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| Joined: | 05-01-2005 |
| Location: | Minneapolis, Minnesota USA |
| Posts: | 575 |
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Judson, if the figures you're quoting for post-10480 cams are from the shop manual, General Specifications pg. 2, then it's my understanding that the exhaust cam timing shown as 66 BBDC / 21 ATDC is a typo, and that the correct figures are 66 BBDC / 26 ATDC. According to section A52 of the shop manual, and service bulletin 75-9, engines 10480-up were timed at 100 IN / 110 EX using the appropriate gears.
All the cam timing specs listed in General Specifications, pg 2 are for the same cam... what Lotus called the "C" or 101 cam (Cam ID numbers like 101, 104, 107 refer to the last three digits of the Lotus part number). The different timing events listed result from installing different MOP pulleys on the cams.
The middle data set for engines from 4030 represents the nominally correct C-cam timing with two proper 110 MOP pulleys.
The third data set for engines prior to engine 4030 is with 115 MOP pulleys on both cams. The 21, 71, 71, 21 timing noted there is correct.
The upper data set for engines after 10480 reflects a correct 110 MOP pulley on the intake cam and a 115 MOP pulley on the exhaust cam, advancing it 5 degrees. That changes the exhaust cam's 66 BBDC opening / 26 ATDC closing to 71 / 21. The manual indicates 66 / 21 and would be a typo for that combination.
However, as Mark notes, on page A52, the manual contradicts itself by stating that the cams on engines after 10480 are timed to 100 MOP / 110 MOP. I'm a Lotus guy and not into J-H enough to say which page is correct. But I can say that pg A52 agrees with the Federal Emissions Lotus 907 settings... 100/110.
A 907 with C-cams will run best overall when timed at 110/110. Everything else is a compromise for one purpose or another. For Lotus, the 100/110 timing was used on emissions engines. Advancing the intake cam 10 degrees increased overlap by a like amount. That allowed a little more exhaust gas dilution in the intake charge with a result similar to exhaust gas recirculation... it lowered the oxides of nitrogen and they didn't have to develop an EGR system.
It also gave a little more top end horsepower at the expense of low end torque. Horsepower sells cars. The lean carb and retarded ignition timing settings used on the emissions engines really sapped the power, and this got some of it back. At least on paper. However, the 2.0 907 never had much low end torque and really couldn't afford to give up any in the quest for more top end power. So the car was even weaker off the line and in traffic... it wasn't much fun to drive at anything less than full scream.
For a street car, set the C-cams to 110/110, static ignition timing to 14-16 BTDC and adjust the idle mixture for a strong idle.
On a wide open track with long straights, top end power might be more important. In that case advancing the intake cam alone might be worth it. The 104, 102.5 and 100 MOP pulleys will all advance the intake cam. Or advance the intake a bit and retard the exhaust a bit. 104 MOP pulleys on both cams will advance the intake 6, retard the exhaust 6, and increase overlap 12. That would be a poor man's 104 cam timing without the increased lift and aggressive lobe ramps.
Remember, changes to MOP affect the two cams in opposite ways...
A larger MOP advances the exhaust and retards the intake.
A smaller MOP retards the exhaust and advances the intake.
Later, Tim Engel