VT Commodore Undertray Prototypes (with pics)

[BLSTIC]

I have one of these...

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 7.2, 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.

Aero Development of the VY Commodore

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.

PICS

I looked under the car and was confronted with this.



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



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



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



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Now I need to decide what to do.

First up is that front air dam. It's there for a reason, and that reason is most likely to develop a low pressure area behind the radiator for heat extraction purposes. It would also deflect air around the sump a little, but I can't see that being its major function (why reduce sump cooling?). So what I'm going to do is make a tray that extends from the rear of the bumper to the bottom of that plate, extending out to the side of the car. If this causes any confusion, the second last picture will now look like a piece of real estate sign, and the air that goes into the front opening has nowhere to go other than into the engine or through the radiator. That should cut down on drag a fair bit, and depending on my luck, it might reduce front lift too...

The rear will get a similar treatment. The entire fuel tank will be covered, right to the lip of the rear bumper (hoons may confuse this for a partial diffuser. = Street Cred), except for the exhaust. I don't have the material to make the rest of the tray just yet...

I purchased another pair of wheel covers too. They have a flat surface, perfect for attaching a pizza pan (or something else light and flat). Those will go on the front. The rear will get a complete cover (80's caddilac style). I'm also going to remove the mud flaps, and if I can find some polystyrene, make some deflectors for the wheels.

I'm hoping in that lot to drop from 7.2l/100km to somewhere around 6l/100km on flat roads with all that...

Thoughts?

Ben

[BLSTIC]

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 to follow...

[BLSTIC]

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...

[BLSTIC]

Much revised right-rear section





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[Pinhead]

I like what you've done so far!

That car looks a lot like a Pontiac GTO....

[BLSTIC]

That isn't a co-incidence. The GTO is a left hand drive version of our Monaro. Which is the coupe version of this platform. Engines, gearboxes, some suspension, most front end parts, headlights, tail-lights, instrument clusters... they are all interchangable. It's conceivable that if you wanted a unique car over there in the states, you could import this (or one of the later VY-VZ interpretations) and use GTO parts for the LHD conversion.

If you were building a show car (and were painting and modifying anyway) starting with one of these would be a good way to score originality points with the judges.

[BLSTIC]

Preliminary test results are in.

200km one-way trip

Fuel economy appears to hover around the 6.8-7.8l/100km mark according to my instantaneous fuel consumption display. The average differs from factory highway claim by .2km/l (7.3 as opposed to 7.5). However it is lower than previously noted (around 7.0-8.2 l/100km). I have a new flat ground no wind record of a one-off 6.5l/100km. Which probably means I actually got into lean cruise mode AND required less power.

I had a random burning smell twice in the journey but I believe they were unrelated and possibly from other vehicles (they were different)

That's objectively. Subjectively it's noticeably quieter and it appears to roll that little bit easier.

Note that since my last highway drive I have gone from 3mm toe out at the front to 3mm toe in (overall), so there might have been some gain from that.

To do an A-B-A test I would have to, at some point, install my undertrays in a strange town on the side of the road. So I'm just going to do a B-A test and take the thing off half way through my drive.

I pity the fuel...