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Discussion Starter #1 (Edited)
I would like to encourage a discussion that draws on the vast knowledgebase of our forum membership and unlimitedly arrives at a decision regarding what standard baseline should be used to correct horsepower and track times. I also would like to plant the seed for creating a “sticky” of standards that all could refer to at anytime when computation was necessary which included this standard and many others.

Let me start by telling you how and why this subject is important to me. Since being privileged to watch the Apollo 11 liftoff on television as a kid I have been familiar with the concept that our atmosphere becomes less dense the higher we go. This was later augmented with the fact that our atmosphere is not a static homogonous entity when required to recalibrate my pocket altimeter routinely to keep pace with changing weather conditions while mountain climbing and hunting. Secondary and postsecondary education added to this general understanding with the exposure to formula like the Ideal Gas Law.

So when I sat down to eliminate the effects of the constantly changing atmospheric conditions on my ET times at the track I felt very comfortable in the process. I found an online calculator in short order that outlined the formula and algorithms necessary and was the product of the Society of Automotive Engineers (SAE). My confidence in its answers was complete and I used this calculator with much success in predicting the effects on my ET. Further, the SAE was used by every dynamometer manufacturer that I found. I noticed however that the barometric pressure used by SAE was lower and the temperature was higher than that used by the International Civil Aviation Organization (ICAO) in computing the density altitude, which was part and parcel to the SAE algorithm. I assumed that they were just two ways of expressing the same absolute air pressure because to maintain the same absolute pressure when increasing the temperature one must lower the barometric pressure.

Now, fast forward to this winter when I was tuning my engine with my new Predator. I had completed some major engine modifications and was anxious to find out what modifications worked and which didn’t. One of the ways that I went about this was to find similarly modified Hemi’s and compare our ETs. Because I was doing my testing in some extreme absolute atmospheric pressures it was imperative to not only correct my ETs but to correct the other ETs as well. This all went without a hitch and proved to be very helpful.

Recently I noticed a member had posted his correction along with his ET and it didn’t look right to me. My initial check with my online calculator proved me right. Upon further investigation it appeared that this member had neglected to properly adjust for pressure and appeared to just reduce by the elevation. Even more recently I was corresponding with a member from a different forum via PM and was sent a different online calculator that was a little easier to use. Well much to my surprise this particular online calculator provided me with a different answer than the one that I had been using. So more investigation ensued and as a result I start this thread.

My preliminary conclusion is that in an effort to use the NHRA elevation tables some people (and online calculators) have used a density altitude correction to come up with their relative altitude and then used the NHRA tables to reduce to sea level. At first blush this seems a fairly straightforward and logical process, especially with the assumption that they both use the same absolute air density as a baseline. With further investigation however one realizes that the baseline used for density altitude is different than the SAE baseline because the absolute air densities used for their respective baselines are different! I guess car scientists just have to be different that plane scientists!? The result of this is that if you use just the DA in figuring your correction you will be on a different baseline than if you use the SAE correction and therefore you will be comparing apples to oranges.

Because we are a car forum, the SAE are car scientists, and all dynamometer manufactures use SAE I think it only logical that we use SAE. However, I have done no historical research to date on the habits of car races and don’t know if there is a precedent set already on this.

I would appreciate any knowledge on this subject you would care to share or thoughts you may have.
 

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Very impressive. I'm a freshman Mechanical Engineer, you sound like your either a Mechanical or Aerospace Engineer. However, I do not really have any information that would help you, but everything you said made sense. Using SAE figures would make the most sense, but you never know with these online calculators. I wouldn't trust any of them unless they were on the SAE or NHRA website.
 

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Discussion Starter #3
Very impressive. I'm a freshman Mechanical Engineer, you sound like your either a Mechanical or Aerospace Engineer. However, I do not really have any information that would help you, but everything you said made sense. Using SAE figures would make the most sense, but you never know with these online calculators. I wouldn't trust any of them unless they were on the SAE or NHRA website.
Thanks for the quick feedback. With your major you are the perfect man for the job! Make this your thesis...no, that would take too long..make it your extra credit assignment for the semester.

