A Thought About Relative MPG
 

This might get moved but I think it speaks to the direction we should be moving.

I have two four-wheel vehicles: A 2000 F-350 with a 7.3L diesel and a ZF6-650 transmission; and a 1996 Chevy Impala SS with a 5.7L (LT-1) engine and an automatic transmission (4L60E). Both vehicles are 4x2 and have 3.08: gears.

The Impala weighs a little more than half what the F-350 weighs. (Curb weight forboth).

The Impala has a smaller frontal area and probably a lower coefficient of aero drag.

Both vehicles' tires are aired-up rock hard and both have fresh front end alignments.

I drive the F-350 with very moderate hypermiling (a bit of engine-on coasting and timing of lights) and it pretty much gets 26.5 to 27 MPG routinely.

I hypermile my brains out (coasting, timing lights, drafting, brakes-free operation (it corners much faster)) in the Impala and struggle to get 21 MPG on occasion.

The F-350 gets better MPG in the winter than the Impala gets in the summer.

Does this not speak volumes for the superiority of the combination of diesel engines and manual transmissions or what?

It might have something to do with the gearing of trucks in general as well?

I think it has everything to do with the superiority of a diesel engine with a manual transmission. Since a diesel has a limited rev range (the 7.3 redlines at 3K, correct?) it has to have higher gears. The impala, being a SS model, is likely designed for power and ignoring economy altogether. I'd be looking at a possible transmission swap for one that is geared more toward economy if the one you have is geared for performance. It may just need to be adjusted for economy.

A 350 in an impala screams old school muscle car, not econobox. Is it possible to force the torque convertor to lock up sooner than the speed it locks itself? Some TCs can be manually locked by grounding out the correct wire in the ECU harness.

Does the impala see mainly city or highway?

Does your impala have to have high octane fuel? I find it difficult to believe you couldn't use 87 octane on a properly tuned engine. Lower octane should give better economy assuming you don't have octane knock.

A 350 in a car that could run great with a 3.8 boggles the imagination. The setup described screams out muscle car with no chance of economy. I'd be blaming the gears in the transmission for being geared for burnouts and low 0-60 times instead of high mpg and low emmisions.

Depending on your amount of creativity, you may be able to get a manual from a Corvette in that rig. The new Corvettes have a 6 speed and can fetch 30 mpg on the highway.....of course the variable valve timing and direct gas injection probably helps out a bit. That and the Corvette is a bit more streamlined....

I agree, diesel engines mated with a manual kick rear all over the place as far as mpg. Plus, if you have some leftover grease from your fry daddy, it's free fuel after a straining. Or used motor oil. Or old auto tranny oil......

Hold on a sec, your avatar shows your truck having a fuel saving bed cover. So your Cd is likely significantly lower than stock!!



PS sorry for the double post, thought my original post was lost in cyberspace!

Both vehicles arebeing compared over my normal test route - roughly one-third urban/suburban, one third superslab, one third state two-lane roads.

Both vehicles have 3.08 gears.

The 7.3 diesel can be coaxed up to 3,800 RPM, but it is a dead cat above 3,200 RPM.

The Impala 350 runs 87 octane. The LT-1 uses reverse-flow cooling (cold coolant flows first into heads. This supresses knock, so the engine has over 10:1 compression.

There may be a T-56 six speed manual in this car's future.

The listed Cd for the Imp is .34. I'm sure my efforts have improved my Cd, but I doubt I got it lower than the Impala. FWIW, the Impala coasts magnificently.

With a diesel and a stick, this is a 33-35 MPG car, easy.

Problem is that is not a clunker. It is pristine.

Diesel engines have been superior to comparable gasoline engines in term of fuel economy, but gasoline engines have been closing the gap. That is especially true as we currently see gasoline direct injection engines go mainstream, while diesels are toned down by emissions requirements (which is fine with me).

One wonders when we will see DI gas engines in the US.

Will they be reliable?

There are already. Hyundai's new Sonata has a 2.4 liter GDI engine for example.

What I'm seeing is maybe a 3-5% improvement over conventional MPI, not the huge advatage diesels have. They don't seem to have the big low-end torque that diesels have. That big torque is needed to push efficient gearing.

Plus there are a few Ford EcoBoost engines on the market in America. Of course, we'll miss out on the two and three cylinder versions.

automatics
 

Dave,I don't know if this is germane to the issue,but back in the 1970s,B.Dayman Jr. of Jet Propulsion Lab published a SAE Paper on coastdowns conducted at Edward's AFB,and he claimed that driveline losses were so great with an automatic transmission car in neutral,that they had to add a special clutch to dis-engage the propeller shaft from the tranny to normalize effects with a standard transmission car.
Could still be valid,don't know.If true,this might explain some of the 'discepancy' between the Impala and the F-350.

I don't know if that's still true. While an automatic with the TC unlocked is pretty inefficient, they don't coast badly.

