SC Valve Clearance

[Kevin McAndrew]

I need to replace six broken dilavar head studs. Motor details are: engine no 63D2723, crankcase Number 930.101.103.4R with 930/10 stamped on the fan housing. This is as far as I know from the numbers a 1983 3.0 litre SC. Following Wayne’s Book I thought that I would verify the valve timing before removing the cam shafts. The overlap should have been 1 mm but was 1.5 mm on both sides. From the book this is not the setting for the 10 motor but others listed in the book.
This is not the original motor. I believe it was imported from Japan in a cabriolet
Could the settings be different for the Japanese market? What figures should I use for reassembly

[Eagledriver]

Depending on year, the same camshafts were timed anywhere from.9mm to 1.7mm. Our experience with race engines using these cams indicate that timing towards the lower end will give you the most HP albeit at higher RPM. I would do what Porsche did and match the cam timing to the intake port. 34mm port go with 1.5, 39mm port go with 1.0 mm.

-Andy

[Kevin McAndrew]

Thanks Andy

[targa72e]

My experience with cam timing is that the most advanced (most lift at tdc) results in the most power under the curve. You can get a little more hp in the retarded position but lose hp everywhere else.

dyno tuning results

john

[Neil Harvey]

I come on here often and like to read about what others are doing. When I can, I try to offer some suggestions on how we approach this work.

There is no "set" cam position on any engine. The only critical factor is the distance between the valves and the pistons.

The way the cams are set on Porsches, (lift at TDC) is a simple and effective way to set the cam settings for those that do not have a degree wheel or some sort of electronic device.

The device often discussed here that is electronic, just sets the TDC within a decimal of true TDC but does not give the actual centerline of the lobe. I have watched the video and there are procedures that are shown that are good, but do have a large amount of possible errors also. Turning the engine backwards is not a good way to split the TDC. Dead stops are not accurate as you have piston rock and engines should always be turned in their running direction. In the Porsche engine, they rotate clockwise.

Another factor to remember is this.

911 Rocker arms are quick acting, so if the assembler tries to measure the cam timing with some valve lash and its off by half of one thousands, (0.0005") which is easy to do, they will get inaccurate cam timing numbers.

The easiest and most accurate way is to set the valve lash at zero and them measure the opening and closing crank angles at 0.012" valve lift (the hot lash point). Measuring it at 0.004"/1.0mm cold lash would also give you big durations that would be confusing.

Add in rocker arm geometry often modified by re grinding the rocker faces in a quick repair just to get a good surface will change the cam timing at the valve by large amount also. This often changes the rocker ratio which has a change in lift at the valve.

So, there are a lot of factors here that will give erroneous results. The best way is to assemble the engine knowing you may have many changes from the original designed position, try the engine performance, then either advance or retard the timing position to obtain the best performance result.

Assembling the engine should include finding the piston to valve clearance so any changes in cam position later can be done knowing its safe.

Too much is placed upon a certain cam timing number used, when there are many factors that can change the performance. Nothing wrong with using a setting known for its performance, just remember it doesn't mean its the best in your engine.