Thanks for writting back, John, but I'm gonna have to call ya on this one
:)
----------
> From: John Neff <jndneff@texas.net>
> To: Dakota Mailing List <dakota-truck@buffnet.net>
> Subject: RE: DML: Shorty vs. Long-tube headers (kinda long)
> Date: Friday, July 17, 1998 10:51 AM
[snip even more]
> Uhhhhh,
> Nope, sorry, you have it backwards.
> The longer the header pipe length, the lower the RPM's because you
are
> timing how long it takes for the exhaust pulse (slug) to reach the
collector
> to create the vacuum for the other tubes joined at the collector. If you
> shorten the tube length then the pulse will reach the collector faster
> (shorter length of time) and you have to have an exhaust cycle beginning
in
> one of the other cylinders in order to take advantage of the vacuum
caused
> by the pulse entering and expanding in the collector. I understand your
> career and education, but I have 10 years experience in hands on, getting
> major results in tuning 2 stroke exhausts. Now before anyone tries and
tells
> me that a 2 stroke and our Dak engines are apples and oranges, and that
> there is no way of comparing them, stop and a look at the physics.
[cut]
This is exactly an apples & oranges comparison. A tuned pipe is made so
that the exiting exhaust slug creates a vacuum behind it to suck in the
intake charge (the intake and exhaust ports on a 2 stroke are open at the
same time on one of these). The tuned pipe gets larger and larger in
diameter as the slug goes down the pipe. Then, the slug gets to the end of
the pipe. The pipe necks down very rapidly at this point, reflecting the
exhaust pressure wave back at the cylinder in an effort to keep the intake
charge inside the combustion chamber. Now, because the timing of all of
this is very dependant on engine RPM, two strokes tend to be extremely
peaky. They're kinda lazy until they get "on the pipe", then LOOK OUT!
Our 4 strokes don't depend on this bouncing effect at all.
Now, the reason a shorter 2 stroke tuned pipe works better when longer for
low RPM is because the exhaust slug is moving much slower. At higher RPM,
you need the exhaust pressure wave to come back much sooner, necesitating a
shorter pipe.
>there is actually more than one exhaust pulse
> in each header tube at any given time. When the car/truck is racing. This
> only compounds the scavenging effect as not only is each cylinder helping
> the others, but it is helping itself as well.
[cut again]
Yes, true, there is more than one slug in the pipe at a time. I
intentionally left that out cause my post was getting too long as it is and
I didn't want to muddy the waters. But as you mentioned, the longer headers
also feature equal length pipes, and this helps out a lot in getting
everything synched up.
> John
> 98 Dak 4X4 CC Sport, 5.2L, auto, 3.92LSD, T&H Package, Power Everything,
> Nerf Bars, Billet Grill, Rhino Liner, Snug Lid, DDBC, home made FIPK 2
> (pictures available soon)
>
John, thanks for the post. Clearly you have some good insight into exhaust
sytems.
Everyone else, I'm not trying to suggest that shorty pipes are pure junk
(even though that's what I said. Sorry.). Shorty pipes are a pretty good
deal, because of ease of installation. However, they are slightly less
optimal than equal length (longer) headers. The way I see it, shorty
headers cost about $400 or so. Longer headers might be around $500. The
thing I don't want to do is get shorty headers, then change cams and then
wish I had bought equal length headers. I've had equal length headers on my
Dart, and yes, they were a huge pain. Less ground clearance, difficult to
install, etc. But it was all worth it to me when I got to the dragstrip.
Really, weather long or short headers are best for you depends on what you
want to do with your truck. I want lower ETs, and I'm willing to put up
with some grief, so I'm holding out for the longer headers.
BTW, this brings up a question: The longer headers also usually have a
mounting flange at the collector, allowing you to unbolt the exhaust when
you get to the track. If I were to try this, would OBD-2 be able to adjust
to this fast enough to allow me to race? Open headers can be worth up to
half a second in ETs. Oh, but then that would take the O2 sensors out of
the loop and the computer would have a fit. It wouldn't work. Nevermind.
-Mike Miller, 98 RC R/T
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