Date: Sun, 1 Sep 2002 12:12:47 +0100
Reply-To: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Subject: Re: Exhausts...tuning.(was loud identity crisis)
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Well, I drafted this and put it by, but seeing as others have contributed
now.... I'll get stuck in.
...and its a fact that nearly all vehicles exhaust are quite well thought
out and 'tuned', but to give a broad spread of torque and hence economy.
Manufacturers can't afford not to, the vanagon system is not just pipework
to fit the space, but suit the boxers firing order and low revband.
Relatively small bore pipes also keeps gas velocity and hence low speed
torque up. Balancing pipes are really 'tuning' pipes too.
Kim,
I'm a fussy sod as everyone has found out and certainly not always right (!)
but I'm gonna pick your synopsis apart anyway, sorry...
> And it isn't a matter of pure length, but the ratio of volume of
> gases passing through it to the length.
#Top hole stuff!
> The wrong length pipe can cause a standing pressure wave in the exhaust
> system, this means that is is harder for the engine to get the burnt fuel
out
> of the pistons #cylinders!, and causes a loss of power.
#Correct length pipes create standing waves too, the stronger the better -
thats the point!
>Block your exhaust pipe and you'll see what I mean. If the exhaust
> can't get out, the engine won't run for very long. ON the other paw,
> if you have just the right length, the exhaust system acts as a partial
> vacuum and actually aids in removing the spent fuel from the pistons.
#cylinders I think#
> Why? Harmonics. You have pulses being caused by the pistons. These occur
> rhythmicly and as with any other rhytmic system, you can get reinforcement
>of the waves in a positive or negative fashion. In this case the waves are
> pressure points in exhaust gases. High pressure is a positive
reinforcement,
> low pressure is a negative reinforcement.
#Yes, but not only at exhaust closure! The point being to squeeze some fresh
mixture that has been tempted down the exhaust pipe back into the cylinder
just before the valve closes and /or increase fill pressure. But the whole
ART is to 'also' get a low pressure across the port at exhaust opening. When
these two events coincide, and also resonate with any inlet tuning, torque
really goes up - but the more tuned it is, the more torque and economy drop
either side of the 'peaky' revband as scavenging gets worse and spent gases
even get stuffed back in.
- over here (Europe) the Vauxhall Senator 3 litre straight 6 used a very
sophisticated variable inlet tuning system to spread the torque gain wider -
they really were quick for a big car but have gone out of production (UK
police forces used them a lot).
Fast 2 strokes live or die by exhaust tuning (fully tapered expansion
boxes) - hence their amazing mid-range grunt. Its because of the 2 strokes
per 1 of a 4 stroke that, contrary to impression, they can't be tuned to run
at such high rpm as 4 strokes - the speed of sound in hot exhaust gas
(higher than cold air) is still something we can't change.
>
> All of this explains the large market in sports car after market exhaust
> systems. It isn't just about noise, but various people trying to get a
>little extra "free" power out of their engine, by tuning the exhaust.
#and rarely rejetting!
>
> Back to my original point. Depending on the size of the hole (and where it
> is) you will probably end up with more power because of a less restrictive
> path for the exhaust gases.
Almost always a good torque spread is destroyed - its not black and white,
but the genereal concesus is power is reduced. Small holes in the exhaust
unbalance the system, don't change the 'back-pressure', whatever
backpressure is, you won't find your brother using this term if he's an
engineer or physicist as its a purely lay term.
Clive
'88 Syncro Transporter
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