Edwards & Young Racing
Seattle, WA.
NHRA 6035 & G603
© 2004 - 2013
February 2004 edition - Air Density & Fuel Mixtures by Roger Young:
Preface: There is much yet to cover in engine tuning but I want to tackle this subject at this time because it is one that can help many once you have your air/fuel mixture and timing close. Also, with spring coming up you may want to make some changes or purchase some measuring equipment now before the season starts and decided to post this edition early. I've attempted to make this as simple as possible and hope some gather some new insight! - Roger
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In short, even with the proper air/fuel mixture as the air density decreases (or higher the corrected altitude reading), the internal combustion engine will produce less horsepower because of less oxygen in the air. Conversely, as air density increases (or the lower corrected altitude reading), you will produce more horsepower.
As air density increases, the engine likes more fuel - less as it decreases.
Air density & jetting: (Air density or density altitude gauge required)
First you need to establish a base-line setting. For example; one day you are running at 98.0 percent air density (the number doesn't matter), and you have your engine running at peak efficiency with your timing, air/fuel mixture & plug heat range on the numbers, record all such engine data and the performance data you collect from your time slips, and what track you are running. This will be your base-line.
General rule - 4150 & 4160 Holley 4bbls:
Conversely if the air density fell below 93.0 percent, to minimize the power lost, you would decrease all your jet sizes by one. The key figure is a 5% change from whatever your base-line air density reading was. Even with the proper air/fuel mixture, you will lose power as the air density decreases.
The reality of this general rule: Seldom does the air density swing plus or minus 5% from your base-line unless your home track is at or below sea level, and you decide to run at Denver. The carburetor via the vent tubes also compensates jetting to a certain extent itself. So, who cares??
Well, if you are talking about an aircraft with a carbureted engine, you care a lot! I won't bore you about "pressure differential" carburetors since it does not apply, but they exist. Computer-controlled vehicles automatically compensate for air density changes, but let's get back to racing your door slammer and how all this mumbo-jumbo can help you get that trophy!
If you are racing in a class where ultimate horsepower is a primary concern, you make small jet changes (or main "high speed" air bleeds) and might make a slight timing adjustment for maximum performance as the air density changes. Not only that, but as the air density increased from our base-line of 98.0 to 100, we would increase two of the four jets (stagger jetting) one jet size per carburetor, and conversely two of the four jets leaner as the air approached 96%.
NOW we finally are getting to the point of this edition, and that is how air density effects your dial-in's!
1) Keep and record all engine data, air density, temperature, & time slip performance, track & lane for each run. Make certain you keep your gauges in a shaded and open area, and off heated objects or the ground that will drastically impact their readings.
2) As you collect all this data, you will learn what your vehicle will run at a certain air density (or altitude) & temp at a given track and lane. Unless you have made changes, or have some problem in the engine, fuel system, traction problems - you will soon know what your car will run for a certain engine/chassis combination you are running that day. This information and more gathered by those who run the super categories is how they make the fine adjustments to run those 9.902's on a 9.90 index.
3) For bracket racing, a computer database or other equipment available will allow you to store and sort all this data thus allowing you to make a confident decision for your dial-in. This is why today, many run so close to their dial-in run after run. This compiled data can also forewarn of unknown engine problems before it destroys itself - if your car suddenly and unexpectedly doesn't run the numbers, you best start looking for reasons why.
4) For all racing, a properly sealed air dam & functional hood scoop will improve air density seen at the carburetor by lowering the temperature of the incoming air (less engine heat) and some increased pressure going down the track - combined they increase air density!
5) Always re-check your plug readings for proper air/fuel mixture when adding a scoop and air dam. Also be advised they have a down side - such as small 'objects' being ingested by your engine while waiting for the car in front of you that's doing their burnout. An air cleaner mounted above the air dam inside the scoop will eliminate such foreign objects, but negates most of the pressure gained from using a scoop.
6) Pray you never drop your air density gauge - all that data you've been collecting is virtually useless. No two gauges read the same!
7) Another major advantage of keeping such a database is that when you do make a change say over the week, and the air density is different the next time at the track, you will be able to make a valid calculation if those changes were a real improvement or a step back - that the air density change is not the reason for the increase or decrease in performance.
In Review:
If air density improves and your mph drops, you are most likely too lean. (Either the main jets are too small, or you have a fuel delivery problem).
High humidity robs horsepower, as the moisture displaces oxygen in the air.
Recording data will help you predict your next dial-in.
If you have screw-in air bleeds, rather than changing main jet sizes, adjust your fuel curve with the main "high speed" air bleeds to compensate for atmospheric changes from your baseline. Air bleeds are a finer adjustment!
"Our Guide to Weather Stations and Predicting" written by Peter Biondo
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