Subject: Re: [s-cars] A/C

John Petersen pete3633 at
Thu Jun 27 00:14:38 EDT 2002

I will propose 1 assumption, AC has a constant torque demand.
AC runs off a belt connected to a pulley on the crank.
This pulley rotates at engine speed.
engine speed increases as you accelerate.
AC speed increases as you accelerate
rotational speed times torque=hp.

HP demands for AC increase as you run at higher RPMs.

I personally believe that 5hp is a fair assumption at cruise (2-3k) speeds.
However, people who are familiar to old underpowered Japanese cars might
suggest that, as they can do nothing but cruise with the AC on, the AC
takes up every one of the available HP (which admittedly, is not much).


At 10:58 PM 6/26/2002 -0400, Bruce wrote:
>Taka and I did an interesting comparison on the way back from the track last
>Strictly unscientific in many ways, but with following results.
>He estimates his car at ~280HP, I estimate my car at ~310HP. Now mine has
>bigger turbo, with more lag, and better (I hope for all the friggin money I
>spent) top end.
>Both of us in 4th gear, 3K RPM, steady throttle showing 0 vacuum on boost
>gauge, right next to each other on highway.
>We are both on the cellphone (OK, we're idiots, I admit it), and hit the gas
>at as close to the same time as possible. With A/C on in both cars, I pull
>away slightly as the RPMs increase, sort of a steady pull. Same test with
>Taka turning off his A/C, and the cars are dead even.
>So that tells me maybe A/C saps more than 5HP?
>p.s. Did I mention that the V1 in my car saved our ASS*S twice on the way
>back. license flashed before my eyes.
>----- Original Message -----
>From: "Wayne Dohnal" <dohnal at>
>To: "S-car list" <s-car-list at>; <pizzoman at>
>Sent: Wednesday, June 26, 2002 9:33 PM
>Subject: Subject: Re: [s-cars] A/C
> > I'll dust off the rusty engineering skills and take a crack at answering
> > the A/C is run off of a belt.  I've heard that the A/C takes about 5 HP
> > the engine.  Starting with this assumption, that means an electric motor
> > would need 3730 watts, which at 14 volts is about 266 amps.  But since the
> > motor won't be 100% efficient, it would draw over 300 amps.  IF we had an
> > alternator that could handle this draw, it would just transfer the drain
> > the engine from the A/C pulley to the alternator pulley, but require more
> > engine power because of the losses in the alternator and wiring.  To
> > this, the system could use a hermetically sealed compressor like most
> > electrically-driven refrigeration units, which would give us a huge
> > in reliability and I'd assume a more efficient system.  A car A/C system
> > considerably more powerful than a typical window A/C because of the
> > tremendous amount of solar heating through the windows.
> >
> > Wayne Dohnal
> > 1994 S4
> >
> >
> > <parts of post snipped>
> > >My next question is that since our compressors
> > >are so ancient and will probably need to be replaced
> > >anyway, has anyone come up with a solution that is
> > >less power draining than having it run off a belt?  I
> > >was thinking of an electrical source rather than a
> > >pulley, being drawn from the alternator?  Just a bit
> > >of curiosity here and frustration at losing some
> > >performance when at WOT on hot days...which I probably
> > >shouldn't do before I load up on a nice big
> > >intercooler, anyway.  Anyone BTDT?  There is the space
> > >in our bay, but can it be done?
> >
> >
> >
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> >
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