starr@unmvax.UUCP (04/22/85)
I have seen some hypotheses flying around about the causes of the inherent
lag of turbochargers. Allow me to offer my theory...I apologize in advance
for any errors regarding thermodynamics, I have been away from this subject
for about 15 years (mercifully).
Anyway, when one is cruising at part throttle, the turbo impeller is also
cruising at maybe 20,000 rpm. When one suddenly increases the throttle
setting to maximum, the impeller must accelerate to its peak speed, maybe
90,000 rpm. Well, folks, the impeller has inertia, and it just doesn't
accelerate instantly. It takes *time* to achieve this velocity, and this
is the lag you perceive. A number of things can be done to reduce this,
but I just want to point out the main reason it happens.
One of the methods to reduce the lag is to keep the working fluid (exhaust
gas) as hot as possible when it enters the turbocharger. The theoretical
efficiency of a heat engine (of which the turbo is one) is directly related
to the inlet temperature of the working fluid, e.g. exhaust gas. So if can
keep the exhaust gas *hot* you get more efficiency, implying more torque,
implying less lag and higher peak boost, etc... This is why you want the
turbo close to the exhaust manifold, to keep the exhaust gas as hot as
possible.
Now this has been known to readers of Hot Rod Magazine since around 1963,
when a GM engineer named Dick Griffin won the '62 NHRA Nationals with a
1962 Corvair Spyder. Dick ran in G/Stock, and cut an E.T. of 15.3 sec at
around 89 mph. One of his modifications (the one that helped most) was to
wrap the exhaust pipe leading from the manifold to the turbo in sheet
asbestos which had been dipped in sodium silicate (water glass), then to
wrap in aluminum foil. This insulated the pipe (which was around 2 feet
long) and kept the gases hotter. And boy did it work!! The lag was cut
in half (to maybe one second) and the peak boost just about doubled. And
you could *really* feel it! I know, I performed the same mod on both a
'62 and '65 Spyder.
"Corvairs forever...Ralph Nader be d****d"
Greg Starr
Robotics Research Lab
University of New Mexico, Albuquerque
{lanl, ucbvax, gatech...}!unmvax!starrpauldan@hou2e.UUCP (P.SAUNDERS) (04/24/85)
Yes, inertia is a main cause of turbo lag, and keeping the turbine close to the exhaust manifold does help to decrease lag. But you forgot to mention another problem that causes lag-- the turbine's distance from the *intake* manifold. This is a major problem, especially when adding an "air-to-air" type intercooler to the system. If there is a large amount of tubing at the output of the compressor, then there is a large volume of air to be compressed, which takes a while. Many tight bends in the tubing are undesirable as well, since they add resistance to the system. The bottom line is, for minimum lag, keep *all* plumbing to/from the turbo unit as short and straight as possible! Or maybe not... Dan Masi