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

[roflwaffle]

Originally Posted by MetroMPG

Really?

The fancy drag & rolling resistance formulas disagree by a pretty wide margin.

How so? The Cd of a bunny is .45 with around 20ft^2 of reference area IIRC. Weight is ~2300lbs with a driver, and the middle of the Crr range is ~.01 (.006 for LRRs to .015 for snow tires). This calc gives 17+hp at 60mph and 21+hp at 65mph. Since that's at the wheels, we still have losses from the trans/CVs. Assuming the driveline is ~90-95% efficient, then the engine will need to make ~18-20hp@61mph.

Course, in terms of engine efficiency, there isn't much to be gained by an increase in load thanks to hills for example, going from ~220g/kWh to ~200g/kWh, but on the way down BSFC would drop into the ~300g/kWh area, so that's gonna hit mileage unless they have mostly flat driving or P&G on the way down.

[roflwaffle]

Originally Posted by dremd

In my limited experience (1 Vehicle) just saying that your vehicle has a Diesel in it is enough to avoid California Emissions testing.

We told Cali. that a Mustang had a Powerstroke in it and it never again had to be emissions tested (I haven't followed the car in the last 2 years, but for the first 5 it worked).
That car was actually highly modified Gasoline.

Shhh, don't tell everyone!

[roflwaffle]

Originally Posted by eyesoreracing

In my experience engine swapping in CA, the ref won't let you do any swap that changes the emissions category. No truck engines in cars, for example. That means you can't legally put a B20 engine in an Integra originally powered by the nearly-identical B18, because the B20 is from a CR-V which is laughably classified as a truck.

I haven't specifically asked about a Diesel swap, but I would be shocked if that was allowed under these same rules. Are you speculating, or speaking from experience?

That's almost right, but according to the rules an LDV engine of the same year or newer can go into an LDV so long as all the stock emissions equipment is there and functioning, and the same goes for MDVs/HDVs. You can drop a newer/same age B20 from a CR-V into whatever other car, even a F-150 truck (but not a F-250, LDV/MDV/HDV classification depends on GVWR IIRC), as long as it has all the OE emissions stuff and can pass smog. You can also drop a 500ci BBC from a Cadillac into a CRX if you can make everything fit. A diesel swap only has to pass visual inspection since they aren't smogged, although 97 and up diesels are going to start being smogged sometime in the near future (Thanks to dumbass pickup truck drivers belching smoke on CA roads after plugging in "performance" chips). This info is from personal experience both in terms of a swap and talking to the state refs.

Originally Posted by eyesoreracing

I would be offended by this comment if it wasn't true...

At least it's a fun project, and in the end, aside from penny pinchers like myself, that's what it's about.

[Deezler]

Originally Posted by roflwaffle

How so? The Cd of a bunny is .45 with around 20ft^2 of reference area IIRC. Weight is ~2300lbs with a driver, and the middle of the Crr range is ~.01 (.006 for LRRs to .015 for snow tires).

Cd of 0.45? That seems a bit on the high side; I used 0.35 in my calculations but suspected it was a little optimistic. Do you have a source? I couldn't find any data on this.

As for weight, this site lists 1775 lbs dry. Perhaps not the best source.
Volkswagen Rabbit, 1981 Rabbit Pictures, News, Reviews, and Info | Conceptcarz.com - Pictures, Wallpaper, Pricing, Information, History

Should be considerably less than 2300 lbs even with a driver and fuel. I would expect these guys to rip out most of the interior anyway.

Anyway, I listed all my #'s used, you can see from my plot that this site is calculating just 13.7 HP to maintain 60 mph, with all losses factored in.

Dremd, putting down 40 HP at 60 mph in almost any car would give acceleration.

In the end I think we can all agree that this thing is going to hit it's mileage target with ease. Provided they tune it properly once they start going after power mods; Dave, did you get rocketchip on board? I want to see some well documented dyno testing!

