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| Joined: | 03-12-2005 |
| Location: | Kingman, Arizona USA |
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115 IN / 115 EX -- Vehicles made in 1973 (and presumably 1972 as well)
110 IN / 110 EX -- Vehicles made in 1974
100 IN / 110 EX -- Vehicles made in 1975 and 1976 (both JH and GT)
IIRC the federal emissions standards were based on model year for high-volume cars and actual date of production for low-volume ones, while the California standards were based solely on production date. This results in a complex regulatory muddle that only a bureaucrat could love. In any event, the 100/110 setting really should be used only on engines with catalytic converters and air pumps, and not all cars notionally belonging to the 1975 model year were so equipped.
If you closely examine the cam gears found on most JHs, you'll note that the markings on the two faces are somewhat different, as indicated in the attached photo. If you place the gears side by side, a bit of experimentation will make it apparent that flipping each gear so one face or the other is upward, will give you four possible ways to align an 'IN' index line on the left gear, with an 'EX' index line on the right gear: 110/110, 115/115, 110/115, and 115/110. These are the four cam timings possible with these gears.
CAUTION: The engine generally will not run, and may be damaged, if one uses an 'EX' index on an intake cam, or an 'IN' index on an exhaust cam, to set the timing.
A stock 907 engine should run with any of these four combinations, but if memory serves the 110 IN / 110 EX setup gives slightly better low-end torque at the cost of top end power, while the 115 IN / 115 EX combination gives slightly better top-end power at the cost of low-end torque. (Or perhaps it's the other way around.) In any event, since even the California smog nazis do not check cam timing (yet), I'd suggest using whichever setting works best for you. Most folks (myself included) seem to be happy with the 110 IN / 110 EX setup.
I've found through experience that the cam gear markings are generally hard to read when the gears are installed on the engine, and consequently it is quite easy to repeatedly mis-set the cam timing. This eventually becomes terribly embarrassing. I strongly recommend marking the desired indexes with daubs of paint, permanent magic marker, or the like, so that proper alignment will be obvious.
Historical note: There are three types of JH cam gear, plus additional types used only on Lotus cars. Early JH gears were marked only IN and EX and had no numbers. Mid-range JH gears were as described above. Very late JH/GT gears were similar to the mid-range gears but permitted 100/110 timing instead. Earlier Lotus cars may have used any of these. Later Lotus cars may have used gears with a variety of colored pimples and/or dimples that indicated their timing. And, as seems typical of Lotus products, there were various minor and generally undocumented variations.
Attachment: engine cam gears annotated.jpg (Downloaded 113 times)