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Old 01-06-2008, 12:47 AM   #11 (permalink)
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Quote:
Originally Posted by jazzie604 View Post
...you may look into something that will allow some flow out of the back of the hood. cheap racer trick is to add a couple of washers to the mounting brackets(between the hood and the pivots) so that the hot air gets pulled out of the back of the hood.
I think that popping the back of the hood would not suck flow out of the hood. Generally the base of a windshield is a high pressure area, while the under hood area is low pressure. So most likely, popping the hood (especially with a undertray and airdam) would pull air down into the engine, not out. Tufting would ultimately tell you what was happening.

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Old 01-06-2008, 09:54 AM   #12 (permalink)
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hmm. didnt think about the high pressure situation. goes to show that the average racer doesnt know much. though maybe with open grills and undercarriage maybe there is enough pressure to create some flow :whoknows:
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Old 01-06-2008, 11:07 AM   #13 (permalink)
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One more time - this time with feeling!

Here it is in pictures, showing relative lift forces on an RX7. Note the positive pressure at the windshield base.



Source: http://www.autospeed.com/cms/article.html?&A=2455
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Old 01-06-2008, 03:31 PM   #14 (permalink)
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that would explain why the large vent on my evo is center front on my hood. allows the extra heat to be drawn out.
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Old 01-06-2008, 04:59 PM   #15 (permalink)
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This is from another post I did and have already been challenged on it but here it goes:

That fast back crap won't help a bit, it will only add weight. A pickup truck already gets that effect by the turbulant air behind the cab with no extra weight.

If you want to better the aero in the bed area: use a tonneau cover and put a 6-8" piece of aluminum extending the roof off the back of the cab and tonneau cover over the tailgate. that should get you up to 2mpg at highway speeds with minimal weight. A truck cap with the extension off the back would also do.

Plug any holes in front, lower truck, add a front air dam, remove trailer hitch and replace rr bumper with roll pan, remove all extra weight. Get smaller mirrors. Use under-drive engine pulleys. tune up the truck. Increase gearing.

*Keeping weight down is absolutely necessary!!!!! Fuel economy is impacted most during acceleration where weight is the largest factor. Every vehicle gets decent mileage cruising. My 8,000 pound diesel truck (non ecomodded) gets 27mpg cruising at 85mph but accelerating from stops or slower speeds reduces the average to 18-20mpg.

Don't get me wrong, drag is important, but under 35-40 mph it is minimal compared to weight. We have done studies on taking 1200 pounds out of a "C" sized car and with the stock drivetrain and stock aero it got over 85mpg in EPA testing. The EPA rating was 32mpg before.
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Old 01-06-2008, 10:16 PM   #16 (permalink)
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Quote:
Originally Posted by Smoky View Post
This is from another post I did and have already been challenged on it but here it goes:
Prepare to be re-challenged!

Quote:
That fast back crap won't help a bit, it will only add weight. A pickup truck already gets that effect by the turbulant air behind the cab with no extra weight.
That's an extreme statement, unqualified, & a massive generalization to boot. At speeds where aero matters: tapered bed cover > tonneau cover > open bed > full width&height cap.

Quote:
A truck cap with the extension off the back would also do.
If you mean one of these, that's not true, unless your extension is tapered in plan & taper, like the fastback "crap".



In fact it'll likely get worse fuel economy than an open, uncovered bed because the flow separates from a point that is nearly the maximum cross-sectional area of the vehicle, with nothing downstream to reattach to. So the trailing wake is almost as big as it can possibly be.

I shouldn't have to say this, but note: I'm talking highway speeds, where aero resistance dominates as the largest proportion of energy requirements.
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Old 01-06-2008, 11:43 PM   #17 (permalink)
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Quote:
Originally Posted by Smoky View Post
...*Keeping weight down is absolutely necessary!!!!! Fuel economy is impacted most during acceleration where weight is the largest factor. Every vehicle gets decent mileage cruising. My 8,000 pound diesel truck (non ecomodded) gets 27mpg cruising at 85mph but accelerating from stops or slower speeds reduces the average to 18-20mpg.

Don't get me wrong, drag is important, but under 35-40 mph it is minimal compared to weight. We have done studies on taking 1200 pounds out of a "C" sized car and with the stock drivetrain and stock aero it got over 85mpg in EPA testing. The EPA rating was 32mpg before.
DrEMHmrk2 didn't mention speeds, and %highway. So essentially "everyone is right", since the problem isn't defined enough to determine if the aero improvement is worth the weight gain. Or more importantly at what weight the aero improvement of a tapered bed cover would not be beneficial.

DrEMHmrk2, is the 2001 Tacoma bed metal? I've heard the '06 Tacoma beds are "plastic" (most likely because of the aforementioned impact of weight). If it is metal, you might be able to "pad" your weight budget by replacing the bed with the newer plastic one (depending on cost of replacement/weight/fitment concerns, of which I have no knowledge).

BTW, what are the dimensions of the Tacoma bed? Specifically, length and width at the top of the bed. *And* the height from the top of the bed to the top of the cab, and the width of the cab at it's top? That can help us define the potential weight impacts better.
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Old 01-18-2008, 08:16 PM   #18 (permalink)
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DrEMHmrk2, is the 2001 Tacoma bed metal? I've heard the '06 Tacoma beds are "plastic" (most likely because of the aforementioned impact of weight). If it is metal, you might be able to "pad" your weight budget by replacing the bed with the newer plastic one (depending on cost of replacement/weight/fitment concerns, of which I have no knowledge).

'01 Taco beds are steel. Since the redesign a couple of years ago, though, mounting points may have changed that new beds won't cross over.

My '87 has extra steel innerwalls; if yours has those and you don't do a lot of destructive loading, you could pull yours and save yourself twenty pounds. I have to have mine.

Here's a think: the spare is mounted belowdecks; pull it and stash it in the bed. That moves that mass inside the wheelbase for improved handling, and now you can close over that space with a partial bellypan.
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Old 01-18-2008, 09:46 PM   #19 (permalink)
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Lookin' to do something like this?

http://www.fokisd.org/gallery2/main.php?g2_itemId=508

I have a banjo-rigged "fastback" fairing that I am looking to convert into a more usable topper this summer. For me, the "fastback" has been worth 3 MPG improvement over the open bed.

I certainly can't quibble with basjoos' success, but for me I've gone with the air dam. There are just too many places under the truck I need to get to so I minimize air flow under the truck with an air dam. Actually the air dam was removed for the winter, and I am making an "Air Dam 2.0" that will be more rounded in the plan view. also side skirts are coming.

IMO, reducing weight is not a big help to pickup MPG because a truck is a truck and it has to haul stuff. If you shave off weight, it just comes back as payload.

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