Twin vs. Single Turbos Re: <VV> new thread- dual turbo on a corsa

[FrankCB@aol.com]

In a message dated 1/13/05 11:03:32 AM Eastern Standard Time, Levair@aol.com 
writes:
The only theoritical advantage to 
dual turbos is that the extremely small size size, if correctly sized, leads 
to 
quicker spin up due to the smaller flywheel-inertia effect of the turbine 
blades---hardly measurable. 
Warren,
    Well, I beg to differ.  According to A. Graham Bell's "Forced Induction 
Performance Tuning", "The reasoning behind the twin turbo system is quite 
simple -- a big turbo takes a long time to spool up to speed so turbo lag becomes 
an issue"  (This is particularly true for a street driven car, especially "stop 
light to stop light".)  Again Bell says "Also, in Vee style and boxer 
engines" (that's exactly what the Corvair engine is) "a single turbo invariably has 
to be located off to one side.  This, in turn leads to a long crossover exhaust 
from one side of the engine to the turbo."  (Exactly what the Corvair has)  
"That long pipe loses a lot of heat energy that should have been put to better 
use accelerating the turbo. <SNIP> Two smaller turbos can be mounted up close 
to the exhaust ports, minimizing heat loss and miximising exhaust energy.  
Smaller turbos have a lower moment of inertia so they spin up to boost more 
rapidly, thereby minimising lag.  Additionally, turbine wheels accelerate better 
when hit by distinct and powerful exhaust pulses.  More cylinders into a common 
exhaust sees exhaust flow activity even itself out and pulse intensity being 
diluted.  Fewer cylinders exhausting into a single turbo spaces the pulses 
further apart but each pulse has a greater intensity peak to give the turbine 
wheel a good solid bang, so it accelerates faster, producing more boost more 
rapidly."
    Of course Corky Bell thinks that twin turbos have no advantage over a 
single larger one.  But he also thinks that water injection is a "Band-Aid" and 
of very little use.(:-)
    So whether or not to go with the increased complexity of parallel twin 
turbos depends on the type of driving you intend to do.  For road racing or 
other events where you can keep the rpm up above 3000 and the engine load up above 
1/4 throttle, the cost and complexity of the parallel twin turbos is probably 
not justified.  But for street driving and drag strip driving where it's 
critical to get high boost as soon as possible and it's hard to keep everything in 
the exhaust system "warmed up" I would definitely consider using twin turbos. 
 Consider mounting them on the ends of the exhaust manifolds (switched so 
that they exhaust out back) to completely eliminate having to heat up the long 
stock exhaust pipe upstream of the turbo.
    Reminds me that one of the early Corvair draggers (maybe Dick Griffin) 
used to win his races by pulling up to the starting line at FULL throttle using 
the brakes to control his speed.  If the race began promptly he usually won.  
If, however, there was a delay, causing his exhaust system to cool, he usually 
lost.  Notice that in all the turbo 4 cylinder cars, the turbo is put on the 
end of the exhaust manifold to maximize the heat into the turbine wheel.  When 
I insulated the piping from the manifolds to the turbo in my 180, the boost 
came up much quicker and increased from 8 psi to 10 psi.  Unfortunately, after 
6 months of highway driving, the stock elbow disintegrated due to overheating, 
but the concept of conserving heat was proven.
    Frank "if it don't go, blow it" Burkhard