[Walter Atkinson]

Gentlemen:

Based on the hard data rather than people's opinions, there is a tremendous amount of misinformation in this thread concerning HP determination and intercoolers. I will try to address as many of the items as I can remember form the read:

1) HP when ROP is NOT affected to any meaningful degree by fuel flow in the usable mixture ranges. HP when ROP is a function ONLY of mass air flow. By definition, any ROP mixture has extra fuel already. Adding or subtracting fuel as long as one stays ROP does not change mass airflow, so FF has no effect on HP…. until one gets so rich as to slow the rate of combustion, changing the thetaPP and reducing HP to the crankshaft. The HP difference between being barely ROP and as much as 150-175dF ROP is extremely difficult to measure and would be within the error in reading an airspeed indicator. We can measure the differences on the test stand and they are less than a HP or two.

2) Best Power is found at 75-80dF ROP, not 50dF ROP… no matter what any POH might say. These are the laws of physics.

3) Best economy is not found at Peak EGT, no matter what any POH might say. This is not open for debate. This, again, is a function of the laws of physics. Best Economy is found at BSFC(min) between about 20 and 90dF LOP, depending on the power being produced. At very high power settings, Best Economy is at about 90dF LOP. At very low power settings it can be as close to peak as about 20dF LOP.

4) Intercoolers are very important and helpful in operating a TC engine. They reduce the IAT and widen detonation margins. This results in lower CHTs in many situations. The increase in O2 allows for more mass airflow when ROP and, therefore, more power, albeit at a higher FF at the same ROP mixture. Best Power with an intercooler will require more FF to match the increased mass airflow.

5) An intercooler when operating LOP will allow one to be either further LOP with cooler CHTs at the same FF or push more fuel LOP for more power with the same CHTs.

6) HP LOP is a function of FF ONLY. Mass air flow plays no part as long as the mixture is LOP. For the engines you are operating 13.75 x FF = HP. Period. MP and RPM play no part as long as the mixture is LOP. So, a setting of 26/2400/13gph produces the exact same HP as 28/2450/13gph as long as both mixtures are LOP.

7) Intercooler recommendations to reduce MP 2" with an intercooler installed are mathematically flawed. While it is true that the intercooler results in more O2 and therefore greater mass airflow and the resultant increased HP, what they forgot to include was the increased exhaust back pressure which negatively impacts the volumetric efficiency of the cylinders. These factor offset on another so closely as to be a wash. There is, therefore NO scientific reason to reduce MP with an intercooler.

I will post a paper by George Braly on Intercoolers in the next post. If anyone would like any of the above comments expanded upon, I will be happy to do so.