cam timing - advance or retard?

[jhugh]

So I've gotten to the cam timing. I'm working on #1. According to Elgin, who reground my stock 3.2 cams to C2 spec, my target measurement at TDC is 1.29mm. I'm working with a dial gauge borrowed from Ted Robinson. It's in thousandths of inches. So my target in inches is 52 thousandths.

I've been bugging Ted all day, so maybe you guys can help me. I'm at a bit of an impasse because I can't move the pin any finer (choosing between two contiguous positions) and I'm getting either 48 thousandths (retarded) or 60 thousandths (advanced) in those two positions. Which one should I take? 48 is actually closer. Does that matter?

Bruce Anderson says that conventional wisdom is that people take the retarded route for high-end and the advanced route for low-end, but that in his experience neither have much effect. Any real world experience out there?

Thanks!

[john walker's workshop]

you should be able to get any setting you want. you can even rock gear back and forth without removing the pin to get a fine adjustment. keep trying, you'll get it. i prefer the advanced setting, but there's such a small amount between +\- that it's doubtful anyone would notice the difference. annoying to have to use a gauge in thousandths.

[Early_S_Man]

Hugh,

The increments for cams to be considered advanced or retarded are somewhat larger than the values you have cited. The fact is that every cam has a range over which it may be timed at and still be considered in the acceptable nominal range, that is, neither retarded or advanced. It is very important to get both sides of the engine set as close as possible to each other, ideally within 0.001" of each other for the #1 and #4 valves. At one time the factory used to set the value at the 'high end' of the cited range of lift values, i.e., 5.4 mm for the early 'S' cams ... so that chain wear would retard the value back to the 'center' nominal value of 5.2 mm when the engine was broken-in.

BTW, my HP calculator says 1.29 mm is 0.0508" ...

So, I would recommend using the advanced setting of 0.060" ...

There was a board member that had his SC cams retimed inadvertently by his mechanic from the full advanced setting of 1.4 mm back to the retarded setting of 1.0 mm ... and he DID notice a loss of bottom-end torque and responsiveness, and wondered what had happened during a tune-up. After the cams were retimed to the advanced setting, the power and torque returned!

[HawgRyder]

Advanced = more lower end torque
Retarded = more HP at top end.
Bob

[GIBSON]

I jumped into a post on this subject some time ago when the issue of adjustable cam timing came up. I've thought about it quite a bit since and changed my thinking somewhat on the subject.
I imagine that Porsche designed the cam sprocket arrangement as a solution to the problem of how to time various cam profiles to the crankshaft position. I'm sure they carefully calculated the number and size of the holes on the sprocket and then created an allowable range for each camshaft profile produced. The result of their design does allow some flexibility in positioning the cam in relation to the crank. I'm still having a hard time calling it an adjustable cam sprocket (although someone has introduced an aftermarket system similar to those made for other types of engines-most noteably the VW8 and 16V overhead cam engines- designed to allow more fexibility in adjustment for Porsche cams, the company is MPD Motorsports).
I think it's safe to say that some 911 cam profiles would offer more benefit than others to advancing or retarding a cam's timing.
It may also be true that the more radical the cam charateristics, i.e. the greater the lift and duration, the less likely you are to find much improvement in altering the advance/retard of the cam.
I know that timing an "S" spec cam offers very little deviation from spec before piston interferance and even within the spec range I really wonder if any advantage that could be noticed or measured would be gained.
So, not all cams are created equal. Perhaps the SC cam offers more advantage in terms of setting the timing to advance or retard depending on your intentions. These cams are milder in that the CIS system will not tolerate much or any overlap. It may be that they respond better than other profiles to adjusting within the range specified.
While it's interesting and fun to experiment with cam settings (and I think Warren has mentioned that using new chains, setting cam timing toward the advance side would allow some chain stretch and a return to a more "in between" setting), it still seems to me that these trade-offs may be of little real-world benefit in many engines. I think cams timed identically using the middle of the range settings probably will work best for most engines.

Ok, let me know where I'm wrong and I'll rethink the whole thing.

[jhugh]

On John Walker's advice, I discovered the "feel" to this adjustment. Thanks John! I've got the vibe. I decided to be a little on the advanced side. My target was 52 thousandths of an inch. I got the left at 56 and the right at 57. It took me about 7 adjustments on the left before I got a value I felt comfortable with. I got that value on the right the second time.

Thanks so much, you guys. THIS is the beauty of this board. I'm up to my eyeballs in engine rebuilding arcana, and on a Saturday night I get this kind of advice. Awesome.

Cheers,