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Old 07-08-2010, 05:36 AM   #1 (permalink)
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Aero mods to VT Commodore (with pics)

Doing some flat undertrays for my car, amongst other things...

VT commodore

It's about 9 foot tall and a mile wide, but I'm looking at it from an economy point of view. It already gets a creditable 8l/100km on the highway (factory claim is 6.5, but that's in the rarely reached lean cruise mode), but I want better.

Data on the aerodynamically damn-near-identical VX car points to a Cd of 0.329. The later model VY commodore achieves a 0.319 due to a sharp cutoff of the rear, a 3% improvement for something that can be done (DIY style) in about 5 minutes with some plastic. It also gets 33% less lift at the rear.

Autospeed - Aerodynamic Development of the VY Commodore

I'm thinking about an undertray arrangment as well. This would work well with the independent rear end (cutouts for adequate suspension travel, not a massive gap). Flat wheel covers will have next to no effect (the factory hubcaps are so flat it's not funny), however modified guards or rear wheel covers might. I'm not entirely sure, but given the mega-smooth transitions of the windscreen, roof, and rear window, airflow will probably stay attached. However for my purposes I can also make up vortex generators if wool tuft testing proves otherwise.

I have a very long drive coming up (1800km of smooth, hill free outback roads with no traffic, or rain) so I can trial various things. My fuel consumption display shows down to 0.1l/hr, so I can see with reasonable accuracy.

What do you think I should do?

*Edit*

I looked under the car and was confronted with this.



Uploaded with ImageShack.us

Behind it is the radiator exit and sump. Like so.



Uploaded with ImageShack.us

In front of it is a hole below the entry to the radiator (this pic taken between the front bumper and that flat plate)



Uploaded with ImageShack.us

And finally the rear of the car. Could deal with improvement. Other side is the same, but less exhaust.



Uploaded with ImageShack.us

So now I need to think about the reason that front deflector is there. I think it's to cause a low pressure after the radiator and a slightly higher pressure in front. A flat cover between the front *should* give less resistance and have a similar effect through sealing the front of the radiator. It may also lower front lift (yay).

The rear, well that's just messy coz it's cheap, but the rough aero shape is there. Time for plastic sheet and duct tape. Oh zip ties. Can't forget zip ties...

** Edit: Front undertray***

Centre piece



Side Piece



"Complete" (taped and tied) front piece.



Yes it has a downwards slope. If I was to retain the function of that plate, there had to be. However I don't think that will cause lift, as the air coming through the front is now trapped, airflow should be a lot higher on the bottom of that piece, sucking it downwards enough to cover the slope... It would have been better if I was starting with an 'S Pack' front bar, as they are lower to start with.

Rear





I'll have to do something about that lip. The rear section is fine but the transition to the fuel tank is a step. And the RH side needs some kind of cover, it's acting as a scoop.

Preliminary testing (i.e. I couldn't wait and wanted a number) indicates 6.5-7.5l/100km for the short flat sections we have around warwick, and it seems a little quieter. So it's *slightly* better than last time I went on a trip. However more to the point, nothing fell off at 130...

Much revised right-rear section





Last edited by BLSTIC; 07-11-2010 at 01:38 AM.. Reason: pics added
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Old 07-08-2010, 06:13 AM   #2 (permalink)
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Actually I just read the article again. And noting the high effectiveness of the factory VY spoilers (and the identical roof line and door profiles) it's safe to say that airflow IS attached all the way to the end of the boot... So I won't need to worry about vortex generators, just the flow separation point...
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Old 07-08-2010, 08:27 AM   #3 (permalink)
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Though being a yankee I am unfamiliar with the car, I can still make a few general recommendations which I think might well help:
1. If the car has an OBDII diagnostic plug under the dash, as U.S. cars beginning in 1996 must, then you should get a ScanGauge so that you can figure out how to drive the car for best fuel economy. You will never know what you are doing rightly or wrongly without some sort of instantaneous fuel consumption gauge.
2. Underbelly smoothing definitely helps on most cars. Do as much as you can.
3. A block heater will help if you are in parts which get cold on July nights ;-)
4. A grill block will help with both warmup and aero. But do watch your water temperatures carefully until you get the hang of how it works.
5. Rear fender skirts will help some.
6. Air the tires(tyres) to the maximum listed on the sidewall.

In the U.S. aeromodders frequently use a corrigated plastic sign board material called "coroplast" to make aero pieces such as fender skirts, underbelly pieces, and grill blocks. It is probably available from sign companies in Australia. Good luck.
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Old 07-08-2010, 09:26 AM   #4 (permalink)
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I already have an accurate fuel consumption and temperature meter built into the dashboard of the car (accurate to 0.1l/hr and 0.5*c respectively) thank to poor feature hiding by holden. Kind of helpful that one (has a voltmeter, and digital speedo/fuel guage too), and the tyres are already at 42psi hot.

I was considering blocking the grill, but then again I'm driving through the outback. It's really hot and dry here in the sun, even in winter.

