what does DA: 5016ft mean?

[mtiberio]

Its on my timing slip from last weekend

[Dynoroom]

That is the "Density Altitude" of the on site SCTA weather station. It is there to assist you in future tuning.

[mtiberio]

I thought it had some relation to altitude, or air density, just couldn't figure it out from the value and units...

So when this goes up, the air density is down?

Better to know before the run, wouldn't you say?

m

[Peter Jack]

Not necessarily. If you know the density altitude that the engine was tuned for and you know the actual when you run, you can then tune with that information for future runs.

Pete

[Milwaukee Midget]

It will vary startlingly during the day.  I've got runs logging anywhere from 5800 to 7200 feet.

Better to know before the run, wouldn't you say?

If you're able to make quick adjustments in line, post a team member over by the timing shack and just ask what the DA is.  We changed out jets on the Weber 3 times in one day in 2013.

If you're running EFI, for most systems, it's automatic, and not a big concern.

But it's a very useful tool with carbs, if you're looking for that last 1/10th.

[mtiberio]

... post a team member over by the timing shack and just ask what the DA is.  We changed out jets on the Weber 3 times in one day...

good idea on getting the info from the shack before the run. so a 10% increase in altitude translate into a 10% smaller jet?

m

[jimmy six]

It means your going run shitty without a blower or fuel....

[RansomT]

It means your going run shitty without a blower or fuel....

But you will push through the air easier....

[GH]

I'm with JD, stick a blower or turbo on it.

[Stainless1]

While that seems like a poor DA for EM, it would be welcomed at Bonneville.

good idea on getting the info from the shack before the run. so a 10% increase in altitude translate into a 10% smaller jet?

It would be nice if it really worked that way, but I don't remember that ever working for us back in the carb days.  The way to do it is data.... lots of data written on charts.  I remember one day we changed jets 3 times in line at the salt. 
If you want a better DA, run earlier in the day 

[SPARKY]

"While that seems like a poor DA for EM, it would be welcomed at Bonneville"



Hear Yee! Hear Yee! Francis and I are now working on our air intake system to try to get rid of about 1000'of those DA---we are hopeful  that we are not the DAs .

[Rex Schimmer]

Stainless said: "If you want a better DA, run earlier in the day" and it is true that the cooler air will be more dense and you motor "should" make more power but the down side of the denser air is that it also will increases aero drag which many times can be greater than the hoped for horse power increase. We have all watched record runs in the early morning and the car will actually go slower than its qualifying run, could it be the denser air aero drag increase? Probably need some input from Woody here.

Rex

[Dynoroom]

To add another twist to what Rex is saying, as the day warms, the water table rises and the salt can become even more damp creating drag on the tires.
This of course depends on how wet the season has been before an event.

[jacksoni]

Stainless said: "If you want a better DA, run earlier in the day" and it is true that the cooler air will be more dense and you motor "should" make more power but the down side of the denser air is that it also will increases aero drag which many times can be greater than the hoped for horse power increase. We have all watched record runs in the early morning and the car will actually go slower than its qualifying run, could it be the denser air aero drag increase? Probably need some input from Woody here.

Rex

I have seen this slower in the morning phenomenon many times and have considered Rex's explanation as the likely answer but Dynoroom's comment above also makes sense and adds to the puzzle- but would make things faster in the morning. 

[Milwaukee Midget]

I know my little brick ALWAYS ran faster in the thinner air of the afternoon, and that was normally aspirated.