No I am not a Mechanical, I'm not that smart, I own a Civil & Survey business. Hang tough with your studies. The math is just a threshold you must pass over to weed out the weak of heart. It gets a lot easier in that regard after school, but you will still need it so pay attention.:)
 

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already...

Bruce,

You already know how I feel. Corrected times are exactly as you described them, HYPOTHETICAL.

While we need to understand atmospheric corrections, we also need to understand they are conjecture, not facts.

I also would like you engineering types to express your views on the various forms of supercharging, turbo, mechanical, and nitrous/chemical. Please advise us how we factor these into said corrections.

Nitrous is especially interesting with its unique combination of cooling and pressurization of the manifold with no parasitic losses. I'd like to see that quantified!
 

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Discussion Starter #5
You must really mean finally! don't you?

Bruce,

You already know how I feel. Corrected times are exactly as you described them, HYPOTHETICAL. Hypothetical if you consider gravity hypothetical.:) Or a boost meter to determine additional pulse duration on FI hypothetical. It is just counting oxygen molecules.

While we need to understand atmospheric corrections, we also need to understand they are conjecture, not facts. If you measure the absolute pressure then you know how many oxygen molecules there are. If you then state that you will be correcting a certain percentage more of less of the oxygen content, you can make the a linear correction with a very high degree of certainty, can't you? This is of course with the limits of reason and the adaptability limits of the PCM to richen or lean the mixture. For the purposes of this example the engine is always running at 100% of potential.

I also would like you engineering types to express your views on the various forms of supercharging, turbo, mechanical, and nitrous/chemical. Please advise us how we factor these into said corrections.The same rules would obviously not apply as you would be artificially altering the manifold pressure. Although I think it possible to create an algorithm that could be used to predict how either an FI or nitrous engine would perform at a given absolute density. I would guess at this point that the correction would depend on the amount of boost (mechanical or chemical) and it would not be near the correction factor that the NA motor would benefit from.

Nitrous is especially interesting with its unique combination of cooling and pressurization of the manifold with no parasitic losses. I'd like to see that quantified! You have peaked my interest now as well. This will be next on my list following the resolution of which baseline to collectively use.
So Steve, which baseline is appropriate for we 1/4 milers to use, the SAE or the ICAO. (Hint, neither is not an option!:) )
 

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whew!

Wow, Bruce, you make my head hurt! In a good way of course.

My "hypothetical" comment applies to the extrapolation of quarter mile times and speeds from math. While I'll agree the math is absolute, the car, driver, and track have so many variables they are not easily quantified. If we say we are assuming all other variables are moot then we can proceed!

Thanks for opening this one. It's a personal fav of mine.

BTW, I'm all for SAE!
 

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Discussion Starter #7
Wow, Bruce, you make my head hurt! In a good way of course. My goal from now on but only if you make mine hurt back!

My "hypothetical" comment applies to the extrapolation of quarter mile times and speeds from math. aha...I see where you were coming from now. While I'll agree the math is absolute, the car, driver, and track have so many variables they are not easily quantified. If we say we are assuming all other variables are moot then we can proceed! Absolutely agreed.

Thanks for opening this one. It's a personal fav of mine. My pleasure.

BTW, I'm all for SAE! Your vote is much appreciated. That make 3 nil in favor of SAE so far.
You are a much more seasoned member of this forum that I am, can you lend a little advise. Where do you think the members that would both know the subject and be willing to participate tread water? You SRT8 guys might seem to be more inclined to dabble in this type of thinking? Do you notice that most of them frequent this section? My assumption so far is that the performance section is the appropriate spot for such a discussion but I'm not sure?

And secondly if we actually get a legitimate consensus on the subject and I would like to pursue a "Sticky" that this would be part of who should I hunt down to make it happen. This forum is somewhat a microcosm of society and our local, state and federal governments have a Department of Weights and Measurements along with corresponding societies so it seems only fitting the our forum have a similar standard of reference.

Finally, if such a "Sticky" was created what sort of standards might you want to add in addition to atmospheric correction factors? Or asked in a different way, what subjects over time have you noticed create endless debate because there is no standard to prove the validity of one or the other debaters point of view? (i.e. rotating mass, forward acceleration of a rotating mass, wind friction coefficients, rolling resistance, horsepower per mph gain curve, stopping distance per weight/tire area etc.) I would envision including all the standard conversion for weight, speed, distance and so forth as well.
 

