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 Post Posted: Sun Oct 14, 2012 7:36 pm 
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The way I understand it. Of course I will probably be wrong. When the pressure is high it allows fuel to run through the carb faster to where in good weather (DA) it will pick up ET (just right per set up.). In bad air (DA) It will slow down (too Fat no adjustments made to carb).

Pressure low bad air = just right lower ET. good air = lean pick up ET. Does that make sense?

The reason I'm asking is I saw the pressure go from 29.43, 29.34, 29.15, 29.09. The kestrel works off of station pressure so these are real time readings. What effects would this have on a carb that you don't change anything on.

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 Post Posted: Sun Oct 14, 2012 8:13 pm 
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As barometric pressure goes down "most" engines will slow down as DA usually goes UP when that pressure is low. I say "most" because if you're running LEAN to begin with then bad air could make it go faster rather than slower.


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 Post Posted: Mon Oct 15, 2012 1:01 pm 
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A carb doesn't adjust for a change in barometric pressure, it meters fuel based on air volume. When the barometer is higher (more air pressure) the air is denser but at the same airflow (volume) the carb signal is the same therfore the fuel flow from the carb is the same but now it's not enough for the denser (more mass) air which will make it leaner. Fuel:air ratios are by weight or mass.

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 Post Posted: Mon Oct 15, 2012 5:55 pm 
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#44- Summertime Blues: How to Live with Bad Air
Category: Tech Talk —

Published in National Dragster

Written by David Reher

As I write these words, we’re loading our Speedco Pontiac Grand Am into the transporter for the annual trek to Denver. Preparing to race at a mile above sea level brings home the reality of racing under adverse conditions. While we make many adjustments for Bandimere Speedway, one of the items that is definitely not on our to-do list is to change carburetor jets.

Some racers just about wear out the threads on their carburetors trying to adjust for altitude and weather conditions. The truth is that you really can’t compensate for bad air by changing jets. When it comes to carburetor-equipped racing engines, you can’t fight Mother Nature.

It is a misconception that you must lean out a carburetor at high altitude. The fact is that a properly tuned engine will use the same jets in Denver as it does at sea level.

So why do cars run so much slower in bad air than they do in good air? The obvious answer is that the engine is making less power. When an engine is tested on a dyno, a correction factor is applied to the raw numbers to adjust the observed power to standard conditions. This allows us to compare the dyno test results that are made at different times of the year and under very different conditions. But when you are running a car down a race track, the correction factor is irrelevant. The only power that is available to accelerate the car is the engine’s actual output at that particular moment in time. If the engine is producing more or less power than it would at standard conditions, that’s what you’ve got to work with.

In this age of digital everything, carburetors have an undeserved reputation as low-tech devices. In fact, a racing carburetor is a very ingenious system. A carburetor responds to differential pressure, and therefore it self-compensates for changes in barometric pressure. The gas in the float bowl is always subject to the prevailing atmospheric pressure; the jets deliver fuel in proportion to the differential between the pressure in the float bowl and the pressure in the induction system. So when the barometric pressure falls, as it does so dramatically in Denver, there is less pressure differential and therefore fuel flow is reduced accordingly.

You don’t have to go to Bandimere to experience the effects of thin air. Even if you don’t travel, the changes in your race car’s performance at your local track from February to August will be substantial. On a typical summer day with 90-degree heat, the relative altitude can easily approach 4,000 feet. The unfortunate fact is that there is very little you can do to regain the missing horsepower by tuning the engine. While you might have a zero correction factor in January, it’s common to see a correction factor approaching eight percent in the summertime – and in Denver, we see 22 percent!

The harsh truth is that you’ve got a car with less horsepower in the summer, so you must figure out how to race it. What can you work on? You can work on the car – the torque convertor or clutch, the transmission ratios, the rearend gears, the tires, and the chassis – to work around the power deficiency. That’s really what we do in Pro Stock, and that’s why you seldom see Pro racers working on engines at the track aside from routine maintenance. We simply take the power we’ve got and try to make our cars use it as efficiently as possible.

In general, drag racers tend to be more engine-oriented than racers in other forms of motorsports. Perhaps that is because we spend relatively little time on the track compared to oval-track and road racing drivers. In my infrequent visits to NASCAR events, I find that most teams regard the engine as a small variable at the track because their engines are developed and tested at the shop. They spend the majority of their track time adjusting the chassis and working on suspension setups. I’ve seen competent teams qualify their Busch Series cars faster than a Nextel Cup car – despite the fact that the Busch Series engines have about 110 horsepower less than the Cup engines!

