[Greimann]

Advanced? Retarded? or just developmentally disabled?



After the recent discussion about how the Variocam works, I started to doubt my own perceptions about what I have read from Porsche publications, what enthusiasts have written, and what I have learned about performance cam timing from building small block Chevy motors.



Probably the heart of the confusion is what does the Variocam do and why does it do it? Secondary to the confusion is the terminology regarding the movement of the system. Advance? Retard? In reference to what component?



I decided to forget anything I have read, sit down with the cam specs and figure it out for myself, from scratch.



The fundamentals of tuning a camshaft for ANY motor should apply to the 968 because I'm pretty sure that Porsche doesn't have their own special brand of physics built into the system. That said, the very basics are this:



A cam with a large spread between the exhaust and intake cam lobes (the lobe center angle or LCA) will have a smooth idle and good emission characteristics at low RPM. A cam with a narrower LCA will have a bit rougher idle, but will produce more torque in the mid rev range.



Additionally, advancing the intake valve opening produces more low end torque, and retarding the intake valve opening produces more torque at high RPM.



Web Cams has a great summary of these principles here (scroll to the bottom): http://www.webcamshafts.com/cam_glossary.html



So what does the 968 Variocam do exactly? First of all, only the intake cam timing is changed, the exhaust cam timing is fixed directly to the crank shaft. I think we all know this. If you can believe the ad copy in the green sales book, at 1500 RPM and below the system is in "Basic" mode where the LCA is wide and the idle is smooth and clean burning. Between 1500 and 5500 the system switches to "Torque" mode where the intake lobe is moved closer to the exhaust lobe by 15 degrees, effectively narrowing the LCA. Above 5500 RPM the system switches back to "Basic" mode to optimize the intake timing for high RPM power.



Now here is the where I think Porsche's publication goes wrong. They claim that the intake cam retards in the 1500 to 5500 RPM range. Clearly the intake cam advances in terms of crankshaft degrees because its opening event happens earlier in the cycle. The only thing I can imagine they interpret retarding as the lobe center angle is reduced from 112.37 degrees to 104.8 degrees. Either that or it is an outright mistake or a misinformation conspiracy!?



But enough of my technical mumbo jumbo, allow me to demonstrate:

The attached picture is a graphical representation of the cam lobes superimposed upon themselves and the relationship in various modes. Considering that camshafts rotate at 1/2 the crank speed, the entire circle represents 720 crank degrees and 360 cam degrees

968_cam_lobe_diagram.pdf Size: 88.53 KB  Downloads: 150

So to summarize. at IDLE, the intake cam is retarded to spread the LCA and to keep the idle smooth and emissions clean. Between 1500 and 5500, the intake cam advances to close the LCA, and make more midrange torque. Above 5500 the intake cam retards again to eek out the last bit of high rpm power.





The next part is for the motorhead technogeeks like me. The rest can mosey on to the next thread!



The raw numbers of the cam timing breaks down like this,



BASIC MODE:



IVO = 7.5° ATDC

IVC = 52° ABDC

EVO = 31° BBDC

EVC = 1° ATDC

Overlap: -6.50 degrees

Intake Duration: 224.50 degrees

Exhaust Duration: 212.00 degrees

Intake Installed Centerline of 119.75 degrees ATDC.

Exhaust Installed Centerline of 105.00 degrees BTDC.

112.37 LCA





TORQUE MODE:



IVO = 7.5° BTDC

IVC = 37° ABDC

EVO = 31° BBDC

EVC = 1° ATDC



Overlap: 8.50 degrees

Intake Duration: 224.50 degrees

Exhaust Duration: 212.00 degrees

Intake Installed Centerline of 104.75 degrees ATDC.

Exhaust Installed Centerline of 105.00 degrees BTDC.

104.8 LCA



My Brain hurts now.

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[flash]

looks like dave dragged out his spyrograph - cool



nice info dave - that's kind of what i expected - i was having this conversation, and indicated that my suspicion was that it was retarded at idle and while "advancing" above, did not necessarily mean that it was advancing like we would do in an older setup



so, it seems that a failed variocam would leave the timing retarded in the 1500-5000 range, and could therefore expain the problem in cloud's car, especially since it seems ok if he can get past 5k



thoughts?