[tasdrouille]

Don't forget that RR is not just the tires, but the wheel bearings, seals and brakes too. Add more or less 15% for those.

I don't think they'll get a .25 Cd, not with the shape they're starting with and I've a feeling they won't go all out on the aero. I bet we'll mostly see quick stick or screw on stuff.

Nevertheless, with the small frontal area and light weight, they'll hit their numbers, but not by a huge number if you ask me. The killer will be friction and parasitic losses from the 1800 rpm. That thing will bust 100 mpg at 40 mph though.

[MetroMPG]

Originally Posted by roflwaffle

How so?

OK, I blame deezler! And my laziness. I thought he researched the figures.

Previously, the only numbers I'd seen were in Hucho's book, and he optimized that car's aerodynamics. He talks about the Mk1 Golf/Rabbit "styling model" having a Cd as low as .36 at one point. I guess "styling models" don't come with cooling systems or rough underbodies.

FYI, a quick search turns up Road & Track having published a figure of Cd .42 for the MK1 Golf/Rabbit. Source: Fluid Mechanics - Google Book Search

This page also gives the .42 figure.

[roflwaffle]

Originally Posted by Deezler

Cd of 0.45? That seems a bit on the high side; I used 0.35 in my calculations but suspected it was a little optimistic. Do you have a source? I couldn't find any data on this.

Mayfield has a similar year Quantum (wagon) at .4, and those are longer/more streamlined than the short/fat box that is a bunny, so .45 seems reasonable. Owners report .4-.48, with .42 and .48 supposedly being from published books.

Originally Posted by Deezler

As for weight, this site lists 1775 lbs dry. Perhaps not the best source.

That sounds like the dry weight for a early bunny. Car and Driver has an 83 GTI at 2100lbs wet. Toss in a 150lb driver and we're at ~2250lbs, and the extra 50-100lbs from the TDI and associated will probably put it over 2300lbs, depending on where they start of course. Add ~25-50lbs for a four door, maybe drop some w/o AC, etc... It really depends, but IME they'll probably end up around ~2300lbs w/ driver unless they do some weight reduction.

Originally Posted by Deezler

Should be considerably less than 2300 lbs even with a driver and fuel. I would expect these guys to rip out most of the interior anyway.

They could, but I don't think there would be room in their budget. They could tear everything out, but in that case it wouldn't be very bearable. Trust me on that one.

Originally Posted by Deezler

Anyway, I listed all my #'s used, you can see from my plot that this site is calculating just 13.7 HP to maintain 60 mph, with all losses factored in.

I've done the same, with sources. Since I've owned a couple I've been up and down the interwebs, and a TDI in a stock VW with a ~150lb driver should come in at ~2300lbs, plus the horrible drag coefficient, will need ~18-20hp at the flywheel, depending on how efficient the driveline is, something I actually haven't seen any figures on.

[Deezler]

Damn! I accidentally did not save my calculations that led to the previously posted plot. I will run the numbers again tomorrow with the .42 (and maybe a .3) set of Cd numbers.

I tried to research the Cd, but didn't have much time. Blame away. :P

[MetroMPG]

Originally Posted by tasdrouille

Don't forget that RR is not just the tires, but the wheel bearings, seals and brakes too. Add more or less 15% for those

I wondered out loud about that at one point (here), and Phil suggested the additional losses are insignificant compared to the tire losses.

the SAE considers bearing losses to be so low,they are basically ignored with respect to RR.

[roflwaffle]

Originally Posted by MetroMPG

FYI, a quick search turns up Road & Track having published a figure of Cd .42 for the MK1 Golf/Rabbit. Source: Fluid Mechanics - Google Book Search

This page also gives the .42 figure.

.42 sounds right, although according to what I've read that even depends on the wind tunnel, so while it may be .42 compared to a car of that era, iono if Cd measurements age well. Didn't Ford or GM get all hissy w/ Toyota about that?