Accessory drive speeds -2-??

mike_starling · 06-12-05, 10:44 PM

OK Guys ... no takers on the last question. I'll try to be more specific.

The Powermaster site indicates that for circle track racing one would like a 1:1 pulley ratio; for drag racing a 2:1 ration and for the street a 3:1 ratio (where the alternator rpm = ratio*engine rpm)

Looks like the high continuous rpm of circle track racing allows a relatively low alternator rpm.

Now ... lots of varying philosophies about w/p pulley ratios. Underdriving by 20% or so can save some hp if you can tolerate a lessening of cooling. Probably not a really good idea for a GT40. I was thinking that a 5-6% overdrive might be worth trying.

That leaves the a/c compressor. Mine is a Sanden 508 (slightly large) and has a max sustained rpm of 6000. There fore one must underdrive this puppy just a little in order to stay within the design constraints.

The question comes down to how much to underdrive the a/c compressor? Looks like a 4" main pulley would give a max rpm of abt 7200 while maintaining a 6000 rpm rotational rate for the a/c compressor.

With that pulley, the a/c compressor will be turning about 2000 rpm at 70 mph with my G50/52. Sound reasonable?

I will check with Vintage Air tomorrow to see what they say and pass the info along.

Thanks,

Ron Earp · 06-13-05, 07:40 AM

Mike,

I would not skimp on the AC drive pulley speed. Nor on the alternator, but you can get away with it there IMHO.

I've fooled with Mustangs and other 302 powered things for years. Just about everyone, sooner or later (me included) slaps a set of underdrive pullies on their 5.0 to reclaim some of that lost hp. And, them everyone figures out that "Hey, it is hot outside and my AC doesn't work as good as it used to!" Most take them off and go back stock then, some die hards stick with them. I contend that on the AC it doesn't matter since it has a WOT AC cutout switch anyhow, so it won't be robbing power under WOT. Maybe you have the same, or will have the same.

I cannot remember anything about what Sanden recommends, although I looked it up a few years ago for the RF to make sure it would cool well. I want to say my last 5.0 in the Cougar had the AC turning at 2800 RPM when the car was rolling along at 60mph, with an engine speed of about 2200 RPM. I tried pullies on it too, and the compressor speed dropped to around 1600-1800 or so, and it didn't cool for squat on the highway, and barely did anything at all at idle. I can't remember the exact figures, but I think these are close. The wife was not pleased. That is with a Ford compresor though, and I have no idea how it would compare to what you have.

Be interested to hear what Vintage has to say too. AC is a must where a lot of us live.

mike_starling · 06-13-05, 04:38 PM

Vintage Air was very helpful.

I chose to use a slightly larger pump than the smallest they make. It is a Sanden 508. It has a max continuous rpm of 6000 and a downshift max of 7000 rpm.

The Vintage Air folks indicates that it is fully functional (as an a/c compressor) at 1000 to 1200 rpm.

Now, this leads to something of a small problem in that arranging the ratios of the c/s pulley to the fixed a/c compressor pulley and the existing w/p pulley will always be a compromise. Only way to get it perfect is to make two of the three.

At any rate, seems to me that a 4" c/s pulley would yield function of the a/c at 1200 engine rpm and not exceed the max continuous a/c compressor rpm until a crank rotation rate of 7150 rpm. With the existing w/p pulley this arrangement would yield a 6+ percent overdriving of the w/p.

Is a 6% overdriving of the w/p OK to do in a GT40?

Thanks,