Altitude adjustments for AFR?

[John Wallace]

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

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

You didn't know this was happening?





Congrats.

[USMC_Spike]

John,

Yeah but was kinda earlier than planned.

Wondering about the classes in the big white dyno.

I need to go back and start measuring the frontal area
of my 69 Chevelle. For a production class, or a modified class
using gas, there are some pretty good records. I'd need some
pretty big HP numbers just to even get close...

...that and a pretty low numerical ratio. Any one know what
you need? No sense maxing out at 160 MPH - 200 MPH on
a big block.

It will take some pretty big numbers....

I'll need to also calc some of the other numbers...
I'm trying to find how Smokey tightened up his Chevy's
front to make less drag.

Cheers,

Spike

[Dave Koehler]

Congrats, now and only now will you understand the word "budget" but it will be fun most of the time.

[Strange Magic]

1 jet number per every 2000 to 3000 feet of movement.

[USMC_Spike]

I think so Dave. : )

Magic, is that actual elevation change or the
DA change?

Cheers,

Spike

[Strange Magic]

DA change.

[USMC_Spike]

Okay,

Back on again at least for now. Got the date screw up by several months.

Hopefully, I'll mkae it to Bonneville.

Cheers,

Spike

[dieselgeek]

Spike, sent you a reply. Hit me back via PM if you have further questions. IMO frontal area takes a backseat to yaw stability when it comes to Aero. You're gonna need a rulebook, $10 isn't too bad.

In A/Pro you'd need to beat our 237ish mph. It's doable on Chevy power I am sure! Classic production is a few mph slower.
-Scott

[David Redszus]

Given the following two sets of condtions:

Dallas
29.35 in-Hg
80 degree F
50 rel humidity
H2O = 1.634%
Air density = .07101 lbs/cuft
Viscosity = 0.01645

Bonneville
25.78 in-Hg
90 degree F
40 rel humidity
H2O = 1.618%
Air density = .06125
Viscosity = .01926

Going from Dallas to Bonneville has caused the following change:
air density = -13.75%
fuel req = -13.76%
power = -15.19%
The engine will require 13.76% less fuel and will make 15.19% less power. If a jet size change is equal to 5% fuel, then you would need to go 2 1/2 -3 jet sizes smaller. Or you could simply remap the fuel injection map.

The change in water vapor is neglible and can be ignored. While the density decreased substantially, the air viscosity actually increased by 17%.

[USMC_Spike]

Diesel- PM you back. thanks.

David,

okay thanks for the info. Can you clarify for me.
When we aare talking Jet Size, that is the absolute size
or the hole size right?

Jet size and jet numbers are different, but they
track each other as I understand it.

Ah the joys of tuning.
The old fashioned way, by jet size
or the new way by remapping.

Cheers,

Spike

[David Redszus]

Diesel- PM you back. thanks.

David,

okay thanks for the info. Can you clarify for me.
When we aare talking Jet Size, that is the absolute size
or the hole size right?

Jet size and jet numbers are different, but they
track each other as I understand it.

Ah the joys of tuning.
The old fashioned way, by jet size
or the new way by remapping.

Cheers,

Spike

For a given pressure drop, fuel mass flow is determined by orifice area, flow coefficient, fuel density and fuel viscosity.
While it is possible to construct a fixture that could determine fuel flow rate, it is much easier to ask the carb manufacturer whether the jets are indicted by area, flow rate, or some other metric.

When you get to Bonneville, please remember that you will be required to run their fuel which will have its own density and viscosity specs as well as different stoich values. Now a lambda sensor becomes very useful.

[Dan Timberlake]

http://www.watcon.com/Catalog_Pages/Air ... _Gauge.htm

[David Redszus]

When attempting to adjust fuel mixture to atmospheric conditions, neither weather stations nor density gauges have much real value and can be very inaccurate.

This is because the air density (BAR, T, RH) that matters is found under the hood in the induction tract which can be much different for a moving car than for a dyno or stationary weather measurement device.

MAP and MAT sensors are much more accurate and reliable.