OK, for the goal-oriented readers, here is the calculator image, the real McCoy is at the end of the post in XL format. You get to change the yellow variables, i.e., the Scalar and the IAT1 temps. The scalar is dimensionless, and the IAT1 temps are in Fahrenheit. You choose a scalar to adjust the boost figure to what you want and then read the pre-IC, post-compressor IAT2 temps directly.
To provide the IC design engineer with the necessary data to properly size your IC, he will also need to know the mass of air being cooled, along with the IAT2 post compressor, pre-IC inlet air temperature, and the target outlet air temperature. The first number is the bottom right black box above, and the second number is what you
want for a cooled charge temp.
The air mass is relatively easy to calculate if you use Black2003Cobra’s Volumetric and Mass-Air-Flow Calculator
, which you have used previously. There is an additional benefit you can pull out of the calculator, and that is estimated horsepower.
It turns out it takes ~10 lbs of air per minute or ~600 lbs of air per hour to make 100 hp. If we go to the Mass Air Flow Rate Section
of the calculator and step the boost from 22 psi to 30 psi at the top of the first table, it will update the boost figures in the bottom table. They will look like this:
I believe you indicated you wanted a 6500 rpm or possibly a max 7000 rpm operating range for your engine, Kevin. With an air consumption of 10 lbs of air per minute for 100 hp, and your engine processing 89.4 lbs per minute at 6500 rpm and 22 psi, your FWHP will be ~ 890 hp. When you step the boost up to 30 psi, your FWHP will increase to ~1090 hp. Because we think in terms of RWHP and the commonly used factor for drive line loss is 15%, the 1090 number, in this example, would approximate ~925 hp at the tire.
There are caveats! The obvious one is being properly tuned. The not-so-obvious one has to do with IAT2 temps and the need to pull timing to avoid detonation at elevated IAT2 Temps.
Let's dig into the compressed charge air temps a bit. In the attached calculator, I have used dimensionality constants to allow us to input the air temp and pressure in the more common Imperial System Units that we use daily and read the results in the same familiar Imperial System Units.
To get the temperature of the compressed charge in the intake, we need to use a restated version of The Combined Gas Law
, which (I am sure) everyone remembers from High School Physics and Chemistry as;
With a little algebraic sleight of hand and a few assumptions to solve for T2
, this can be restated as;
is temperature measured in degrees Kelvin (˚ K), p
is pressure measured in Newtons per square meter (N/m2), and V
is volume measured in cubic meters (m3). While useful in the SI
system for engineering, mere mortals like us tend to use a more familiar Imperial System. The Imperial System represents T
, in degrees Fahrenheit (˚ F), with p
in pounds per square inch (psi) and V
volume in an equally proletariat cubic feet (ft3) or, for us automotive types, cubic inches (in3). I have hidden metrics like the ratio of the molar heat capacities of the gas being observed/measured. ‘Stuff’ like that does not need to be visible here.
Soooo … this calculator is constructed to use our commonly employed temperature, pressure, and volume units rather than the SI units more common in the engineering space. It is a good approximation, but an approximation because we are attempting to measure what is a quasi-static adiabatic compression, which is the result of a compressor with an unknown adiabatic compression efficiency that is less than 100%
Some accommodations become necessary for simplification and calculator development. When you use the calculator, you change manifold pressure by changing the scalar value. I have provided a table of scalars in the calculator to help you find the scalar necessary to specify your boost level and discharge temp. While not exact, the calculator is more than adequate to help the IC engineer calculate IC parameters to cool the charge. It is also a good barometer for determining when too much is no longer smart.
A couple of closing thoughts. While we can continue to increase boost, almost
without limit the interesting dynamic that begins to play a progressively larger role as boost increases is intercooler charge cooling effectiveness. This is what I was referring to in the previous paragraph. IC performance is size limited, and in large part, that means by the physical space we have available in the engine compartment for the IC.
Once we run out of physical space, we can do things like further chilling of the coolant in an air-to-water unit or increasing coolant flow through the IC, but we have essentially run out of IC cooling capacity. These sorts of window dressing changes tend to be marginal improvements in the bigger picture. So what to do?
If you still want to pursue additional boost and power, it means it will become necessary to reduce engine timing, which, as luck would have it, costs power. This quickly becomes an exercise in diminishing returns. As we back off the timing to accommodate a higher boost and its associated charge temperatures, that the IC system can no longer as efficiently sink away, we also take away throttle response and, of course, some power also. This sort of dance with the devil has an increasingly disappointing seat of the pants and emptiness in the checking account feel that is worth paying attention to so you can avoid it.
The seat of the pants dyno (or at the track, et slip) usually begins to show smaller improvements with each additional increment of boost. This is a very important warning sign! Continued exploration down that path will lead to detonation and broken parts. How badly can detonation hurt the engine? Here is a pic of a Teksid block that detonated too much. BTW the fuel wasn’t gas; it was Methanol!
Here is a view from the rear of the block;
Detonation is not a far-fetched ethereal sort of event. It is as real as a heart attack and equally devastating, except for your engine and wallet. Even more significant is the fact that the only fuel more detonation sensitive than gasoline is nitromethane — think about that one for a moment!
Remember, as you engage with your individual builds, you are not involved in a laboratory experiment
. This is real-world stuff, and it is only inches away from you (and your wallet) on the other side of the firewall. Don’t abuse it, and it won’t abuse you. There is no trophy for a pile of expensive but broken parts — that didn’t need to break.