixtures over 95% of the time, a ten-dollar word that just means that all the fuel and all the available oxygen are converted to heat. I consider any engine using lambda to be perfectly “modern”, and emissions tests show that the measured pollutants differ by only a few percent from newer engines.

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That leaves differences in how efficiently the various engines convert the heat of combustion to usable work, and what are the ancillary losses, mostly valvetrain and pumping losses. Those differences are also exceedingly small between engines of similar displacement.

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So, since the same amount of work needs to be done and the engines chosen are nearly the same as to how efficiently they convert fuel to work, it’s kinda nonsensical to claim that the popular gasoline swaps will automatically deliver an overall fuel efficiency gain that the owner could actually measure by the crude method we all use, mileage between fill-ups. Variances in filling level, driving style, weather, traffic conditions, tire condition and inflation pressure, payload, and other factors can cancel each other out, or stack up in one direction. Basically, it’s not really possible by that method to reliably measure changes of less than about 10%, or about 2mpg in a roughly 20mpg platform like the Vanagon. Consequently, almost everyone who reports their comparative mileage before and after their Vanagon engine swap says the change was 1 or 2mpg; that’s if it was better, if it was worse they don’t say anything. The actual difference in cruising hp per pound of fuel if you compared the two engines on an engine dyno under controlled conditions and using a fuel flow meter would be in single digits at best, in most cases under 5%. The same goes for emissions, since both engines are using the same feedback strategy to trim mixture dynamically for the overwhelming majority of the operating hours. So it’s always been funny to me how confidently people anticipate higher gas mileage and lower emissions after their swap, while the anecdotal reports and tailpipe tests don’t bear out the claims.

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More reserve power? Sure, the most popular swaps, the Subaru 2.2 and 2.5’s, have a larger reserve power margin ( being power available in excess of what’s needed for typical cruising speeds), but to hear these same people talk about their dissatisfaction with the stock engines, it sure seems like an awful lot of them never use the reserve power their engine already offers, judging by how many I see claim, with evident pride, that they only use the low and mid-range rpms (often claiming that the engine is being harmed somehow or simply “doesn’t like it” if they rev any higher ). A small 4-banger like a stock wbx’s reserve is modest, to be sure, but if US drivers used these engines as they are intended, where their reserve power is made at higher rpms, I think fewer would be complaining about the lack of available power.

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If I had to guess why, I would say in many cases it comes from growing up on Detroit iron. Until more recently, Detroit took the opposite approach of the European and Japanese manufacturers. Instead of smaller, lighter engines with good midrange flexibility but that are designed to make their best hp at higher rpms, US carmakers built heavier, larger displacement engines with short cam duration, where reserve power is available even at low rpms by a small push on the pedal, coupled with automatic transmissions that allow almost guilt-free engine lugging. So the driver drives exclusively with their right foot, and even if the transmission downshifts for a hill they are barely aware of it. Those engines top out at lower revs (with a short cam there’s nothing more to be had by revving higher) which works OK because the low-end torque works well with the tall gearing, so many US drivers are blissfully unaware of what gear their rig is running in or what the engine is doing at any given time.

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Then these folks get in an imported small 4-cylinder, cars that demand but reward more driver engagement, drive it like their American ride, and complain that they can’t keep up with traffic. It doesn’t work. You see it on the forums all the time, people who claim that they always keep their wbx under 4000rpm, or sometimes even lower, while asserting that that’s a good thing. They clearly state that they never use the engine in the rpm band where it makes the most power, and I can safely infer that they are lugging the little 4-banger a lot of the time. So as they believe they are treating the engine well, they are actually abusing it.

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“But the newer engines have more hp for similar displacement, doesn’t that mean they’re more fuel efficient?”, some may ask. It’s a good question, but the difference isn’t the result of efficiency, not more than a very small percentage, it’s because the newer engine is able to burn fuel at faster rate to make more power. People almost always compare engines by just the hp number and ignore the rpm that power is made at. An engine only makes torque, while hp is an abstract comparison of the work the engine could do in one minute based on how many times per minute the torque is applied ( hp= (torque [ft.lb.] x rpm) / 5252). Compare the hp AND rpm specs and you’ll always see that the engine of the same displacement makes its higher hp at a notably higher rpm. It might even make less peak torque than the lower-hp engine, but it’s ability to apply its torque more times per minute earns it a higher hp number. But each application of torque uses a fuel/air charge, more charges burned per minute equals more work done, and more fuel burned to do it. It takes the same fuel to do the same work. There’s no cheating the 2nd Law.

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And of course the higher hp being available only at a higher rpm may make that extra power of little value if it is at too high an rpm to be usable within the gearing ranges of the vehicle it’s installed in.

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I’m not against engine swaps at all, I really don’t care what anyone does to their own car, life’s full of choices and they should do whatever makes them happy. I differ, though, with the justifications people present for their decisions, because they are often founded on nonsense and as such mislead others into making their own uninformed choices. But it’s easier to change an engine and clean out a bank account than to change someone’s mind.

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