LoneShark

Hi all, new 928 owner here. I'm trying to bring back to life an adopted '87 S4, and am hoping the brain trust here can help me with my latest bump in the road. So far I have done timing belt, water pump, tensioner rebuild, tension check, belts, oil, coolant, plugs, and have been able to drive it to work for the first time (woo-hoo!).

Next task is to (obsessively) adjust cam timing. I'm using the workshop instructions with dial gauge, which specify "drop" of 1.8mm on #1 and 2.0mm on #6 intake. If I follow directions and measure between TDC and 20deg ATDC I average 1.42 and 1.54 mm respectively, which would both be low. However if I measure between -10deg (BTDC) and +20deg (ATDC), I get 1.50 and 1.91mm, implying the valves are opening before TDC. Tech Specs say input valve should not open until 11deg AFTER TDC, so I would expect zero drop until after TDC. So...

1 - Should lifter start to move BTDC, or +11deg ATDC?

2 - Is there some kind of slack or takeup that causes the lifter to move before the valve, and would explain the discrepancy?

3 - Do I advance the cam to bring the 0to20deg value in spec, and have the valve open even Earlier BTDC? (yikes!)

Thanks for any help or advise!

blown 87

Get the CD set from Roger, on one of them it has the factory training video which will show you how to do it.

76FJ55

It is common for intake valves to open BTDC and for exhaust valves to close ATDC. This is known as valve overlap.

jcorenman

LoneShark,

The 1.8 and 2.0mm valve-openings are arbitrary numbers, based on an arbitrary crank angle (e.g. 20 deg BTDC) for the specified cams. You are off a bit, but it would be a mistake to try to turn those numbers into real valve-timing figures. If you measure from valve-opening (usually taken as a 0.006" opening), there is some overlap at TDC.

The other option is Porken's "32V'er" tool (www.liftbars.com) which Roger sells (email Sales@928srus.com), along with the WSM and tech-info CD's. Ken's tool is a simple gauge to measure cam timing directly from where the sprocket mounts to the cam. If the cam is machined properly (which seems to be the case) then it gives accurate measurements, much more easily.

Good timing!
Jim

Vilhuer

LoneShark

Thanks for all the advice so far (esp. Jim!). So any ideas on why the Porsche Technical Specifications wouild spell out "Intake opens... 11 deg CS (?) after TDC" if the valve (or at least lifter) is already moving ~10 deg before TDC?

20 deg difference seems significant to me, as does the +/- 0.1 mm spec on lift.

jpitman2

If timing is given as 'inlet opens at xDeg BTDC', it requires some judgement as to exactly WHEN the valve starts to open. So here they are saying 'Valve should be lifted a specific amount(2mm?) at x degrees ?TDC'. this takes out the need for judgement, or skill in observing movement.
It is VERY common (normal) for there to be lots of overlap in inlet and exhaust valve opening overlap, especially in high revving engines. This is due to the effects of inertia of the air being ingested, or the exhaust being expelled, and the non-linear movement of the valves. Optimum valve movement would be close to "instantaneously fully open, pause, instantaneous close", but its close to impossible to achieve with camshafts as we know them, so we have to compromise. Maybe this is what F1 engine valves do with hydraulic pressure controls.
jp 83 Euro S AT 53k.

LoneShark

Thanks, very much, jp. Sounds like I need to quit over-analyzing and just dial in the 1.8 and 2.0 mm per the manual and dial gauge approach (which I have in hand).

For next time, does anyone know if this Porken tool dials in "absolute" zero advance (+/- tolerance stack)? Seems like the pdf instruction I scanned referred ominously to "advancing/retarding from the current setting" (or something like that). Not having to remove the valve covers would certainly be awesome.

jcorenman

Yes, within the tolerance of the parts, which seems to be quite good. The 32V'er attaches to the camshaft by way of the distributor-rotor "spider", and references the camshaft angle to the notch in the rear timing-belt cover. The spider indexes precisely to the cam, and the sprocket is adjustable against the cam & spider.

So with the crank fixed at TDC, adjust the cam relative to the (fixed) sprocket so that the 32V'er pointer is lined up with the notch. That sets the cams to the correct rotational angle for TDC. There are some nuances which the manual explains, but that's the basic theory.

Cheers,