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

[eyesoreracing]

Originally Posted by MetroMPG

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.

MetroMPG,

We started with BSFC because DSFC would require that we know a lot more about rolling and aero drag than we could ever possibly know, considering the car wasn't built yet. Correct me if I'm wrong, but it seems to me a DSFC map could really only be made after the car exists and its drag properties are known.

Besides, just looking at BSFC smacked us up against the tallest gear available, so there didn't seem to be any purpose nerding out over it any longer. Does it really matter if the ideal gear ratio is 10% taller than the tallest we can get, or 15% taller?

Educate me if I'm just being dense.

-Dave Coleman

[dremd]

Originally Posted by eyesoreracing

MetroMPG,

We started with BSFC because DSFC would require that we know a lot more about rolling and aero drag than we could ever possibly know, considering the car wasn't built yet. Correct me if I'm wrong, but it seems to me a DSFC map could really only be made after the car exists and its drag properties are known.

Besides, just looking at BSFC smacked us up against the tallest gear available, so there didn't seem to be any purpose nerding out over it any longer. Does it really matter if the ideal gear ratio is 10% taller than the tallest we can get, or 15% taller?

Educate me if I'm just being dense.

-Dave Coleman

In a Nutshell, that BSFC (Brake Specific Fuel Consumption Chart) and all others measure Fuel used at RPM with Max Load; so unless you guys are planning to tow something rather boxy around you will be turning the ALH far to quickly.

If you check out the Gear Threads on TDIClub you can get a idea on what gears do what (atleast with the MKIV body).

However with (what I can only assume) will be a low drag shape you will want to turn the ALH much slower, unless you just like P+G.

[MetroMPG]

Hey Dave -

Originally Posted by eyesoreracing

a DSFC map could really only be made after the car exists and its drag properties are known.

You're right there, for sure.

I think there's a misinterpretation about what the BSFC map is telling you, in terms declaring an "ideal" cruising RPM.

The engine produces power most efficiently at 1800 RPM, but that's when it's making about 55 hp. In other words, if you need 55 hp, 1800 RPM is the place to be. That's the RPM & load combo where the engine produces the most hp per unit of fuel.

Best BSFC RPM doesn't necessarily = lowest fuel consumption at lower engine loads. Since you're probably only going to need roughly 12 hp to go 60 mph on level ground, lower RPM would be better. Tasdrouille figured the sweet spot is more like 1250.

But since, as you say, you've already stuck the tallest possible OEM 5th in there, it's kind of moot (as I admitted in my comment above); any further discussion is just so much fuel economy geekery.

Unless of course you wanted to make a new 5th ratio... like this guy, or this guy.

Congrats on the project, and the really good writing. Can't wait to see what you guys did in Cali with the aero doctor.

Darin

[eyesoreracing]

Originally Posted by MetroMPG

Hey Dave -

You're right there, for sure.

I think there's a misinterpretation about what the BSFC map is telling you, in terms declaring an "ideal" cruising RPM.

The engine produces power most efficiently at 1800 RPM, but that's when it's making about 55 hp. In other words, if you need 55 hp, 1800 RPM is the place to be.

Best BSFC RPM doesn't necessarily = lowest fuel consumption at lower engine loads. Since you're probably only going to need roughly 12 hp to go 60 mph on level ground, lower RPM would be better. Tasdrouille figured the sweet spot is more like 1250.

But since, as you say, you've already stuck the tallest possible OEM 5th in there, it's kind of moot (as I admitted in my comment above); any further discussion is just so much fuel economy geekery.

Unless of course you wanted to make a new 5th ratio... like this guy, or this guy.

Congrats on the project, and the really good writing. Can't wait to see what you guys did in Cali with the aero doctor.

Darin

What we did in California? Try what we do in California... This thing is real-time. What you see in the stories is only a couple of days behind reality. It's going up as fast as we can write and process. We start fabbing aero parts tomorrow.

I'm pretty sure we found a BSFC map that showed several different load conditions and made our conclusion from that. We weren't really 100% certain the map was from our engine, though... Unfortunately, I've lost the map. As you can imagine, working on this timeline is a little hectic.

Glad you're enjoying the project, thanks for the support.

-Dave

[MetroMPG]

We start fabbing aero parts tomorrow.

Woohoo! The Aerocivic is available for taking moulds. ("Consumer acceptance" wasn't listed as one of the project criteria, was it?)

[Clev]

BTW guys; Dave Coleman was one of the guys who brought you "How to Get Your Econobox into the 14's."

VWvortex Forums: How to get your econobox into the 14's-A must read.

[Deezler]

Originally Posted by dremd

In a Nutshell, that BSFC (Brake Specific Fuel Consumption Chart) and all others measure Fuel used at RPM with Max Load

Not quite dude. The following chart posted earlier by Tas shows fuel consumption from zero load to full load across the entire rpm range. The left axis, BMEP (brake mean effective pressure) is directly related to engine torque. The single line across the top shows the full load engine output, but everything in between that and the x-axis is fuel consumption data at part load. Someone had to take a lot of data points to generate this plot.

Originally Posted by tasdrouille

Dave! Didn't realize this was your project. Always enjoyed your nerd-columns in SCC back in the day. You should be pleased to note that the above BSFC chart is from your engine, the VW TDI ALH. When you guys get done aero-modding, I would guess you'll need around 15 HP to maintain 60 mph. If you look between the 10 and 20 HP lines you get an idea how the fuel consumption will be at various engine rpm. As Tas noted, there are dips in the 210, 220 and 230 g/kW-hour BSFC lines at 1250 rpm. This would unquestionably be the optimal engine speed for any load between 10 and 20 HP. But as you stated, who cares, since the gearing is as low as you could get it anyway. You might consider oversizing the tires to help also, which would also fill up the wheel wells a bit more.

I'm basically taking on a tamer version of this same project on my daily driver so its fun to watch your progress. You should have no problem with your goals!

[MetroMPG]

Originally Posted by Clev

BTW guys; Dave Coleman was one of the guys who brought you "How to Get Your Econobox into the 14's."

VWvortex Forums: How to get your econobox into the 14's-A must read.

I've seen that one too. Instant classic!

I would guess you'll need around 15 HP to maintain 60 mph.

No need to guess. Check the link to the calculator tool in my post above. If you know the Rabbit's specs, you can plug in the figures. The default values in the form are for my car, stock (which had similar Cd, A & weight characteristics of the Rabbit, though you'd have to adjust the engine efficiency value, obviously, if you want it to spit out MPG estimates). Figure some moderate aero mods are going to knock a few points off Cd, and your power requirements drop even more.

But as you stated, who cares, since the gearing is as low as you could get it anyway.

Custom 5th gear, ftw! After $4k for an engine swap... why not?

(Oh, it's SO easy to armchair engineer someone else's project!)

[MetroMPG]

I keep forgetting to add: I have a particular soft spot for this project since I had a fleet (hutch?) of Rabbit fixxer-uppers when I was in university - including an '81 (gas) with the steel Jetta rims, just like project Sipster.

[Deezler]

Originally Posted by MetroMPG

I've seen that one too. Instant classic!

No need to guess. Check the link...

I know, I'm planning to nerd-out over lunch and calculate some things for this car. Supposed to be working right now, so...