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Originally Posted by JackG
A couple of points...
I beleive you're overplaying the role the alternator would have on the test performed. The car's battery has one primary function... to start the car. Once started, the alternator has two jobs... to top off the battery's charge, and to power the car's electrical system.
Topping the battery back up after starting would be a small load, and is essentially repeatable. The large HP drains you're quoting would only be present if you were running many peripherals such as headlights, brake lights, ventilation fans, rear window defogger, etc. As long as the test were done with the same loads on the alternator each time, the difference should be insignificant.
In fact, as the car is shut down and restarted for subsequent pulls, the battery would lose some of it's charge each time, and the load on the alternator would be larger on each pull. If that's the case, he should have seen the HP go down on the later pulls as the alternator works harder to charge the battery back up. Since the HP went UP, that seems to bolster the case for the snorkle slightly restricting the airflow, and its removal making a small, but measurable difference.
The fact that diodes (which are the same things as rectifiers) generate heat as they pass current has absolutely nothing to do with whether or not snorkle removal makes more power. Non sequitur.
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Hi,
In theory, you, Maxter, and YellowJacket have some excellent points. But, perhaps less so in practice.
First @ Jack - Your theory about Alternator draw increasing as subesequent testing continues is fine, but only if you assume one thing, well several actually, but one in particular, and that is the state of the Battery prior to the initial run. An assumption, about an inknown.
The tester mentions that he just acquired the car. He does not mention replacing the Battery, so we can reasonably assume it is a used battery, to whatever degree. Such degrees can range from a relatively new Battery (which may or may not have sat on a Dealer lot for some time - and we know how Boxster Batteries respond to non-use) to a very old battery which may have broken or shorted internal plates and be incapable of holding a full charge, or if so, not for very long.
So, it is quite feasible that the Alternator draw was maximized on the 1st run. But, as I keep saying repeatedly, the issue here
is the unknowns. I never said the Lister's findings actually were faulty, only that there is a good possibility they
may be.
BTW, I know that Diodes and Rectifiers are the same thing, or rather that Diodes are Rectifiers. But, in
Automotive Speak, I have always heard the Output Diodes referred to as Rectifiers and the Field Diodes referred to as simply Diodes, so I use this terminology (although technically, perhaps incorrectly). I also realize the non-sequitur of bringing up the heat produced by the Alternator as affecting the snorkel vs non-snorkel debate. I brought it up to point out the inefficiencies of the Alternator and why it requires so much CHP to produce power.
@ Maxster, I realize what you are saying, but I don't believe you've allowed sufficiently for the mechanical inefficiencies involved. All the data I have read indicates that Alternators do draw the amounts of crank horsepower I've stated (though these amounts are not for the Bosch Alternator specifically, but accepted norms). Additionally, the Boxster uses a 120Amp alternator, not the 70Amp one described in your example, so any effect is nearly doubled (at least assuming linearity).
@YellowJacket, I appreciate your input, you make several good points. But, we're not modeling here and while statistics do accurately support trends, they may not always apply to a specific example, especially when the gains reported are so small. I honestly believe that without further data points (such as those already described), with so small a % reported gain, we're operating within a degree of error which makes any results inconclusive.
I'm not saying the Lister is wrong, not at all, he may very well be right. But, I cannot unconditionally support this view when so many variables which could swing the resultant conclusion remain unanswered. The convincing factor to me would be to show that alternator output (and consequently it's mechanical draw) was the same for each of these runs. I'm not merely being stubborn, I doubt that Industry or the Military would support such conclusions given the methodology used. They would require that all possible variables which could be controlled, are controlled, before comitting to the result. Peace!...
Happy Motoring!... Jim'99