mslovak1

VarioCam ---

With the stand alone engine mngt, you can start controlling the VarioCam at a very early rpm, 1000 - 1500, where you need it to give you the off-the-line torque. As I understand, once the Turbo starts spooling up, the vario cam is slowly changed to the other position, advanced (or is it retard)?

I think the new Porshce Turbos use a VarioCam also?

M

Sach951

Present gen turbos use a variable valve timing system. I do not know if it fucntions in the same way as VarioCam, as VarioCam is a very specific, early implementation of the concept.

Anyone know how exactly variocam is controlled and whether it can thus be configured for different amounts of overlap / timing and so forth? Is it a truly variable implementation or just has two or three points at which the cam mapping shifts over? Can the cam mappings be modified?

Thanks,
Sach

PorscheG96

Variocam is a chain tensioner that uses hydraulic [oil] pressure to keep the cam chain from slackening on both the top and bottom ramps. The chain is driven by the exhaust cam and is used to turn the intake camshaft. Between 1500 and 3000 RPM's [depending on load] the DME sends a 12 volt signal to the Variocam tensioner and the ramps shift downward 6mm. With the cam chain turning in a clockwise rotation, activating the Variocam shortens the chain length at the top between exhaust and intake camshafts. What this does is advances the phase of the intake camshaft relative to the exhaust camshaft by 15 degrees.

Then at 5500 RPM's the 12 volt signal is discontinued and the tensioner returns to its default position [up 6mm] resulting in 15 degrees of intake camshaft retard.

Dynamically, when the intake camshaft is advanced between 1500 [under load, 3000 w/ no load as in freely revving the engine in neutral] and 5500 RPM's the intake valves open while the exhaust valves are still open [several degrees of valve overlap] which doesn't offer the best performance for a turbocharged engine. Normally aspirated however takes advantage of the early intake CLOSING by preventing reversion of intake gases back into the intake manifold before the intake valves close. This is called the TORQUE setting.

When the Variocam signal is turned off [idle-1500, and 5500 to redline] the intake cam is retarded so that it closes later. Dynamically at high engine revs each cylinder has less time to fill...closing the intake valves later allows 15 degrees of extra cylinder filling time before the valves shut. This is better for horsepower.

I think what Vision did with a 550 horsepower 3 liter engine of theirs was allow the Variocam to activate at the same time [1500 RPM's] but they deactivated at 4000 RPM's when the turbo hit full boost. This way the torque setting with increased overlap didn't allow boost to be blown through the exhaust valves before compression and ignition take place. Aside from changing the RPM at which the activation/deactivation signal is supplied by the DME there's no other way to modify the Variocam.

Variocam PLUS is completely different...it works more like VTEC than first generation Variocam. I think it uses different cam lobes to make changes to valve timing, duration, and valve lift in varying RPM ranges. It's a much better system.

BoostGuy951

As far as I know, it functions similar to Honda's Vtech. A ECU controlled solenoid functions as a toggle switch between two cam profiles. I doubt you could change the overlap. It probly just increases lift and duration by having a bigger lobe. I don't think you can change the lobes' relation to each other to eliminate overlap.

Edit: disregard this post... PorscheG96 knows much more about it

What years did the 968 have VarioCam plus?

PorscheG96

Variocam PLUS didn't come out until 996 era. '98 maybe? All 968's use the same early Vario.

Sami951

edit: I'm too slow to write, PorscheG96 seems to know his stuff!

Sach, if I've understood 968 variocam correctly, it could be used to advance one cam slightly and retard the other... the cams are connected with a chain, which is then kept tensioned by this variocam thing (in 944S and S2 there's just a simple tensioner instead), essentially by pushing the slightly-too-long chain up- or downwards from the middle.... if I drew a picture it would be a lot easier to explain anyway, as you can imagine when this chain is pulled upwards, the cams rotate slightly towards each other, and the other way around they rotate away from each other, thus changing the timing!

Hmm, now that I think of it, since the exhaust cam is driven straight off the crank, variocam probably doesn't do squat to it's timing... it would only affect the intake cam. But in any case it's truly variable, not two or three points like VTEC ... but there's only so much you can do with it, I guess total amount of retard/advance is maybe five degrees or so?

If I have this completely wrong, please, educate me

btw IIRC Ferrari uses some really wicked 3d cam profiles to accomplish this. I imagine a design like that allows smooth transition in all parameters, lift, duration and timing... nice niiiceee

Sach951

So if I understand correctly VarioCam actually hinders true output below 5500 RPM loaded, at which point it switches over to a performance cam setting versus the driveability side of things? Has anyone removed VarioCam from a 968 16V head? How much more does the 16v head flow than the 8v 2.7 head? And does the 8v 2.7 head flow more than the 951 8v? Would you be able to do extensive head work on one of the 8v 2.7 heads to get it to flow like a 968 16v a la the sort of thing that Tony G has done with his 951 head?

Thanks,
Sach

mslovak1

I think the VarioCam, with a Stand Alone Engine Managment system, can be matched to give a good performance increase until the Turbo starts kicking in. Then the management system will put the advance/retard in the proper position for optimal Hp. All programmable by the tuner.
I think Huntley has a software program that allows them to input the VarioCam variable and see what the results are in a simulation.

M

fast951

You don't have to use a Stand Alone, a good PiggyBack such as the SMT6 can control it...

Sach951

So in truth with standalone EM variocam isn't much of a concern? Other than that the major concern with using a 16V head would be compression... I thought that low compression pistons were to go into production recently designed for use with the 16v head versus the 8v?

Thanks,
Sach

pete944

From what I've gathered here the new Mahle pistons are only for 3.0L 8V motors.

Duke

Well it's one thing to say a certain standalone is capable of doing it, to really DO it and prove it works as a completely other thing!

Chris White

Controlling the variocam is no big issue with the TEC3, what that will do for you is a different problem! I don’t have the 968 cam duration specs on hand so I can’t see it you can et the kind of low overlap that you need. I would like to see if it is possible to retrofit the hydraulics of the variocam to a S2 head….
Yes, the new pistons (Mahle via Andial) are designed for the 2.7 2v head. JE does make a nice dished piston for the 3liter, but you need to sleeve the motor.

Chris

mslovak1

Chris,

If the duration did not permit the needed overlap, could'nt the cams be replaced?
If sleeving the motor, you could go to 3.2 liter also for the same cost!
Are there any detriments to doing the sleeving? You could choose your desired compression ratio also.

Can the AluSil block handle the higher temperatures that 400 - 500 Hp will be producing?

Chris White

There aren’t many options for 16v cams – and a set of 16v s2 cams can run $1,500 to $2,000. But I guess the answer is yes, its just expensive.
THE down side of sleeveing – if you use the correct clearance you may hear a little ‘piston slap’ when the motor is first started (cold). The aluminum pistons on a nodular iron bore need more expansion room than the aluminum /Alusil combination.
You can up the diameter a little but not much as it makes the sleeves thinner and thinner. I think you would get into an area where you are adding more displacement but weakening the bores a bit. I would not do it on a race motor but a street motor would be OK.
I have not had any cooling issues as long as the rest of the system is in good shape. The GT3 car does have a larger custom radiator but the 370rwhp car can run at full load for 45 minutes at a time (Watkins Glen/VIR/Mosport) without any signs of heat issues. Typically the temp gauge does not go over the first white mark.

Chris White