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Old 09-17-2020, 02:10 PM   #31 (permalink)
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After installing the Scott's solid metal undertray on my Insight (instead of the two smaller pieces that cover 1/3 of the area under the engine each), I found the car took longer to warm up and had a harder time maintaining coolant temperature under lean burn conditions on the highway.

I ended up having to partially block the lower grill area to compensate.

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Old 09-17-2020, 04:26 PM   #32 (permalink)
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Quote:
Originally Posted by samwichse View Post
After installing the Scott's solid metal undertray on my Insight (instead of the two smaller pieces that cover 1/3 of the area under the engine each), I found the car took longer to warm up and had a harder time maintaining coolant temperature under lean burn conditions on the highway.

I ended up having to partially block the lower grill area to compensate.
If you were to go bigger with the front undertray (including filling some of the front wheel air exit openings), you'd then find that radiator cooling flow starts to decrease. That's what happened on my Insight.

So a good example of cooling airflow being controlled by altering the exit openings.
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Old 10-11-2020, 05:55 PM   #33 (permalink)
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Originally Posted by Nautilus View Post
If deflectors before front wheels really give a delta Cd of 0.01, this drops the overall Cd from 0.32 in the 0.28 range, which is actually very good for a daily-driven car.
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You cannot just ascribe Cd reductions to a car based on what the change did on another car. (Same as you cannot just add Cd reductions together to get an assumed total reduction.)
I've also used the downloadable Excel Drag Coefficient calculator.

Frontal area for a Mk I Seat Leon has been published as 2.011, 2.20 or 2.28 sq m. Used an average, 2.16 sq m.

Weight of the car, with all bits and bobs as of Oct 2020, falls around 1364kg. Added driver weight and fuel weight (gasoline density average 0.748 kg/liter) for a total weight of 1471kg.

Tire rolling resistance for passenger car tires may be from 0.007 to 0.015. Tires were rated "E" in fuel economy, so I've assumed the worst and used 0.015.

After filling in the speeds with car coasting down in neutral, Excel formula gave a Cd figure of 0.29 (actually 0.290031, but sixth decimal place figures are too small to matter in Real Life).

As we can see, I had been too optimistic pulling a 0.28 figure from the hat, but my labors over the last few years had not been in vain.
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Old 10-11-2020, 06:00 PM   #34 (permalink)
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Originally Posted by Nautilus View Post
I've also used the downloadable Excel Drag Coefficient calculator.

Frontal area for a Mk I Seat Leon has been published as 2.011, 2.20 or 2.28 sq m. Used an average, 2.16 sq m.

Weight of the car, with all bits and bobs as of Oct 2020, falls around 1364kg. Added driver weight and fuel weight (gasoline density average 0.748 kg/liter) for a total weight of 1471kg.

Tire rolling resistance for passenger car tires may be from 0.007 to 0.015. Tires were rated "E" in fuel economy, so I've assumed the worst and used 0.015.

After filling in the speeds with car coasting down in neutral, Excel formula gave a Cd figure of 0.29 (actually 0.290031, but sixth decimal place figures are too small to matter in Real Life).

As we can see, I had been too optimistic pulling a 0.28 figure from the hat, but my labors over the last few years had not been in vain.
I think that coast-down tests are completely inaccurate - it's one of the two things that Aerohead and I agree on.

Putting invalid data into an Excel spreadsheet suddenly doesn't make the data correct.

At minimum do some testing windows up / windows down and see if the difference makes sense in terms of calculated drag. It doesn't when I do this with multiple runs and averaged results.

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