[DML] Re: Gaining HP/Losing Torque

["content22207" <brobertson@xxxxxxxxxxxx>]

The big theory is that smaller exhaust pipes cause the gas to move
more quickly, and thus at lower pressure. This will reduce the chance
of any getting sucked back into the cylinder during the brief time
that intake and exhaust valves overlap. 

There's also supposed to be a "scavenging" effect of the *VERY* brief
period before the piston starts moving back down, in which momentum of
the exiting exhaust gas is actually creating a vacuum and pulling
itself out of the cylinder. 

In actual practice, smaller pipes invariably create an exhaust
bottleneck. Gasses actually move SLOWER than they would through larger
diameter pipes, totally negating any pressure drop. Plus the engine
has to work harder simply to shove its own exhaust out!

I've never heard of scavenging through a standard cast manifold.
Header manufacturers tout it, but I've also read that you really need
6 feet or so of custom length pipe for each cylinder for it to make
any noticeable difference (more space than is available even on
longitudinally mounted engines).

Some of the confusion arises over casual use of "backpressure" when
people really mean "delta pressure" (drop in pressure between two
points). "Backpressure" means "resistance to air flow" -- never good.

Small car enthusiasts promote supposed "benefits" of backpressure, but
I think they're sour grapes from vehicle size limitations (simply can
not fit a dual 3" or 4" exhaust system under a Honda) and government
emissions mandates.

Bill Robertson
#5939

>--- In dmcnews@xxxxxxxxxxxxxxx, Jim Strickland <ihaveanaccount@xxxx>
wrote:
> DOM, I believe you are correct.  
> The explanation I recall is that the correct 'backpressure" can
> positively affect low RPM horsepower (and therefore peak torque), but
> will impede high RPM HP (peak hp).  
> 
> Length of intake also has a similar correlation- and is the reason for
> toyota's T-VIS : variable induction system.  Basically, it changes the
> length of the intake tract at a set RPM.
> 
> I believe the Mazda cosmo has a variable exhaust system that changes the
> flow of exhaust depending on speed/rpm.  
> 
> Jim
> 
> PS.  Torque can not be measured independent of mass, distance, or
time.  
> 
> 
> 
> On Fri, 30 Jul 2004 19:46:42 -0000 "content22207"
> <brobertson@xxxx> writes:
> > Torque versus Horsepower is a common debate in another DeLorean
> > newsgroup. 
> > 
> > My contention: since Torque is one of the variables required to
> > calculate HP, yet torque itself can be measured independently of any
> > other variable, the entire debate is framed incorrectly.
> > 
> > Others feel differently, and argue their perspective just as 
> > strongly.
> > 
> > The discussions can become "spirited", to say the least.
> > 
> > Contact me off List (brobertson(at)caolina.net) and I'll give you 
> > that
> > newsgroup's URL.
> > 
> > Bill Robertson
> > #5939
> > 
> > >--- In dmcnews@xxxxxxxxxxxxxxx, "Dom Diaz" <dom_diaz@xxxx> wrote:
> > > This is just a technology question but I thought having too little 
> > 
> > > restriction on the exhaust may gain HP but will cause you too lose
> > torque? 
> > > Am I COMPLETELY wrong or did I not once learn that?
> > > 
> > > Can I get an amen? Excuse my rambling, must be the crack pipe(s).
> > Still, I 
> > > would like to hear thoughts on this.
> > > 
> > > Dom
> > > 
> > 
> > 
> > 
> > 
> > 
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> > 
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> > 
> > 
> >  
> > 
> > 
> 
> 
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