TopGear.com America high mpg project (regeared, aeromodded diesel Rabbit)

[dremd]

Awesome to see progress!!!!!!!

Originally Posted by MetroMPG

Haha. Gotta love tales of epic road trips. Good writing.

VW's naturally make for the best epic road trip tales. Someday I'm going to figure out why.

I have a buddy who drove one through the Kaiber pass on his way from London to India; But that's another Thread.

[tasdrouille]

Good thing they went with the 02a trans that was in the mk3 TDI. The 02j found in the mk4 have a shorter geared final drive to compensate for the change in wheel size (14 to 15).

[giff]

Originally Posted by MetroMPG

Thanks for the update, giff, "wide-eyed young edit-slave".

hahah, good catch!

Today brings another installment!
Things are starting to get fun, I think you'll all enjoy this one.



Video;
http://www.topgear.com/us/vide...heart/

Feature;
http://www.topgear.com/us/feat...alism/

[MetroMPG]

Awesome. Thanks for the update!

[MetroMPG]

Somewhere in cyberspace we found an old scan of a chart that purported to show efficiency of a Volkswagen TDI engine, and if we believe this chart (What choice do we have? It came from the Internet) the TDI is most efficient at 1,800 rpm. Our target, then, is to achieve a tolerable cruising speed at 1,800 rpm.

This sounds to me like the classic misinterpretation: "best BSFC RPM = most efficient cruising RPM". But I could be wrong, not knowing the TDI well.

Tas, care to comment on that bit?

Regardless, it sounds like they've also sourced the tallest top gear (top gear, get it?) they could fit in their tranny (from a Euro Transporter), so it's kinda moot. They're going as low as they can, RPM wise.

We also get a tip off about the context they're working with for their 70 mpg target:

we should be able to cruise at 61 mph right on our 1,800-rpm sweet spot.

Sounds to me like they're saying 70 mpg @ 61 mpg = success.

Somehow I don't think that's too much of a stretch for a TDI with tall gearing. They probably aren't going to have to Aerocivic the crap out of the thing to meet that goal.

[MetroMPG]

You've gotta watch the video. Last scene is a teaser to future episodes... sure looks like the Rabbit burning through the corkscrew at Laguna Seca !

[Clev]

Originally Posted by giff

hahah, good catch!

Today brings another installment!
Things are starting to get fun, I think you'll all enjoy this one.

Hey giff, just noticed that you guys got Jared and Dave on your team. I used to work with them at an Unnamed Magazine Conglomerate; they're great writers.

[tasdrouille]

I just looked at the vid and read the progress report. $4000 for the engine!?! That is a lot just for the engine, especially in Canada, but anyway.

You're right Darin, bsfc map misinterpretation. The correct wording would have been most efficient rpm per hp produced.

Looking at the map we can see that at the kind of load they will be putting on the engine, the most efficient theoretical rpm is more around 1250 rpm.



That's why I like dsfc maps much better, but those are even less frequently seen.



I hope they go with tall tires on regular steelies too, to stretch the gearing even more and fill the wheel arch for aero.

70 mpg @ 61 mph is gonna be a piece of cake in that rabbit. Stock the mk4 does 55-60 mpg at 60 mph.

[Deezler]

They only got the rpm down to 1800 @ 61 mph?

Even if they kept the MK4 transmission, simple math shows that a 0.658 5th gear swap combined with the 3.16 pinion final drive would get the rpms down to ~1678 @ 60mph. Of course, that is with the MK4 sized wheel/tire (say, a 205/60/16). I don't know how much smaller the rabbit's tire diameter would be.

This is a fun project to watch.

[tasdrouille]

Deezler,

Stock 02J (mk4) ring & pinion is 3.39. 3.16 was in the 02A from B4 Passats and mk3 golf and Jettas.

So if they are at 1800 rpm at 61 mph with the 02A trans and .658 gear, that means they have kept the 14 inches wheels. Stock size on the mk4 is 195/65/15.