maths challenge, working out lobe displacement angle
TheKingofBling - 10/2/08 at 10:12 AM
On a theroetical (for now) 8v camshaft, I have the following open/close figures...
Inlet opens 46deg BTDC, closes 78deg ABDC
Exhaust opens 74deg before BBDC, closes 43deg ATDC.
What would the lobe displacement angle (in camshaft degrees) be for this camshaft?
I have already worked it out, but would appreciate confirmation. I been trying to time in a camshaft that appears to be considerably different to the
timing figures supplied. It has already been sent back to the supplier, who insist it is fine. If someone could take the time out to see what they
come up with, it would be greatly appreciated.
I would rather not mention further detail as yet.
Volvorsport - 10/2/08 at 10:34 AM
Lobe Centre angle ?
anywhere from 102 - 120 .
centre of inlet cam to exhaust cam .
NS Dev - 10/2/08 at 10:53 AM
For that cam from those numbers, 120.5 degrees..................BUT!!!
the numbers can be pretty meaningless as obviously they depend on how the start and finish of the lift was measured, was it quoted/measured from a
given lift, or "off the seat"??
Stu16v - 10/2/08 at 08:50 PM
quote:
Originally posted by NS Dev
For that cam from those numbers, 120.5 degrees..................BUT!!!
the numbers can be pretty meaningless as obviously they depend on how the start and finish of the lift was measured, was it quoted/measured from a
given lift, or "off the seat"??
Evenin Nat 
The numbers are meaningless in terms of trying to ascertain the actual duration, but as far as the lobe displacement angle goes, they have to remain
constant, whether measured off the cam or at the valve. Think about it - max lift (and the point at which it occurs) will still be max lift regardless
of whether there is a 40thou gap (for example) from the base circle of the cam lobe or not
The only fly in the ointment is assymetrical profiles 
Way out with your calcs too mate - answer is 105.75deg!
NS Dev - 10/2/08 at 09:14 PM
Hi Stu, errrrr what can I say, DOH!!!!
hmmm humble apologies, it would appear I can't count!!!
Edited cos I really can't count and beer has really confused me now, but yep, have just got 105.75 too ((105.5 + 106)/2), right lines? or am I
being really stoopid?
[Edited on 10/2/08 by NS Dev]
[Edited on 10/2/08 by NS Dev]
MikeRJ - 10/2/08 at 09:38 PM
I make the Lobe Separation Angle 105.75 degrees, using the traditional calculations. However, I never understood how this could be accurate if full
lift did not occur exactly in the middle of the opening and closing angles, i.e. if the cam shape is asymmetric, which is surely not unusual?
code:
Inlet Duration: 46 + 78 + 180 = 304 degrees
Exhaust Duration: 74 + 43 + 180 = 297 degrees
Inlet Lobe centre angle: ( 304 / 2 ) + 46 = 106 degrees
Exhaust Lobe centre angle: ( 297 / 2 ) + 43 = 105.5 degrees
Lobe Separation angle: ( 106 + 105.5 ) / 2 = 105.75 degrees.
Stu16v - 10/2/08 at 11:40 PM
Nat, no worries mate. KoB posted here as an extension of a thread I have running on WSCC. I tried to fit a cam to his car, timed using the quoted
figures. After not being convinced all was well, I checked the timing via the exhaust valve, only to find it a country mile out!
Mike, spot on, that was the figures I came too as well. Either that or we are both wrong! I considered the issue with asymetric profiles too. But this
cam has already been sent back to the manufacturer, tested, and sent back to us with 'actual' figures. Interestingly, when using the
'report' figures, the lobe cnetreline works out to within a degree of the measured centrelines. But the best bit - the LCA has been measured
at 118 degrees! Slightly different to the cam we 'thought' we bought... 
No wonder it wont 'time-up' correctly...
[Edited on 10/2/08 by Stu16v]