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[Cloud9...68]

Interesting stuff, and it definitely challenges some of my perceptions. As for the behavior of my car, it doesn't really smooth out above 5000 rpm; once it starts hesitating, it pretty much keeps doing it. The only way it doesn't is if I floor it when the engine is in a very "unloaded" situation, like cruising at a steady but high (>4000) rpm on a flat road.



But how would I go about determining if my hesitation IS caused by a malfunction of the variocam? As I said in another thread, it behaves identically whether the variocam solenoid is connected or not. If I had to make a call, I'd say maybe it's a tiny bit worse with the solenoid disconnected, but that's probably more imaginary than real. What other symptoms would a defective variocam exhibit? Maybe it will be easier to tell once I fix my stray spark problem.

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[flash]

agreed- one thing at a time



one thing for sure - the descriptions of what is giong on seem to change a bit as we go along - it's important that we ahve accurate details or we can easily be chasing the wrong thing



spark, fuel, compression - verify those first - then look for the controls - you got the fuel thing handled - you found a spark problem



first, follow occam's razor



then



"When you've eliminated every other explanation as impossible, whatever you're left with, however improbable is the answer."

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[ds968]

[quote name='Greimann' date='Jul 1 2005, 10:06 AM']The next part is for the motorhead technogeeks like me. [right][post="6668"]<{POST_SNAPBACK}>[/post][/right][/quote]



Oh yeah, because the preceding thesis was so simple ! <img src="/forum/images/smilies/968/dry.gif" class="smilie" alt="" /> <img src="/forum/images/smilies/968/wink.gif" class="smilie" alt="" />



And YOUR brain hurts ? Ha! My brain exploded and leaked out my ear half way trough the second segment of the post, attempting to descifer those hyrogliphs <img src="/forum/images/smilies/968/blink.gif" class="smilie" alt="" /> <img src="/forum/images/smilies/968/tongue.gif" class="smilie" alt="" />





{ insert multiple "I'm not worthy" bows here..}



**Your wish is my command ** DG

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[RS Barn]

    I don't know where all the Porsche specs come from but I have tested the Variocam and it only changes at 4200-4500rpm. The intake advances 15 degrees.

While trying to develop a performance cam for 968's this caused a considerable problem. We can get improved top end but with a drop in torque at 4200-4500rpm where the cam timing changes. We are working with Elgin cams on an improvement over the whole rpm range.

This isn't a basic motor, everthing matters.

Pete

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[Greimann]

[quote name='RS Barn' date='Jul 1 2005, 05:59 PM']I don't know where all the Porsche specs come from but I have tested the Variocam and it only changes at 4200-4500rpm. The intake advances 15 degrees. 

[right][post="6703"]<{POST_SNAPBACK}>[/post][/right][/quote]

And that is where I think Porsche has published some misinformation / propaganda. There is no data in the service manual that tells how exactly the Variocam is supposed to work, and therefore how to troublele shoot it. The only Porsche published info on the swtiching points is the sale brochure.



Apparently the switching points are established by the computer based on a number of variables. Oil temp, water temp, throttle angle, etc...



During your testing of the Variocam RS Barn, Have you rigged a telltale light or some other diagnostic that spells out the switching in real world driving?

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[rustech]

Who would design Variocam to only advance for 300 rpm?



Is the amount of DME controlled variability really that broad?

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[LivingDust]

[quote name='Greimann' date='Jul 1 2005, 12:06 PM']The next part is for the motorhead technogeeks like me. The rest can mosey on to the next thread![/quote]



Hi Dave,



I am not near the motorhead that you are, but I am a bit of a technogeek, and I really appreciated you going through the trouble to both think this all out, and to educate us. I feel so much smarter now.



So if I understand what you are saying, idle is really in Torque mode, not "Basic" mode, yes?



Is there a way to know if the variocam is working correctly? I am finding that [color="purple"]my engine really dogs between 1500-3000 RPM[/color]. About 3000-3500 RPM I feel a massive power boost. I am suspicious that my cams are either locked in "Basic" mode, or that my engines's brain is "retarded"...

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[BruceWard]

Dave, thanks for sharing your research! Unfortunately this area of the 968 is new to me but I am sure that I will understand it someday.



