How I think GT3 Variocam works

[Cupcar]

I posted looking for someone who could tell me the lobe separation angle of a 2004 GT3 and nobody responded, but I did a little measurement and found it is 122.5 degrees as cam is installed and at idle.

Factory literature says the intake cam (Variocam only on intake in this engine) can be advanced 22.5 degrees at the cam, this is a whopping 45 degrees of crank angle.

So, the lobe separation varies between 122.5 degrees at idle, decreasing, depending on load, to a full load angle of 100 degrees between 1000 and 5000 RPM and then increasing again to around 116 degrees between 5000 and 7800 RPM. See graph below showing curve in degrees of crankshaft angle.

The Cup race cars do not have Variocam and are set at a static 105 or 110 degree lobe separation angle.

Anyone else find this interesting?

[Bill Verburg]

Originally Posted by Cupcar

I posted looking for someone who could tell me the lobe separation angle of a 2004 GT3 and nobody responded, but I did a little measurement and found it is 122.5 degrees as cam is installed and at idle.

Factory literature says the intake cam (Variocam only on intake in this engine) can be advanced 22.5 degrees at the cam, this is a whopping 45 degrees of crank angle.

So, the lobe separation varies between 122.5 degrees at idle, decreasing, depending on load, to a full load angle of 100 degrees between 1000 and 5000 RPM and then increasing again to around 116 degrees between 5000 and 7800 RPM. See graph below showing curve in degrees of crankshaft angle.

The Cup race cars do not have Variocam and are set at a static 105 or 110 degree lobe separation angle.

Anyone else find this interesting?

I'm fascinated, the difference is probably due to the needs of a street vs race car. Gearing I'm sure makes a huge difference in the torque curves that you are looking for.

[kenikh]

Totally fascinated! Ah, the benefits of separate intake and exhaust cams...

So you are saying that the GT3 alters lobe spacing to optimize overlap to engine speed. That is cool. IIRC, Honda VTEC systems also use multiprofile lobes on the cams that switch between cam profiles using an actuator pin at a specific RPM...does the GT3 happen to do this, as well?

And then there is Varioram...does the GT3 use variable intake tract lengths?

[Bill Verburg]

Originally Posted by kenikh

...

And then there is Varioram...does the GT3 use variable intake tract lengths?

No but they(997 versions, 996 versions used only 1 flap)) do use a variation on that w/ 2 tuning flaps opened and closed to enhance the resonances in the intake

[jcge]

Further details can be found on the INA Schaeffler website under Camshaft Phasing Systems (for Variocam) and additionally Switching Tappet (for Variocam Plus)

Also US Patent 7284516 and US Patent 6742484 amongst others

[jcge]

(C) INA Schaeffler


(C) INA Schaeffler

Engine management system controls the oil pressure solenoid and the assembly acts as a rotary hydraulic servo (with feedback via a mag/hall position sensor not shown) and is positioned according to the maps posted by Cupcar in the first post.

[euro911sc]

What I find interesting is the steep jump at 2.5-3k rpms and then its just flat. I would have expected something more linear or smoother. I wonder if you can re-map this??

What would be cool would be to super impose a dyno run on those curves at the appropriate load level. Obviously the top curve is your typical full out dyno run, but I'm talking about an abnormal run. I wonder if you get a huge spike in power delivery with that large angular change... my guess is yes. I have no idea what this knoledge would get me, but I would find it fun to know

-Michael

[Cupcar]

Below is an image from the excellent Comp Cams website here Technical Specs & Information along with a summary of the types of change one sees when changing item 10 in Camshaft Diagram below, the "Intake to Exhaust Lobe Separation".

I used to own an Acura Integra Type R which had a cam shift mechanism (VTEC) that came into play at 5200 RPM, this number because the EPA test cycle actually does not test above 5200 RPM and I suspect this may have something to do with Porsche changing the cam advance curve at that point - maybe because they don't need the natural EGR effect to pass after this? Also the improved torque in what would otherwise be a peaky engine is a nice touch.

The wide lobe separation at idle I suspect is to improve idle quality as well as to decrease cranking compression.

Camshaft Diagram Showing Lobe Separation Angle



1) Max Lift or Nose Lobe
2) Flank
3) Opening Clearance Ramp
4) Closing Clearance Ramp
5) Base Circle
6) Exhaust Opening Timing Figure
7) Exhaust Closing Timing Figure
8) Intake Opening Timing Figure
9) Intake Closing Timing Figure
10) Intake to Exhaust Lobe Separation (LSA)

Tighten (smaller LSA number)

Moves Torque to Lower RPM
Increases Maximum Torque
Narrow Power band
Builds Higher Cylinder Pressure
Increase Chance of Engine Knock
Increase Cranking Compression
Increase Effective Compression
Idle Vacuum is Reduced
Idle Quality Suffers
Open Valve-Overlap Increases
Closed Valve-Overlap Increases
Natural EGR Effect Increases
Decreases Piston-to-Valve Clearance

Widen (larger LSA number)

Raise Torque to Higher RPM
Reduces Maximum Torque
Broadens Power Band
Reduce Maximum Cylinder Pressure
Decrease Chance of Engine Knock
Decrease Cranking Compression
Decrease Effective Compression
Idle Vacuum is Increased Idle Quality Improves
Open Valve-Overlap Decreases
Closed Valve-Overlap Decreases
Natural EGR Effect is Reduced
Increases Piston-to-Valve Clearance

[Bill Verburg]

Originally Posted by Cupcar

...
The wide lobe separation at idle I suspect is to improve idle quality as well as to decrease cranking compression.....

Yes and for emissions, as the wider angle reduces valve overlap to zero, reducing the raw mixture going into the tail pipe.

[Cupcar]

It is interesting that the Variocam ECU has to keep track of both the left and right side cams to make sure they are precisely in sync with each other as well as in sync with the engine cycle.

To do this there are 2 Hall sensors on the street engine, one for each intake cam, and the street cams each have Hall sensor discs on them that have 4 tits, each different in shape, giving 4 unique pulses per cam revolution, thus both engine cycle and cam position can be very precisely monitored and controlled.

On the other hand, my racing Cup cams, built for use without Variocam, only have a Hall disc with one tit on it.

This is because, since the camshafts are fixed, the race engine only needs a sync signal for engine cycle from the Hall sensor and thus also only uses a Hall sensor on one camshaft getting a single pulse signal per camshaft revolution.