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Discussion Starter #1
1. With a carburetted engine, does the mixture get leaner or richer at higher altitude?

2. With a fuel injected (FI) engine is there an effect of altitude on mixture? Why or why not?

Bonus question ! What are the Bernoulli and Venturi principles and how do they apply to carburetion?

:-D

Al
 

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Hey Al

1. carburated as you go up in altitude your mixture goes richer. The air becomes thinner (why people can't breathe when the go from florida to Colorado for a ski trip) therefore it takes less fuel to atomize. I recently moved from Savannah GA roughly 9ft elevation at my apartment, I run tripple webers and had it tuned right on the edge of being lean. I drove it up to Asheville, NC were I moved (roughly 2800 ft elevation) and the thing would hardly idle, started blowing a little black smoke basically loading up. Turned each one of my mixture screws in 1/8 turn and it smoothed back out.

2. FI - the effect is the same, however thats what the O2 sensor (as well as others) and the computer is for - to maintain the optimum mixture. So if everything is working correctly then you as the driver should not notice any of this happening.

3. I can't remeber the difference but it seems like the venturi principle relates to how carbs meter the mixture based on vacumn and the Bernoulli deals more with Mass air flow - But you probably got me here cause I really can't remember.

I haven't taken a pop quiz in a while!

Matt
 

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FI automatically compensates for any changes in air density and/or elevation by the use of the mass air sensor (little box between air cleaner and throttle body
this actually measures the amount of air particles passing through it at any moment.

The bernoulli principle states that air moving faster over a surface on one side than another will create a lower pressure than on the side where the air is moving slower(aka aircraft wings. low pressure created on the top side for lift)

a venturi is nothing but a circular wing with the lower pressure side in the center. air moves faster in the center and creates a lower pressure ( in carbs it is used to draw the fuel out of the float bowl)
 

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Discussion Starter #4
Good, Good, but...

no gold stars today! :-(

1. Yes, richer at higher altitudes. SU dudes screw your jets up to reduce excess richness at higher altitudes. Why? Because a carb is basically a Venturi flowmeter which measures volumeteric flow, not mass flow. So at a higher altitude given the same volumetric air flow, there will be less air mass and the fuel ratio will get richer. Airplane carburettors must have altitude compensators to keep the mixture within acceptable limits, but auto carbs get by without.

2. Holowrench gets the nod on this one. FI systems use an air MASS flowmeter so that the ECU can proportion fuel to air mass for the desired ratio. Matt's response is good but not quite right. The O2 sensor does provide feedback to the ECU for it to adjust fuel for proper ratio, this will compensate for drift in the AFM and fine-tune the fuel feed, but Holowrench has the better answer.

3. Bonus. Bernoulli is really the main man here. He found the basic principle of flow in a pipe with varying internal diameter and elevation. This shows that for a given flow in a pipe, at the larger diameter section pressure will be higher and velocity lower while at the smaller diameter section pressure will be lower and velocity higher. The Bernoulli principle that can be extended to understand lift on an airplane wing. This also explains why you suck more exhaust fumes into the Z cab when you roll the window down! Hats off to Mr. Daniel Bernoulli (1700-1782)!

Venturi applied the Bernoulli principle to a specific device, the Venturi meter, which is fundamental to all carburetors. Most carbs have fixed Venturi sections while SUs, Hitachis, and Strombergs use a "variable choke" but the idea is the same: the higher velocity air in the reduced section will suck fuel up out of the float chamber via the main jet.

Later all!
Al :-D
 

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Not so fast...

If we are speaking strictly of our Z cars then I would agree with the FI, mass air flow meter replies.
However, plenty of FI cars do not use air flow meters. These systems (speed-density), use a BARO (barometric pressure sensor) to read the changes in altitude and barometric pressures. They also use a MAP (manifold absolute pressure) sensor to read the absolute pressure in the intake manifold. The basic calculation is BARO - MAP x RPM = air flow. Engineers realized at some point that the BARO sensor was not needed because when the ignition is first turned on (during starting) the pressure in the intake manifold = barometric pressure (no vacuum). This poses problems when driving the vehicle through altitude changes without ever stopping and restarting the car. The computer does however take a new BARO reading if the car goes to WOT (wide open throttle). But how many people ever do that?

Just some useless info begging to get out of my head
 

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Re: Good, Good, but...

Al
aircraft carbs have the mixture controlled by the pilot
also aircraft carbs come in both standard and pressure carb varieties
then there is FI for aircraft
in all the pilot controls the mixture setting and watches the EGT and CHT guages (if not he gets to pay me)
 

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Cool, but the MAF isn't all that important

It may be on certain cars but I know for a fact I can drive my Mustang without the MAF even being hooked up or installed goes to a closed loop system and the A/F ratio is determined by manifold pressure and the O2 sensors. The car actually puts down more power like this, mainly due to a restrictive MAF.

I'm not disagreeing, its just another example.

I like learning something new everyday!

Matt
 

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Discussion Starter #10
Helowrench, Do You Copy?

Interesting, interesting. I recall seeing an altitude compensator device in some antique aircraft engine manual. Is this the same concept as a "pressure carb" you mention? Something new every day!

Note that a venturi flowmeter is a pipe section where the diameter funnels down to the throat and then opens up again usually at a smaller angle. So the Venturi in a carb is simply the reduced diameter or area section. The circular (donut shaped) wing in many carbs is, you could say, a Venturi in a Venturi.

BTW, my longstanding fantasy is to get hooked up with an old taildragger with a P&W Wasp or Wasp Jr radial. These are engineering marvels! I can always dream!

Later,
Al
 
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