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Bruce,
I'm a fairly well read and educated guy, and I'm having trouble understanding exactly what you propose. I agree that just adjusting down to sea level elevation assuming a standard atmosphere is not going to give someone a very accurate time and speed correction.
You probably know as well as anyone that our atmosphere is far from "ideal" with respect to expansion and contraction versus temperature because the volume cannot be held constant, and air is compressible...and our atmosphere has it's pressure ramp up amlost exponentially as you approach the surface of the earth from space.
I very much admire your desire to accurately calculate corrections and the desire for extremely accurate measurements is noble, but unless we are all using the same pocket barometer, there is enough instrument error introduced to make the whole desire for accuracy moot - with different transfer equations for different instruments, and different instruments using different means to measure quantities.
The various DA calculators are just fine, so long as we all use the same one...which is what I think you are really getting at. Be it SAE or the one that pops up when I google the subject which seems to be originally intended for aviation purposes. I usually calculate the DA to tell myself if the air is "good" or not, but don't use it for much else.
I'm sure Stevesrt8 is not out hitting the track for personal bests or times he'll boast about on the internet in the middle of July during the afternoon down in sunny Florida. Comparing adjusted times with people across the counrty seems to be more about internet arguments than anything else, if we want that to be an SAE correction I'm more than happy to use it. Please post a link to one when you get the chance.

Eric
 

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jealousy

I'm sure Stevesrt8 is not out hitting the track for personal bests or times he'll boast about on the internet in the middle of July during the afternoon down in sunny Florida. Comparing adjusted times with people across the counrty seems to be more about internet arguments than anything else, if we want that to be an SAE correction I'm more than happy to use it. Please post a link to one when you get the chance.
Eric

HA! Bit jealous are we? Yes, I agree the air is good here at times, but remember I'm all about the TRAP SPEED! I just love that you noticed!

BTW, if I'm "boasting" with my sig, then what are you doing? My time was run under ideal conditions, but bone stock and brand new. Others on this forum have done much better.

The main interest I have in this thread is comparing the D/A adjustments between N/A cars and boosted cars. I maintain that boost or nitrous "create their own" air and thus are not entitled to the same correction factor that an N/A engine is.

What do you think?
 

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Oh no - I'm boasting right there with you brother - I didn't intend that to be taken as a bad thing. My run came at an estimated DA of about 0 or sea level...pretty good conditions for the Ohio Valley.

I don't know much about boost applications, but what you are saying sounds right to me.

Eric
 

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no foul

Oh no - I'm boasting right there with you brother - I didn't intend that to be taken as a bad thing. My run came at an estimated DA of about 0 or sea level...pretty good conditions for the Ohio Valley.
I don't know much about boost applications, but what you are saying sounds right to me.
Eric
No harm, no foul. I did not take it bad at all. Just proud of my car. I'm sure we all are! I could not BELIEVE that a 4200 pound sled could move like that, and that's why it is remarkable to me. Still is. And trust me on this one, it's MUCH faster now!

Regardless of what you know about boosted motors, almost anyone would agree that when you pressurize the intake, the HP goes crazy. And I used to drive a turbo car, it behaved very differently in the mountains of Kentucky than my N/A hot rod did!

So all I'm trying to get across is that if you're going to correct the time there should be some sort of "correction" for nitrous or boosted cars since they make their own "air".

Any thoughts?
 

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Discussion Starter #12
Eric:

I was hoping that you would step forward and show your metal. Thanks for the response and I think that your summary of my goal is accurate. I want to establish a standard that all forum members can use. Should we use the SAE baseline or ICAO baseline while making our atmospheric correction?

The underline incentive for me to pursue this goal is so that everyone is just that much more accurate when comparing their engines performance from different parts of the world.

You bring up a different point as well, and that is just how accurate the correction is. And I think you have already touched on the answer which is that it is only as accurate as the measurements. The formula, the math is absolute. Whether the SAE calculator gives us a good or bad number depends on whether or not we enter good or bad data.

