While obtaining a headline power number from a dyno session might be a key objective, for many, the dyno's most helpful feature is the help it provides as to tuning. Of course
the information comes at a significant cost both in $$s and effort. Having dynoed my FD since 2003 with a very helpful tuner my experience has been that it has been worth the cost.
Numerous things have changed since 2003 and the newer ECUs now provide all the data necessary to calculate power and torque at any RPM. Just as different dynos will give you
slightly differing numbers, the product of the following calculations may differ from a recent dynos but this method is a constant and will therefore generate a direction as
to power from one run to the next. Make a change, do a run and a calculation and you will immediately know if you are moving in the right direction.
The foundation of the formula is that there is a known relationship for the turbo 13BREW between air consumption and power. We get to the air quantity by accurately factoring the
amount of fuel. Since we know the Air Fuel Ratio we can calculate the air and therefore power/torque.
Let's get started:
Pick a particular RPM from your log, in this case 7311 RPM.
Start with the total base fuel available at 3 BAR/43.5 fuel pressure. In this case
Six 1350 CC injectors.
1350 X 6 = 8100 CC per minute.
8100 X .473 (Injector Duty Cycle at 7311) = 3831 CC per minute
We do need to check the fuel pressure (you do have a digital fuel pressure sensor i hope)
My Differential Fuel Pressure (that's the actual reported pressure minus the boost) was 41.5 PSI. since i had less fuel pressure than the 43.5 i need to adjust
downward the actual delivered fuel. delivery varies with the square root of the difference up or down.
41.5/43.5 = .95, so about 5% less. the square root of 5 is 2.2 so about 2% less flow.
3831 X .98 = 3755 cc per minute adjusted for lower fuel pressure.
We know the relationship between gasoline and air when calculating turbo rotary power. Some of us are running a blended base fuel, generally ethanol.
I was running 37% ethanol so let's do the calculation to adjust for a lower energy (BTU) content.
Gasoline contains 116,090 BTUs per gallon
Ethanol contains 76,330 BTUs per gallon.
.63 X 116,090 BTUs = 73,137 BTUs from gas
.37 X 76,330 BTUs = 28,242 BTUs from ethanol
total BTU content per gallon of my fuel = 101,379
101,379 / 116,090 = .873 BTU adjusted.
3278 CC per minute X .873 = 3278 BTUs adjusted as it were all Gas.
I also use two 630 CC per minute AI nozzles and inject 100% methanol.
Methanol has 56,800 BTUs per gallon. 56,800/116,090 = 49%
1260 CC X .49 = 617 CC of gas equivalent
Total gasoline adjusted fuel consumed:
3278 + 617 = 3895 CC per minute
We now go to Google and type: 3895 CC per minute to gallons per minute and we find 1.03 Gallons Per Minute.
Gasoline weighs 6.35 pounds per gallon.
1.03 X 6.35 = 6.54 pounds of fuel per minute.
Multiply that number by the air fuel ratio:
6.54 X 11.04 = 72.20 pounds of air.
We multiply 72.2 X 7.5 and have our horsepower number which is 541.
Torque is horsepower divided by RPM/5252
541/ (7311/5252) = 541/1.39 = 389 TQ
If you are using water or a water meth mix you will need to do a BTU subtraction for the water. Just as BTU content sets power potential, negative
BTUs are subtractive.
Let's assume you are running 400 CCs of water to protect against knock and keep your intrnals clean. How much power does the water remove?
An easy calculation:
400 CCs is .106 Gallons per minute. There are 8087 negative BTUs in a gallon of water.
.106 X 8087 = 857 Negative BTUs.
If we take the numbers from the above calculation ( 541 rw rotary hp ) required 3895 CCs/Min.
3895 CCs = 1.03 Gallons Per Minute or 119,572 BTUs.
we divide the 857 negative BTUs by the positive BTUs
857/119,572 = .007%
.007 X 541 rwhp = 4 hp.
you can use the above and adjust it for your power level. obviously the lower the level the higher the cost since the water penalty is constant.
for example:
400 rwhp... 400/541 = 74%
.74 X 119,572 = 88,408 BTUs
857/88,408 = 1%
400 X .99 = 4 rwhp... penalty of 400 CC of water at 400 rwhp level
8000 BTUs per gallon.
i have done hundreds of these helpful calculations and you will find after you do one or two they are really easy. Many of the above calculations don't have to be
repeated, such as the methanol as it is a constant.
NO DYNO? NO PROBLEM