Herman K

https://rennlist.com/forums/928-foru...g-a-968-a.html

docmirror

This tool was developed to allow very accurate setting of the cam timing without the usual removal of the cam cover. The 928 is similar in head/cam drive design so that once Herman gets a baseline setting for a cam that is dead on timing at TDC, we can alter this tool to use on the 968 as well. It provides very detailed cam timing, and adjustment based on the index of the cam bolts and the cam profile.

dougs968

Interesting. Just finished reading and re-reading the "new and improved" cam timing procedures. When I did mine I used the workshop manual method with the two dial indicators, air pressurized the the system as per instructed. I know for a fact that I was within less than .001 on the intake cam dial indicator tolerance when I set it. Now, that being said, it now seems quite possible that if there was a drop in variocam, how could anyone have caught it if we didn't know that possibility exists? Although my car seems to have good power, I am not opposed to rechecking it at some point with another method. No dyno available to me so my results would be subjective to my "seat of the pants"driving. I suppose if I was very far off there would be a noticeable improvement, otherwise I have no basis of comparison.

So in summary, the preferred new method would be to come off one of the #1 exhaust lifters and not worry about the intake lifters or pressurize the variocam unit if I'm understanding this correctly. Then use the mentioned device to extra fine tune it from there.

I do appreciate any and all technical information on our cars. Herman, thanks for the post and the link. I need to study this more.

tamathumper

That's awesome!

Cloud9...68

Doug,

You couldn't have expressed my sentiments better. There has been passionate debate about this topic over the years, and I like you timed my cams using the WSM method, and have since learned that there may be flaws with this method, specifically the fact that using the #1 intake cam lobe is subject to significant inaccuracy due to the "droop" that can result from a worn variocam. I also read through the posts on the 928 forum, where the use of the exhaust cam was recommended by someone who seems to have a lot of expertise on the subject. But I haven't found a description about exactly how to modify the WSM procedure to use the exhaust lobe. Obviously it eliminates the need to pressurize the Variocam, but beyond that, I have no idea how to proceed. Are you aware of a step-by-step description as to how to time the cams using the (I assume No. 1) exhaust cam lobe? Also, this leaves me wondering why the manual recommends using the intake lobe, given the complication of needing to pressurize the variocam, and the potential error caused by the possibility of the intake cam moving during the process.

Like you, I need to do a lot more research on this topic. But first, I need to figure out why my power steering isn't working...

dougs968

Yea, more research is needed. I think I understand, but want some clarification. Kind of makes me wonder if it is possible to service or rebuild the variocam actuator. Probably not, but it's a thought.

Got a new PS pump kit and PS rack kit from Travis at Rennbay a couple weeks ago so I'm doing mine pretty soon.

Cloud9...68

As far as I've been able to gather, here are the different methods people have used to time the cams on these cars:

1. The WSM method, using the intake cam - works fine on a new engine, but with a worn variocam unit, it's likely that the compressed air will leak by the piston, not allowing it to extend fully. This causes the chain to not be taught, which can throw off the relationship between the cams by well over ten degrees.
2. The cam block method. This removes the need to tension the chain via the variocam being actuated with compressed air, but it has the disadvantage of being dependent on the ability to very accurately machine the block. There may be other problems with this method, but that's the only one I can think of.
3. Use oil instead of air to activate the variocam. Great in theory, but I have to idea to rig up an oil pressure delivery system to the variocam inlet.
4. Time the cams using the exhaust cam. This seems to be the preferred method, but I don't know how it's done, yet.

This is a very interesting topic, and I'm looking forward to learning more about it, but as I said, I first need to fix my power steering (still no assist whatsoever following my re-seal of the pump and rack), and then drive the car enough to break in the freshly rebuilt engine, before I go back in and re-time my cams.

Rob in Oz

Help me get my head around this…

From my reading on the subject, and much peering at the setup on my 968, I can deduce that the 968’s cam timing is determined by the exhaust cam timing in relation to the crankshaft position, and muddied in regard to the intake cam by the intervention of the Variocam. There is no means of adjusting the relativity of the intake and exhaust cams apart from the Variocam, unlike some other twin-cam engines, which have a vernier adjustment for each cam.

