Note: This post was originally written for 250cc 2-Stroke Grand Prix Roadrace Engines. However, it is applicable to any highly-tuned engine (although 4-strokes are usually less finicky).

Once your bike is set up and running perfectly, your ongoing challenge is to keep it that way. A relative air density (RAD) gauge may provide the information you need to alter the jetting for different atmospheric conditions.

RAD increases with an increase in barometric pressure (e.g., going to a lower elevation) and/or a decrease in ambient temperature. Conversely, RAD decreases with a decrease in barometric pressure and/or an increase in temperature. Standard temperature and pressure (STP) is defined as a temperature of 59 degrees F with a barometric pressure of 29.92 inches of mercury. STP is one point at which 100 percent RAD exists. Although bike tuners typically deal with RADs that are less than 100 percent, a RAD of greater than 100 is possible. (Operating in a cold environment, snowmobile tuners regularly work with RAD numbers greater than 100.)

If your bike is jetted spot-on at a RAD of 100 and makes 100 horsepower, it could make 110 horsepower at a RAD of 110 with the proper jetting. Similarly, at a RAD of 90, the best you can hope for is 90 horsepower.

Making horsepower is all about providing the appropriate amount of fuel for the amount of combustion air available under the prevailing atmospheric conditions. Neglecting volumetric efficiency, each time a piston descends it pulls in the same volume of air, but the mass of that volume of air varies with temperature, barometric pressure (elevation) and humidity. Chemically speaking, it is the mass of the air (oxygen portion) that determines the mass of fuel needed for optimal combustion efficiency (horsepower production).

Your job as a tuner is to jet the bike for the air density you encounter from track to track and day to day (and sometimes even from hour to hour). Typically, tuners using a RAD gauge will produce a copious notebook (developed by trial and error) detailing the proper jetting for a given RAD.

Why trial and error? Firstly, a change in RAD does not dictate a directly proportional change in jetting. (For example, a 5-percent change in RAD does not equate to exactly a 5-percent change in jet size.) Secondly, identical RAD numbers can be achieved under vastly different atmospheric conditions. The same RAD number can be seen in a location with higher temperature and higher barometric pressure as one with lower temperature and lower barometric pressure. For example, when it is 100 degrees F at Willow Springs (elevation of 2600 feet) the RAD will be about 84.5. At Pikes Peak (elevation of 5400 feet) the same RAD would occur at a temperature of 45 degrees F. Although the RAD numbers are the same, theoretical optimal jetting would be one size different for a nominal 350 main jet. You'll see why shortly.

A simple mechanical RAD gauge can yield accurate results under the proper conditions. "Proper conditions" are low or constant humidity and, additionally, the elevation must be constant or the temperature must be constant. Although changes in barometric pressure due to weather are taken into account by a RAD gauge, their effects are usually minimal -- a much greater factor is a significant change in elevation.

Most of the time, you can achieve satisfactory results with a mechanical RAD gauge and copious notes. But, a mechanical RAD gauge is not the panacea it is made out to be, because it only gives a "bottom line" number without allowing you to see the individual contributions of the two factors -- temperature and barometric pressure -- that comprise that number.

The best way to jet a bike is to treat separately the two or three factors that influence RAD. Why two or three? Humidity sometimes enters the picture. When it is considered, we call the resulting value "corrected" RAD.

Because mechanical RAD gauges do not take humidity into consideration, many racers use spreadsheets or tables to account for it. In some parts of the country, humidity is a negligible factor. However, in large parts of the U.S., humidity is a factor. For example, at Brainerd, 90 degrees F with 90% humidity is not unheard-of. Under these conditions almost 5% of the available "air" is displaced by water vapor. This results in a corrected RAD which is 4 smaller than the uncorrected value (displayed on a mechanical RAD gauge).

Jetting must change in direct proportion to absolute temperature. A 10 percent change in absolute temperature necessitates a 10 percent change in jet size. Absolute temperature is a scale referenced to "absolute zero" rather than to the temperature of freezing water. To convert Fahrenheit into absolute temperature, add 460. (This scale is called Rankine for those of you who care. It can be use to confuse your friends, as in "Boy it's hot out today -- 555 degrees R!) For example, the difference in absolute temperate between 70 degrees F and 90 degrees F is not quite 4 percent (90+460) / (70+460).

Jetting does not change in direct proportion to barometric pressure. A 10 percent change in barometric pressure necessitates only a 7.7 percent change in jet size. This is because a change in barometric pressure also affects the pressure exerted on the fuel in the float bowl. Therefore, a change in barometric pressure will automatically alter the fuel-air ratio somewhat. How do you go about achieving an X-percent change in jetting? Fortunately, the number stamped on Mikuni hex-head jets represents a nominal flow in cc per minute at a particular test pressure. Therefore an X-percent change in jetting is just an X-percent change in jet number. For example, a 360 jet is about 3 percent richer than a 350 jet.

As a side note, round-head Mikuni jets aren't as simple to use because their numbering scheme is based on aperture size instead of flow rate. Because flow is proportional to area, it takes an extra step to convert aperture diameter into area. The formula is: area = pi x radius x radius. For example, a 107.5 jet has a nominal diameter of 1.075mm. The area of this jet is about 0.91 square mm. A 3 percent richer jet would be a 109 (which does not exist, so a 110 would have to be used).

Incidentally, the barometric pressure reported on a TV weather forecast is not the actual barometric pressure -- unless you happen to be at sea level. Reported barometric pressure is always corrected back to the pressure it would be if you were at sea level. Thus, in Denver for example, when the barometric pressure is reported to be 29.6, the actual air pressure is more like 24.6 (You can figure about 1 inch of mercury for each 1000-foot change in elevation, but the table below is exact).

