[ROVERNET - UK] exhaust and back pressure

Gundry, Kenneth KG at dolby.com
Thu Aug 12 17:12:41 BST 2004

Thank you, everyone.  I have decided for the present to leave well alone!

Yes, Ron, this is my Rover 16/50 whose engine was rebuilt by my cousin (rebore, new pistons and rings, new bearings) and then had new exhaust valves installed by me (a new and rather alarming experience, dismantling and reassembling the engine) when one of the originals cracked.  So it is very clean.  The only "tuning" has been by me, playing with ignition timing and carburetor settings, following where possible the guidance of the instruction manual that came with the car.  I tried a few months ago to find a firm with a chassis dynamometer in the San Francisco Bay Area, hoping in particular to see whether the power output came anywhere near the specification (50 HP at 3000 rpm, if I remember rightly), but in vain.  I have a contemporary review which gives a curve of speed versus time accelerating from 10 to 50+ mph in 4th gear, but haven't yet found a long enough flat and straight stretch of road with no speed limit and no traffic to be able to try to reproduce it.  I don't think anything can be dramatically wrong because the fuel consumption is comparable or perhaps slightly better than what my father used to claim it had been from new and was when we used it for summer vacations in the 1950s (it's doing slightly more than 12 mpg US!)

By "sign", Paul meant polarity.  A sudden constriction in the pipe will give a reflection of the same polarity, that is, a pulse of increased pressure will be reflected as a pulse of increased pressure.  A sudden lack of constriction (for instance the open end of a pipe) will give a reflection of opposite polarity, that is, a pulse of increased pressure will be reflected as a pulse of decreased pressure (a rarefaction).  By the way, this is exactly the distinction between stopped and unstopped organ pipes.  An appropriate smooth increase in pipe diameter (like a horn loudspeaker) or appropriate sound absorption might give no reflection at all.

Oh, isn't this fun!

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

----- 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
> The benefits at one speed can be reduced and the rpm range increased by
> pulse spreading.  Megaphones do this (racing motorcycles), and
> 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.
> -----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,
> 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
> 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
> 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
> Smith) that most standard length systems work pretty well for
> tuning at around 2500rpm.  This is based on the time taken for the
> pressure pulse to traverse the pipe to atmosphere, and return as a
> 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
> 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
> the effect of the muffler.
> -----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,
> could that damage something?  Alternatively, is there some way of
> the back-pressure, and what value should I expect?  Incidentally, I
> 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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