As we’re adjusting the timing of the exhaust cam to achieve a result on the intake cam, I’d assume that the exhaust cam timing is considered less critical than that of the intake cam. That is, in an engine with some wear in the cam sprockets and chain, we are degrading the timing of the exhaust cam in order to achieve “correct” timing on the intake cam.

In an “as new” factory setup, the timing on both the exhaust and intake would be as per Porsche’s specs and the camshaft adjustment gauge would fit perfectly. I can see that the cam block tool is useful for determining if the cams are a tooth out relative to one another, but apart from that, any adjustment in relativity is via the Variocam, which is controlled by the DME and engine oil pressure. As the relativity between the cams is set by the Variocam, when is the camshaft adjustment gauge applied? With air pressure? Without air pressure?

I’m wondering how reproducible the movement of the Variocam is when operated by compressed air, and how the uncertainty in this compares with the accuracy apparently required with the 2 dial gauge method, or any other method for that matter? Is the uncertainty of timing in the Variocam such that measuring the opening of the inlet valve to 0.001” is a waste of time?

Am I understanding the system?

Cloud9...68

Rob,

I can by no means answer all of your questions, but in looking at the instructions in the workshop manual about the use of the cam block, it is to be installed without the use of air pressure on the variocam. The idea behind it seems to be that it allows you to set the relationship between the two cams without the intervention of the variocam. Of course, that's just my semi-educated interpretation.

jsheiry

Any more info on this setting cam timing up with exhaust cam? Getting ready to do mine soon.

Cloud9...68

None that I know of. I've IM'd the people who seem to understand the procedure, but haven't heard back yet. I'd sure like to learn how to do this, because I suspect my cam timing isn't optimal, considering my variocam, like the rest of the car, is 21 years and 115K miles old.

Vilhuer

Its used only for very rudimentary somewhere at right ballpark setting. It makes sure relationship between cams is correct thus making sure intake cam isn't one or more tooth off in either direction.

In both 928 and 968 can adjustment is done against intake cam. This should take chain tensioner wear into account and make sure intake cam relationship compared to crankshaft is as factory intented. Depending on wear in related parts exhaust cam ends up wherever it ends up. In 968 pressurised air is needed to extend variocam tensioner so that it will take up slack from chain. If additional wear on tensioner changes cam timing when variocam is turned on factory method cannot compensate for that. Air just does same as engine oil when variocam isn't turned on. I believe this means adjustment is done on position which is same as low rpm setting for variocam.

If someone wants to do adjustment when variocam is on that would require also electicity to be applied into tensioner to make it move to other extreme. Though not sure if tensioner work properly without oil pressure. In any case factory adjustment value would be meaningless. Only way to know what would be correct value is to do factory style adjustment on fresh engine and then see what measurement it gives when variocam is applied. Porken's brilliant tool will not help there but it can be very helpful when someone wants to try what general +/- x degrees timing change will do.

Cloud9...68

You make several good points, and you seem to have a good overall understand of how the valve timing works on these engines. The concern I have is over the fact that several people from multiple forums have pointed out that attempting to use air to pressurize the variocam on anything but a new variocam unit (which costs about $1300) will allow air to leak by the worn seals, which will not allow the chain to fully extend, giving the infamous "droop" phenomenon, which throws off the relationship between the intake and exhaust cam, leading to an error in the overlap timing which can be up to many degrees. There are several potential ways around this (i.e. using oil instead of air to pressurize the Variocam, using a degree wheel on the exhaust cam to set the timing, etc.), but these also have their drawbacks (no idea how to set up an "oil pressurization" rig, not enough room to attach a degree wheel with the engine in the car).

A few people have said they know how to "time the cams off the exhaust cam", but despite requests for an explanation as to how exactly, step-by-step, to do that, this remains a mystery, at least to me.

Cloud9...68

Sorry, I forgot to ask you this specific question - how exactly does doing the cam adjustment against the intake cam take chain tensioner wear into account?

Vilhuer

No matter how worn tensioner, chain etc are crank and intake cam will be in same position compared to each other as what is measured is intake cam movement vs crank movement. As you say, thats of course only true as long as air pressure fed into tensioner keeps chain tight enough to simulate whats happening with pressurized engine oil.