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| Joined: | 03-12-2005 |
| Location: | Kingman, Arizona USA |
| Posts: | 532 |
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1. The vacuum capsule retards the timing by about 5°, so the static timing of 8°BTDC becomes dynamic timing of 3°BTDC at idle. My JH ran about 20°F hotter with the retarded timing.
2. By actual test, emissions were generally worse with the retarded timing. Two CA smog tests, performed sequentially on the same machine, gave the data below. The only change was ignition timing. The tests were close enough together in time, that the coolant temperature remained substantially the same (i.e. within 10°F).
3°BTDC:
HC 498ppm, CO 0.30%, CO2 10.30% at idle (940 rpm)
HC 031ppm, CO 0.10%, CO2 11.80% at cruise (2704 rpm)
8°BTDC:
HC 152ppm, CO 0.45%, CO2 10.80% at idle (985 rpm)
HC 078ppm, CO 0.53%, CO2 11.80% at cruise (2375 rpm)
The difference in idle HC readings is huge, favoring the more advanced timing. It is possible that the change was due to a radical improvement in gas dynamics, and/or effective compression, between the two engine speeds, but I prefer the explanation that the advanced timing resulted in more complete combustion before exhaust valve opening.
The cruise HC and CO readings are higher at -8° and 2375 rpm, than at -3° and 2704 rpm. Lacking further evidence, it seems reasonable to presume that this is due to slightly different fuel/air ratios at the two speeds, rather than to the change in ignition timing.
I do realize that my data isn't conclusive, but as I seem to be lacking my own personal emissions tester, it's all I have to work with. I'd love to see whatever studies Lotus may have performed when the 907 engine was designed....