RS_Sam

Quick question to the sea of knowledge
I have "inherited" an aftermarket engine management system in my '97 2.5l Boxster and have to program the rpms at which the system must actuate the variocam. Does anyone know at which rpm's it "engages" and then "disengages" again? any links or directions to the answer or a direct answer would be greatly appreciated.

Macster

I have various sources of this info and they are not all in agreement regarding the rpms at which this cam timing is changed but even in what direction.

I do not have the Porsche "Technik" document ( I think this is the name of the document) for the earliest Boxster which probably gives this in detail. I have one article from an Excellence magazine in which I'm pretty sure the author (the late Jim Pasha IIRC) used this Technik document I mentioned above or something similar as his source.

You might email Bruce Anderson at Excellence magazine and ask him if he has any info.

My belief is that this manipulation of the valve timing is not solely an rpm thing but the Ecu relies upon other inputs as well.

You might have to get a hold of a stock 2.5l car and characterise the camshaft timing activity by monitoring various OBD2 parameters (rpm, coolant temp, intake air temp, engine load, etc.) and noting what these are when the camshaft timing is changed and in what direction.

Sincerely,

Macster.

RS_Sam

Thank you Macster.

Adding up all of your sources what is your feeling on ballpark RPM's where the cam timing changes?
I must say that I don't know what the chances are of finding another same spec Boxster which the owner would let me analyze.
I am hoping it is a simple RPM function.

Sam

Todd Holyoak

It is almost, but not quite a simple rpm function.
On the early cars, if the following conditions are fulfilled, VarioCam is activated

1 - Engine oil temperature between -3°C and 133°C
2 - Engine speed > 1,300 rpm
3 - Throttle opening > 5%
or

1 - Engine oil temperature > 133°C
2 - Engine speed > 1,480 rpm
3 - Throttle opening > 3.9%


Variocam is also deactivated when

Engine speed > 5,120 rpm

-Todd

RS_Sam

Thanks Todd

Does this mean that the variocam only has an effect below 1300(1480) rpm and above 5120 rpm?

Sam

Macster

Looks like Todd has access to the info I suspected existed but that I didn't have access to.

Sincerely,

Macster.

Todd Holyoak

Just the opposite. The timing is advanced (vaiocam actuated) between 1300 and 5120 and returns to base timing otherwise.

-Todd

RS_Sam

Would there be any significant implication if I neglected the oil temperature and throttle opening requirements or would it mainly affect emissions?
Can anyone give more information on the exact mechanism(engine scavenging) as I thought that with low air momentum(low revs) through the engine you wanted less intake outlet overlapping and at higher revs more. Why would you need it between 900 and 1300 rpm?

Macster

Well, if you ignored the throttle opening parameter then engine might stall from the excessive intermixing of the exhaust gases of the recently combusted fuel/air mixture with the incoming fresh air/fuel mixture.

Excessive intake valve deposits could arise. The engine controller might log misfire error codes.

Similarly if the oil temperature ignored upon operation in cold weather the engine may not run properly or at all.

Yet another possibility is by ignoring some of the sensor data the engine controller could get into a destructive feedback loop in which it constantly oscillates between enabling and disabling the valve timing feature to the point the mechanism prematurely wears out or fails from the stress. This of course provided the oscillation is not so bad the engine's behavior renders the car undrivable.

Essentially you are seeking to discard operating condition considerations that many highly trained and highly experienced engine designers with considerable lab equipment and thousands of hours of lab testing and road testing at their disposal have implemented and fine tuned over the years.

I don't believe they arrived at using what that use by accident or because it was while not necessary looked impressive.

The change in valve timing at low speed is to help keep emissions down. It allows some exhaust gas to mix with the incoming fresh air so combustion is not as hot and Nox which is generated at higher combustion temperature is reduced.

It also feeds unburned hydrocarbons to the converters which combined with air from the secondary air injection promotes the burning of these hydrocarbons in the converters which warms them up quicker.

The quicker these warm up the sooner the engine controller can switch to closed loop operation and more precisely control the amount of fuel to the engine. Lower emissions is one benefit. Less engine deposits another less wear/tear from having too much gas present that washes away oil from the cylinder walls and accelerate ring/cylinder/piston wear is another. Additionally there is less gas to contaminate the oil. Oil contaminated with gasoline is not as good at providing protective lubrication to vital engine parts and this can result in accelerated wear/tear of the engine and its internals.

After 1400 rpms the timing is advanced to improve cylinder filling and the engine makes more power, more torque which improves the car's drivability. It is this feature that makes these cars so nice to drive around town, so tractable. And yet so exciting when one has the room to let the engine stretch its legs, so to speak

I have ridden in cars with modified engines that have lost their low rpm tractability and they are horrible cars at low speed operation that is so common for many of us.


Sincerely,

Macster.