The 40MPG Crown Victoria Challenge

[roflwaffle]

Originally Posted by 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.

Do your Impala and truck have the same gearing relative to their engine size and to a lesser degree transmission type (auto versus manual)? If not there's no way you can reasonably compare engine efficiency based on your own experience. If you have a manual trans in the Impala and gearing high enough for the engine displacement, which is cutting it very close w/ a 4+L engine in a stock car, then maybe you could get a comparison of engine efficiency. If you don't there's no way.

[Big Dave]

The car has a 5.7L LT-1 and 3.08 gears.
The truck has a 7.4L diesel and 3.08 gears.

The car makes 250 HP. The truck makes 235 HP but more torque.

The car is an auto. The truck is a stick.

And the truck gets 25% better MPG, despite all the disadvantages.

[roflwaffle]

What are the OD ratios? Is the truck stock or chipped? Where are you getting a Cd of .4 for your truck? What's your driving route like?

Your Impala is rated at 33mpg highway (unadjusted) according to fueleconomy.gov so you should be getting pretty close to that unless your route isn't mostly highway, in which case your truck is probably doing better because of the manual trans/lower idle fuel consumption or because there's something really off w/ the Impala.

[rmay635703]

Originally Posted by roflwaffle

What are the OD ratios? Is the truck stock or chipped? Where are you getting a Cd of .4 for your truck? What's your driving route like?

Your Impala is rated at 33mpg highway (unadjusted) according to fueleconomy.gov so you should be getting pretty close to that unless your route isn't mostly highway, in which case your truck is probably doing better because of the manual trans/lower idle fuel consumption or because there's something really off w/ the Impala.

His old v8 350 Auto Impala SS is rated at 33mpg? That seems rather strange.

[Big Dave]

Read the avatar text. My Impala is the old B-body Impala, not the glorified Cavalier sold as an “Impala” today. EPA Combined: 19 MPG.

The truck’s net double overdrive ratio is 0.57:1.
The car’s single OD is 0.71:1.

The Cd is like most Cds posted here – a guess.

My route is one third urban/suburban, one third two-lane state roads, and one-third Interstate.

The car has relatively LRR Goodyear Assurance fuel Max tires. OD = 27” The truck has OEM-spec E-rated Firestones. OD = 31.7”

[roflwaffle]

It looks like your truck, accounting for displacement, is geared about 15% taller than your Impala, and if it's chipped then a comparison would need to be stock or if the Impala was also running something to lean it out. I'm thinking your truck's Cd is closer to .3 than .4 if someone like Phil Knox is at .25, but that's just a WAG. How long is your route btw? If you had something where you could cruise at 55mph in OD for a couple hours (minimize warm up/trans losses), had the same gearing, and both vehicles were either stock or chipped, then maybe you could get a decent comparison, but as it stands I think there are too may confounds.

Originally Posted by rmay635703

His old v8 350 Auto Impala SS is rated at 33mpg? That seems rather strange.

Yup, 33mpg unadjusted (~55mph) highway for the Caprice/Impala. I wish I had that kind of tall gearing available for my Camry, although w/ a 5.7L I imagine it's about as tall as it can go. What would be interesting to see is what kind of mileage a T-56 and 2.73 rear end would get w/ the 4.3L V6 in an Impala/Caprice.

[comptiger5000]

If you were to gear the Impala taller (adding a gear vendors would get you to a 0.55 double OD), and maybe slightly taller rear axle gears too, it would probably get a lot closer to the truck in mpg. City mileage would still be worse, due to worse thermal efficiency at light load. However, if you can get the gasser loaded heavily enough on the highway, it will be almost as efficient as a diesel in a similar vehicle.

[Big Dave]

My Imp has 27” OD tires (746 revs/mile). A Crown Vic would have much the same rubber. With 3.08 gears and a 0.71 overdrive that’s roughly 1,650 RPM @ 60 MPH.

To go to 2.73 gears and a T-56’s 0.5:1 overdrive the 60 MPH engine RPM would drop to about 1,000 RPM. Doable for a L92 engine but probably beyond the capabilities of a 4.3 V-6.

L92 crate motors and T-56 transmissions are available off the shelf. I’ve considered this change. The L92 gives you a very driveable and reliable engine and mated to a T-56 where some meaningful hypermiling is possible, I could add some aero work and maybe get 25-28 MPG.

1,000 RPM operation would probably work OK for a Ecoboost V-6 but there are no crate motor Ecoboost engines unless you are well-connected.

But high-load operation at 1,000 RPM is right in the sweet spot of a Cummins B engine.

Sooner or later you have to slow down I town and the throttle starts to rip the efficiency of a gas engine. The diesel is much less affected.

BTW, chips dramatically help diesel HP and torque, but only barely help MPG. I have an outstanding MPG special chip and it gained me 0.5 MPG.

By no means could I claim to have Phil Knox’s low Cd, or even close. All I have today is a tonneau cover and a too-short air dam. Phil ruthlessly hunts down every cubic foot of air.

My truck’s superiority comes from the diesel/stick combination.

[roflwaffle]

Why does everything think larger displacement V8s can handle 1000rpm but the vortec V6 can't handle 1000rpm? Is there something wonky about the engine family or something?

[Christ]

Seems to me like the 4.3 can do it...