930/16 Cam Timing Questions for the Experts
 

Here is a brief background:

About a year ago I bought a running 930/16 motor with good compression from a wrecked '83 911SC. It will be converted to run 40IDA Webers and headers and then will be installed in my '72. Before the Weber conversion, I'm doing a light refresh on the motor. In terms of "light refresh" I'm referring cleaning, resealing common oil leaks, valve adjustment, upgrade to Carrera chain tensioners, and general maintenance such as alternator belt and ignition (plugs, wires, cap, rotor).

I'm presently in the process of doing the Carrera tensioners. After reading Wayne's "How to Rebuild your 911 Engine" book, I'm sensitive to verifying that the cam timing is correct before proceeding. So, I decided to check the "Intake Valve Lift at overlap TDC with 0.1mm (0.004-in) valve clearance." I followed the process in the book to check the left and right sides of the motor.

I repeated the process several times and got consistent results. Both side are measuring the same at 0.9mm (0.035-in). Incidentally, this appears correct (albeit on the low side) for a 930/10 motor. However, Wayne's book indicates that Intake Valve Overlap for a 930/16 motor (my motor) should be 1.4 - 1.7mm (0.055 - 0.067-in).

So, finally to my questions for the experts...
Is it OK to run my motor with the smaller Intake Valve Overlap of the 930/10 motor?
Is there a reason that someone might have done this intentionally?

Thanks!

I just did the timing on my SC motor yesterday, but i'm using DC60 cams and my values were different. I'm not going to comment on if your car is fine to run with slightly retarded timing, but will ask why do you plan to run CIS cams with the Webers? Seems like a great time to install something new.

SC and Carrera are all the same cams
Spec on the cams from the factory. Change according to application, compression rating, and use on the road.
The set of .9 mm on a 930/10. Makes more power at the higher RPNs.
The 930/16 is a 9.3 cr with a setting of 1.7, giving more response light to light.
As soon as you get rid of the wave piston surface the cam can work out as far as 2.5 I think

Bruce

@3literpwer: point taken and I'm definitely going to consider a cam change. At the moment I'm anxious to get the motor in the 72 and start shaking it down. Was just trying to avoid the slippery slope but you're right that now is a good time to consider the change. I also associate cam changes with piston changes. If I'm not changing the pistons, is there still power to be unlocked with a different cam in concert with the Webers?

@Bruce: So if I'm understanding, shorter Intake Valve Overlap shifts power to the higher RPMs? Or said a different way, retarded cam timing provides more power at higher RPMs?

Sounds like there are no issues running the motor the way it is. I suppose if anything it's "safer" that at TDC the valve is slightly further away from the piston.

Thanks guys for your input

@3literpwer: point taken and I'm definitely going to consider a cam change. At the moment I'm anxious to get the motor in the 72 and start shaking it down. Was just trying to avoid the slippery slope but you're right that now is a good time to consider the change. I also associate cam changes with piston changes. If I'm not changing the pistons, is there still power to be unlocked with a different cam in concert with the Webers?

@Bruce: So if I'm understanding, shorter Intake Valve Overlap shifts power to the higher RPMs? Or said a different way, retarded cam timing provides more power at higher RPMs?

Sounds like there are no issues running the motor the way it is. I suppose if anything it's "safer" that at TDC the valve is slightly further away from the piston.

Thanks guys for your input

Isn't the exhaust valve closer?

chis_seven, based on figure 7-14 in Wayne's book my understanding is the that intake and exhaust valve lift is equivalent at TDC overlap.

Here is an old thread which I hope explains.


http://forums.pelicanparts.com/porsche-911-technical-forum/79012-piston-valve-clearance-explained-excel-graph.html

chris_seven, thanks for cool thread link - very helpful