The 40MPG Crown Victoria Challenge

[Big Dave]

40 ain't gonna happen with any gas engine or any automatic, no matter how much you massage the speed/load.

[roflwaffle]

The only information I've seen indicates that something like a cummins B series is going to be within spitting distance of most common gas engines in terms of BSFC. The 6BT for instance was tested at ~230-250g/kWh at 75% load/1800rpm, and a gas engine at high load will perform almost as well.

[Big Dave]

Step back from the graphs a second and think about what you are saying.

How is it that a Dodge Ram with a Cummins engine gets 50% better MPG than the same truck with the same transmission and the smaller and lighter Hemi?

How is it that the VW Jetta with a manual transmission gets 33 MPG where a gas engine in the same car with same transmission gets only 22?

I think there are a lot of “apples and oranges” in the published BSFC graphs.

The only performance downside to the Cummins four is its weight. That engine weighs about the same as a Ford gas V-10.

[roflwaffle]

Where's the data indicating that a cummins get 50% better mileage (city, highway, combined?) on the same test in the same truck with the same gearing and (roughly) the same displacement gas engine?

[Big Dave]

The EPA does not test these trucks. I go by what people tell me.

My Ford (International engine) gets 24 on a bad day. No V-10 Ford gets over 12.

What's your answer about the VW? That's closer.

[comptiger5000]

Dave - That's MPG at cruise. If you load them both heavily, the mpg gets closer. However, the gasser has less torque, so it needs more gearing. If they were both geared as tall as they could get away with, there would be a difference still, but not a massive one, when cruising empty.

Basically, you could probably see 20 out of a V10 Ford, but you'd have to gear it to turn 800 rpm at 60 or something ridiculous to do that. Plus, you'd probably need to mod the engine a bit for it to be efficient enough at that rpm.

[aerohead]

If you're at 25 mpg highway and around Cd 0.36,with the full 'Template' tail you might see Cd 0.12 and that would only fetch you 33.33 mpg hwy at 55 mph.

[roflwaffle]

Originally Posted by Big Dave

The EPA does not test these trucks. I go by what people tell me.

My Ford (International engine) gets 24 on a bad day. No V-10 Ford gets over 12.

What's your answer about the VW? That's closer.

The problem with going by anecdotes and other models is that there isn't enough data to come to any conclusion. Anecdotes don't wok for obvious reasons, and different manufacturers may not have gas/diesel engines in the same car with the same gearing and same displacement. For example, the 2011 jetta wagon has a FD of ~3.1 for the 2.5l gas engine while the diesel otoh has fds of ~2.8/~2.3 w/ a 2l engine. I imagine most of the difference in mileage is because the smaller diesels are geared much lower. If the gasser was geared proportionally lower than the diesel, or vice versa, then you could make a case that X engine is really A% more efficient than Y engine, but there's no way anyone could get a decent comparison on the epa combined w/o tweaking the transmissions. BSFC maps at least are objective.

It could be that the dodge gas engines have much worse BSFC than the cummins diesels, or it could be that they have comparable bsfc. Compared to gasoline engines in general, given the only data I've seen, it looks like cummins diesels have similar, albeit slightly better, BSFC. Unless large displacement gasoline engines have really poor BSFC, then I doubt there's going to much of an advantage (maybe ~10% or so) to a diesel swap. If you happen to have some BSFC data showing that the 4.7l/5.7l dodge V8 gassers are real pigs feel free to post it up.

[Big Dave]

Gas and diesel engines are mature technologies. You could throw a fairly small spread of overall efficiency over the whole market. If one were noticably poorer, they'd never sell it at half of today's fuel prices. Conversely if one got noticably better MPG, people would snap them up - just as they snap up VW TDI cars.

My diesel truck, weighing 7,000 lb gets 25 MPG on average right now. My 4,500 lb Impala SS gets 19-20. Same roads. Same driver. Same amount of hypermiling. The truck has a larger frontal area and a Cd maybe a little south of 0.40. The car has a smaller frontal area and a Cd of 0.34.

The car should get better MPG, but it doesn't get close.

The diesel/manual combo washes out the car's advantage of frontal area, Cd, and weight. That difference must be powerful.

[Galane]

Something to check on that era of Ford 4.6 is if it's one with weak spark plug threads. Some penny pincher decided that only 4 threads in aluminum heads would be sufficient. Uh, no. There's been many instances of those engines blowing plugs out.

Fortunately there are fixes. Skip the Helicoil for this. The good fixes drill out the plug holes and thread them for a metal insert with more threads and a new spark plug seat.

Trick Flow Specialties makes Twisted Wedge heads for the 2-valve 4.6. Trick Flow Cylinder Head Upgrade For 2 Valve 4.6L Ford Modular Engines - Hot Rod Magazine

The big difference from stock is the TFS intake valves are angled toward the center of the engine while the exhausts are left at the same angle as the stock heads. The TFS heads are designed to directly replace the stock heads without changing anything else.

The TFS heads can use the stock Ford cams and followers, gaining efficiency through optimized intake and exhaust flow. (I dunno what a stock ECU would do with these heads, leaving everything but the heads, valves and springs stock.)

With the "modular" engines you have to know which plant they were built in, because unlike previous Ford engines there are significant detail differences depending on which plant built the engine. I dunno why, it's as stupid as the wimpy spark plug threads.