[200q20v] Re: Lifting Stains... and Gains

QSHIPQ at aol.com QSHIPQ at aol.com
Mon Jan 28 13:06:36 EST 2002

Javad writes:
>Rob, in a supercharged application which is getting boost from crank speed
not >exhaust speed, greater overlap will cause power loss due to lost boost. 
So yes, no >need to go to a more aggressive cam really unless you are looking
to take some >power off the bottom and add it up top for greater peak power.

Not with you Javad, few turbo guys would be.  Increases in combustion energy
happen the same way in EITHER a supercharged application or a turbo charged
one.  If you add overlap you have decreased the amount of effective
compression, the lower the effective compression ratio (all other things
equal), you lower the power of the engine.  The reason super/turbo charged
cars use low overlap is that you have boost pressure to help scavenge the
exhaust from the cylinder bore.  Add overlap, you add to the reversion of the
pressure from the turbo back to the cylinder bore = lower velocity of the
exhaust gasses out of the cylinder bore, lower power out of the turbo.

>In a turbo applicaiton, however, increased overlap isn't quite *as* bad of a
thing.  >Unlike a supercharger where boost out the exhaust heads down the
exhaust, in a >turbo application boost heads back through the exh turbine,
thus creating more >boost energy on the intake side.

Not with you.  Exhaust energy drives the turbine, which means the combustion
with the highest energy gives the fastest turbine speed (or in a
supercharger, more power to overcome the crank losses that work the pump).
In any Forced Induction (super/turbo/comprex pressure wave) the combustion
with the highest energy is the one with the most trapped boost pressure. Open
the valves as fast as you can, as high as you can, and slam them shut as fast
as you can.  Increased overlap is <less> of a factor as rpm's rise, but what
you find on the dyno (or seat of the pants, btdt) is that you get the same
boost as quickly, but less power results from it.  At higher engine rpm's
this becomes less of a factor (for several reasons), but low overlap is key
to torque on any boosted motor.  The same principles apply to overlap on
either the super or turbo application.

>I'm more of a proponent for agressive overlap (relatively speaking) in turbo
>applications than most since unlike a supercharger, a turbo can actually
help spool >itself up with a little overlap.

I don't agree at all with that statement, in theory or practice.  Just about
every turbocharger book tells you to open the valves and slam them shut
Javad, that's how you get the most exhaust energy.  Using the cold side to
spin the hot side isn't a valid argument.  Audi used a fuel injector in the
manifold to spin the turbine on big turbos.

On my 10vt for instance, comparing the stock to the schrick 272 (more lift
than the stock cam, but more duration), I get 5psi at the same speed and same
rpm with the 272 as the stocker, but the car has no torque (not to mention
the lumpy idle that CIS can't handle).  At about 4000rpm the effect of the
overlap is less, or more specifically the effect of the lift is more, and all
hell breaks loose in the drivers seat.

Lot's of documentation exists on cam profiles for N/A vs super/turbocharging.
 Even the really tweeked I520vt cars (race) use mechanical lifters because
the lift is high and the duration so short on the turbocharged car (avoiding
hydraulic vavle float).

I'd sure love to read more on how increased overlap on turbo cars helps
Javad.  I have nothing in my library that remotely supports that contention.
In fact all principles of turbo and supercharging are the same in terms of
combustion and airflow.  For simple reference try pp 163 of Sir Bell:
"Make no mistake in the fact that turbo performance cams are very different
from atmospheric performance cams.  The characteristics of long duration and
high overlap  for atmo cams are unwelcome in a turbo system.  The street
turbo, which is generally small, operates with exhaust manifold pressure
somewhat higher that intake boost pressure.  This situation, when presented
with long duration, high-overlap cams creates a huge amount of reversion.
Thus the "turbo cam" tends to become a low-duration, very limited overlap

If your argument is of the "tradeoff" variety, you are better off trading
shim service of mechanical lifters with hiL/lowD turbo cams, than you are
accepting more overlap.

Rob, wrt your question, I'd suspect that you could do well with more
effective valve area, and/or reducing the overlap of the N/A cams that are in
the car.  Since I haven't the pleasure of a supercharger customer yet, I have
no data point for you, other than my local dyno got 338hp at the wheels for a
30v2.8 supercharged car.

Mike, wrt flow numbers, some comparative data for you on a set of tweeked v8
heads:  220intake/155exhaust @ 400lift

My .02

Scott Justusson
QSHIPQ Performance Tuning

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