Originally Posted by Richard

Thousands of 67-70 trucks built with who knows how many still on the road. Even my 94 Chevy truck has a similar grill opening with the bar in center. If you like the 71-72 grill and want it, go for it. My favorite as well. IMO there will be no cooling difference.

Originally Posted by Keith Seymore (Post 6329036)

It's gonna be somewhat tough to prove out in the real world.

I should preface this by saying I was the engineering manager on the Chevy Volt program (both domestic and export; and related variants: Opel Ampera, Vauxhall Ampera). CdA for the Volt was .284 which at the time was the lowest GM had ever done for a midsize sedan. We spent a ton of time in the wind tunnels (we have two: the aero tunnel that the NASCAR boys borrow, and a Climatic tunnel for hot/cold operation).

The problems with doing a truck program on the outside as I see it are:

a) Airflow over a vehicle is not intuitive. You don't really know what's going to happen 'til you smoke it. Example: We introduced a duct at the front of the vehicle one time, looking for a ram air effect. It wasn't working like we expected so we brought it into the tunnel for evaluation. With the smoke, you could see the air was actually hitting the inside corner of the inlet, scooting across the opening and shooting out the outboard side of the inlet and around the vehicle. No air was actually going into the duct and in the hose. We would never had known (or even guessed this) without the smoke and video. The whole "tailgate up vs down" and bedcover debates that just won't die (in spite of objective data) are a symptom of this.

b) You need to be able to focus on one small area. On the road testing incorporates the overall vehicle, which is fine for a final "capping run" evaluation, but for development work you need to isolate one area and gage the effects that can be attributable to those specific changes. Sure - the NASCAR and Bonneville guys to some testing on site with duct tape and cardboard - but if they're smart it's after spending hours and hours iterating and developing inside a tunnel like ours.

c) Things change while you are looking somewhere else. So - the bad news is while you are focus on one area of the vehicle the other end can be changing without you knowing it. Hopefully you find out and can nuetralize the effects but if not it can generate a lot of confusion.

We had been working for years on prototype and preproduction vehicles so we were quite anxious to do a "sense check" on the first saleable vehicles off the line. We put them in the tunnel and the results were .289 - not good because it was higher than expected but also because the media reports the values rounded to two decimal places = .29. We very much wanted the .28 value reported, so we had to be below .285 to achieve this.

After much angst and diagnosis it turns out the cars were building 5mm too high. This change in trim height was affecting our test scores, the advertising numbers and our fuel economy certification. Dropping the cars down 5mm brought everything back into line. Note that's 1 count of aero drag for every 1 mm of trim height, in this case (your results may vary).

With my duct example above, we could've changed something at the rear of the vehicle, changed the flow under or around the truck and suddenly our front duct could've started working. You never know.

d) Small changes have big results and big changes can have small results, like the example above. My boss used to joke that he would give his finger for .001 CdA (one "count" of drag). Little scallops and skegs and flanges that look like styling features can actually be aero enablers. Things like mirrors and door handles and grille openings are fairly significant aero disenablers.

Originally Posted by Keith Seymore (Post 9391401)

The other thing is this: the airflow is not intuitive.

We decided one time we were going to use some parking lamp openings for an air inlet. Looked like a nice spot for some "ram" effect.

We ran the tests and the made no difference. We couldn't figure it out until we ran them using smoke: the air was going in the outboard edge of the inlet and coming right back out the inboard edge of the inlet. It wasn't even entering the ductwork.

You are not going to know anything until you smoke it.

K

Originally Posted by Keith Seymore (Post 9391401)

...<snip>...

You are not going to know anything until you smoke it.

K

Originally Posted by 70STOVEBOLT (Post 9391421)

I always smoke first...

Originally Posted by howzzzit (Post 9391454)

Take your grill out and see if there's a big difference. Might not be your problem.

Originally Posted by 72c20customcamper (Post 9391452)

Thought it was customary to smoke afterwards?!

Originally Posted by 72c20customcamper (Post 9391452)

Thought it was customary to smoke afterwards?!

Originally Posted by 68isgreat (Post 9393461)

The 68 grilles are the worst. Most of the 68 trucks blow up because the grilles are so horrible. Please send all those ugly 68's to me and I'll make sure to dispose of them properly. :lol: