Don Terrill’s Speed-Talk

Maybe someone can help me figure this out.

Say I'm here in Tejas and will be going to Bonneville.

Here in Tejas were at 300 feet elevation.
Particulars are: 29.515 In-Hg = 1000 Millibar.


Bonneville is at 4219 feet elevation.
Particulars are: 25.78 in-Hg = 873 Millibar.

Correction factor for Bonneville
air temp and rel humidity for altitude
at 90 degrees F, 40 percent humidity = 1.085

So 873 * 1.085 = 947.2 Millibar

Temperature only correction factor
at 90 degrees F, 947.2 Millibar
(per deg F, add algerbraically)
add -.003 per degree for 15 degrees.
add a neg number is subraction:
-.003 * 15 degrees F = -.045

So we end up with 902.2 Millbar.

So how do I use this information for a standard holley type carb?
If I going from 1000 Millibar to 902 (say 900) Millibar,

Will my N/A car run 10 percent richer at altitude than adjusted here in flatlander country?

If so, how many main jet sizes will I need to change?
Is my logic correct?

Is there a correlation between jet size and Millibar?
I've been lookiing but can't find it.

Cheers,

Spike

REF: Pressure Barometer, Millibars, ML - 102 - D, U. S. Army Signal Corps, 17 - DAY - 44, FORM No.1766.
Manufactured by Wallace & Tiernan Procucts, Inc. Belleville, New Jersey, U.S.A.

I don't have a definitive answer, however a carb will compensate for changes in airflow amount, but not for air density. I might expect a jet change would be in order to the tune of a jet size per 2000-3000 ft, but then it really depends on the difference in the air density. You can have over 2000ft DA at sea level, and likely less than what the actual altitude in some cases in places like Bonneville. I have also seen more than -400ft DA in Florida.

I have a question, if someone would be so kind as to answer it.

I keep seeing this abbreviation "DA" in terms of altitude, and I would like to know what that means.
I just have had no exposure to that abbreviation before.

Thanks in advance.

DA=Density Altitude

Thank you!

Mark,

So you might change one jet size per 2000 - 3000 feet?
Ballpark.

TWL

DA = Density Altitude. I was wondering the same thing so don't feel alone.

It seems like everywhere in the world people assume we know the acronyns
or initialisms automatically. It is always handy to identify them at first use.
Doing so takes a little more time, but will save everyone more time in the
long run. For example, when writing about Density Altitude (DA) after this
first use we'd just abbreviate it DA and all would know.

Once I new what DA was I could then search for an article;
I'll burn through it when I get the time.

http://www.hotrod.com/techarticles/engi ... ewall.html

I didn't have to wade through links for District Attorney and others.

Examples:
AWACS (Airborn Warning And Control System) is similar.
But glad we know that EFI is Electronic Fuel Injection and
MTBF which is Mean Time Between Failure. The last two are
pretty important to racers and auto entheusiasts.

Also for us old timer's we know that NACA is the forerunner to NASA;
that is, NACA - National Advisory Committee for Aeronautics and
NASA - National Aeronautics and Space Administration.

Here is a link that has the same problem just described.
When looking for a link to a NACA duct, NASA is not defined
in the same article.

http://en.wikipedia.org/wiki/NACA_duct
Here is a link I found that explains it from Hot Rod,

"Density Altitude" is an abstract used to rate the present density of the atmosphere, RIGHT NOW. It is derived from all of Temperature, Humidity AND Barometric Pressure, (Altitude). There is also actual grains of moisture in the air to consider.

Just knowing the Altitude means almost nothing when there is a 50% humidity difference and 70f degrees temperature difference, also.

From near sea level to Bonneville usually wants four jet sizes leaner. his will get you real close I know for a fact. Your actual horsepower will have something to do with the jet change required. That is what I saw on a 990 Hp small BB chevy seeralyears ago.


JOE SHERMAN RACING

For Jetting Calculator

Try this calculator:

Jetting Calc

We see around 7000-8000' density altitudes at Bonneville. For EFI, I see it changing between 11-14% from my near-sea-level tunes in Fort Worth. Not sure how that translates to carb tuning but hope it helps. We are the #97 car this year, drop by if you have time to chat! ('69 charger daytona A/BGALT)

holley recommends 1 jet size for every 2,000 ft elev,but i guess ya gotta check wat the air is when ur at ur track..for instance iam at sea level now er 500 ft wen i go to race this summer in denver at bandimeres drag strip 6,000 ft actual elev,i will jet down 4 because theactual air at track reads 9,000 ft always,i will have to check with the rich racer folks with the fancy air meters to c







keep the shiney sideup
an dont let ur meatloaf
.... ......

Thanks ya'll.

The calc works pretty well.

So based on my calculations the old fashioned way
they cross checked pretty closely I think.

Dallas
29.35 in-Hg
80 degree F
50 rel humidity

Bonneville
25.78 in-Hg
90 degree F
40 rel humidity

I went from DA in Texas 2,537 ft
to the DA at Bonneville 7,580 ft

Does that sound about right?
My jets would change as follows:

Jet Size Pri/Sec
from 76(.084)/84(.099)
to 74(.081)/83(.095)

Using the published specs from Holley Carbs.
Is it worth cheating on size a bit and leaning out a bit more?

Cheers,

Spike

Check with others at Bonneville about what the DA is that day
you do not want to go lean on the long white dyno
ease up to it, read the plugs,
are you gas or alky?
early AM will be different than late PM
as the old timers, everyone will be very helpful

This can clear up all the mysteries.
http://www.koehlerinjection.com/racing- ... -book.html
Bad news: I ran out again.
Good news: More on the way. Perhaps call to reserve a copy?

Okay,

Well plans changed. Will have to wait for next year.

Besides all the automotive stuff that didn't go according to plan.

I did end up with something pretty special.

See sig for details. : )

Cheers,

Spike

Once I can remember how to change it again.