928Belgium

Hello everyone,

I just done a head job on my 928S from 1981 after a problem with the head gasket (see my other post) but now that I'm putting the timing belt on I'm having trouble with aligning the timing belt marks . The mark on the camshaft sprocket on the passenger side is half a tooth off. Can this be a result of the machined heads (They skimmed the head the minimum amount necessary => 0,1mm which equals 0.004 inch) or is there something else I'm overseeing?


I'm installing everything new (timing belt from GATES, rollers from INA ,waterpump from LASO), I first put the belt on the crank, then oil pump sprocket, driver side camshaft sprocket (mark is perfectly aligned over here) , waterpump and then the passenger side camshaft procket, made sure every part was tight and then I tensioned the belt and checked it with a new Kempf-tool. Even after some turns the passenger side sprocket is still not correct.

Thanks for your input

Wisconsin Joe

That is normal.

It's also about 1/4 tooth, not 1/2. 1/2 tooth would put the mark on the top of the next gear tooth. Full tooth would put it in the middle of the space.

With the non-adjustable cam gears on the 16v, you just get it as close as it will go.

SwedeInSiam

Mine is same and that's normal for our engine

jpitman2

WSM says to retard the cam (turn anti-clock)until belt drops in to place. As the engine warms up the block expands and will pull the timing on that side back to spec. I asked the same question on my first belt.

jp 83 Euro S AT 57k

GlenL

Google "offset woodruf key"

Charmingly works it does.

928Belgium

Ok thank you all for the confirmation!
Firing up the engine for the first time will be stressfull enough.

Dmhager

I will not be following the WSM procedure for setting timing going forward on the 2 cam cars (although I dont generally think that I know more than the Porsche engineers, Im not afraid to improvise). Here is what I found with my 83 US spec 5-speed when I replaced my belt: passenger timing gear notch in perfect alignment, driver side notch off by a little less than 1/2 tooth so I followed the WSM and rotated the driver side cam CCW until the belt dropped in, which meant the driver side was now retarded a little more than a half a tooth. I adjusted the timing to 24 degrees at 3000 rpm/no vacuum advance (spec is 20, but I prefer 24-25 and I still need to deal with CA smog testing so I cant run the car richer and dont like the timing any higher either). The car ran ok, but the loss in low end torque was noticeable and the engine didnt run as smooth (very slight vibration, just enough to cause a little buzz in a loose door card pin - probably not noticeable to most), but mostly I did not like the shift in torque curve. After all, when I drive, while I enjoy running it up near redline to shift occaisionally, I generally live in the 2500-3000 rpm range when driving/cruising and the car just did not have the torque to respond like it used to. Another thing, I checked the timing marks hot and even with a hot engine, expansion did not move position at the timing mark at all so dont count on that (I only use Gates belts so its possible the rubber band Contis might be different, but I doubt it - expanding aluminum still generally wins). I suspect the WSM procedure was just written for simplicity and well within manufacturing tolerances. Keep in mind that the US spec cars dont have the torque or cams of the Euro cars and need all the torque you can squeeze out of them for regular driving (and after all, who doesnt like torque). Also, my 83 is a really nice car, Ive had it for years, its perfectly maintained, and Im probably a bit more sensitive to small changes in driving characteristics.

After tinkering with the ignition timing (exploring more advance and less), I concluded that it would be best to adjust that driver side cam so that its a little advanced rather than retarded (if the timing marks dont line up). I made that change, reset the ignition timing to 24 at 3k rpm and bada boom, bada bing, my sweet 83 was back the way I like it (this was the way it was set up on the old timing belt) - with this setup, the engine is buttery smooth, has good low end torque and is a joy to drive.

So, going forward, my rule of thumb will be this: if the timing marks dont line up, I will err on the side of advancing the cam until the timing belt teeth match the timing gear (rotating them CW rather than CCW as the WSM calls for), unless the timing mark is off by less than 1/4 tooth (then I would go CCW on the cam until the belt fits). Why 1/4 tooth?? Sounds good to me ... maybe Ill change my mind in the future, but Im good with that for now. An alternative would be to get an offset woodruff key to bring it into perfect alignment, but I dont think that is necessary - I love the way my car runs, Ive owned a bunch of 83-84 cars (I really like these years) and will follow this approach on others.

jpitman2

Having trouble visuallizing how you get the driver side cam off a little (assuming LHD?) and pass side on the mark with the harmonic on OT. My 83 Has NEVER shown that - always pass side (car's RHS) that is off ~1/3rd tooth, while other side is on the button. Usual effect of changing valve timing is Advance improves bottom end at expense of top end, and retard does the opposite.
jp 83 Euro S AT 57k

jcorenman

Advancing or retarding cams changes the indicated compression, because you are changing when the valves open and close. So a simple way to tell if you have the left and right cams set the same is to run a simple compression test. If one side shows significantly higher compression on average than the other, then the cams are set differently. (I think an advanced cam shows higher compression, but don't quote me on that-- it is late here).

buccicone

I always say a Hail Mary before I turn the key.

Dmhager

I am guessing the driver side (LHD/US spec car) timing mark is slightly off due to manufacturing tolerances. I dont know how far off it has to be to affect compression numbers (its also possible the key is machined slightly off, but the cam timing is correct). IIRC, my compression numbers (from a couple years ago) were very consistent ~155 (all within 5%). Regardless, my thinking is that its better to err on side of being slightly advanced, rather than retarded due to the effects on the torque curve. And FWIW, I always say a short prayer (and search for leftover parts and tools) before turning the key after surgery ... two Hail Marys on the 4 cam cars.

thepurpleblob

I'm in the middle of changing my belt (and a mountain of other stuff). Although I'd only run it for a couple of hundred miles after I bought it, it seem to run fine. However, on lining everything up it was a whole tooth out on one bank. It's now absolutely spot on with the new belt. I'm probably a couple of weeks away from starting it up but it'll be interesting to see if it makes much difference.

I am not a religious person