Both in neutral and in gear, at lower speeds (25 - 30 mph) where aero drag isn't an issue, my Jeep will coast practically forever. Then again, at those speeds, the thing actually coasts as well, or slightly better, in gear up to about 25 - 28 mph, as the engine is turning at, or just above, idle, and not dragging the Jeep down at all. Basically, that means that it's more efficient to coast in gear than to NICE-ON coast at low speeds, as the engine is using about the same idle fuel (as indicated by the instant mpg display), and still providing some motive power, rather than wasting it away in neutral.

I have often held that the torque converter is the tool of OPEC.

Even though both have 3.08 gears, your diesel still has higher overall gearing. The gearing in the tranny is different and you have taller tires on the truck.

Anywho, another datapoint for you: My truck is a 4.8L V8 Auto, 2wd with 4.10 rear end. On good behavior (no hypermiling tho) I'm a hair under 20mpg. Weighs in a 5400lbs with me and a full tank of gas (according to the scales at the dump). I'd love to have a manual instead, but I couldn't track one down. They were like hens teeth apparantly in '03 (and no longer offered at all).

The 3.6L Cadillac CTS/STS engine is also DI, IIRC.

Do these DI gas engines have throttles?

Big Dave -

Ay, that's the rub. If you converted the Impala to manual, would you be able to "reverse it" to OEM if you so chose, or would there be irreparable harm, aka an extra hole for the clutch pedal or something? Would the carputer get upset?

CarloSW2

We've had Mitsubishi GDi engines for a while but they were designed for the super clean fuel available in the Japanese home market, so some had carbon issues over relatively small miles - some people relearning the art of carbon removal :

I have seen evidence of Petrol engines getting better but nothing which is close to matching Diesel for overall efficiency, and I haven't see any evidence of Diesels being restricted in terms of torque/hp and efficiency.

In 2000 state of the art in Europe was 90hp / 150 lb/ft 45mpg, 100mph. Now its 160hp / 290 lb/ft, 55 mpg and 142 mph - both from 2 litres, Peugeot and BMW.

Pretty much only the injection changed, they still need to control the AFR. You can think of it somewhat as an IDI vs DI diesel.

Yes.

An Otto cycle engine has to have something restricting airflow, as that is the only way to ensure a homogenous air/fuel mixture. The Diesel cycle does not have this requirement and can take in un-throttled air.

If you can figure out how to keep gasoline from detonating when the mixture isn't homogenous, then you can get rid of the throttle.

Darcane:
Your point about wheels/tires is taken but it actually reinforces the superiority of the diesel/stick configuration. The truck has tires with a 31.7 inch outside diameter while the OD on the Impala tires is 27.0 inches. Since rotational moment of inertia goes up with the square of radius, I would expect the truck tires to have 38% higher rotational moment.

We have been over this before, but in the real world where stops lights and signs enforce stopping the higher rotational moment of inertia penalty of bigger OD tires rapidly overwhelms the lower engine RPM. With trucks the equation is always the same: Bigger tires = Lower MPG. Now that’s conventional drive trains I’m talking. It might be different with hybrids, but I notice that both Toyota and Honda put small-diameter tires on their MPG flagships.

One mitigator: The Impala has efficiency-robbing EGR where the truck does not. There is a EGR-delete kit for the LT-1, but I’ve been too busy/lazy to do it.

I hear you about the disappearing manual transmission. Let’s face it, ecomodders/hypermilers are a rare breed. The vast majority are just lazy. Talking on the cell phone is more important to most people than being aware of how you are driving. Also, I think an automatic is a way for the manufacturer tio hold the engine into a regime where NOx emissions are minimized. The very low RPM – high load regime that give optimum MPG tends to have high cylinder temps that generate a lot of NOx. Hence the rev-happy way most cars and trucks drive these days.

CFG83:
There are kits out there to convert the B-body to a Tremec T-56 (the manual available in Corvettes, Vipers, Camaros, and Mustangs). It is not a project for the faint of heart (although a cinch compared to Bennelson’s Chevedes project). Yeah, you have to cut sheetmetal. There may be such a project in this car’s future, but hopefully not this year.

As long as the gas engine has to tolerate a throttle, it will never catch the diesel for efficiency at part-load operations.

I agree there is a penalty for the higher moment of taller tires... but I don't agree it outwieghs the change in gear ratios. This winter I changed to 5% taller tires and saw about a 5% increase in mpgs. It's the simplest way to change your overall gearing ratio and generally taller tires has meant higher mpg in the posts I remember reading.


The other issue that increases emissions is letting off the throttle in between shifts. My wife's Vue "solves" this by having the drive-by-wire throttle stay open for a second or so and slowly close after you let off.

Agreed. The 6-cylinder Impalas in the EM Garage get around 30 mpg. A turbo would help get some of that 350 performance back while allowing better mileage at cruising speeds.

Or, like was already mentioned, a 6 speed with a tall sixth gear.

The T-56 swap gives you a 0.50:1 top gear.