Re: head gasket, any mods while I am in there?

["Graydon D. Stuckey" <graydon@apollo.kettering.edu>]

Bruce,

	Some interesting points you make here.  I havealways understood 
that there are two distinct sets of tunings going on in the exhaust.   
The exhaust gas itself has a pulse which can be reflected, as you discuss 
below, but there is also the noise pulse which travels at the speed of 
sound (in the exh gas, whatever that new speed is).  My understanding is 
that this noise pulse travels much faster than the exhaust gas itself and 
can have a significant impact on exhaust tuning.   Am I confusing things?

On Mon, 8 Jun 1998, Bruce Bell wrote:

> In normally aspirated motors those exhaust pulses (the low pressure ones) 
> are actually used to scavenge the combustion chamber and help the intake
> gases begin to flow.  Hence NA motors benefit from cam timing where there
> is an overlap in the time the exhaust and intake valves are open. If you
> reflect theses pulses away with an antireversionary setup you have just
> negated the value of a well tuned exhaust and the cam overlap.
> 
> On the other hand, pulses aren't very tunable in the short distance to the
> turbo, there is little to help scavenge the combustion chamber. As you say,
> pressures are high in the manifold, and I maintain the antireversion steps
> are necessary to help keep the exhaust out of the CC. Note also that turbo
> motors have cams with very little or no valve overlap because there is no
> scavenging effect. 
> 
> One last thing, Flow is great but more isn't always better. I doubt
> maximizing flow at the exhaust port is the whole picture when your are
> trying to spin up a turbo. I would expect as you let that gas expand it
> will lose energy just before it does the work you want it to ... spin that
> T.  My son has a '77 Westphalia which someone had converted from fuel
> injection. They used two huge Weber carbs that will flow like crazy - one
> 40 mm throttle bore for each cylinder with a 37 mm intake valve.  Flow is
> great (on a bench); but, the port velocity is so low bus drives like a dog
> here at 5300'.  Hey, If you thought these things were slow..... oh, well
> .. those Webers do look cool :-) 
> 
> 
> Bruce
> Looking for '77 Bus manifold to use "one" Weber 40 IDF carb.
> 
> ----------
> > From: Kevin Ford <cheryl_ford@bc.sympatico.ca>
> > To: quattro@coimbra.ans.net
> > Subject: Re: head gasket, any mods while I am in there?
> > Date: Sunday, June 07, 1998 1:20 PM
> > 
> > The "antireversionary step" from exhaust port to exhaust manifold is 
> > benifical in normally aspirated engines, (preventing some of the 
> > exhaust pulse echo from entering the port), but with exhaust manifold 
> > pressure reaching 25 psi or more in a turbo engine, the pressure 
> > difference in the pulse is much lower. I think the slight benefit of 
> > the antireversionary step is outwayed by the gain in flow by port 
> > matching the exhaust port.
> > 
> > Generally, turbo engines are ported with gaining the most possible 
> > flow, above all other concerns.
> > 
> > Kevin Ford
> > Chase, B.C., Canada
> > email: cheryl_ford@bc.sympatico.ca
> 


Later,
Graydon D. Stuckey

"There's alot more to Jazz than just wrong notes"