Lotus Elan

Valve timing

PostPost by: l10tus » Fri Apr 24, 2020 8:10 pm

Using Miles Wilkins Lotus Twin-Cam Engine book as initial reference, in regard to how to establish the 'True' MOP for the camshaft timing:-

Q: is it possible to work this out without knowing the actual cam specification, make or valve angles, (as Miles calls it)?

If the exact history of the engine is unknown, there may have been cam swaps or reprofiling carried out by previous owners.

Q: Is there any laymans way of establishing the valve angles, of an unknown Cam setup?

Also, Miles doesn't go into detail about the possibility of finding/using, two differently profiled cams in the same engine,

Q: Is the use of identical or matched cams always required for best results, in a given engine?

I'm trying to establish the details in order to check and set the cam timing correctly.

Any detail or thoughts most appreciated.

Regards,
Philip.

UK '72 Sprint DHC

Sometimes my Lotus makes me cry.
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PostPost by: Chrispy » Fri Apr 24, 2020 8:52 pm

Q1 - Yes, measure the cam duration and lift with a dial gauge and a degree wheel mounted to the crank and a pointer. Cam duration is normally measured at 0.050" lift - but that depends on the cam manufacturer. There will be lots of videos on youtube on how to do it. Here's some info from Summit Racing:
https://help.summitracing.com/app/answers/detail/a_id/4700/~/camshaft-duration

Q2 - As above

Q3 - Not always. Used to be not uncommon in the japanese turbo engine tuning world to have mismatched cams. Haven't heard of it much on naturally aspirated engines though. Meausure what you have and you should be able to make a decision on if re-grinds or new cams are necessary or you can work with what you have.
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PostPost by: Davidb » Fri Apr 24, 2020 9:11 pm

Yes it can be done but it is time consuming if you do it fully. I was once provided with a camshaft for a vintage engine-I was not given any specs including valve clearances. By measuring every 5 degrees of cam rotation I was able to figure it all out. But it took time!
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PostPost by: rgh0 » Sat Apr 25, 2020 5:03 am

See my answers (for what they are worth :D) to the questions below. Hope they help:

cheers
Rohan

Q: is it possible to work this out without knowing the actual cam specification, make or valve angles, (as Miles calls it)?

Answer - You can measure the actual MOP of an installed cam easy enough but determining what is the correct MOP for a specific cam in a specific engine build is not so easy. In general MOP is related to cam duration but there is lots of wriggle room and you can play with MOP setting to alter the engine characteristics i.e. more torque versus more power. Cam suppliers will give a recommended MOP setting for their grinds but these may not be totally correct for a specific build but they are a good starting point.


If the exact history of the engine is unknown, there may have been cam swaps or reprofiling carried out by previous owners.

Q: Is there any laymans way of establishing the valve angles, of an unknown Cam setup?

Answer: I normally just send a cam to my local cam grinder to get it measured up in full detail if needed which he can do quickly for a small fee and produce a printout results. If you want quick measurements to try to identify what type of cam you have based on lift and seat to seat duration (at a nominal 0.10" lift above the base circle this can be measured by mounting the cam in a head or in a lathe and rotating it to measure with a dial gauge and degree wheel when the opening , MOP and closing events happen


Also, Miles doesn't go into detail about the possibility of finding/using, two differently profiled cams in the same engine,

Q: Is the use of identical or matched cams always required for best results, in a given engine?

Answer: The Lotus Twincam in fully developed race engines appears to benefit from a slightly more restrictive exhaust cam ( i.e. a little less duration and a little less lift) than inlet when you start getting into the 0.45+ lift and 290+ degrees seat to seat duration for the inlet cam. This helps mid range toque while not significantly affecting top end power. In less developed road engines with cams up to around 0.44 lift and 285 degrees seat to seat duration the need for this does not appear to be there but equally it would also probably do no harm. I have never built an engine with for example a sprint cam for the exhaust and a cam like a QED 420 for the inlet but I suspect you would get a similar result to having two QED 420 cams and save yourself the cost of buying two new cams.
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