Another cam timing question
 

hey all

I am doing some baselining of my engine before i rip her apart for a head stud reapair. Up until this point my car has been running spectacularly. With a lot of Jump , no oil usage and great overall performance.

I have a stock 3.0 lite 78 SC . I did notice the case had been split bfore i ever got her so maybe a little more investigation is in order.

The reason for this post is as follows.

I have done my valve overlap and for a 78SC it is suppose to be 1mm or 40 Thou.

when i check it i consistently get 25 thou. I checked the intake of both cylinder 1 and 4 and get the same result. i even went as far as setting up a second dial indicator on the opposite cylinder/piston to measure TDC accurately and the results are the same.

i also verified the valve clearance before i started on both sides and also checked the cam partnumber a it is stamped on the front.

I guess my existing cams could have been reground and i would not know that.

is there any particular reason anyone would set this up intentionally this way ?

Anyway just lookin for input. i think when i put it back i will set it up at the required 40 if i cant come up with a resonable explanation.

Cam Timing..........
 

Did you check the valve clearances? If that number is not "right on" it will affect your cam timing specs. On the other hand, the cam timing could have been set incorrectly when the engine was apart earlier!

perhaps chain stretch or maybe even sprocket wear. more likely the chains.
if they stretch the valves would open later making the lift at overlap a smaller number.
this would retard the cams giving you better bottom end.
or, maybe they were set that way on purpose. porsche moved the cam timing around on teh SC so check the specs for other SC's, maybe they were set for another year,

hmm that is a reasonable explanation .. i will have to think about this some more. i was hoping to do this repair without splitting the case but fully under the understanding that depending what i find I may end up there.

Per my remembering what Bruce Anderson says there is a range of lift at overlap TDC. One year they set it at the front end of the range, next year at the back of the range (talking loosely here) and by the time they got to the 3.2's they just set it in the middle.

I will scan Anderson's book when I get to the shop to see if I can find the info.

My gut feel is that it has been adjusted to those specifications because it's spot on.

yup i was think it was set there intentionally also since both sides are identicle.

i am going to pull the chain when i get there and check it for stretch. there may be a way for me to indirectly measure IMS gear wear also.

bob, that is what they did. i did not remeber the years and the specs
advanced, retarded then in the middle. so someone (someone like me) might have changed the timing to one of the others years.