Diesel vs. Hybrid vs. Gas: Who Wins?

[Big Dave]

Re; Posts #3 and #5

Imagine what he'd have if Basjoos had started out with a VW TDI with a lower numerical final drive and them did his thorough aero improvement program on it.

125 MPG?

But, the Civic was paid for...

[Deezler]

Big Dave I wonder that all the time. I wish I had the guts or time to do it. Well, really, I just wish someone else would do it.

[energy0uth]

random post but I just saw a built dodge with 700hp or so getting around 50 mpg or something ridiculous around 3psi. Diesel power mag...numbers aren't exact but amazing

[roflwaffle]

Originally Posted by Big Dave

Re; Posts #3 and #5

Imagine what he'd have if Basjoos had started out with a VW TDI with a lower numerical final drive and them did his thorough aero improvement program on it.

125 MPG?

But, the Civic was paid for...

I think anything w/ a TDI, barring a swap, has a greater reference area and curb weight, so even w/ the aero treatment it'd probably get the same mileage as the smaller Civic, give or take ~5-10mpgs depending on conditions.

[Big Dave]

I hear you. Wish somebody would do it.

I talk projects all the time with guys in various car/truck communities.

It seems to me that good projects start out with a vehicle you can or don’t mind cutting on. That’s why almost all projects start with junkers of some sort. Salvage or rust-buckets. You don’t have anything in them so you can merrily whack away at it. If you screw it up early (when most screwing up gets done) you junk it. It was junk anyway.

This is why the sport-truck guys love S-10s. They are throwaways to start with. The body-on-frame allows you all sorts of scope for aero work. Sport truck guys step and lower the frames all the time. No reason you couldn’t narrow the rear frame to support a nicely tapered (10-12 degrees) bed. With a seven foot bed you could reduce your wake area by at least 60% and never go more than 11 degrees. You can get Chevy axles narrowed to keep the tires inside the 2.7:1 teardrop that a 11 degree taper allows – a al early Insights. Nice thing about the S-10/sport truck approach is that you are not constrained by the need to put a 4x8 in it.
You can buy off-the-shelf suspension drops. For road use in the Midwest, I wouldn’t go beyond 3” drop.

Another thing that militates to the S-10 is that is supports all sorts of engine/transmission swaps. The simplest mod might be a OM617 Benz 5 cylinder diesel with the Benz 4-speed. This is the last word in reliability and longevity. Old Benz engines are easy to work on, and there are plenty of parts available (although they are eye-wateringly expensive). All you have to do is fab up the revised engine/transmission mounts and have a driveline shop make a drive shaft to fit. Put on a Chevy axle with 2.73:1 gears (common as dirt) and you have a fine economy setup. The drawback is that OM617s are IDI and not as efficient as more modern engines. Maybe better would be to get an adapter and put your VW TDI of choice. I’m no fan of the T-5 transmission but it should handle the torque of any VW engine and it gives you an overdrive. A T-56 would be massive overkill and NV4500s are too heavy.

Simply modding a VW would involve finding the right car and doing “stick-on” body work much as Basjoos did. If I were going to all that trouble, I’d also opt for the numerically lowest gearing you can find for the VW. Whether you use a VNT turbo, a simple turbo, or a normally aspirated diesel (really old diesel Rabbits), I agree with tas. Every now and then get her hot and burn out the carbon.

Keep in mind that at some point a project ceases being an economic exercise and it becomes a “hot rod.” Somewhere along the line you jump the shark and further improvements are made for the sake of tiny improvements in MPG, and you don’t care. That last MPG becomes your Holy Grail.

Phil (aerohead) and I have discussed moving my truck beyond the aerolid approach and making up a full aerobed assembly. I’m a little squeamish about cutting up the truck too much because the darned thing is so rock-solid. What I need to do is take the entire bed off it and start making a “plug” that a fiberglass layup guy can make a body out of. I suspect that even with an optimized bed assembly, I maybe get another 1.0 MPG. But my truck ceased being an economic exercise long ago and has been a “hot rod” of MPG since.



Energy0uth:

Cummins guys are famous for that stunt.

You gotta realize, these big diesels have enormous torque even at an idle. The engines will idle much slower than the factory idle but they idle at 600-700 so the engine-driven accessories (alternator, power steering, A/C, etc) work. With my long-legged gears, I can idle down the road in overdrive at 30 MPH. I mean IDLE. Foot completely off the pedal.

They'll bump the thing up to about 15-20 MPH or so for a short distance (usually downhill). They idle along at a speed that would drive a hypermiler crazy and make a quick measurement. Technically 50 MPG, but in reality a lie. Some even disconnect their accessories and reset the idle to 300 RPM for this short stunt. I once pulled the alternator fuse and just lived with the power steering and det the idle for 350 and did it with no problem.

To me, real MPG is that you get at the posted limit over a distance of at least 1,000 miles. Anything else is EPA-style guesswork.

I've invited them in the past for a meaningful MPG showdown, say coast-to-coast loaded to GVWR at the posted limit. Never any takers.




BTW, I agree with tas about blowing the carbon out from time to time. With my simple turbo I go out once a month and blast up a 7% grade in top gear, pinning the EGT at 1100 degrees. About three trips up and she runs real good for a month. With an VNT or VGT I would agree it should be done more often. Having a pyrometer really helps one do it safely and effectively.