I think the general principle at work here is exhaust gas speed. You
need an exhaust system appropriately sized so that the exhaust moves
fast enough to aid in cylinder scavenging on the exhaust stroke. If the
system is too big for the manifold/header/head design, then cylinder
scavenging is incomplete ("exhaust" still left in the cylinder) as the
exhaust gas is moving too slow to "suck" exhaust out and power suffers
(remember that the piston doesn't go all the way to the "top" and "push"
all of the exhaust out of the cylinder, you need a bunch of "sucking" to
get the job done). Too small, and there isn't sufficient flow available
in the upper RPM ranges to "clean out all the exhaust", so again the
cylinder is left with some residual exhaust in it, and upper RPM power
suffers..
This is the reason that some exhaust/header systems hurt low end power;
they make more power in the top end as there is sufficient gas speed for
scavenging (as the motor is turning faster) and improved flow, but at
low RPM, there just isn't enough gas speed and the cylinders don't get
emptied. People often say "oh, there is not enough backpressure so low
end torque has suffered", but thats not really the issue...
Clear as mud?
Craig
P.S. If the "bap, bap, bap" is occurring under deceleration with
fractional throttle openings, then its not a sign of anything other than
a "loud" exhaust and a disrupted exhaust flow (which makes squat
difference since you're not making any power at that point). Now if you
hear "bap, bap, bap" under acceleration, then thats a problem :-)...
____________________________________________
Craig Baltzer - Anjura Technology Corp - Ottawa, Ontario
Craig.Baltzer@anjura.com - (613) 727-1411
_____________________________________________
> -----Original Message-----
> From: Klaus Wilkens [SMTP:klaus@buffnet.net]
> Sent: Friday, February 20, 1998 10:29 PM
> To: dakota-truck@buffnet.net
> Subject: Re: DML: Gibson exhaust installation
>
> Joe,
>
> I don't know where your getting your information but it is contrary to
> my
> experiences with Otto cycle engines. Generally an engine which
> performs better with
> back pressure has an underlying problem causing this condition.
>
> KW
>
> Joe Borg wrote:
>
> > Yes, the 2.5 offers more resistance than a cat back 3". Engines
> NEED a
> > certain amount of backpressure to operate efficiently.
> > My point: if you are NOT installing headers, new y pipe AND hi
> flow
> > cat in a TRUE dual exhaust setup you dont gain much and may
> even lose
> > due to turbulance and resonance.
> > You are absolutely correct in stating that aftermarket or
> custom
> > mandrel bent pipes are better than stock. Joe
> > An indication of "over exhaust conditions" is a very noticable
> "bap,
> > bap bap bap" sound during decelleration. Ever have a muffler
> fall off
> > a Vega??
> >
> > ______________________________ Reply Separator
> _________________________________
> > Subject: Re: DML: Gibson exhaust installation
> > Author: <dakota-truck@buffnet.net> at internet
> > Date: 2/20/98 9:16 AM
> >
> > Joe,
> >
> > I also have to disagree with the info you got from Flowmaster. The
> stock
> > 2.5" exhaust may be "adequate", but it still presents more
> ristriction
> > on the exhaust than an aftermarket single 3" or dual 2.5". Exhaust
> > restriction is a cumulative effect due to the fluid resistance
> between
> > exhaust gases and the sides of the pipes and any obstructions. The
> > resistance values for all exhaust components are added together to
> > calculate a flow rate for the system. In other words, even with a
> 2.5"
> > openning through the cat, you will have a higher flow rate with a 3"
> cat
> > back than a 2.5" cat back.
> >
> > There is a point of diminishing returns, however. I agree that the
> duals
> > don't do much for you from a performance increase standpoint vs. a
> 3"
> > single, unless you have other mods to take advantage of them.
> >
> > My biggest objection to leaving the stock exit pipe (muffler back)
> is
> > 2.5" with crimped bends. The crimped bends produce increased exhaust
> gas
> > turbulence and flow resistance. The aftermarket pipes (even custom
> bent
> > jobs from the local muffler shop) are mandrel bent, which provide a
> nice
> > smooth curve and less restriction.
> >
> > jeff
> > 98 RC 4x2 V8 5spd 3.55SG flares
> >
> >
> > Klaus Wilkens wrote:
> > >
> > > Joe,
> > >
> > > Flowmaster is trying to sell Flowmaster. What youwere told doesn't
> compute. If
> > the Y
> > > pipe and Cat are the major restriction opening up the exhaust
> system after the
> > cat
> > > would have negligible effect.
> > >
> > > I have a 94 Sport with the 5.2, 5spd, and 3.90 gears. I've had the
> truck since
> > new
> > > and have had it to the track a few times. The most marked single
> improvement
> > was the
> > > Gibson cat back exhaust with a set of JBA shorty headers which use
> the factory
> > Y
> > > pipe. I picked up 2 mph at the strip and 2 mpg on the road with
> this
> > modification.
> > >
> > > A few years ago CC did a 2.2 turbo package called the Super 60.
> The
> > development was
> > > done on a dyno and with mufflers and the Dynomax Ultra made 20HP
> more than the
> > > Flowmaster.
> > >
> > > KW
> > >
> > > Joe Borg wrote:
> > >
> > > > Jeff, I recently called flowmaster. They stated with minor
> mods, the
> > > > stock exhaust system is more than adequate when a
> flowmaster 2 chamber
> > > > muffler is installed. They stated 3" pipe does not have
> enough
> > > > backpressure and although sounding "meaner" woulsd actually
> slow you
> > > > down. The concept of single in dual out mufflers is pure
> show and no
> > > > go. An exhaust system can only be as efficient as its
> weakest point -
> > > > the 2 into 1 y pipe and cat. Unless you install 2 cats with
> a true
> > > > dual setup you are gaining nothing but sound and
> appearance. Joe Borg
> > > >
> >
> > jeff
> > 98 RC 4x2 V8 5spd 3.55SG flares
>
>
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