Air density and jetting

 For the first time karter, and even for those that have been partaking in the sport for some time, carburettor jetting for optimum performance can sometimes appear to be a bit of a black art or even a hit and miss affair.  This chapter is intended to impart a better understanding to those who think they will never really understand what’s going on because they consider themselves too technically challenged.

Basics of air density

By now we all know that for combustion to take place you must have oxygen, and for maximum power you need to have the correct air/fuel ratio.  Air density affects the performance of a kart’s engine (any internal combustion engine actually) because it alters the air/fuel mixture going into the engine, and that directly translates into how much power gets to the rear wheels.  Air density is a combination of three major factors viz. air temperature, air pressure, and humidity i.e. how much water vapour is present in the air.  So, we’ll have a look at them in isolation to see how they affect things.

Let’s look at temperature first.  Higher temperatures reduce the air density and the amount of available oxygen, so it’s fairly obvious that hotter days require smaller jetting.

Air pressure.  Lower air pressure such as at higher altitudes, reduces the air density and the available oxygen, so a lower pressure requires smaller jetting.

Humidity.  Moist air is actually less dense than drier air, which may seem contrary to popular belief.  Let’s explain - a given volume of air will contain a certain number of molecules and it just happens that water molecules are relatively light compared to nitrogen or oxygen (the main constituents of air).  As a result, the more moisture present in the air the lower the density, so a higher humidity requires smaller jetting.

Calculating air density

OK, so we see from the above that these three major factors combine with each other to arrive at the number for air density.  One can then calculate the actual density using the formula for the Ideal Gas Law, but that takes a bit of doing with a scientific calculator, and it’s what’s commonly known as ‘pita’, or for the uninitiated, a pain in the arse.  In short, don’t even attempt the calculation!!

Relative air density

To allow us to get easy to use tuning information from the air density data, it is better to compare our value to a standard set of conditions.  This is based on the international standard of 0m altitude, 15ºC temperature, 1 013,25mb air pressure, and 0% relative humidity.  At any point in time our value at the track can be compared to this standard and the result is expressed as a percentage known as relative air density, or RAD for short.

Jetting based on RAD

Why is knowing the RAD important, you ask.  Well, firstly recall that we want at an optimum air/fuel ratio to achieve best performance.  So, armed with your trusty RAD meter and the Mychron/Unipro on the kart that gives you lap times of your driver, you’ve done some testing and experimentation with jetting.  You have established that under conditions when the RAD reading was “X%”, the best times were achieved when you had a jet size of “Y”, so let’s call these values your baseline.

The numbers you see on the carburettor main jet can be one of many things viz. based on diameter, based on the cross sectional area of the hole, based on flow, etc.  We are very fortunate in that for the jets we use in karting the number is near enough the diameter in millimetres e.g. a 126 jet would be ~1,26mm in diameter, a 75 jet would be ~0,75mm, etc.  A change in the RAD requires a proportionate change in jetting so that we keep the air/fuel ratio the same.  Knowing that our jet numbers are based on the diameter, then we need to calculate the orifice area and use this to approximate the practical flow change.  The following formula is simplified from the fact that area =π/4 x diameter² so we can neglect the π/4 because that is a constant value.

New jet diameter = √(Old jet diameter² x required flow factor)

In this equation, the flow factor could have been for example +4%= 1,04 or even a negative value such as -7% = 0,93 etc.

Let’s look at a practical example.  Your baseline numbers tell you a 60 jet works well when the RAD was 94%.  Today, the RAD reading is 86% so the change required is as follows:

Change = (86 - 94)/94 = -8/94 = -0,085.  This means a change of -8,5%, or a flow factor of (1 – 0,085) = 0,915.  The minus simply means the air is less dense so we need less fuel and a smaller jet is required.

OK, so we put this into the new jet diameter equation and that gives us the following:

New jet diameter = √(0.6² x 0,915) = √0,329 = 0,574 which equates to a jet that is 0,574mm diameter, and the closest you will have to that is a 57 jet.

To make life easier for you, because there is always something that needs to be done in the pits before the next session, you can develop an Excel spreadsheet that can do most the drudge work for you, or you can even find some halfway decent ones on the web.  Speak very nicely to me, and I’ll let you have one for free.

RAD meter

So the obvious question is, where does one find a RAD meter.  There are a number of overseas suppliers and Kestrel make a very good inexpensive handheld unit shown on the RHS.  As far as I’m aware there isn’t a local representative, so if you intend getting one you will need to do so online via the internet.

Jets

A word of advice.  The hole diameter is not the be-all and end-all when it comes to jet sizing, so don’t get carried away with attempting to measure that.  That’s because the shape of the cone leading to the parallel section, the diameter of the parallel section, the length of the parallel section, the shape of the exit, and a number of other factors, all play a part in how much fuel flows through the jet.

Jets from the more reliable manufacturers are flow tested before they are stamped, so as an example, a jet that is stamped 96 may well measure as little as 0,94mm or as much as 0,98mm, but it delivers the same flow rate as the manufacturer’s 96 ‘reference’ jet.  If you can locate them and can afford them, then rather purchase jets from these suppliers in preference to those from some cheap knock-off shop on the corner.

Fine tuning

Great stuff, so now you think you are on the road to the silverware.  Wrong!!  All this stuff requires a bit of fine tuning because although you thought you had it all correct, your lad was a bit off on the day you established your baseline numbers.  Then, once you have a better baseline established, something else like the kart setup changes.  Maybe the toe-in changed because there’s a bent tie rod you didn’t spot, or there is too much grip and you’re bogging in the corners, or the engine needs a new ring, or whatever it might be, and you’re blaming the jets instead.  Well buddy, welcome to kart racing where winning is never quite so easy.

Emile McGregor - MSA Technical Consultant