Altitude, Pressure, Delta
(feet), (" Hg), (" Hg)
0, 29.92, 0.00
1000, 28.92, 1.00
2000, 27.82, 2.10
3000, 26.82, 3.10
4000, 25.82, 4.10
5000, 24.90, 5.02
6000, 23.92, 6.00
7000, 23.02, 6.9
8000, 22.22, 7.70
9000, 21.32, 8.60
10000, 20.52, 9.40

Table from __Haynes Automotive Heating and Air Conditioning__




Now that I've totally confused you... remember you can always fall back on this simple rule of thumb:

When tuning a TZ250, for every change of 3 on the RAD gauge, change the main jet by 1 size. If the RAD has increased, install a larger jet. If the RAD has decreased, install a smaller jet.

I've never read or heard a good explanation... so I can only share what
I've learned over the years. If you understand what "piston wash" is, then
it's pretty easy to know what to look for and what to look out for.

The first thing you need to realize is that the top of the piston is not
a uniform temperature. The area in front of the exhaust port that is
exposed to all of the hot gases exiting the cylinder is naturally the
hottest. The area in front of the transfer ports that is cooled by the
fresh air and fuel entering the cylinder is obviously going to be the
coolest. And, the center of the piston will be somewhere in between.

The second important thing is that there is some small range of
temperatures where the piston is hot enough to char the oil that comes in
contact with it, but cool enough not to melt the piston.

Putting those together, you have "piston wash".

When the engine is jetted very rich, most of the piston (except near the
exhaust port) is too cool to char the oil; and you will have large areas
spreading out from the transfer ports that appear to have been "washed"
clean of carbon by the air and fuel entering the cylinder... that is "piston
wash".

If you jet down leaner, the temperature of the whole piston will
increase... and more areas will be hot enough to char the oil. The "washed"
areas will be large circular areas, located just in front of the transfer
ports. At this point, the center of the piston is quite hot; and is
actually charring the oil on the underside of the dome, right in the center.

Jetting down further will cause the whole piston to get still hotter.
Now, nearly the entire piston is hot enough to char the oil... and the
"wash" will be just a small area about the size of a half of a dime, right
in front of each transfer port. The bottom side of the piston will be
charring the oil over a large area by now also. At this point, the area
near the exhaust port is getting almost hot enough to melt. This would be
considered (at least by me) to be jetted correctly... but near the "edge".

If you go leaner yet, the whole piston will be hot enough to char the
oil on top of it and there will be no "washed" areas left. The entire
piston will be covered with carbon, and the aluminum at the edge of the
piston, right in front of the exhaust port will actually be slowly melting
away and smearing onto the rings... more than a few seconds of this and it's
time for new pistons.

That, for whatever it's worth, is my own twisted look at piston wash.

Some things to keep in mind include the fact that the size of the
"washed" area depends somewhat on the upward angle of the transfer port. If
the port is angled flat across the top of the piston, there will be more
cooling and more wash... in spite of the fact that the area in front of the
exhaust port might be just as hot as it would be on a motor with upward
angled transfers that shows much less wash. The upward angled transfers
don't cool as much of the piston top.

Also, the size of the "washed" area has to be somewhat proportional to
the size of the piston. A "half a dime" sized wash area on a 600 triple
(very small piston) means the piston is a lot cooler than a "half a dime"
sized wash area on an 800 twin (with coffee cans for pistons)! What you're
really interested in is how much of the piston isn't clean, because that
tells you how much of the piston is hot enough to char the oil... and you
know the exhaust area is hotter yet.

All this really means is that you can't just say, "Every motor should
have a half-dime sized wash area." You have to correlate the size of the
wash area to other indicators and engine specs... and use it as just one of
your tools for jetting.

Did you ever wonder why it's so hard to get people & businesses to "sponsor" your race program?

Thank about it from the sponsor's point of view. If you sponsor somebody for $1000, you get to reduce your net income by $1000 for tax purposes... you don't get to take $1000 off of your tax bill.

The actual end-cost would depend on what your tax rate is. If you own an S-Corp or LLC you get taxed at your personal tax rate, so if you're total tax rate is 30%... the sponsorship still costs you $700!

Somehow, in order for you to break even on the deal, that racer needs to provide you with $400 in advertising value, $400 in useful product feedback, $400 in something!

Otherwise, why would you give up $700 in profit to save $300 in taxes... just to spite the government?

Let's say I have $1000 in profits for the year. I can:

1) pay $300 in taxes and keep $700 for myself... costs me $300.
2) pay $1000 out in sponsorships and get $300 off on my taxes... costs me $700.
I need to get $700 - $300 = $400 in "value" to break even on the sponsorship.

For large corporations that pay a flat 35% federal income tax plus say 9% in state income tax, the picture is a little different.

If they have $1000 in profits for the year. They can:

1) pay $440 in taxes and keep $560 for themselves... costs them $440.
2) Pay $1000 out in sponsorships and get $440 off on their taxes... costs them $560.
The large corporation only needs to get $560 - $440 = $120 in "value" from their sponsorship in order to break even.

Think about how much radio or newspaper advertising you could by for $400... does a sticker on the side of this guys car or bike get you at least that much exposure?

A lot of racers I know don't give back nearly enough "value" to their sponsors, because they think the sponsorship is a no-cost deal for the sponsor... but that's just not the case.

If you want someone to sponsor your race program, you need to find a way to show them that you can provide enough "value" for them to at least break even on the deal. Whether that means you get their name on TV or in the newspaper, you bring your family & friends into their restaurant or bar to spend money, you provide testing & feedback on their products... whatever you have to do to provide some "value" for the sponsor!