[ROVERNET - UK] exhaust and back pressure

[Paul Smith]

Nope!
It goes from positive pressure to negative pressure, or vice versa.
Longitudinal wave mechanics.

PVS

-----Original Message-----
From: Slatskars [mailto:slatskars at comcast.net]
Sent: Thursday, 12 August 2004 2:10 pm
To: rovernet at lyris.ccdata.com
Subject: Re: [ROVERNET - UK] exhaust and back pressure


I believe that you meant sine not sign Paul?

Slats
----- Original Message ----- 
From: "Paul Smith" <Paul.Smith at auroraenergy.com.au>
To: <rovernet at lyris.ccdata.com>
Sent: Wednesday, August 11, 2004 4:27 PM
Subject: RE: [ROVERNET - UK] exhaust and back pressure


> I'm an EE too.
> The velocity of sound is raised due to the temp & pressure rise in
exhaust.
> The benefits at one speed can be reduced and the rpm range increased by
> pulse spreading.  Megaphones do this (racing motorcycles), and
intermediate
> expansion chambers.
> Note that a reflection off a closed end has the same sign as the original
> pulse, an open end reverses it.  Think microwaves.
>
> PVS
>
> -----Original Message-----
> From: Gundry, Kenneth [mailto:KG at dolby.com]
> Sent: Thursday, 12 August 2004 9:16 am
> To: rovernet at lyris.ccdata.com
> Subject: RE: [ROVERNET - UK] exhaust and back pressure
>
>
> 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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