strange fuel economy pattern on Tracer Wagon

[93tracerwagon]

I recently completed another long highway trip with my wagon. I am not sure, but it appears that the wagon gets much better mileage when heavily loaded. I have seen this on previous heavily loaded highway trips. My best tank and the only one that was almost all highway was 37 mpg, and this with 30 minutes sitting in traffic to move only 4 miles and a typical speed of about 65mph. I noticed road warrior also had this happen and couldn't explain it. Any others have this with their escort wagons?
Vehicle has 80,000 miles, full upper grill block, all joints sealed on front fascia, 12 inch front chin belly pan, Radiator air intake all from just above splitter and space between fascia and radiator enclosed, headlight brackets etc. sealed to direct all air to radiator. All hardware removed under airbox (everyone should do this)
Also of great interest, I had been running with warmer underhood air and reinstalled my custom cold air intake baffle to the factory airbox and saw a 2 to 3 mpg jump under all types of driving. These engines seem to prefer cooler air, probably because the throttle stays further closed, which keeps the fuel mixture leaner.

[Jay2TheRescue]

Throttle position does not control the mixture. The computer in conjunction with the fuel injectors and O2 sensors does that. Actually, under load, an opened throttle has less pumping losses, and you were probably operating the engine closer to its optimum torque, which explains the better mileage.

[theholycow]

Agreed about mixture, but I disagree about the optimum torque/better fuel economy. While you're certainly getting more work out of the engine per unit of fuel, it still takes less fuel to do less work slightly less efficiently.

There is no shortage of possible other explanations given the parameters described, though; there are still PLENTY of variables in play. For one thing, there are so many completely random variables not under our control and often not even possible to measure that we can never be sure. There's fuel quality, pump click-off affecting measurement, traffic, weather, etc. Then there's more easily observed/quantified things like elevation change or other qualities of the road, if you usually take your heavy loads in one direction; changes in shifting (whether manual or auto), speed, driving style, etc related to load; and all manner of things I've forgotten.

That's not to say that I think it's impossible. For a highway trip without an increase in elevation I don't believe changing only weight can make a measurable difference in terms of the extra weight causing extra drag except a slight addition of rolling resistance. I know it's popular to think you'll gain 10% just by removing your spare tire and back seat but I don't buy it, my experience and research don't back it up. OTOH a car with soft suspension could have a significant change in its aerodynamic qualities. It doesn't take much weight in my trunk to make my car's rear drag really low. A Tracer/Escort wagon with the rear riding low could gain a shape more like a Prius or a modern crossover, with the roof sloped gently down as it goes back. The angles of the front end and windshield could also be better. A small aerodynamic advantage could make a significant difference on a highway trip.

[93tracerwagon]

However, when under heavy throttle, the computer richens the fuel mixture. I am now getting consistently better mileage with the cooler intake and probably a more closed throttle. I am thinking the increased pumping loses are being overcome by the leaner mixtures. As you can guess, power is noticeably up too.

I think when unloaded the wagon roof slopes upward and when loaded, down, so aero changes are probably making an impact. Also, with the upper grill blocked the angle of attack is much higher, especially in a wagon. With the backend sitting low, that angle is reduced, especially if the roof is now sloping down instead of up. I have been thinking for a long time of trying to create some boundary air turbulence at the back end to see if I can get the air to wrap down the back end. The radiuses are well within the data if have seen to get the air to follow the back glass.

[Jay2TheRescue]

I would think if the rear was riding low, that you are probably getting more air under the vehicle than you probably want. Scooping up and forcing the air into the unaerodynamic underbelly of the beast.

[theholycow]

Originally Posted by 93tracerwagon

However, when under heavy throttle, the computer richens the fuel mixture.

Very heavy, like 95%+.

[ukrkoz]

1. define "much better" mpg. What is exactly that YOU consider MUCH better mpg?
2. for how long did you keep your logs and how accurate are they?
3. there is a known fact, that long term mpg is showing about 10% variable per refills. 10% up for many may appear as kick .ss improvement while in reality, it is well within the percentile.
4. otherwise, like a guy with the cow shot with RPG said - too many variables to consider. I seriously doubt that at speeds around 65 mph any outside drag changes will do much over only 4 miles trip. And truly, how exactly did you manage to measure mpg for that trip only?
5. I can only see overall clearance reduced due to high load and possibly lesser drag from under the body. But again - you load any car, and it takes MORE petrol to propel the load, period, end of sentence. That's how ICE-s operate. This is why EVERY manufacturer and racer in the world is trying to REDUCE vehicle weight.

[CapriRacer]

You are aware that cars get better fuel economy on long trips, right? Less stop and go!

And you've factored that into your comparison, right? Long trip vs long trip, not long trip vs stop and go.

[93tracerwagon]

1) 10% is quite significant in my mind. The vehicle regulars runs out a lot of highway miles, but not heavily loaded, so I have some reasonable comparison (about 30 to 32 on average) In addition, I have taken this same trip heavily loaded several times and the best previous mileage was 32 to 33 mpg ( at 75 to 80 mph with A/C on), always the best the car gets. This trip pulled 37 with the following tanks at 34 and 33, with second and third additional tanks seeing higher speed, A/C use, some city driving and hilly terrain.
2) I've been keeping track of mileage for nearly 5 years on a tank by tank basis.
3) Your 10% rule is about the variation I have seen also. In this case, however, it was followed by two additional very high mileage tanks.
4) I agree about the variables, but road warrior also saw this same increase in mileage when he had his Ford Escort twin wagon heavily loaded. That really got me thinking something is happening when escort/tracer wagons are heavily loaded.
5) Once mass is moving it only takes enough power to overcome rolling resistance and especially in this case, wind resistance. Weight has very little impact in and of itself on an open flat roadway once moving. Racers try to reduce weight to increase responsiveness of handling, breaking and acceleration. So a heavy vehicle with the same wind resistance as a light vehicle will only see some increase in rolling resistance i.e. larger tire contact patch, increased resistance in roller bearings. My E250 van gets nearly identical highway mileage whether empty or carrying 3500 lbs. City mileage, on the other hand is a different story.