question about aero

[Red_Liner740]

^^ You're asking for imperical data of which is better than what when we have no idea what you drive or what your car specifically looks like.

all those modes u listed can have different impact from one car to another...

[Hubert Farnsworth]

From the Front, Same car mentioned on the left hand side of my posts as well as in the fuel log in the signature.
Although I was asking in general which is more important aerodynamically, sealing off the undercarriage, or redirecting more air around it by effectively lowering the front end with an air dam and possibly blocking the grille opening. It didn't have to be empirical data, just a rough estimate would suffice for the theory, as my current knowledge is more in tune with the powetrain side of things.

[Frank Lee]

I nominate this for one of the weirdest threads.

[donee]

Hi All,

At speed, with a quiet engine, the car will seem to be slower. As the aero vibration in the chasis is not sensable, and the noise is very low.

Down a hill, however, the car will seem very quick. That is not the same as fast, but many people confuse the two. The car will take off like a bandit, with no additional accelerator pressure.

And accellerating, in a Prius, the car will seem slower, as the engine will run slower for a given speed and/or accelleration.

[aerohead]

'Feel' is a subjective evaluation and can't be quantified.
Actual changes to aerodynamics will produce quantifiable changes in pitch,yaw,roll,lift,top speed,wind noise,and mpg.

[aerohead]

Originally Posted by Hubert Farnsworth

Again this is where the part open throttle engine curves get really messy when talking about bsfc, its easiest to represent them as contour maps or islands of efficiency, speaking of which no one has located one for the 4a-fe either, but that's not relevant to this topic, however at reduced loads the engine makes less power but may not be operating at its most efficient state, other optimizations non withstanding. However does lower power output lead to lower fuel consumption if the combustion events are still occurring as frequently as ever? I guess as a mechanical engineering student who has only just begun to get into the exciting powertrain classes this is still a bit fresh theoretically vs physical experience.

Also my car probably has the C50 5-Speed Manual Transmission
Gear ratios for this transmission.
1st 2nd 3rd 4th 5th Reverse Final
3.545 1.904 1.310 0.969 0.815 3.250 3.722
At least it was used on the equivalent corollas and some of my research indicates that it may be the same as in my car. that 0.82 overdrive is why its still spinning quicker than is desired, peak torque is closer to 2-2.5k iirc.

Hubert,I think the aero/BSFC info you need was covered in Hucho's book and earlier by Gino Sovran at GM in his SAE Paper.
Both works deal with maximizing mpg after drag reduction by gear-matching to maintain the engine on the island of highest BSFC.
As early as 1934,Chrysler's Carl Breer realized that to get the biggest bang for the streamlining buck,a car would need to have taller gears to get the 'load' back up and prevent over-revving of the engine.
The best BSFC is generally said to occur at the torque-peak rpm.If so,then,after aero mods you'd want your 'cruise' rpm to reflect that engine parameter.

[aerohead]

Originally Posted by Hubert Farnsworth

Controlling the fuel intake effectively throttles the power output of the diesel engine yes, but I was referring to the throttle plate on the intake air charge, which modern diesels typically don't require and are effectively at wot all the time on the air side ie the volumetric efficiency goes up, and the precise amount of fuel required can be injected just prior to the top of the compression stroke. This increase in volumetric efficiency coupled with the higher energy density of the diesel fuel helps lead to higher fuel efficiency/lower fuel consumption

Historically,a Diesel engines mpg advantage diminishes with speed.As an Otto cycle engine car gets into higher load situations ( faster ) the throttle is moving closer to WOT all the time.Honda 3-barrel carburetors were designed to have both the idle and the #2 barrel throttle plates at WOT at highway cruise speeds for nice volumetric efficiency,something modern EFI fails to do.
The Diesel will still have the chemical energy density advantage over gasoline.You can argue the cost/benefit ratio.
Presently,my gasoline pickup is returning better mpg than the VW Diesel Rabbit at higher speeds.

[aerohead]

Originally Posted by Hubert Farnsworth

From the Front, Same car mentioned on the left hand side of my posts as well as in the fuel log in the signature.
Although I was asking in general which is more important aerodynamically, sealing off the undercarriage, or redirecting more air around it by effectively lowering the front end with an air dam and possibly blocking the grille opening. It didn't have to be empirical data, just a rough estimate would suffice for the theory, as my current knowledge is more in tune with the powetrain side of things.

You might block the upper opening with cardboard and tape.The lower grille is at a higher pressure.See if anything shows at the pump.Watch your coolant temp. If you can make the inlet airtight,even better.
You'll notice,in late model cars,that getting air around,rather than under the car is preferable,so the airdam isn't a bad idea.