RE: Tailpipe Probing of EGO

[Mike Rolica <mrolica@meridian-mag.com>]

	Pardon e'moi,
	I tweaked the subject to segue to the more techo side.
	Gar
	On Wed, 28 Jun 2000 18:46:43 -0400, "Jason Bonie"
<gmecmml@cfl.rr.com <mailto:gmecmml@cfl.rr.com> > wrote:
		>quick question about the probe shown in the pictures.  does
it have
		>to have that much "pipe"? sticking out of the exhaust?
what if you 
		>wanted to take a spin while watching the meter?

	Well, yeah, that particular probe WAS used to take quite a few
spins. :) Some of them in that twin-turbo Mazda were pretty dizzying! :)
Why would you think not? BTW, that particular length we settled on for
conservatism's sake is only 16" long. Might look longer in the pix than it
really is?
	Here's the deal about the "aft end length" of any proboscus (I
posted about this before, but 'oh well'). You MUST make it long enough aft
the pipe that you get NO reversion from exhaust pressure pulses. I've seen
some other vendors shorten theirs to near 6", and all I can say is, "fine,
make it as short as you can, but make sure you see NO difference if it's 6"
or 26"". After all, it's a MEASUREMENT we're after here, so it wouldn't do
to wank it up by skimping on the aft-length and suck reversion. Personally,
until it really gets in the way, I'd make it as long as possible. As long as
that probe might seem to be, all's that's happened in MANY test runs is we
get quite a LOT of "oogles" on the road. In fact, I've raised up the Meter
to eye level on a couple occasions to answer some VERY curious looks from
other proformance "afficiandos". They seem to instantly sense some tuning
operation is going on. :)
	I can tell you from measuring "in the bung" near the exhaust port or
header pipe, compared with using the proboscus, that except for that narrow
band around STOICH, the CAT really IS outta the picture (thank you, Dr.
Heywood). Sure, you cain't hardly see the closed-loop cross counts, and
that's good proof that the cat IS working as it should, but for any OTHER
range of AFR, outside 1AFR centered around STOICH, you most definitely CAN
use a temporary proboscus to get your numbers for OL tuning, because the CAT
during those runs IS outta the picture. Nice to be able to "switch" it out
by the press of the pedal, eh?
	WELL, in fairness and for fun's sake, I MUST mention another
exception to the above. There IS a very real value to at least looking at
the AFRs pre-CAT. We had occasion to tune an opposed 4-cyl experimental
aircraft engine (water-cooled) that was mounted in an auto for testing, and
were able to see a mixture imbalance from side-to-side at the conjunction of
the Y-pipe just prior to CAT, from the way the AFR jittered backNforth at
ALL rpms (it was worse under load; ick :).

	Me thinks it may be a subaru engine No??... buit quite a few of
them.....the induction system kinda sucks, as ds cyliners at front seem to
run leaner than the rest......




	 This says that despite questions about the sensor's response time,
it most certainly is adequate to see if you have cyl-to-cyl AFR variation,
if you look just aft the collector. BTW, testing on another exact same XA
engine but with different induction, no jitter. We were able thereby to
point the finger at the different/"newNbetter" induction setup, which was
why one side was running leaner than the other. OTOH, looking at the same
aggregate mixture at the tailpipe, the gas had mixed and averaged to the
point where we got the same AVERAGE readings for AFR, and you couldn't see
the jitter any more.
	Soooooo, to make a short story long, like I said some time back, we
don't INSIST you use EGOR with a probe, just that once you've sniffed the
mix pre-cat, you CAN practically go back to sniffing the tailpipe for most
of the rest of your tuning. It's alot safer for the WB sensor if you baby it
and treat it as part of your "instrumentation gear".  We'll supply you with
the schematics for building a "stoich-crossing" simulator to drive your OEM
ECU, if you INSIST on keeping your WB sensor "in the bung" perpetually to
replace the OEM O2 sensor, but frankly, we see NO reason to do so at this
time. When they're (the Honda/NTK sensors) priced like Pez candy (hee), then
sure knock yerself out and run the WBO2 sensor all the time, but at
$90+/pop, it presently seems "ill adviced". :)
				HTH,
				Gar


	
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