View single post by Esprit2
 Posted: 11-03-2005 12:55 am
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 288
Mark Rosenbaum wrote: The data in TDA.pdf is an enormously valuable supplement to the JH shop manual -- the ignition timing and cam timing areas, in particular, caught my attention...  (Snip)... My thanks to Tim for providing this information.


You're welcome.   Glad to help.

With regards to other data in TDA:    There's a bit of a caveat for J-H owners using Lotus data...  cam timing specifically.   I'm probably getting off-topic for this thread,  but...

UPDATE 05/08/2007:  The info I posted originally was wrong.   It was deduced from the belief that the published MOP values were accurate, but one is not.   The 100 MOP is really 97 MOP.   No one seems to know why it was rounded up before publishing, but it was.   All the other MOP values are accurate as published.

Using 97 MOP instead of 100 MOP,  the numbers all fall in line, confirming there is no keyway location difference between the J-H and Lotus versions of the camshafts and pulleys.   They're interchangeable.

There is a small difference in cam pulleys after (?) 1979 that changed the hub offset to better center the pulley on the belt; however it's not a works/ doesn't work sort of change.   J-H and Lotus pulleys of the same MOP can be interchanged.

I'll leave the original next paragraph,  but don't get sucked into the bad info.   It's marked.

There is a difference between early J-H cams & pulleys and the Lotus versions of the same parts.  (No, there's no difference)   In matched combinations (JH/JH  & Lotus/Lotus) the nominal timing events are the same;  however, the hardware is different by a small change in the angular placement of the keyways (nope, same-same).   The same difference is applied to both the cam & pulley in each pair so that the J-H pulley & cam together give the same nominal cam timing as the Lotus cam & pulley give together (nominal being 110°).   But installing a Lotus pulley on an older J-H 907 cam will increase the MOP 7.5°,   and installing a J-H pulley on a Lotus cam will reduce the MOP 7.5°.  (no, you can interchange the J-H and Lotus cams and pulleys of the same MOP)


On either engine,   Increasing MOP Advances the exhaust and retards the intake.   Reducing MOP  retards the exhaust and advances the intake.

J-H used a dual MOP pulley that gave 110° or 115° MOP depending upon which side of the pulley faced forward.   Similarly,  the Lotus dual MOP pulley gave 110° & 100°.   In order to produce those different combinations,  different keyway locations were required on both the cams and pulleys.   Later J-H engines also used the 110° / 100° pulley and from that point on have full cam/pulley compatibility with Lotus.   Prior to that time, care is required not to mix and match similar looking sibling parts.

The early J-H cam/pulley's timing events match those of what Lotus called it's C-cam.   The Lotus C-cam and D-cam both use a nominal 110° MOP (the alternate 115° & 100° MOP's are both for playing emissions games and not desireable) and would both be at some risk of an MOP-error if used with early J-H pulleys.      All other Lotus cams listed in Section TDA  have different MOP's and require different matching pulleys (E-cam/ 102.5° pulley,  107-cam/ 104° pulley,  104-cam/ 104° pulley),  so there's little chance of mixing-n-matching there.

UPDATE 05/08/2007:  The 115° MOP reduced the overlap 5° per cam for a total of 10°... from 52° to 42°.   That reduced HC emissions readings but cost power.   The early test engines met Federal specs up thru 1974, but just barely passed '74.   The change was made to give a larger margin for error/ wear, not to meet specs.

In later 907's,  Lotus switched to a 100° Intake MOP (really 97° MOP) for emissions engines.   The point there was to increase the overlap.   That allowed the intake and exhaust charges to co-mingle a bit more, giving a result much like Exhaust Gas Recirculation (reduced oxides of Nitrogen).   The extra advance also increased the horsepower, replacing Hp lost to overall emissions tuning.   Unfortunately the high end power came at the expense of low end torque,  and the overall effect was a weaker, more difficult to drive engine.   "Horsepower sells cars", but the net performance was way down.

Neither emissions cam timing MOP (115° or 100°/97°) is desireable.      The engine will run better with both of the stock cams timed to 110° MOP.

Other than that,  there shouldn't be any land mines hiding in Lotus Section TDA waiting to pounce on J-H owners.   Indulge in good health.


Last edited on 05-09-2009 05:39 am by Esprit2