Would it be possible to design a replacement for the variocam that would allow for greater adjustment, perhaps fourteen settings (every 500 rpm) instead of the current two settings? I am assuming that more frequent adjustment to the variocam would produce a stronger torque curve. I am also assuming that such a system would be very expensive but I wonder if it could be an alternative to the super / turbo charger systems available.

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[flash]

that's a lot like what i've been told the new systems are like - i've been wondering lately if something could be retrofitted

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[Greimann]

[quote name='LivingDust' date='Jul 5 2005, 09:38 AM'].....

So if I understand what you are saying, idle is really in Torque mode, not "Basic" mode, yes?



Is there a way to know if the variocam is working correctly?  I am finding that [color="purple"]my engine really dogs between 1500-3000 RPM[/color].  About 3000-3500 RPM I feel a massive power boost.  I am suspicious that my cams are either locked in "Basic" mode, or that my engines's brain is "retarded"...

[right][post="6867"]<{POST_SNAPBACK}>[/post][/right][/quote]

At idle the system is in basic timing mode, or off. This is verified by the fact that you can disconnect the plug with the engine running and nothing happens. No change in idle speed or vacuum reading.



You can drive around with the plug disconnected to see if there is any relationship between your problem and the variocam system operation. Simple and conclusive diagnostic. I just did this and the car runs fine. Really can't tell much, if any difference.



Now here is where the plot thickens. I disconnect the plug at idle and and note that there is zero change in how the motor runs. So reason suggests that if I probe voltages in the connector, I should get zero volts - WRONG. I get battery voltage at the plug at idle! WTF?



Plug and unplug the connector. Zero change idle speed or vacuum. Jump the variocam solenoid to the battery with some jumpers, the thing stutters and dies due to significant cam timing change.



So that means that the variocam system must operate on a variable current system perhaps, and the action is subtle, not on and off like a switch. I don't quite understand it yet. Maybe it is a pulse like an injector? I do know that it is supplied with constant positive voltage and the negative side is controlled by the computer.



I think the only way to tell "reverse engineer" the switching is to physically monitor the movement of the system itself. Rigging a telltale light or volt meter won't do it.



To be continued. ......

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[LivingDust]

[quote name='Greimann' date='Jul 5 2005, 07:00 PM']At idle the system is in basic timing mode, or  off. 

[right][post="6913"]<{POST_SNAPBACK}>[/post][/right][/quote]



Doesn't this imply that "Torque Mode" is the Basic Mode, and that the the intake cam is adjusted to a "High RPM HP Mode" at the crossover RPM? (Either by on/off or a by variable change in voltage...)



Can you test for voltage change by connecting up your voltmeter and adjust ing the RPM up to and beyond the crossover RPM?



It would make sense that the cams would be by default in the idle and low-rpm torque mode, and need a signal to switch to high-rpm horsepower mode. Otherwise electrical failure would result in a engine that would not run. (Or at least not very well.)



Where is the electrical connector for the VarioCam so I can unplug it and test if it changes my condition. I will try this. If unplugging it has no effect, then my problem is not likely due to the variocam. But if unplugging it makes it run poorly, then I would conclude that the Variocam is getting the high-rpm signal, and that my engine has been tuned to that cam position? (It does idle fine)



Do you think I should start a different thread about my low power at 1500-3000 RPM problem?



Scott

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[Greimann]

[quote name='LivingDust' date='Jul 6 2005, 09:08 AM']Doesn't this imply that "Torque Mode" is the Basic Mode, and that the the intake cam is adjusted to a "High RPM HP Mode" at the crossover RPM?  (Either by on/off or a by variable change in voltage...)[/quote]

In the data sheet they list two sets of timing specs and they call the one with the widest lobe separation angle (read best idle characteristics) "Basic" mode. This is where the variocam is spring loaded "home" in the de-energized position.



The "torque" timing specs show a narrow lobe separation, which is good for mid RPM torque. So I am just staying consistent with the datasheet terminology.

Can you test for voltage change by connecting up your voltmeter and adjust ing the RPM up to and beyond the crossover RPM?

Further observations need to be done for sure. My previous test was with the connector unplugged and test leads stuck into the socket. That may have been incorrect electrically to observe the function because if the system needs the resistence of the solenoid coil to work properly, then I wasn't observing the actual real-works situation.

Where is the electrical connector for the VarioCam so I can unplug it and test if it changes my condition.

