IDI engines: fuel economy comparison

[lyeinyoureye]

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.

Depends on how much you'll do. If ya do like this and go w/ taller tires/wheels, I'd say ya could do an easy ~80-100mpg@65mph. Less extreme aero improvements will result in smaller mpg increases. Rolling resistance, aero resistance, and gearing will see greater gains than fiddling w/ the engine base on what I've read.

[Ziptar]

Anyone know aerodynamically what the difference between the small bumper and big bumper numbers are? is it very large drag wise?

Getting ready to send the big bumper car to the scrapper but, maybe I should hang onto the bumpers. Is it worth the cost of getting them painted to match?

On that note, I was planning on saving the stock springs from the big bumper car to swap onto the 89. The 89 has Eibach lowering springs, which i normally hate.

Now I am starting to think leaving the lowered springs on and a big front bumper might give me better MPG, less frontage area and air under the car.

[Mark(The Miser)UK]

Larger exhaust seems to be a factor in better fuel economy maybe 20% or so :shock:

[lyeinyoureye]

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.

I should also add that when i say a little less than 50mph average speed I mean door to door average speed. That includes ~10-20 miles of stop/go in the city, ~20-30 miles of highway, and ~55-65 miles of freeway. My speed depends on conditions. The lower my average cruising speed is w/o having to excessively brake while optimizing engine efficiency, the better my mileage is. That being said, I tend to average ~30-35mph in the city, ~45-55mph on the highway since traffic is generally sparse, and ~55-65mph on the freeway depending on grade/winds. One section of the freeway portion is a significant uphill grade into constant headwinds. Here I can tuck in behind one of the slower semis and go ~45-55mph. On the way back, I'm going downhill w/ a tailwind and pass semis at a ~60-75mph average.

To calculate your average speed, you take your distance and divide it by the number of hours you spend getting where ya went. Unless you literally fire up the car and get on the freeway, and your destination is literally right off the exit, I doubt you actually average 65mph if you stick to a crusing speed of ~65mph. The best piece of advice I have about mileage is that keeping it under 65mph is best, and the slower you go, down to ~20mph average speed, the better your mileage can be provided you can optimize engine efficiency and can stay off the brakes.

Anyone know aerodynamically what the difference between the small bumper and big bumper numbers are? is it very large drag wise?

Now I am starting to think leaving the lowered springs on and a big front bumper might give me better MPG, less frontage area and air under the car.

I'm pretty sure the difference between big and small bumpers are insignificant. Lowering otoh will reduce reference/frontal area like ya mentioned, and more importantly imo, reduce the amount of chaotic air flow under the car.

[Ziptar]

Great! I am growing to like the small NA bumpers.

[deejaaa]

i am running 185/80r13's @ 35 psi, cruise at 60-65, shift on the shift light, 95% highway driving w/ no other mods and have pulled in 59 mpg but usually it's between 50-55. will be putting an exhaust cut out on soon w/ no muffler for the highway and cruise control and will be doing the pump mod on the spring soon.

[MaxHedrm]

Well, if I ever get the Jetta on the road, I plan to play with some underbody aero mods. & I already picked up a GTI air dam (primarily to cover the slightly mangled radiator support) which should also help. Then to pull off the luggage rack on the trunk and perhaps add a small spoiler to the rear (the sort that goes on the vertical part of the trunklid, rather than on top I think).

This should be handy as well...
http://www.instructables.com/id/Measure-the-drag-coefficient-of-your-car/

[blackbird82]

I have a 94 1.9 IDI (AAZ) and have switched my trans over to Lucas full synth 75w90, done a couple light pump mods (boost limiter switch and re-wired the timing retarder solenoid(for emissions).  Still need to do new rear bearings and I drive mixed city and highway at 65mph. I can and do pull a solid 950kms to a tank, and never really pushed it too deep into the red.  Oh yeah tires are at 44 psi all around.  And I have the CHE transmission.  (60mph, 2600 rpm)

I found this car last december with 60,000 miles on it! bought it for 3800, and it had already had the TDI crank mod done!

[burn_your_money]

Sounds like you got a good deal and a very healthy motor.

My car (91 Golf NA) with a Giles pump, AGS tranny and stock everything else gets me 1000 kms / 46 liters usually if I drive 110 on the highway. I haven't done a full city tank yet. My motor has 250 PSI all across the board. It will be getting replaced soon.

[blackbird82]

yeah the only thing that came up recently is it got hard starting if it sits overnight.  I am still diagnosing, most likely a glow plug, but when I shut it off their is a bubble that pulls back on the fuel supply line, almost line a back vacuum is backsucking.  I a have replaced everything under the hood for lines, and the the lines to the tank are good, pulled out the fuel pick-up, clean and smooth. new filters, and all. there are two little lines goin to the pick-up in the tank.  in front of them look like 2 little check valves. Is that what they are? Trying to figure this out.  I even took off both banjo fittings on the pump and re-annealed them.

[Smokey Eddy]

good god 72 cents a litre in 2004 (i only read the first post)
and we're paying $1.50!!!
diesel came DOWN to $1.36 the other day i was so surprised i got a full tank. Still cost me about 70$ though

[macka]

bump 

[deejaaa]

wow! it's been so long. kind of weird to be reading something from so long ago. i have a few ideas for economy but it will take some bodywork and i don't have the time right now. i've now got 2 kids. i avg 45 mpg.

[gldgti]

my new mk3 TD aaz just got 6.38l/100km on its first tank with me, gunning it around trying to see how fast it goes. it runs 205/45 r16's

my old '79 gld n/a 1.5:

best - 4.5l/100km
worst - 6.6l/100km
average - 5.4l/100km

with big exhaust, cold air intake, 185/60r14 sticky rubber and aggressive alignment.

cheers

aydan

UPDATE - of my old figures - its been 2 years!

My '94 mk3 with aaz now gets between 5.2-5.6l/100km every week doing 900km, 3000rpm cruise.

New car - '91 cabriolet with 1.5D (sooo slow) gets 5.7-5.9l/100km with reco'd engine, good injectors, and AUG trans.

My '77 LS Golf gets 10.5l/100km (1.8 8V gasser with Dellorto carb and euro cam) but its fast :-)

[Wayland]

Pump timing seems to have quite a profound effect on mileage. On my 93 Passat Eco-diesel, bumping the timing up from .95 (I think it was) to 1.05 took me from 47 mpg to 53.