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 Post Posted: Wed Feb 29, 2012 10:04 pm 
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Hello All,

What is everyone's take on this, I have always been taught that the changes in atmospheric pressure will adjust the carb as needed. The way I understand it is when you go to a higher altitude, there is less atmospheric pressure on the fuel in the bowls, thus less pressure pushing fuel through the jets, which would lean the mixture. Reading a thread on speedtalk, some of these guys are saying they see a decent gain with changing the jets 4-8 sizes.

Most of my on track tuning has taken place at the lower altitudes and I haven't messed with it too much at the higher altitudes. I plan on doing some test n tuning this weekend to see what happens.

So what is everyone s experience with this?

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 Post Posted: Wed Feb 29, 2012 10:48 pm 
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Just answer this question. When you go from low to high altitude does your data logger read a change in AFR?.


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 Post Posted: Thu Mar 01, 2012 4:52 am 
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It does show richer, guess I need to do some tuning.

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 Post Posted: Thu Mar 01, 2012 7:46 am 
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Don't forget to recalibrate your o2 for the higher elevation.


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 Post Posted: Thu Mar 01, 2012 8:34 am 
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Scott, Explain why the 02 needs to be recalibrated...

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 Post Posted: Thu Mar 01, 2012 9:15 am 
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http://www.adlers.com.au/oxygen.php

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 Post Posted: Thu Mar 01, 2012 1:02 pm 
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I understand there is less oxygen, but wouldn't the change in barometric pressure change head pressure on the jet and reduce flow? Maybe its not linear, but I would think there would be some compensation there.

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 Post Posted: Thu Mar 01, 2012 2:20 pm 
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The sensors need to be calibrated at altitude. It is recommended from Klaus (the Innovate designer) to recalibrate for 10,000 ft change. You can do it for 5,000 ft if you want, it wont make a reading error that you could detect though. The sensor operates on pumping O2 and the pump is an electronic process. The energy needed to pump the O2 is affected by the partial pressure of O2 in the air. So when you change the partial pressure by changing altitude, you alter the necessary electrical energy needed to pump the O2 in or out of the cell. The controller reports AFR based upon electrical energy thus if you dont recalibrate it the controller reports the wrong AFR.

When an emulsion carby operates at higher altitude the relationship of air pressure to viscosity changes. The viscosity of the fuel doesnt change with altitude but the pressure forcing the air into the emulsion system does so the balance of emulsion alters. The viscosity is important in all jetting calculations. The main jet flows according to pressure head and viscosity.
Air decreases viscosity with altitude increase, the viscosity of the air determines the pressure gradient across any venturi and because the booster is sensing the pressure gradient at the center of the main venturi the booster signal is different with altitude change. The air flow through the booster may be turbulent at low altitude and less so or laminar at high altitude. It depends upon the size of the main venturi compared to the booster venturi.
At higher altitude the booster signal ratio to the total flow is reduced. You may be thinking that requires a larger main jet but you have to remember that there is also less air traveling around the outside of the booster that isnt sensed. We are looking at a ratio here, not a total flow CFM. Because the total mass flow is less at altitude there needs to be less mass flow through the jetting systems. The easy way to achieve that is to make the main jet smaller. General rule of thumb for a holley is 5000ft equals one main jet size leaner.


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 Post Posted: Fri Mar 02, 2012 12:08 am 
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I ran some numbers on my Density program by Patrick Hale (RSA). Density Altitude is a good indicator for jet changes. Here are a couple different DA examples and the recommended jet based on an optimum jet on the baseline. The only change I made was the track elevation. Note that a change in track elevation will cause the DA to change more than the elev change.

Looking at these a change in DA of 5000' needs 4.2 jets smaller from an 80 jet according to this program. I have not changed this much. With my Camaro and the 1050 I ran 94's in the summer or high DA >2500' and I ran 95's when DA <2500'. Most all racing between 1000' and 4000'.

baseline
Track Elev: 1000'
Baro Press: 29.92 (corr)
Temp: 70
RH 50%
Jets: 80
Density Altitude: 2314

Track Elev: 3000'
Same Temp, RH, Corr BP
Best Jets: 77.9
Density Altitude: 4791 (Increase of 2477'DA)

Track Elev: 5043'
Same Temp, RH, Corr BP
Best Jets: 75.8
Density Altitude: 7314 (Increase of 5000'DA)

Your mileage may vary ...
Rick


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 Post Posted: Fri Mar 02, 2012 4:21 am 
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If you use a WB you will generally find that the AFR doesnt change anywhere near as much as the programs say. When you go off of density altitude its supposedly the same as racing in that altitude, but its isnt. All these calculators dont know anything about combustion and they dont know anything about how the energy balance is occurring in your engine. I see engines that go faster when the density altitude goes up due to heat of the air not a reduction of pressure. I see engines that react in line with the "rules" and go faster when its lower density altitude due to pressure. I see engines that go faster when the humidity reaches 65% and drop off either side of that. I see lots of things. Everybody has to learn their own thing.


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