Re: [alfa] head porting, flow reversal

[bearbvd@xxxxxxxxxxxxxx (Greg Hermann)]

At 10:13 PM 10/4/04, AlfaNeely@domain.elided wrote:
>In a message dated 10/4/2004 7:24:33 PM Central Daylight Time,
>owner-alfa-digest@domain.elided writes:
>Date: Mon, 4 Oct 2004 13:15:26 -0400
>From: Joe Elliott <jee@domain.elided>
>Subject: [alfa] head porting, flow reversal
>
>
>>  It's a given that to leave a step on the exhaust port can help
>>reduce flow reversal with cams that have a large amount of overlap.
>>Surely then the same would apply to the inlet port? Also the sudden
>>pressure drop at that point may induce more flow than a gradual
>>taper would.
>
>Is what they say about the exhaust port/header interface true?  If
>so, which way are they talking about stepping it--bigger header than
>port, or vice versa?  And why does this reduce flow reversal (which I
>assume happens because the exhaust manifold is at a higher pressure
>than the intake manifold).
>
>Thanks,
>Joe Elliott
>'82 GTV-

'Anti-reversion' headers are fairly  common in the world of hot-rodding.
Their effectiveness depends on many factors, particularly cam timing and
compression ratio. Steps are from smaller to larger going downstream. A
step from port to manifold is one place, some do steps partway down the
tubes. A tri-wye header design, such as used on Alfa fours is , in some
respects, a variation on this theme.

Short version is that it works (sometimes) because resistance to flow is
much less across the step going from small to large than it is going from
large to small.

Greg

>-------------------------------
>
>    The Anti Reversion headers sold by Jack Beck www.orionmotorsports.com
>have a short pipe that exits the exhaust port about an inch.  Surrounding
>that is
>a huge pipe that tapers back down to the nominal diameter of the exhaust
>port.  That creates a chamber that looks rather like the megaphone exhaust
>pipes
>you see on some motorcycles.
>    When the sound wave created by the exhaust reaches the end of the pipe,
>it "echoes" back down the length of the pipe.  If the reversion wave reaches
>the exhaust valve at exactly the wrong time, it can push exhaust back into the
>cylinder.  It creates a high pressure at the wrong time.
>    A "tuned" exhaust can and will help pull exhaust out of the cylinder.
>That is when the reversionary pressure wave returns to the exhaust valve at
>exactly the correct instant to create a low pressure that will help draw the
>exhaust out of the cylinder.
>    The problem is, a tuned exhaust only works at a certain RPM and multiples
>of that RPM.  An anti reversion exhaust creates a huge chamber where the
>exhaust reversion is killed before it can reach the valve.  That kills the
>tuned
>exhaust effect, but makes the exhaust work across a wide RPM range.
>
>    The street headers sold by several of the catalog vendors are crap.  The
>front of the exhaust pipe shrouds the exhaust port on all four cylinders.  If
>you can easily get a standard exhaust nut to fit the headers, then the exhaust
>is shrouded.
>    These pipes all have a step built into their design.  The problem is the
>step goes the wrong direction.  I know the headers from IAP and Centerline had
>that problem.
>    The headers from Alfa Ricambi used to be great, but I am not sure they
>are in business this week.  I will probably buy from Paul Spruell or Jack Beck
>next set I buy.
>
>
>Ciao,
>Russ Neely
>Oklahoma City
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