[DayDreamer]

I've read references to retarding or advancing the timing on the 94/968 range of engines. I'm a bit confused, as from what I have seen from my cars is that the distributer is mechanically driven off the one cam which is driven off the crank. Basically, its all mechanical. The ECU does not have direct control of the ignition spark other than the influence via variocam that 'shifts' the cam and thus effects the distributor/spark. This 'control' is not in the hands of the user, its purely ECU driven.



Am I thinking right? And if so, can I assume these guys are talking of third party 'Sparky' systems that do not use the distributor.

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[flash]

this topic has been moved to the proper location - please consult the Tutorial for a list of categories



you can adjust overlap timing by rotating the exhaust camshaft



as for ignition timing, it is indeed ECU controlled, but that is adjustable if you know how, and have the necessary tools

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[tamathumper]

Or you could get different cam profiles, but that's a "forklift upgrade".

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[chudson]

Hey Flash - bat this ball with me. How would you change the valve overlap (cam timing) anything less than 1 full tooth since the intake cam is chain driven by the exhaust?



Just to insure we are discussing the same subject, I'm referring to valve overlap that occurs at exhaust stroke TDC.



For those reading this that don't understand the subject matter, valve overlap increases volumetric effieicncy in the cylinder. Here's how.



When the piston is at top dead center (highest point of travel) during the exhaust stroke the camshaft begins opening the intake valve before the exhaust valve fully seats. During the short period both valves are open the exhaust gas rushing out of the combustion chamber helps "pull" the fresh air / fuel mixture in. The Design 1 supercharger makes this process even more efficient by adding pressure to the intake system so the fresh mixture is pushed (rather than sucked) in.



Sorry, I tend to ramble........

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[flash]

there are slots on the exhaust cam gear - pull the rotor and you can see them

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[chudson]

Yes - agree, but that changes (adjust) the timing of both cams simultaneously. My question is how can you change overlap as earlier stated. To change the duration of valve overlap one has to change the position of the cams in relation to each other. I.E., the timing of one cam would have to be advanced or retarded with the timing of the other remaining unchanged. This is the only way overlap can be altered. The POINT at which overlap occurs can be changed (which would change valve opening / closing timing also). Is this what you were saying?

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[DaveN]

For those of us who need more info here is an excellent article on all things valvy.

http://www.mechadyne-int.com/vva-referen...ssions.pdf

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[flash]

i would have thought the 2 cams were locked together too, due to the fixed length of the chain, but pete was very clear in telling me that it only changes the exhaust cam in relation to the intake cam - the only thing i could surmise is that the variocam allows for a change in distance between the 2 cams

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[chudson]

Agree. By pushing on one side of the camshaft drive chain (which shortens the other side) the variocam does indeed rotate the intake cam to new position to change overall camshaft profile. The exhaust cam, being physically held by the timing belt gear, cannot change position in relation to the timing belt gear with engine running. I was reading into your statement that there was a way to manipulate camshaft timing (in a static position) to alter valve overlap duration. I now realize you were referring to being able to change overlap timing which is, in effect, what you are setting when you degree the cams in a 968.

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[Cloud9...68]

Dave,



That looks like a great article; I'll try to read it when I have some time. It never ceses to amaze me how much has to work in harmony to accomplish the seemingly simple task of propelling an automobile down the road. And to think that simply attaching a battery to a DC motor will accomplish much the same thing... But the devil is really in the details.

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[DayDreamer]

Ok... so if I have it right, the advance/retard is purely cam/valve related. I always thought it was to do with the moment of spark. Yes?

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[chudson]

The variocam mechanism changes camshaft overall profile by rotating (repositioning) the intake camshaft at specific times.



The engine control module (aka DME) has complete control over spark advance and fuel delivery. It determines proper ignition timing by monitoring several sensors and, after analizing those imputs, referencing a "map" that specifies optimial ignition timing and fuel delivery.

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