Re: electric supercharger for an E-28

[ChrisBourk@xxxxxxx]

In a message dated 98-11-07 12:44:17 EST, you write:

<< Date: Sat, 07 Nov 1998 11:46:09 GMT
 From: John G.Burns <john@domain.elided>

 BMW Car has an add for a gizmo that cuts the alternator load under 
 high engine load and claims up to 5bhp gains. I imagine it works off
 the throttle switch. It claims to still maintain enough battery charge
 and costs 99 GBP. >>

<<I think Bret hit it on the head when he said  " the Bosch alternator in an
E30 to drain over 5 hp from the engine when it is under high load.  Therefore,
chances are good that the electric supercharger could actually cost power.">>

A couple of things to remember:
1) 1 HP = 746 watts
2) amps x volts = watts
3) 65 amp alternator charging at 14.2 volts = 923 watt output

Alternators and not nearly efficient so if we factor copper, iron, and
machanical  losses into the equation and say 1 hp x 746 =  746 watts  = 52.53
amps at 14.2 charging volts out of an alternator that's taking 5 engine hp.
Ok so far,  now, what will this get us?  I think a 1hp electric air compressor
*might* be able to deal with .4 psi at 270 cfm (I'm using 270cfm [cubic feet
per minute] as an example for a 182hp engine - think E-28)

This is what I *think* we might get:

182 hp (stock) naturally asperated - 14.7 psi (one atmosphere)
therefore: 182 hp / 14.7 psi = 12.38 (we'll use this as a factor)
14.7psi + .4 psi (electric supercharger *boost*) = 15.1 psi
15.1 psi x 12.38 (our factor) =186.93 HP

Best case = .7 hp loss.
Now from this we should determine the effect the electric compressor has on:

1) weight - plus 50lbs?
2) intake air turbulance
3) reliability?

This is what I think - I'd be interesting in counter points but please don't
tell me about dyno charts - heck,  I've got an SI board that'll tell you my
engine is running at 3,000 when it's at idle.

Christopher

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