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

["Bruce Bell" <bbell04@ibm.net>]

Are you sure you don't have this backward?

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