areodynamics

[camshaftgsxr]

which creates more drag a flat bed cover or an open bed with nothing in it

[dan76]

Are you referring to a cover fitting over the bed rails? If so, just about every pickup I've seen run at Bonneville has a bed cover installed. These guys won't use one if it didn't aid in the aerodynamics.

A few years ago I read a test study conducted by a university on the benefit (if any) of running a pickup with the tailgate down vs. up. The conclusion was no measurable benefit realized with it down. I'd suggest if one thought there was a MPG benefit to running with the tailgate down, it would take just one impact while backing to wipe out years of potential savings.

[camshaftgsxr]

sweet cause the bed cover looks sweet and now i can be happy with keeping it

[Keith Seymore]

Originally Posted by dan76

Are you referring to a cover fitting over the bed rails? If so, just about every pickup I've seen run at Bonneville has a bed cover installed. These guys won't use one if it didn't aid in the aerodynamics.

A few years ago I read a test study conducted by a university on the benefit (if any) of running a pickup with the tailgate down vs. up. The conclusion was no measurable benefit realized with it down. I'd suggest if one thought there was a MPG benefit to running with the tailgate down, it would take just one impact while backing to wipe out years of potential savings.

True.

Although, when we ran the Don Stringfellow driven S truck at Bonneville we got the best results with a "half" cover: covered the rear portion of the box forward of the tailgate, but with the front half of the box open. God only knows why - I've found you really can't tell what's going to happen unless you put it in the tunnel and visualize it with the "smoke". It's probably a function of anticipated top speed as well as a host of other variables.

Your second point is also valid: we designed the aerodynamics to be most effective with the tailgate up; I've heard guys swear they get better fuel economy with the gate down but we can't support it with empirical test data.

K

[Prerunner1982]

Keith... just wanted to thank you for your input. It is great to have someone around who was there and involved. I always look forward and enjoy reading your responses...

[Kinghat]

Just saw a Mythbuster episode about tailgate up and down, and they came up with the tailgate down idea actually hurt fuel mileage. Even showed a mock wind tunnel that demonstrated.

[Keith Seymore]

Originally Posted by Prerunner1982

Keith... just wanted to thank you for your input. It is great to have someone around who was there and involved. I always look forward and enjoy reading your responses...

Thank you; I am reminded of a test we ran one time on a GMT400 version. We decided it might be neat to take the parking light and turn it into a cold air duct, for the engine.

We cobbled up a design and the powertrain folks tried it, but they didn't see any improvement. We were a little stumped, so as part of our development process we "smoked" it in the wind tunnel.

When we watched the video, we couldn't believe it: The air was impinging the inboard side of the duct opening, traveling across the back (over the ductwork) and back out the outboard side of the duct!! It never even went inside....

I wouldn't have believed it if I didn't see it....

Originally Posted by Kinghat

Just saw a Mythbuster episode about tailgate up and down, and they came up with the tailgate down idea actually hurt fuel mileage. Even showed a mock wind tunnel that demonstrated.

I'm encouraged to hear that...

Incidently, these little "vanes" and "bumps" are all there on purpose. Working on the Volt program, we can add a little tweak here or there which is worth .001 (pronounced "one count"). My boss has told me he would give his little finger for "one count", so we have started calling them "a one pinkie" change, or a "two pinkie" change. (lol)

The scary thing is that you can unintentionally make a change that takes away one or two counts; it's very easy to do.

K

[dan76]

Originally Posted by Keith Seymore

True.

Although, when we ran the Don Stringfellow driven S truck at Bonneville we got the best results with a "half" cover: covered the rear portion of the box forward of the tailgate, but with the front half of the box open. God only knows why - I've found you really can't tell what's going to happen unless you put it in the tunnel and visualize it with the "smoke". It's probably a function of anticipated top speed as well as a host of other variables.

Your second point is also valid: we designed the aerodynamics to be most effective with the tailgate up; I've heard guys swear they get better fuel economy with the gate down but we can't support it with empirical test data.

K

Thanks Keith for your input....brings back some memories.

If I recall this truck was running with a 4.3 punched/stroked to 5.0. And the half bed cover concealed the cooler allowing for cold water to circulate to the air exchanger thereby cooling intake air.

Did this truck ever get into the 200 club?

[piecesparts]

Originally Posted by Keith Seymore

True.

Although, when we ran the Don Stringfellow driven S truck at Bonneville we got the best results with a "half" cover: covered the rear portion of the box forward of the tailgate, but with the front half of the box open. God only knows why - I've found you really can't tell what's going to happen unless you put it in the tunnel and visualize it with the "smoke". It's probably a function of anticipated top speed as well as a host of other variables.

Your second point is also valid: we designed the aerodynamics to be most effective with the tailgate up; I've heard guys swear they get better fuel economy with the gate down but we can't support it with empirical test data.