Look at the geographic center of your valve cover. It is the black electrical plug that goes straight down. Squeeze the wire spring retainer with fingers to disengage.

Do you think I should start a different thread about my low power at 1500-3000 RPM problem?

If you really think you have a problem - yes, but these motors don't come alive until after 3000, I usually never drive below 3000 rpm. No power down there, If you look at a torque curve, torque is just building below 3000 and after 3000 it reaches a respectable level and peaks at ~4200. Drive it like you stole it. It will be happy that way.

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[LivingDust]

[quote name='Greimann' date='Jul 6 2005, 08:37 PM']If you really think you have a problem - yes, but these motors don't come alive until after 3000, I usually never drive below 3000 rpm. No power down there, If you look at a torque curve, torque is just building below 3000 and after 3000 it reaches a respectable level and peaks at ~4200. Drive it like you stole it. It will be happy that way.

[right][post="6973"]<{POST_SNAPBACK}>[/post][/right][/quote]



In my 94 coupe, I did not have this problem. Sure the engine came on stronger at 3K, but there was still plenty of power at 2K to spin the wheels. In my 92 cab, I have to really plant my foot in it, and wait for the engine to rev up before letting the clutch out if I want to take off in a hurry. Otherwise the revs drop and the car becomes gutless. I can count out two seconds for the engine under load to get from 1500 to 3K. Would you consider this normal?



One thing I have also notice is that my car runs a lot better when it is really hot. I don't mean a hot day, I mean the engine has been working hard for awhile.



Any ideas?

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[Greimann]

I think your other thread will cover any input on this issue.

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[LivingDust]

Dave,



Have you done any more investigation on the Variocam? I would really like to know for sure if idle is "basic mode" and not "torque mode". I have not had a chance to unplug my variocam electricity yet, as I have been suddenly hammered with a corporate emergency.



I shared the stuff you found so far with the local mechanic that does most of the work on my car. He was also very interested in what you find out...



Scott

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[Greimann]

[quote name='LivingDust' date='Jul 8 2005, 05:58 PM']Dave, 



Have you done any more investigation on the Variocam?  I would really like to know for sure if idle is "basic mode" and not "torque mode". 

[right][post="7094"]<{POST_SNAPBACK}>[/post][/right][/quote]

Like I say above, the basic mode is the idle mode. The system is OFF. Nice wide lobe centers that make lots of stable, idle vacuum. I am sure of this.



The torque mode is equivalent to a "lumpy cam" that your neighbor's 396 Chevelle has. It wont idle worth a crap, but it sure does pull once you get the revs up.



Disconnecting the variocam plug takes less time than it took to write this post. Squeeze the wire and pull the plug. Easy!



I'll be doing some more testing in the next few days.

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[tsj9197]

Greimann,

nice work on the pdf diagram, but your rotation arrow shows that you've got your engine running backwards! <img src="/forum/images/smilies/968/ohmy.gif" class="smilie" alt="" />

maybe the system would work better if it was going the right way. <img src="/forum/images/smilies/968/tongue.gif" class="smilie" alt="" />



on 968.net they have scans of a august 1992 Panorama article.

looks like some good technical details on the improvements of the 968 over the 944 including the variocam system.



http://www.968.net/pressroom/articles.htm

http://www.968.net/pressroom/images/panora...9208_43_600.jpg

http://www.968.net/pressroom/images/panora...9208_46_600.jpg

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[rustech]

[quote name='tsj9197' date='Jul 11 2005, 09:22 AM']Greimann,

nice work on the pdf diagram, but your rotation arrow shows that you've got your engine running backwards! <img src="/forum/images/smilies/968/ohmy.gif" class="smilie" alt="" />

maybe the system would work better if it was going the right way. <img src="/forum/images/smilies/968/tongue.gif" class="smilie" alt="" />



on 968.net they have scans of a august 1992 Panorama article.

looks like some good technical details on the improvements of the 968 over the 944 including the variocam system.



http://www.968.net/pressroom/articles.htm

http://www.968.net/pressroom/images/panora...9208_43_600.jpg

http://www.968.net/pressroom/images/panora...9208_46_600.jpg

[right][post="7224"]<{POST_SNAPBACK}>[/post][/right][/quote]



The arrow represents cam rotation, not the reversed order of "intake, compression, power, exhaust" stages.

It looks correct.

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