[ROVERNET - UK] exhaust and back pressure

[Gundry, Kenneth]

Thank you, everyone.  You have given me things to think about.

Yes, of course, I hadn't thought of the exhaust as a transmission line, with pressure waves traveling back (and forth) and the silencer as a load (a termination).  If the match was perfect, there would be no standing wave, but in practice there will be a reflection which will arrive back at the manifold either to increase or to decrease the size of the pressure pulses depending on the dimensions, the degree of mismatch of the silencer to the characteristic impedance of the pipe and the engine speed (you couldn't guess that I'm an electrical engineer, could you?).  If the reflection helps scavenging at one engine speed, then it will hinder at a higher speed (50% higher, I think), so I suppose other things being equal, the length should be chosen to optimize near the speed of maximum power (presumably towards the top engine speed).  However, that may be impractical.

Anyway, I take the point that the exhaust system cannot be considered simply as a restriction causing back-pressure that rises monotonically with flow-rate.  Enough technicalities for today!

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

-----Original Message-----
From: rovernet-bounces at lyris.ccdata.com
[mailto:rovernet-bounces at lyris.ccdata.com]On Behalf Of Paul Smith
Sent: Wednesday, August 11, 2004 3:55 PM
To: 'rovernet at lyris.ccdata.com'
Subject: RE: [ROVERNET - UK] exhaust and back pressure


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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