Emission tuning for reindexed cams

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Emission tuning for reindexed cams

Postby h-bot » Sun Jan 26, 2003 6:14 am

From: doctorduke_REMOVE_132296_THIS_@yahoo.com

I think I finished emission tuning today and am tentatively planning
to have my car tested Monday. To review the issues, the reduced
overlap from the narrower lobe center decreases the exhaust residual
and will likely increase NOx. Also, the altered vacuum
characteristics will affect mixture - actually lean it out because of
the increased vacuum at idle and low speed/low load.

With my vacuum advance, which increases manifold vacuum further, I
found a good compromise by biasing the MAP sensor 1/8 turn CCW (rich)
with the ECU idle pot at the third hash mark from the "R" (rich)
limit, which is "richer" than the OEM setting. The OEM idle pot
settings are usually within half a hash mark of the center point in
the adjustment range.

The vacuum advance is disabled for the test, and I ended up setting
the MAP sensor about 1/16 turn CCW, but left the idle pot unchanged at
three hash marks from full rich. I did my tuning primarily by leaning
the MAP sensor to the lean surge point then richening it (CCW)a bit to
get rid of the lean surge plus a hair more, then got out and smelled
the exhaust. I'm sure my neighbors think I'm a lunatic or have some
weird fetish seeing this guy drive down the street, then stop, open
the hood, bend into the engine compartment for a few seconds, then go
back and sniff the exhaust, and repeat the process several times.

To get a little more margin I set the initial timing at 9 degrees. In
prior tests I always had it at 15, which is at the limit of acceptance
of factory spec plus three degrees. The retarded timing will reduce
peak combustion temperature, which will mitigate NOx and increase EGT,
which will help with oxidation of HC and CO with the injected air from
the PAIR and keep the converter hot. A richer mixture will also
reduce NOx, so I want any error to be a bit on the rich side as I have
lots of margin on HC and CO.

The California Acceleration Simulation Mode (ASM) test is done at 15
and 25 MPH and the engine must be between 2000 and 3000 RPM. This
requires the 15 MPH test to be done in second gear (2037 revs) and the
25 MPH test in third (2200 even). My OEM centrifugal springs seem to
be a bit weak as the centrifugal was beginning to pick up before the
specified 2000 rev start point, so I installed a second set of light
springs that are large enough OD to allow them to be installed
concentric with the OEM springs. I then measured 12 degrees total
timing at 2100 and 13 at 2200, so the timing is well retarded from
optimum for best torque and least specific fuel consumption at these
speed/load points, but this is what's required to minimize NOx during
combustion without lots of overlap or an EGR system.

The one thing I'm still mulling over is whether to pay to have a
pretest done. There's a decent shop only a mile away from my place
who will work with me, but his machine is not set up to do pretesting.
He referred me to another place that can pretest, but he's about 10
miles away and it will be more expensive.

I noticed quite clearly that in emission test configuration with the
reindexed cams, the performance is noticeably better in the mid range
and top end than emission test configuration with the OEM cam
indexing. It's still soggy below 2500, especially below 2000, but
that's because of the lazy centrifugal advance. Once there's near full
advance in the range of 3000 to 3500 it pulled more stongly than I
expected to 6500 and wanted to keep going.

As the engine simulations indicated, the more exhaust back pressure
you have, the better the improvement from less overlap.

It still idles smooth and stable at 900, but in emission trim only
pulls 14-15" Hg. vacuum. With 15 degrees initial timing and the full
10 degrees from the vacuum advance, which yields 25 degrees total idle
timing it pulled 18". With that setup I only read about 500 degrees
with an IR gun pointed at the top of the #1 header pipe. I don't have
the IR gun now, but it's obviously a lot hotter and heat just boils
out of the engine bay. That's the difference that tuning for peak
efficiency versus low emissions makes.

Duke






[This is message #10409 by user doctorduke on Yahoo! Group Cosworth Vegas: http://groups.yahoo.com/group/cosworthvegas ]
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