Twin Centrifugals VS Single

[Walter Sherwin]

Once again, thanks!

Walt.


----- Original Message -----
From: "Greg Hermann" <bearbvd at cmn.net>
To: <diy_efi at diy-efi.org>; <gmecm at diy-efi.org>
Sent: Thursday, April 12, 2001 10:34 PM
Subject: Re: Twin Centrifugals VS Single


> At 10:25 PM 4/12/01, Walter Sherwin wrote:
> >Forwarded from another list.....
> >
> >
> >
> >>Here's something I've been tossing around in my mind, for street toy
use,
> >>and I'd love to hear more from the group........
> >
> >>Let's say you have a large displacement stroker motor, and you desire to
> >>artificially stuff it's hunger with centrifugal supercharging (non
> >>manufacturer specific at this >point).  The typical approach today would
> >>be to seek out a largish single centrifugal unit that would do the dirty
> >>job, and pulley it to gain the max desired air flow/boost at >a specific
> >>engine speed.  This would net you a large diameter centrifugal,
operating
> >>at a somewhat less than maximum impeller speed, hung from one side of
> >>your >engine.  This would work, and you would prolly have a cog belt
> >>drive arrangement as a result, if you wanted serious manifold pressure.
> >>Of course this will generate a new >force/belt vector load at the nose
of
> >>the crank, that may or may not affect your front bearings.  One might
> >>even direct the output of this large compressor to a single
> >>>inter/aftercooler device for post cooling.
> >
> >>Let's consider a different approach......
> >
> >>What about mounting two smaller centrifugals, low on the motor, perhaps
> >>directly opposed to the crank centerline so as to cancel the belt load
> >>vectors.  Further imagine >the discharge(s) from these compressors being
> >>directed thru two parallel inter/aftercooler exchangers (ala: Porsche
> >>twin turbo setups).  The twin intercoolers might be a >bit easier to
> >>position within a street vehicle.  At an esoteric design level, twins
> >>could allow a person to fabricate an engine bay setup that has a
> >>symmetric and artful >appearance.
> >
> >>Cost aside, does anyone see any further advantages/disadvantages to
> >>running two slightly smaller compressors in place of one larger unit?
> >>Obviously, the cost will be >higher with twins.  I'm more wondering
about
> >>airflow versus crank speed, throttle response, boost response profile,
> >>parasitic drain at a given combined flow/pressure, >etc.
> >
> >>Thoughts would be appreciated;
> >>Walt.
>
> The problem with a crank driven centrifugal is that boost varies more or
> less with the square of crank speed. Just the basic law of centrifugal
> devices. One can play games with restricting the inlet to keep the curve
> from peaking quite so dramatically, but----this leads to higher pressure
> ratios and even more problems with intercooling in the higher rev ranges.
>
> Had never though about the idea of using two of them, it almost seems like
> a deal where one would have to look at individual compressor curves to
make
> any sense of what would happen in terms of air flow. You have got the
> mechanical advantages of using two pretty well nailed.
>
> It might be possible to flatten out the boost curve to some degree by
> playing with using two different size units, or running tow units at
> different speeds---
>
> I just flat don't like crank driven centrifugals because of the fact that
> you have to go to a bazillion engine rpm to get any real boost. Mebbe a
> variable ratio belt drive??? (Like from a snowmobile??)
>
> Greg
>
>
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