High-Lift Camshaft Timing

[dennisv]

Referencing the How to Rebuild and Modify Porsche 911 Engines book. Can someone educate me on:
1. Why there is a separate set of cam timing instructions for High-lift, High-Duration Camshaft Timing?
- Reduced-lift instructions: rotate till your dial indicator reads desired value. Remove pin. Turn crank to Z1.
- High-lift instructions: rotate crank to Z1. Remove pin. Turn cam to desired value.

Won't these both deliver the same result, regardless of reduced-lift or high-lift ?

2. Why "you need to keep the dial indicator pegged on the desired value while you are tightening it (camshaft nut) down"?
- If the pin is installed in the sprocket, it seems to me the position of the sprocket to the cam, and therefore the camshaft relative to the crank, is effectively fixed.

Thank you.

[Flat6pac]

The pin in the sprocket has a variable
Bruce

[Jeff Higgins]

I think Wayne's book is a bit misleading on this. This is not a matter of camshaft profile, it's a matter of how the drive sprocket is secured to the camshaft. Early camshafts (what Wayne refers to as "high lift") have the protruding center "spud" with a big nut that goes over it and is tightened down to secure the sprocket. Later cams do not have that "spud", nor the big nut. They have a center bolt.

We put a special wrench over that center "spud' to hold it while we torque the big nut. That "spud" also allows us to turn the camshaft separately from the crankshaft. We take advantage of that and set the crank to "Z1", then disengage the camshaft sprocket and turn the camshaft to the timing spec.

Well, we cannot do that with the newer center bolt design. There is no way to turn the camshaft separately from the crankshaft. Now we are forced to use the crankshaft to turn the camshaft to the timing spec. So, we do that, then disengage the two and move the crank back to "Z1".

If we are running "high lift" camshafts in a 3.0, 3.2, or any engine with the center bolt design, we still have to use Wayne's "low lift" timing method. So, it's more a matter of Porsche having upgraded the design of the end of the camshaft where the sprocket attaches than a matter of lift. Kind of misleading the way Wayne describes it.

[stownsen914]

To answer q #2, the pin does more or less fix the camshaft position relative to the crankshaft, but there is some slop. It's enough that the lift value can change by a few 10th of a mm as you tighten the camshaft nut. The guidance to watch the dial indicator while tightening is so you are aware if this happens and can readjust if needed.

[wgirardot]

[QUOTE=stownsen914;12340277]To answer q #2, the pin does more or less fix the camshaft position relative to the crankshaft, but there is some slop. It's enough that the lift value can change by a few 10th of a mm as you tighten the camshaft nut. The guidance to watch the dial indicator while tightening is so you are aware if this happens and can readjust if needed.[/QU

I agree. The pin get's you into the "ball-park", but you still have have to torque down and get your exact value in the process and try to be exactly equal on both sides.

Bill

[dennisv]

Thanks for the replies and insight. It definitely helps to understand the theory behind it.

In practice, I am finding it pretty much impossible to get to repeatable hundredths of a millimeter accuracy, between:
* Eyeballing the alignment of the pulley notch to the case parting line
* Slop in the sprocket pin
* Torque needed for the sprocket nut

I appear to have gotten them both in the specified range, but neither are dead on for the mid-point target. I hope my engine doesn't suffer because of it.

[Dpmulvan]

Originally Posted by dennisv

Thanks for the replies and insight. It definitely helps to understand the theory behind it.

In practice, I am finding it pretty much impossible to get to repeatable hundredths of a millimeter accuracy, between:
* Eyeballing the alignment of the pulley notch to the case parting line
* Slop in the sprocket pin
* Torque needed for the sprocket nut

I appear to have gotten them both in the specified range, but neither are dead on for the mid-point target. I hope my engine doesn't suffer because of it.

If your that worried and have $400 to burn. https://www.stomskiracing.com/products/digidix

[Classic 911]

Thats a super tool, using it when dialing in cam shafts.


Sent from my iPhone using Tapatalk

[mikedsilva]

My thougths on the difference between high and low lift cams, is depending on the cam, it may 'bounce' back on you. To counter this I install rockers in both intake and exhaust. Never had an issue doing this, and I usually rotate till the cam lift is on target, then loosen the nut and rotate crank back to zero.

I just timed a set of higher lift cams on a 3.2 engine a couple days ago.

For TDC, I always use a digital degree wheel like the stomski tool: why? Because I have personally witnessed pulleys that are not always accurate to TDC. Firstly, the pulley can rotate a degree or two with the slop in the locating dowel between the pulley and crankshaft. But in this actual engine, the pulley is off by a couple degrees too.

I swapped the clients pulley with one from my own stock, and my pulley was almost perfect inline with the case centreline. So this just reinforced my thinking to always use the digital tool.

I've been told that I over think this and waste time being overly accurate, but I cannot see how it hurts.

As for the sprocket locating dowel on the end of the camshaft, yes, there is a LOT of slop in that too which allows you to dial in the left and right banks. I typically aim to be within 0.02mm of left and right.