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Old 12-29-2007, 07:36 PM   #1 (permalink)
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Here From TDIClub - looking to increase highway mpg

I don't see many others with diesels on this board :-(

I have a 2002 VW Golf TDI GLS
I've only owned the car a Few months now

5 speed, 16x,000 Miles
Low 50's Most of the time Best tank so far 58mpg (many trips around 100mpg, but highway driving kills that real fast)
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I'd like to get my steady state 70 mph up near 50mpg, it is around 35 now, Thinking aero Mods.

Thanks for putting up this awesome site, I've been reading for a while now and now believe myself to be "up to date".

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Old 12-29-2007, 08:48 PM   #2 (permalink)
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welcome to the board. Sounds like aeromodding is your best bet, we hatchback drivers have our work cut out for us.
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Old 12-29-2007, 09:02 PM   #3 (permalink)
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Hi dremd, welcome.

I guess because diesel models are relatively rare in North America, people who own them tend to do the birds of a feather thing. Thus TDIclub, and clubsmartcar, etc.

I'm surprised more diesel folks aren't into modding (or maybe they are, and I just don't know about it). Everyone on this board knows of VW's diesel 1 liter car, that's for sure.

That said: aeromods and gearing are the way to go for highway fuel economy, if you're dead set on driving @ 70.

And I have read (maybe you can confirm or correct me) that diesel powered vehicles are even better suited to aero mods than gas SI engines. As you decrease load on a gas motor by helping it move through the air more cleanly, it needs less "throttle". Unfortunately closing the throttle probably also has the effect of shifting the operating range further away from the ideal BSFC zone.

My understanding is that because diesels don't have throttle plate restrictions and the associated pumping losses of SI engines, reducing engine load through streamlining will give a diesel car a larger relative return in efficiency.

What about gearing - do you know if you have any options there? (I put a taller final drive in my car, which helps most at cruise on the highway).

Anyway - I'm sure others will chime in.
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Old 12-29-2007, 09:27 PM   #4 (permalink)
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Welcome to the site! I'm looking forward to seeing what you do with it. Not many tdis yet, but hopefully we'll get some more on here...no pressure on you to set a superb example though,
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Old 12-29-2007, 09:55 PM   #5 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
And I have read (maybe you can confirm or correct me) that diesel powered vehicles are even better suited to aero mods than gas SI engines. As you decrease load on a gas motor by helping it move through the air more cleanly, it needs less "throttle". Unfortunately closing the throttle probably also has the effect of shifting the operating range further away from the ideal BSFC zone.

My understanding is that because diesels don't have throttle plate restrictions and the associated pumping losses of SI engines, reducing engine load through streamlining will give a diesel car a larger relative return in efficiency.
I have no Idea about aeromodding Diesel vs Aero modding gassers Interesting I hope someone knows more about this topic.

I'd also like any info about pulse and glide with a diesel, I'm having difficulty trying to find an optimal throttle to pulse at. To much and I spool the turbo (lots of fuel), to little and I end up with similar -> worse mpg than the cruise.

Quote:
Originally Posted by MetroMPG View Post

What about gearing - do you know if you have any options there? (I put a taller final drive in my car, which helps most at cruise on the highway).
We do have a few gear options available to us I can't remember the ratios at the moment, but there are 3 "upgraded" 5th gears for my trans, or swap in a 6 speed (but that's a pricey option)








I am not "dead Set" on driving 70, my reason for driving 70 is that I get within 2mpg any speed for 58~71 mph (turbo starts working at 58ish) and under 58 is just to slow for me on the interstate, If I could do 62~65mph @ 50mpg I'd do it. For reference I get 65~70mpg at 55mph. and 100~120mpg @ 40mph

I don't do a lot of highway driving, but when I do it is usually in a bit of a rush; Typically have to pick up a sick 2 year old 75 miles away from me.


If anyone can't understand my post just speak up and I'll try and clarify.
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Old 12-29-2007, 09:56 PM   #6 (permalink)
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Hi

I am new here also. I have a Prius and a Jetta TDI. The TDI is the wife's car so I can't mess with it to much. The Prius is my driver and I would like to increase the highway substantially.

City driving I am pretty happy with. I can sustain 60 MPG without much trouble, but highway driving I am pretty much stuck at 50 MPG. I figure aero mods is the way to go, so that is why I am here.

Here is a recent pic of my MFD for city driving.
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Old 12-29-2007, 10:47 PM   #7 (permalink)
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Same era engines, the consumption diesel ranges from ~224-358g/kWh equivalent over the same range where the gasser goes from ~250-450g/kWh. Diesels are less sensitive to changes in load in terms of efficiency, so making the vehicle more efficient won't make the engine run as inefficiently as doing the same thing to a gasser would. That being said, taller gearing and aero improvement help both significantly assuming the driver cruises at a lower speed of ~60mph.
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Old 12-29-2007, 10:56 PM   #8 (permalink)
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Good illustration, rwaffle. In fact, I'm pretty sure I'd read that tidbit (about the potential for diesels to benefit more from aero mods) and saw similar maps posted at TDIclub.

But I don't know turbos at all. What's the effect of taller gearing on turbo operation? If dremd's goal is to stay out of the boost, wouldn't reducing RPM through a gearing change raise engine load and cause the boost to come on sooner for a given road speed?
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Old 12-29-2007, 11:04 PM   #9 (permalink)
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Turbos on diesels are generally designed to help out efficiency and power, so staying on boost isn't a bad thing per say. Some, such as variable vane turbos, can make boost at with very little exhaust flow. I think a pressure wave supercharger is similar, but I'm not sure which one has better overall efficiency.
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Old 12-29-2007, 11:21 PM   #10 (permalink)
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AWESOME chart!



Quote:
Originally Posted by MetroMPG View Post
But I don't know turbos at all. What's the effect of taller gearing on turbo operation? If dremd's goal is to stay out of the boost, wouldn't reducing RPM through a gearing change raise engine load and cause the boost to come on sooner for a given road speed?
Hmmm Good point.

The guys who have done the gear swap don't get much of an increase in mpg, maybe 1~3 mpg average; This could well be the reason.

The only "real world" load to fuel consumption data I have is that my mpg's stay virtually the same @55mph when climbing a slight hill. Maybe an indication that I can put more load on it at the lower rpm and not increase fuel usage?

At this point I'm basically considering Cleaning up the underside with aluminum flashing, and probably some wheel covers. I'd like to do a boat tail, but I know if I do that it will either a) never get done, or b) be way over the top.

Offsubjuct: I haven't noticed much discussion about LRR tires on here. Do they not matter much on cars so light? On my supra going from Re050's to Energy +'s gives me around a 5 mpg gain on the highway (sorry no instant reading on the Supra, so thats from full tanks). Any Favorites? Experiances?

And just incase I forgot to say it already; This site is awesome.

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