Twin Turbo Ramblings

Charlie Smith charlie at
Mon Sep 17 00:00:37 EDT 2001

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  614-471-1418      Columbus Ohio   USA     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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