That simply shows how important the chassis setup is in circle track racing – and points out that drag racers could benefit from spending more time on chassis adjustments and less time on carburetor jets when the weather and altitude conditions are bad. The best place to work on an engine is on a dyno; the best place to work on a car is at a race track. As a professional engine builder, that’s a difficult statement for me to make, but I think it’s the truth.

In reality, the first 1/8th mile pretty well determines a drag race car’s elapsed time. If you’re competing in “go-fast” class such as a Quick 32 or Top Sportsman eliminator, the setup that made you fast in February isn’t going to make you a winner in August. When the relative altitude has changed 3,000 or 4,000 feet, you’re not going to be successful using the same convertor and the same gear ratios that you used in winter.

My recommendation for running in bad air is to target the engine’s rpm range and then make the necessary changes that will allow the engine to achieve that range. A racing engine’s peak power and torque is fixed by the intake manifold’s runner length, the airflow capacity of the ports, the camshaft timing and other factors. The engine is going to perform at its best when it runs at the speed that it was intended to run. Therefore if you go to a high-altitude track, or if you encounter high relative altitude conditions, you have to gear the car to allow the engine to reach its optimum rpm. When you’re missing 300 rpm at the top end due to a change in weather, you must work on regaining the engine speed to maximize performance. Maybe it’s a set of shorter rear tires (if the available traction permits), or perhaps a numerically higher rearend ratio. You still won’t run as fast as you did under ideal conditions, but you will close the gap.

Most of us race to go fast. We love to see a good number on the scoreboard. When the conditions are bad, the e.t.s aren’t as satisfying to our egos, but we can use those times to work on the race car and to learn how it responds. Challenge yourself to regain as much performance as the conditions will allow. The payoff will come when the “good air” returns because your race car will be faster and you will be smarter. Gaining an understanding of how a car responds to different conditions will make you a more formidable racer.


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 Post Posted: Mon Oct 15, 2012 7:30 pm 
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Scott Smith wrote:
Quote:

It is a misconception that you must lean out a carburetor at high altitude. The fact is that a properly tuned engine will use the same jets in Denver as it does at sea level.




Thanks Scott
That is correct. I have raced from 5000' to -800' in the one day and not changed jets and had the same AFR reading on the data logger and the same readings on the sparkplugs. The car just changed power thats all.
There is no doubt most drag racers will say that as they lean the jets when the density altitude goes up the engine will make more power than if they didnt do it. The reason for that is NOT that the carby corrects its mixture the reason is that the engine at higher altitude has less air molecules to compress and thus less heat is generated so less fuel can be vaporized in the time period available. Because the burn speed is influenced by the percentage of vaporized fuel gas in order to obtain the optimum percentage it is necessary to put less fuel into the cylinder, thus allowing the lower compression heat to achieve the same vaporization prior to ignition.
If you have a kettle with a smaller wattage element it takes longer to boil the water for your coffee, have a smaller coffee and it will boil it in less time. Smaller coffee is less power in the body, see same thing.

David Reher said " a properly tuned engine" Thats the key. He could have added ' suitably designed'.

Carburetors deliver fuel based upon the pressure differential from the fuel bowl to the venturi and the metering is done by jets. Lower baro pressure results in less pressure differential thus less fuel flow, which is what you need. Correctly designed carburetion doesnt need altering.


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 Post Posted: Mon Oct 15, 2012 10:20 pm 
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I like to throw in there as some sarcastic humor. "When your climbing the mountains in your carburated daily driver do you pull over half way up the mountain and change jets?" =; =;


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 Post Posted: Mon Oct 15, 2012 11:03 pm 
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Back in the day of carbureted daily drivers the engine would run like crap near the top of Pike's Peak (14,000'). Now with auto adjusting EFI they obviously still make less power at the top, but not nearly as much difference and they still run clean and smooth. The carbureted engines run rough etc. Why?

Rick


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 Post Posted: Mon Oct 15, 2012 11:36 pm 
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Hi Rick. There are a few things to consider with that question. With stocker engines the carby's in US are not designed for that altitude change, The simple emulsion carby has to have an air horn thats restrictive to work properly over large altitude changes and those carbys went out of favor once big V8's came along. The Larew book details carbys that can do altitude change correctly but they wont feed the HP range of modern engines without many barrels, too expensive.
If your talking about racing engines then it's what I said in the above post. Mind you some engines are over reactive to air temperature and dont seem to care much about pressure. Some are reactive to water grains but the thing is most racing engines react to some change of atmospherics. They dont just loose power they react with poorer running etc. EFI is a winner in those situation because it can correct but there are carburetors that do that too. But your not racing with those ones.

