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Old 01-03-2014, 11:06 PM   #21 (permalink)
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mcrews,

From the top photo only- it appears the exhaust system is recessed and unlikely to receive airflow from directly underneath. In the front there appear to be vanes which seem to be directing airflow toward the center- but also away from the wheel wells. Also, is there ducting (?) in the front wells (inside, behind the axles) into the exhaust system channel?

The question is "is the channel simply a radiant barrier, does the ducted air to cool the car above the channel, or is it ducted air meant to cool the system".

Since I don't see any "fins" to increase surface area of either the channel or the cat/muffler, I'd say the Audi engineers weren't betting on big chances of conductive losses. Also, and article I just read doesn't suggest any great temperature differences whether at idle or at speed for the warmed up auto. Since conductive losses would improve with the mass of the air that came in contact- I would assume conductive losses to be considered negligible by engineers.

So once again I'm just guessing but I'll throw out some numbers now and make my bet: Mechanism for temps <300C = convection
Mechanism for temp >300C = radiation

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Old 01-04-2014, 02:37 PM   #22 (permalink)
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The Rental - '13 Hyundai i30
90 day: 37.55 mpg (US)

Autocross - '04 Ford Focus ZX3
90 day: 24.76 mpg (US)

The Wifes - '02 Ford Focus SVT
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i tip my hat to you, u absorb the subject matter well. My all encompassing statement about convection was mostly for the engine bay and the cooling system which are around 100C. when you get upto 1000C at a cat converter, radiation will play an effect. I guess that's why they put heat shields on it.

Maybe that can be our rule of thumb. If there is a heat shield on an item (like muffler and cat), then you need to manage its radiant heat
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Last edited by Thenorm; 01-04-2014 at 03:39 PM..
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Old 01-04-2014, 03:33 PM   #23 (permalink)
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Camaro belly

Here's a contemporary Camaro:
https://www.google.com/search?q=Ferr...0%3B1024%3B681
Here's a Ferrari pan:
http://www.tamiya.com/english/produc...enzo/enzo6.jpg
Here's a fella's scale model pan:
http://acmenovelties.net/scale_model..._undertray.jpg
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Last edited by aerohead; 01-04-2014 at 03:41 PM.. Reason: add scale model pan image
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Old 01-04-2014, 05:56 PM   #24 (permalink)
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Thenorm,

Thank you for the kind compliment.

You have brought up another excellent point- did aardvarcus' misadventure give us more than one "learning opportunity"?

I'm very happy he was kind enough to share his experience, and doubly happy he didn't blow himself up. But what makes me most happy is that it didn't happen to me.

Would it be safe to say 50% or more of the "ecomodding" inspired MPG gains are a result of "adjusting the nut behind the wheel"?

Would it be just as safe to say 40% or more of the "ecomodding" inspired MPG gains are a result of aerodynamic mods? And that the remaining 5-10% are mechanical?

I don't use all of the hyper-miler tricks (not sure all are safe, practical or prudent), but I use enough to return me almost 3 mpg more than I was getting in my monster flying brick supercrew. The CAI/exhaust, and tonneau have given me another 2, maybe 2.5 mpg.

And now I'm considering aeromods devoid of any professional automotive safety engineer input that I will personally and incompetently install myself. Fortunately, I feel it's fairly obvious that things like bondo's Aerocap, and garden edging air dam are pretty safe (unless my home made one flies off at highway speed), at least for me.

How many times have brakes or steering failed in an engine off coast? Have their been any unusual fires or system failures that put life or property at risk related to modding. I know I can save a bit more mpg if I remove all of the weight of the airbags and restraints, but should I?

As I said, I don't find all of the hyper-miler tricks to be that wise (for safety issues). Nor do I necessarily find all of the aero and mechanical mods wise for the same reasons.

With more negative feedback we may find a lot of RoT's waiting to be discovered.

I plan on being more active in this forum in my quest for my personal grail- 25mpg, and I promise I will try to bear in mind always your suggestion "maybe we need a rule of thumb" for the safe promotion of ecomodding, since I suspect there is a whole lot more than a single rule that is needed. We should encourage people to publish more of our failures- especially when "learning opportunities" are present (I personally learn more from the failures anyway).

How about even a "Rule of Thumb's" sticky? Putting our bad outcomes in one place would provide an excellent knowledge base.

I'll even suggest Rule #1:

1. Thenorm/aardvarcus Rule (aka the "Make Sure it Can Cool Off" Rule)- All mods should consider the potential to create conditions where unanticipated heat energy may build up, as well as the potential to impair the convective and radiative dissipation mechanisms engineered in the stock, which may result in catastrophic failure.

What do ya think?

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