Twin Turbo Ramblings

[Tom]

The RX7 and mk iv supra use 2 identically sized turbos. Only one functions
at low rpm w/ both coming on in parallel at a higher flow (I think the
control strategy looks for both a minimum boost and RPM when deciding to
engage the 2nd turbo).. common mod for the Toyotas is to "fix" the system so
they both operate at all times to lessen the hit of the second turbo coming
on. With a freed up exhaust and some basic other changes, the 2nd turbo
comes on pretty violently - not unlike nitrous - and can be a handful  :-)

I think a tractor pulling configuration complete w/ a couple intercoolers
could be great fun.. just think of what havoc 30 psi @ 1500 rpm could reak.
Seriously though, there was a recent SAE paper presented this spring that
discussed the advantages of a series / parallel system and control
strategies.  Idea was to run series at low rpm and switch to parallel later
on to fill in bottom end torque while maintaining good flow requirements for
top end horsepower with a broad torque curve.

Charlie, are you going to be at the run offs @ MidOhio this year?

-tom
'87 5ktq paperweight -> WTB: 20vt flywheel, nf/ng lower intake manifold,
non-leaking fuel tank.

> Message: 17
> From: Charlie Smith <charlie at elektro.cmhnet.org>
> Subject: Twin Turbo Ramblings
> To: fast928 at prodigy.net (Andrew Lundy), quattro at audifans.com (Quattro
List)
> Date: Sun, 16 Sep 2001 23:00:37 -0400 (EDT)
> Cc: charlie at elektro.cmhnet.org (Charlie Smith)
>
> Earlier, Andrew Lundy wrote:
> >
> > Sequential turbos...
> >
> > You have two SAME size turbos.  You hook one turbo on each bank of
> > cylinders.  One turbo is set to spool up at an earlier RPM than the
other
> > turbo.  As you indicated, this early "spool" is to reduce turbo lag.
With
> > sequential (and generally twin turbos) the turbos used are small.
Again, to
> > reduce lag!!
> >
> > The first car that I saw that did sequential turbos "successfully" (no
lag)
> > was the 1996 993 Turbo.  It's becoming the norm with twin turbo's now!!
>
> There are two different ways of engineering a "Twin Turbo" setup, not
> getting into tractor pull configurations that actually have one turbo
> blowing thru an intercooler that feeds a second turbo that blows through
> a second intercooler that then supplies air to the engine ...  usually at
> a very high pressure..
>
> The one you describe has the turbos in parallel, with both of them
> acting in unisen.  As in the S4tt, A6 2.7tt, and Porsche twin turbos.
> The advantage of two smaller turbos (one working from each bank of
> the exhaust) is that the smaller turbos spin up faster and there is
> less lag, plus the exhaust path from the exhaust ports to the turbo
> is shorter and the exhaust gas thus has less chance to cool down.
> I don't think this configuration has one turbo spin up before the
> other one, as you noted they are both the same.
>
> The other configuration is with the turbos effectively in series, but
> with a small one acting quickly at lower rpms, and with a larger one
> taking over at higher rpm.  I believe the twin turbo Mazda RX7 has
> this type of setup.  The advantage here is that the smaller turbo
> builds pressure at low rpm without lag, and the larger turbo takes
> over at higher rpm when more air mass is needed without heating the
> intake air as much.
>
>     - Charlie
>
>
>   Charlie Smith   charlie at elektro.cmhnet.org  614-471-1418
>   http://www.elektro.com/~charlie      Columbus Ohio   USA
>   http://www.elektro.com/~audi     photos & technical info
>
>   95 S6 Quattro  -  24 PSI, RS2 6 speed, and other features
>   96 Dodge Ram   -  30 PSI, w/Cummins turbo diesel



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