You know I never knew what coroplast was. We have real estate signs made out of corrugated and coated plastic. It's quite stiff for its weight and thickness, and I'm willing to bet it's the same stuff. It's my prediction that the local realtor is about to have it's bin lightened to the tune of 6 signs. Not that my new front undertray, wheel panels, and bootlid extension would know anything about that...
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Old 07-08-2010, 10:04 AM   #5 (permalink)
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I thought it might be a Holden. Can you give us some basic information on the car, such as weight, engine size, and government listed fuel economy?

Finally opened the link and looked at the article. From the pictures, I'd be terribly tempted to tape a block on that upper grill, and do some testing. You may be surprised. You may be able to maintain regulation water temperatures during your cool months. If the temperature gauge starts to rise, just take a quick moment to remove the grill block.

Of course, you can improve your fuel economy by limiting your speed. In the vast outback, I suspect that one wants to move rapidly, but confining speed to 55mph(90k/hr) would show significant improvement over 100 k/hr and the travel time isn't much different.
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Old 07-08-2010, 10:06 AM   #6 (permalink)
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lovely car, but a bit of a sad story aerodynamically, considdering the opel omega B it's related to, replaced a car with an 0.28Cd! the prev omega had cut off rear wheel arches (semi wheel skirts) and was also more square in the back. I always thought the round corners at the back where part of the cause for the degraded Cd

what i can highly recommend is making a front undertray and next intall front and rear wheel dams(dont overdo thems just look at curent production cars they all seem to have them by now), and perhaps boattails or trakes behind the rear tires (i just screwed a lenght of plastic L frame under the bumer angled slightly inwards... seems to help)

I have been toying with the idea of using some sort of rubber insulation strip (like what you'd use on doors and windows) just ahead of a round edge to create a sharp cutoff. so perhaps your car might benefit from that just ahead, of the taillights but perhaps also on the trunklid

wide tires often seem to be the case of an increase in Cd when data is compared for a same model so if you can go for something more narrow that should also have an impact, although grip might also be degraded so that's a compromise

is the car a turbo? otherwise a warm air intake could work, at the expense of some power, however i have found that by leaving the original air intake in tact and adding an aditional warm air pickup ,(or forcing open an existing on in my case, that's only used during warmup) and adjusting the size of the WAI you can tune the air temperature to where you strike a good balance between drivability and economy.

interestingly engine temps seem slightly lower since i used a WAI (despite this being one of the hottest summers ever over here) so perhaps a little more grillblock could be an interesting side effect but YRMV
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Old 07-08-2010, 11:36 AM   #7 (permalink)
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Basic Specs
1570kg (depending on trim)
3.8L 'Ecotec' V6 (L36? Related to the Buick 3.8), 147kw @ 5200 304Nm @3600
4spd electronic auto
12-7.2 l/100km city/highway (evidently the 6.5 figure was for my AU, which has a Cd of .29) ~1800rpm @ 100kph.

The commodore started out as GMH's interpretation of the 'V' car back in the 70's. After 1988 none of the original parts remained, and only the suspension was similar. When the VT came out in 97/98 it wiped away all memory of the 'V' car except the stud pattern and the badges.

Yes the round corners ARE a cause for concern. With the rear end of the VY (and front end 'not better but not worse either') sharpened only a little drag went from .33 to .32... And they didn't exactly go extreme.

In terms of aero I'm thinking covered rear wheel arches, make a clear separation point, front and rear undertray (I'll leave the floor if I can't get enough material), and maybe front wheel covers. The front wheels (according to the oft-quoted volvo table) make up 13% of the total drag and the rear make up 7%, so if I can find some foam blocks I'll give that a try, along with removing the mud guards.

Math time people...

Original - 0.33
+ Sharp cutoff - 0.32
+ Wheel deflectors 'kit' (half the drag from the wheels through flat caps, covers, and deflectors) - .288
+ Undertray (take 1/3rd off total axle, suspension, and floor drag) - 0.25

So going from 0.33 to 0.25 is 3/4 of the drag.

Assuming the car actually GETS 7.2l/100km now and aero makes 90% of the total drag, I'm looking at 5.6l/100km IF the engine is just as efficient. Mathmatically the engine can't be as efficient so I'm probably looking at 6l/100km (however it will probably stay in lean cruise longer).

1.2l/100km over 1800km = 21.6L of fuel = $28 fuel saving over a really long trip.

So if I spend $28 on parts, I'll have an immediate break even point and will have proven a few theories on the way...

I might enjoy this immensely...
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Old 07-08-2010, 12:15 PM   #8 (permalink)
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Is this VT a front wheel drive? I had a buick lesabre which had the venerable 3.8 that barely spun at 2000 rpms in top gear doing 90. I never really tracked it's economy, but it seems like it did better than 20 mpg (us), but the newer turn of the century versions were capable of 29 mpg. My lesabre was also rather large, and had independent rear, so i wonder if these aren't related vehicles.....
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Quote:
Originally Posted by jamesqf View Post
I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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Old 07-08-2010, 06:08 PM   #9 (permalink)
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12.5 % mpg

If you can get to Cd 0.25,your mpg,say at 89-km/h would improve by 12.5 %.
More at higher velocity.
And this is contingent upon a fixed BSFC for the engine.
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Old 07-08-2010, 06:31 PM   #10 (permalink)
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RWD. Australia has a love affair with large engined RWD cars...

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