Chasing efficiency

I’ve been keeping very detailed records for quite some time, figured I’d share. While I’m interested in fun cars, I’m also interested in efficient cars. That leads me down some interesting paths.

First off, I’ve been analyzing my lube & oil use. There’s a ton of misinformation out on the web about this. I’ll tell you firsthand the things that I’ve learned (due to BITOG and Blackstone Labs). First, find truly important data. Ignore the marketing, and the reputations, and anyone that has no evidence to back up their opinions. For instance, a used-oil analysis on your engine, with wear metals highlighted, is worth more to your pocketbook than millions of well-meaning but ill-informed forum posts.

I’ll use my ’91 318i as an example. As purchased in April ’09, the car had 266K miles. It was in great mechanical shape with the exception of the 4.10 differential. I swapped in a good used 3.73 unit. I also immediately changed the fluids with high-quality & commonly-available coolant, motor oil, diff lubes and filters. The motor got Mobil1 5w30; the replacement diff received Valvoline 75w140 synthetic gear lube. Unfortunately for any comparison, all those upgrades had been done before I put any miles on the car at all. So, as my baseline after all those upgrades, this well-maintained M42B18 5-speed started out at 29.2MPG average. Stock aluminum 14″ wheels with wider summer tires actually provided a slight improvement to 29.8MPG later on, but partial credit goes to high-quality summer fuel (more on that later)

In Nov. ’10 (276K miles), the first significant work performed was fresh air & fuel filters, and a few liters of Valvoline’s commonly-available synthetic ATF in the gearbox. Fuel mileage went up significantly to 30.8MPG. Admittedly, much of this is due to highway driving.

March/April ’11 (280K) saw a new O2 sensor and new NGK BKR6EK twin electrode spark plugs, along with a new thermostat, fanstat and auxiliary fan. Slight loss was noted in economy.

July ’11 (282K): I installed a COP upgrade, along with new silicone vacuum hoses and very thin 5w20 Mobil1 synthetic motor oil. Mileage rose to an excellent 32.9MPG. Later than year, my annual UOA showed uncomfortably high wear metals. Conclusion? There is no such thing as a free lunch.

As a comparison, my 2006 Mazda MPV equipped with a Ford Duratec 3.0L V6 showed no real wear at all, even over a 15K interval, using 0w20 oil (resource-saving Mobil1 synthetic). Tighter engines need lower viscosity. Admittedly, like most 80s motors, the BMW engine is ‘looser’ than the Ford.

At 287K I switched back to 10w40 Valvoline high-mileage synthetic and repaired a fan clutch. Mileage dropped all the way back to 26.6MPG, aggravated by an early cold snap & winter fuel mixtures.

Note the low annual mileage. I tend to average only a few hundred miles per month. Despite all that, I’m still checking the oil annually.

Fun things that I’ve learned about economy is that the best mod you can start with is high-quality fuel. It’s not about octane, either. Note these remarkable effects of the fuels I choose:

  • Seasonal fuel changes, proper octane, alcohol content and refinery quality are more important to economy than any other ‘modification’. Summer & fall fuel ratios for summer/fall vs. winter/spring were 31.5:30.0. Winter fuel imposes a 5% penalty, all else being held equal.
  • Curiously, I noted that octane had a direct effect on the car. Our local octane ratings of 89 : 91 : 93 provided 29.3 : 26.7 : 31.1 MPG respectively. The way-too-high-octane-for-this-POS-car 93 provided a 5% improvement over 89 and 14% over 91. The data oddly shows 91 octane as the worst of the bunch. My hypothesis? Reputable 93 octane is a completely different formulation from the GroceryMart 91 fuel. I also typically pumped 93 on the highway and burned up the cheap 91 octane for local driving. A few gallons of the cheap fuel also evaporated in storage, or went into Mum’s 2-stroke lawn equipment, so the analysis isn’t 100% accurate. While it is something of a “perfect storm” in favor of 93, it’e typically the most economical choice.
  • Gasoline stations are not standardized by franchise. Look for a Top Tier retailer and keep detailed track of your fuel costs. They’ll be a few cents higher, but the franchise name is largely unimportant. Patronize reputable businesses. The 318i MPG ratio for Top Tier vs. local GroceryFuelMart averages out to 30.3 : 28.1 MPG. That’s a whopping 8% improvement in economy, nearly negating the typical 10% cost increase of far superior fuel. However, your results will vary; every vehicle is different. The relatively crude digital fuel injection of my BMW doesn’t compensate for knock nor octane.

Other information I gathered about motor oils:

  • Nearly all brand-name motor oils are very high-quality. So brand name is essentially meaningless, viscosity and oil additive packs are far more important.
  • Most synthetics are only superior to conventional oils if you’re extending oil change lengths.
  • Even a cheap synthetic in a well-maintained engine would likely protect for very high mileages. I’d estimate 25K miles wouldn’t be a problem if it’s the proper viscosity. Filter changes and partial replacement of oil would be required for this method.
  • Significant mechanical wear in carburated engines is caused by fuel contamination, so FI cars shouldn’t be subjected to the same service frequency.
  • Synthetics lubes are also an important consideration; long service life and slightly higher efficiency contribute to a lower overall cost per mile.

Other mods that I’ve seen limited successes with:

Cooling system mods: I’ve also found advantages in running a ‘warmer’ motor with thinner coolant mixes. I prefer a 2:1 water/coolant mix along with some surfactant additive (like Redline Water Wetter). I’ve also found a slight improvement from bumping up the thermostat temperature rating about 10%. Don’t forget a matching fanstat.

Fuel injector mods: Bosch design III injectors were a pretty significant improvement, but much of that is likely due to overhauled injectors being a really good idea.

High performance rubber: Eliminating vac leaks is an important long-term solution for engine wear. BMWs are especially vulnerable to these leaks, primarily due to poor design & materials choices. I’ve adapted silicone and other exotic (toxic) rubbers like Viton/fluorocarbon for that purpose.

Drivetrain mods: Don’t run over-sized wheels. They literally suck power and efficiency. Thinner sidewalls contribute substantial harshness to the car’s ride. Wider tires cause noise, tram-lining and increase rolling resistance. Heavier wheels increase sprung weight, compromising handling. Also try a taller differential. Your passengers, your motor and your ears will all be happier for it.

Other misc. mods: Don’t upgrade your alternator to a higher wattage unit! They’re all pretty inefficient, try to minimize the parasitic drag that your car needs to run. It’s pretty easy to freshen up a Bosch or Valeo alternator with a new voltage regulator & brushes. Some BMW alternators even have the factory equivalent of under-driven pulleys, they’re well worth seeking out. Also be sure your grounds are all in a well-serviced state, especially the battery terminals, primary grounding cable and the big ground strap from the engine to chassis.

In a related note, keep your battery in a well-changed state. A partly-discharged battery will run the alternator pretty hard as it recharges. I prefer designs like this cheap $14 HF solar charger. They come with a bonus feature: they’re surprisingly effective at extending battery life.