Weather stations are very inexpensive and I would hope that most tracts have one to establish their published data, but I do not know. They make and I own a portable weather station that is quite handy and you can hang out the window on your return trip back to the starting line that will give a true track reading. The particular one I have measures both dry and wet bulb, humidity, and absolute barometric pressure. It is quite handy. I have found this the key to performing well in bracket racing. As the day progress and weather systems pass through, or temperatures rise you can make those .01" adjustments to your ET that keeps you in the hunt. Even without the track weather station quite often there is a FAA weather station nearby that will give reliable data for your track. Undoubtedly there will be tracks that have better data than others and some that have no usable data at all.

You touched on another benefit of using corrected data while discussing engine performance and that is the effort made to only race on a cold clear days. Think of how much more fun we would have if we didn't limit our track time to only days that will give us the best times without correction. Members could be openly comparing notes throughout the summer without the cloud of slower ET times.

I am glad that you are OK with the SAE standard. I will mark you down as a yeah for purposes of conduction this Ad Hoc survey. This brings the count to 4 nil in favor of SAE, unless I misinterpreted your post. Here is the online calculator that I use. I think that you will find the discussion on this website interesting and informative in regards to atmospheric correction. Engine Tuning Calculator - dew point
 

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Discussion Starter #13
Steve, you asked for it...

I also would like you engineering types to express your views on the various forms of supercharging, turbo, mechanical, and nitrous/chemical. Please advise us how we factor these into said corrections.
Nitrous is especially interesting with its unique combination of cooling and pressurization of the manifold with no parasitic losses. I'd like to see that quantified!? The main interest I have in this thread is comparing the D/A adjustments between N/A cars and boosted cars. I maintain that boost or nitrous "create their own" air and thus are not entitled to the same correction factor that an N/A engine is. What do you think?
Steve, this turned out to be a lot simpler that I had imagined. It was more of a concept solution than any nuts and bolts, so to speak.

Atmospheric corrections for chemical and mechanical forced induction...

Turbochargers and Supercharger:
Let’s agree that superchargers and turbochargers mechanically maintain an artificially elevated level of air pressure in the intake manifold. This elevated intake manifold pressure is maintained regardless of the ambient air pressure. It follows that a 6 psi supercharger at sea level is going to provide 6 psi at 5,000 feet. It also follows that there would be no correction for altitude for a supercharged or turbocharged engine as the available air for the engines combustion cycle would be the same regardless of the altitude.

This is not to say that there is no correction necessary for varying weather conditions. Humidity affects the percentage of oxygen in air and temperature affects the mass of the air as always. It would then follow that to normalize separate mechanical forced induction engine tests one would still use the SAE formula but run both calculations with an elevation of 0.

Nitrous Oxide:
I think we can agree that the injection of N20 in liquid form at approximately 1000 psi into or immediately before the manifold does several things. It drops to manifold pressure of approximate 15 psi and evaporates causing the surrounding air to cool significantly. It also increases the oxygen concentration of the intake manifold because it is of a higher oxygen concentration than the air. As a result your intake manifold charge has a higher percentage of oxygen content because of the nitrous oxide and an increased mass because of the cooler temperature. This benefit is regardless of altitude, temperature or humidity. If it cools 70 degree air 30 degrees then it will cool 40 degree air 30 degrees as well.

Now here is where it becomes interesting. A N20 injection system commonly doses out its charge in units of horsepower and to achieve that 75 shot or 100 shot it injects a given amount of nitrous per unit of time. Therefore a 100 shot will add 100 horsepower at sea level or 5,000 feet over and above whatever horsepower the engine is making without it at the time. In other words, the amount of available oxygen in the intake manifold will be whatever the ambient amount is plus the additional effects of the N20 injection.

It now follows that to apply atmospheric correction to a nitrous injected engine one must figure the correction factor the same as a naturally aspirated engine but subtract the added horsepower from the nitrous before applying the correction factor to the horsepower.

In summary, it is my opinion following the above thought process that to apply atmospheric corrections to supercharged and turbocharged engines one must use the SAE formula with 0 elevation. To apply atmospheric corrections to nitrous engines one must use the naturally aspirated horsepower of the engine.
 
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