K

I had the opportunity to discuss this very issue with Don Stringfellow, the year that they had the Salt Flat Truck on the HR Power Tour. They used a tapered solid bed cover at the rear of the bed that covered items that was mounted under the box (computer gathering info gear, and fuel cooling box, etc...). They found that the box mounted at the rear provided a couple of items for them (1) it covered the rough edges of all of their stuff in the bed, (2) it provided a cover to remove the positive pressure at the tailgate, (3) the tapered front cover face allowed for a small negative pressure at the back of the cab but moved that open bed space forward enough to keep the air from boxing up into the bed itself (the taper worked to keep the negative pressure at a minimum). Don stated that they tried numerous covers and this one worked the best at the speeds that they were running.

The tailgate being put down is considered a misnomer by those that do things with speed. A full bed cover is a good thing and I have run them for years (soft cover on some trucks and a hard cover on another one), and the gains are not that substantial, but it helps, as well as keeps everything dry in the back of the truck. Ther real causal effect is the negative pressure behind the cab, not the air at the tailgate. The negative pressure works on a basic principal, like the lift on an airplane's wings on take off. It is not so much the air under it, but the negative pressure on the top of the wing that lifts the plane from the ground. The air at the gate does create somewhat of a resistance, but once the opening is filled it becomes a buffer for the most part.

[Keith Seymore]

Originally Posted by dan76

Thanks Keith for your input....brings back some memories.

If I recall this truck was running with a 4.3 punched/stroked to 5.0. And the half bed cover concealed the cooler allowing for cold water to circulate to the air exchanger thereby cooling intake air.

Did this truck ever get into the 200 club?

IIRC - it took a couple seasons but they did finally get over 200 mph.

I always like that truck and was intrigued with the idea of creating a drag truck using that configuration.

K

[Keith Seymore]

Originally Posted by piecesparts

I had the opportunity to discuss this very issue with Don Stringfellow, the year that they had the Salt Flat Truck on the HR Power Tour. They used a tapered solid bed cover at the rear of the bed that covered items that was mounted under the box (computer gathering info gear, and fuel cooling box, etc...). They found that the box mounted at the rear provided a couple of items for them (1) it covered the rough edges of all of their stuff in the bed, (2) it provided a cover to remove the positive pressure at the tailgate, (3) the tapered front cover face allowed for a small negative pressure at the back of the cab but moved that open bed space forward enough to keep the air from boxing up into the bed itself (the taper worked to keep the negative pressure at a minimum). Don stated that they tried numerous covers and this one worked the best at the speeds that they were running.

The tailgate being put down is considered a misnomer by those that do things with speed. A full bed cover is a good thing and I have run them for years (soft cover on some trucks and a hard cover on another one), and the gains are not that substantial, but it helps, as well as keeps everything dry in the back of the truck. Ther real causal effect is the negative pressure behind the cab, not the air at the tailgate. The negative pressure works on a basic principal, like the lift on an airplane's wings on take off. It is not so much the air under it, but the negative pressure on the top of the wing that lifts the plane from the ground. The air at the gate does create somewhat of a resistance, but once the opening is filled it becomes a buffer for the most part.

That's some good info there, Frank. Thanks for posting that -

I do have one other story about this truck: back then, we had an event at the Desert Proving Ground called "the Happening". It was a big shindig, the intent of which was to make the rest of the corporation aware of all the cool things we were doing in Truck.

We had some activities off property - a truck "rodeo" of sorts, with barrel racing, trailer backing, a truck pull, etc. During the course of the week a little rivalry evolved between the C/K truck guys and the S/T truck guys. They started talking smack, mostly from the C/K guys about a particular 454SS pickup they had fixed up and about how good it was running. The S/T truck guys had a project, too, and coerced their opponent to meet them at Firebird Raceway for a best two out of three matchup.

The day came and the two black trucks lined up; IIRC the 454SS won the first run and then (somehow, I'm not sure how) the S truck won the second round. They lined up for the third and final run when, suddenly, the S truck mysteriously backed off the starting line and, in it's place, this Bonneville truck slipped into the starting beam. Amid raucous laughter the tree came down and, even with it's high Salt Flat gearing, the S/T truck shuddered away and then took off like a shot, leaving the big dog 454SS in it's dust.

Even the Full Size Truck guys had a good laugh, because they knew they'd been "had"...



K

[Wild83C10]

With the tailgate down it acts like a "wing" and adds a downforce to the rear of the truck. This adds more rolling resitance which actually does negatively affect fuel economy.
The mythbusters episode on this was great and had some very neat scale modeling and testing in a water tank.

[TwoFiftyShifter]

I have a question. My 96 silverado doesnt have the front valance/air dam. I run a lot of highway and check mpg reularly. Do you think buying an air dam would noticeable affect mpg?

[piecesparts]

Originally Posted by TwoFiftyShifter

I have a question. My 96 silverado doesnt have the front valance/air dam. I run a lot of highway and check mpg reularly. Do you think buying an air dam would noticeable affect mpg?

What is accomplished by the air dam is to create a slight negative pressure under the truck for cooling and wind resistance. The answer is YES, it would help. I can tell when I have my air dam off of my 84 truck, especially at speeds above the speedo's max. My lowered 91 DD still has it's air dam and as I tear one up, I put on another, just for those reaosns.

[spinem]

this has been debated for years...

cover=none-1.5mpg increase
tailgate down= NEGATIVE 1-2mpg
no tail gate=none-1.5mpg