IDI engines: fuel economy comparison

[Jet-A]

With diesel bumping upwards of 4$ per gallon this topic is close to my heart and wallet. I have been tinkering with tires, speeds,weight reduction, synthetics etc. The obvious solution--my lead foot--has the most impact, but I am curious about the drag issue. Does anyone know what the Cd (co-efficient of drag) is for a MK1 Jetta? More significantly at what speed does the drag become a big factor (parasitic drag?) I think I am bumping up against simple physics on my quest to increase mileage.....

[hotroddr]

from the information I found it appears that the mk1 jetta has a drag coefficient of 0.43 while the mk1 rabbit/golf has a Cd of 0.42.  This was the trend from the different places I looked.  The jetta has a slightly higher drag coefficient than the hatchback golf.  

I am going to go out on a limb here and guestimate that aerodynamics will start to make a difference in fuel economy at 40-45 mph.  Im really not sure about that though.  My biggest indication of this is not real scientific.  Drive with the window down and put your open palm into the wind and feel the resistance then you be the judge.  

My rabbit project is still rolling on still.  I have a few ideas so far but I have to wait till I get the engine back in it to get a baseline mpg number, then I will build the belly pan and diffuser.

[tylernt]

Is buying a year's supply of fuel in the early fall when it's cheap (before winterization and the heating oil demand) advisable? I know you'd have to add some kind of anti-gel agent for the winter but it would take a lot of anti-gel additive before you stopped saving a ton of money.

[jtanguay]

i would just add the power service cetane booster & anti gel.  the cetane boost will probably increase your mileage enough to make it economical

[spencebm]

91 jetta big bumpers 1.9TD
i can squeeze over 50mpg outta my ride if i grandma it, thats over 700 miles on a tank running 5% biodiesel

[subsonic]

I vote for a new group  brain storm project.

With the price of diesel and home heating oil going through the roof, and diesel looking like it is going to hit $4.00 a gallon, I suggest we figure out a Milage Master potential build up.

I think the primary goal should be to squeeze out the maximum MPG possible at highyway speeds of 60-70 mph.  0-60 times would not be relevant.  A car built for commuting.  Can 60 MPG be obtained?


We could look at various engine efficencies and modifications, to tires, to transmission modifications. Areodynamic changes, ride height changes, weight reduction, etc.

NA or TD? TDI or IDI?  Perhaps one of each?
   
       Team IDI vs. Team TDI  ??

Theroreticle into practicle into cost savings for all of us.

Anyone interested?  Perhaps we can start this as a new post.

[spencebm]

good plan

[subsonic]

Kind of a different train of thought.  I still plan on finishing my build up for power, but economy is getting very important with fuel costs so high.

I think that as a group we should be able to produce a result that gets far greater mpg than is now achieved.

Just in the engine alone there are countles mods.  Nozzles, breaking preasure, timing, ceramic coatings, friction reducers, porting, etc....

The transmission holds all kinds of gear variations.  

I am sure most of the research has been done on this forum already.  It was just aimed at a different result.  Power.

[lyeinyoureye]

from the information I found it appears that the mk1 jetta has a drag coefficient of 0.43 while the mk1 rabbit/golf has a Cd of 0.42.  This was the trend from the different places I looked.  The jetta has a slightly higher drag coefficient than the hatchback golf.

Do ya have sources? I've heard it was the opposite.

I think the primary goal should be to squeeze out the maximum MPG possible at highyway speeds of 60-70 mph.  0-60 times would not be relevant.  A car built for commuting.  Can 60 MPG be obtained?

Bone stock w/ a 3.89 R&P/.71 OD/155/18R13s we can hit ~60mpg at a little less than a 50mph average speed on open freeway/highway if we drive it right. Any better requires mods for better aerodynamics.

[subsonic]

So what do you think the upper end is for MPG?

I am interested in the MPG rate for around 65 mph.  If I drive much slower then that on I-95 I will get rear ended.

[Jet-A]

Subsonic, I like your idea of a competition/build. It would seem that there are enough gearheads here who could contribute to the idea. It seems me that we are seeking to get better MPG at realistic freeway speeds. (65-75mph) Courtesy would dictate that flowing with traffic would be the first benchmark to attain. The idea of setting up a competition with our TDI bretheren could be an inspiration for workable ideas.
It would require some baseline rules and disclosures to benchmark. I think the TDI club website has something along these lines but we might be able to raise the bar. My personal best was with a 79 Rabbit and a Callaway turbo driving eastbound across South Dakota in the winter with a tailwind and averaging about 55 mph--I got 69.8 MPG. I'm sure that there are some who have done better. Just my two cents....

[Vanagoner]

Great idea- for inspiration check out VW's factory effort, the 1 liter car-
http://www.seriouswheels.com/cars/top-vw-1-liter-car.htm
It looks like that ride is pushing 300 mpg.

I would maximize my mpg if I cut the top two thirds off of my westy, which normally has the drag coefficient of a small office building.  
Short of that (actually taller than that) I will play with a teardrop cover for the luggage cap, and eventually make curved double doors at the rear that give it more of a fishtail.

I would be tickled with 35-40 mpg.

[subsonic]

I am not looking to go off site with this, or have a mine is better type of thing. Just group research, testing with posted results that we can all use.  We can do what auto makers can't because of scale of economics. We can have if we choose to,  a car that gets outstanding highway mileage at the cost of perhaps other performances losses that a corp. would find unacceptable for general sales.  I really do not know what type of diesel engine could get the best MPG.

[VW Smokr]

"Back in the day... " of the first farcical "Arab Oil Crisis", 1973-74, we in the U.S. were told by the feds(Same administration that claimed, "Your President is not a crook!") that 55mph is the point at which the average passenger car's drag from tire rolling resistance was surpassed by the drag from aerodynamic forces.

That said, the average VW R/J/S (MK I series) of the mid 70s-early 80s was still a lot better aerodynamically, than most of the porcine Detroit offerings, so that 55mph mantra may or may not have been true for 'us'.

Our then new, but broken-in, '81 Rabbit 5-door consistently bettered 50mpg on highway trips(70-75mph cruising speeds) even with 4-5 pairs of skis on the roof rack; tips to the rear for best efficiency, please.

My '79 Dasher 1.5L n.a. diesel 5-door sedan did an all-time best of 57.8mpg on a trip, when new tire imbalance limited the practical cruising speed to 60mph(3000rpm in 4th gear). That was with 4 adults on board, and every inch of the car's cabin & trunk & a huge Thule "Pod" on the roof crammed with traveling/camping gear(total loaded weight est. c.3300lbs). That is not too far off the Federal noise about the magic of 55mph, and IMO is a great tribute to the aerodynamics of the Thule "Pod", not to mention our trusty IDI clattermeister under the hood.

Previous high fuel mileage had been 54.6mpg, done at my normal cruising speed of 70-75mph(on relatively level ground!). Even when pulling a trailer with my '80 Dasher 1.5L n.a. diesel station wagon (total weight: 4650lbs!), the highway mileage has never dropped below 33.6mpg.

Side note: A couple of months ago, I wanted to see what our '01 Saturn (1.9L 16V gasser w. auto trans) coupe could do at a (mostly)55mph cruise-controlled pace on I-5: it came out @ 42.3mpg for a 300-mile segment. That says something for better aerodynamics, as well as the validity of the 55mph claim, since this Saturn's 'typical' highway fuel consumption is 33-36mpg(@ 70-75mph). OK; now back 'on topic'...

So, re. the VW diesel-powered vehicles, it seems to me that the earlier suggestions re. cleaning up the cars' aerodynamics with spoilers & belly pans, as well as trying to minimize rolling resistance, while utilizing turbos, intercooling, free-flowing 2.25-2.5" exhausts, careful pump tuning/timing, and 55-60mph cruising should easily get the lighter Mk I cars well into the 70+mpg range, or maybe even higher with some trick gearing or extra cam & headwork. Maybe the cleanest body envelope to start with would be a lowered '74-'76 Scirocco, and build from there with all the tricks. Could be fun.

Seems like this site has some sharp thinkers & tuners among its frequent contributors. It will be interesting to see what emerges as the pump price of Diesel no.2 edges toward $5.00USD per gallon.

Back to lurk mode. Shutting up!

J.R.
SoCal

[Ziptar]

 It will be interesting to see what emerges as the pump price of Diesel no.2 edges toward $5.00USD per gallon.

Aside from that cost being reflected on the grocery store shelves.

First thing you'll see is the masses lamenting that Detroit didn't spend the last 15-20 years focusing on SUVs with giant Leather Lazy Boy Recliner like interiors with all manner of needless/nuisance electronic gadgetry and that the U.S. Government made importing cars like the Volkswagen Lupo 3L TDI with its 78 MPG impossible via cumbersome and pointless emissions and crash test requirements designed to protect the big three.

Just as with the fuel crisis of the 70's The big three will take years to turn production around, make some flops (ie: Oldsmobile 5.7L Diesel).

$5.00 a gallon should also bring alternative fuel into common use. Up to now it's cost has been a stumbling block.