Lifting Stains... and Gains

[JShadzi at aol.com]

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[ Picked text/plain from multipart/alternative ]
Scott, I too believe, and everything I have read, also supports low duration
cams in forced induction.  Your statement below about "all hell breaking
loose" with your 272 in the turbo is what I am going for.

Preferentially, I am not looking for tons of low speed torque, I don't mind
waiting until 3500rpm for the real power rush.  Currently I am running an NG
cam in my 80tq, with the header, t3/t4 turbo, power comes on strong at about
3200, but at 3800 all insanity breakes forth, and the car rockets forward and
I am clawing for the next gear seconds later, its a rush, I like that.
Granted, in a stock application, with a small turbo making lots of low speed
torque (ala k24), low duration for low speed power make a lot of sense, and I
support that.  I am not looking for the "stock application, however".

Much of what I stated about turbo vs supercharging cams comes from a weekend
I spent at Magnussen Supercharging, the North American dist. for Eaton
Superchargers, where they debuted the Neuspeed Superchager kit for VW's 2.0.
Speaking with some of the engineers there who design supercharger systems for
even OEM applications, they described many differences between turbo and
supercharging cam design, which they support with real data and dyno #'s.  I
don't claim to be an expert, however, and there is no denying that what you
describe is the standard and accepted theory.

And, it does come down to compromise.  As good as the MC and stock Audi cams
are, they are pigs above 6k rpm.  Trading a little overlap blow-by for
increased high rpm breathing is a compromise I happily accept.

Javad

In a message dated 01/28/2002 10:06:36 AM Pacific Standard Time, QSHIPQ
writes:

> 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.
>
>