[MNBoxster]

Hi,

As you say, "everyone is entitled to their opinion, but opinions can further become validated with data".

But, I think you're not fully understanding why I think your results are in jeopardy due to the Alternator Output.

I'm not referring to the Voltage variation and it's possible effect on the DME. I realize that it (DME) has compensatory circuitry designed to make it perform through the range of possible voltages it can see. I am referring to the mechanical power draw of the Alternator itself to produce that voltage.

An Alternator's parasitic draw from the crankshaft is variable. It can range to almost nothing (maybe as low as 0.05HP) to as high as 30HP depending upon how much electrical demand it's being asked to supply.

Alternator's are not very efficient so far as power generation in a car is concerned. First, they produce Alternating Current (AC) for a machine designed to use only Direct Current (DC).

In order to convert this AC to DC, the current is passed through a series of Diodes and Rectifiers which results in tremendous losses (mainly as Heat) due to inefficiencies. So much so, that Modern High-Amperage Alternators are now starting to become water-cooled, transferring their Heat to the Car's Cooling system to be carried away. GM has several of these in use today, and they will become standard in just a few years.

But, it's advantage (aside from the fact that it can produce larger amounts of current in roughly the same size pkg. than a Generator can) lay in the fact that, unlike a Generator, which has a constant parasitic loss of CHP (Crank Horse Power) regardless of the amount of demand placed upon it, the Alternator is variable - almost Freewheeling when not in demand, but can consume pretty high amounts of CHP when demand is high. Similar to the power consumption of the AC Compressor - Low when AC is OFF, but rather high when the AC is ON.

This is where it can skew the results of your testing. If the Alternator is really pulling on one run, but not the next, this will have a definite effect on the amount of WHP (Wheel Horse Power) you see on the graph, and well within the gains shown in your results.

Unless you're monitoring the Alternator Output (and extrapolating it's CHP draw from that data), you simply cannot be sure you've duplicated the Test Conditions from one run to the next. This is what I mean when I refer to your methodology, you simply haven't accounted for everything.

If the reported gains were in the 50HP range, then the Alternator draw would have no bearing on whether or not there was a definite increase (only the value of that increase).

But, when you report gains of only 5.78 (CHP) and 5(WHP), you are smack in the range where the Alternator draw can mask your results leading one to draw a possible incorrect conclusion. You could in fact actually have a decrease in WHP and not know it. The graph would lead you to assume otherwise, though incorrectly so. This is why I say that your results are within the degree of possible error which is at least the 0.05-30 CHP the Alternator can draw off before the Chassis Dyno can extract it's data.

Put simply, you're reporting a positive variation of 5 WHP within a test which has a ± variance (or degree of error) of at least 0.041 - 24.6 WHP (accounting for drivetrain losses of the Alternator draw). I say at least, because I believe there are other variables which could affect this as well, such as the fact the Sanden AC Compressor used in the Boxster is also variable, and never OFF (according to Porsche). It too draws a varying amount of CHP.

Also, while I agree with you that an Inertial Dyno introduces the variables of the Tire dynamics into the results. Except for finding absolute values easily, it too can be sufficient to determine whether or not a change in output is observed...

Happy Motoring!... Jim'99