[ROVERNET - UK] exhaust and back pressure

[Paul Smith]

It turns out (read Scientific Design of Exhaust and Intake Systems, Philip H
Smith) that most standard length systems work pretty well for engine-assist
tuning at around 2500rpm.  This is based on the time taken for the positive
pressure pulse to traverse the pipe to atmosphere, and return as a negative
pulse which assists scavenging.
Very short pipes (stubs; piston aircraft used them a lot, drag racers usee
them now) give 0 assist, but aren't detrimental.
So disconnecting a long pipe can make the performance go down.  However it
is rev range specific, tuning only happens for 1000rpm or so.
This is the reason Rover went through several designs for the P6 exhaust
system.  Getting the position of the front expander right affected it
somewhat.  An expander mimics the effect of the atmosphere and reflects
positive pulses as negative.  Eventually they discarded the expander (66 or
so).

You won't damage anything, the extra cooling for the exhaust valves is a
plus, but it may not be the best guide to performance.  The ideal is a
straight through pipe of the same length as the original, but it won't mimic
the effect of the muffler.

PVS


-----Original Message-----
From: Gundry, Kenneth [mailto:KG at dolby.com]
Sent: Thursday, 12 August 2004 1:25 am
To: rovernet at lyris.ccdata.com
Subject: [ROVERNET - UK] exhaust and back pressure


Although my Rover is running reasonably I still have doubts whether it is
developing the power it should. It occurs to me that the exhaust system
(pipe and silencer) was replaced (not by me) and it is conceivable that
there is more back-pressure than there should be. Is that possible? If so,
could I check by disconnecting the exhaust pipe from the exhaust manifold,
to see whether (despite the noise!) the power is increased significantly, or
could that damage something?  Alternatively, is there some way of measuring
the back-pressure, and what value should I expect?  Incidentally, I remember
back in the 1950s a massive back-fire blew the back off the silencer, and
the noise was impressive!

Ken G, 1925 Rover 16/50 (San Francisco)


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