It all comes down to the engine isn't balanced in the energy input methods. Energy is input to the fuel by conduction with walls, compressing the mixture, retaining exhaust gases, Stuff like that. Energy is removed from the fuel by vacuum. In racing engines we do things to eliminate hot intake manifolds, exhaust crossovers, we try to get the exhaust gas out as much as we can, and we try to compress the fuel as much as we can BUT we shorten the compression stroke distance a lot. These factors all conspire against you in the vaporization result. Without vaporization the engine wont run. It is the whole scene, the most important scene and the dominating factor in what an engine is sensitive to with atmospheric conditions.
The vaporization at ignition time is the key. Dont just think of your compression pressure at TDC, What is it at 35 degrees advance. Whats the cylinder pressure then, how much heat is in the chamber then, Seal the intake valve on the seat at 90 degrees BTDC and you only have 55 degrees of compressing before the fire is started. Not much time, so the fire at the start is weak and poor quality. That sets the condition for the rest of the burn. If the engine combo is relying on this short compression because its got no exhaust gas and its all cold in the intake etc then any change in pressure will dramatically effect the running.
Go up the mountain and you get to the top and lean the jets and it runs better because its got less fuel that has to be vaporized so that results in a better mixture at the start (It could be due to there being actually more fuel molecules present than when you were at the bottom of the hill. ) In the mountains you can run the mixture leaner and not melt the engine because it doesnt have enough O2's to generate the same heat as sea level.
Its the Oxygen oxidizing molecules that generates heat, reducing the number of O2's reduces heat so leaning it off increases the heat back to where your familiar with.
See there are lots of ways of looking at it isn't there.


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 Post Posted: Tue Oct 16, 2012 11:09 pm 
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A couple things ...
- So if a race carb compensates properly for changes in density, why does a car that is too lean slow down when the air gets better? Seems if it was too lean to start with and the mixture stayed the same as the air changed it would still improve ET/mph as the air density improves....but it will slow down. What gives here?

- I assume that the humidity effects on Density Altitude DOES require a jet change since the water in the air is now taking the place of oxygen.

Rick


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 Post Posted: Tue Oct 16, 2012 11:31 pm 
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Race carbs dont compensate for changes in density. They simply dont do that. There are carbys that can do it but you guys dont use them. The questions asked here are about barometric pressure not density. 2 different things. Baro pressure is what pushes the fuel through the jets. Air density has nothing to do with it. Air density does changes to the mixture of course.
I predominantly only look at baro pressure and water for direction of jetting changes. This density altitude stuff is just about useless to me.


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 Post Posted: Wed Oct 17, 2012 6:38 am 
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We use DA for ET prediction. And keep an eye on vapor pressure or water grains if running Alcohol.


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 Post Posted: Wed Oct 17, 2012 8:39 am 
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Here is what I get from my weather station...
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 Post Posted: Wed Oct 17, 2012 8:55 am 
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I'm a little different than you guys. Cause I run a restrictor plate in a index class and have to be at 11.00 in any conditions. If the DA is above 2000' the plate comes off. As the DA and or water grains gets higher I lean it out (Jets, squirters and MAB's) and it does pick up. I have data sheets to back it up. This has turned into a pretty good discussion. Everything still points to BP or Station pressure has some effect on ET. I'll post some numbers tonight to show you what we faced Saturday.

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 Post Posted: Wed Oct 17, 2012 10:06 am 
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Here's a calc from wallace that pretty much matches what barry seen with his weather data.

http://www.wallaceracing.com/weather-corr.php

I believe that you can have two DA's the same, but arrive at one cause of cooler temps and the other has high pressure etc, and I bet without changing jets the higher pressure one runs faster.

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 Post Posted: Wed Oct 17, 2012 11:17 am 
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shrinker wrote:
I have raced from 5000' to -800' in the one day and not changed jets and had the same AFR reading on the data logger and the same readings on the sparkplugs. The car just changed power thats all.

shrinker wrote:
Race carbs dont compensate for changes in density. They simply dont do that. There are carbys that can do it but you guys dont use them. The questions asked here are about barometric pressure not density. 2 different things. Baro pressure is what pushes the fuel through the jets. Air density has nothing to do with it. Air density does changes to the mixture of course.
I predominantly only look at baro pressure and water for direction of jetting changes. This density altitude stuff is just about useless to me.


OK, now I'm really confused. Baro pressure is the biggest factor in Air density and in one post you say you race at very large changes in elevation which means big baro pressure and therefore air density change w/o changing jets and you get the same AFR. Then you say density has nothing to do with baro pressure and you only use barometer and water grains to tell jetting direction.

:-k ](*,)

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