Turbo Cams and their optimisation

 

For the uninitiated, here is some basic info on cam dialling, from the bike tuning expert Muzzi (local copy here)

 

Hot cams for turbos

Cam lobes push the valves open, so that gases can flow in and out of the engine. We want out engine to make lots of power, so we want cams that push the valves further (more lift) and keep them open for longer (more duration) - ideally without hitting the pistons or other valves (zero interference).
So far, so good.
Hotter cams will do just that and this works fine for n/a engines, where the Volumetric Efficiency (VE) can be pushed upwards a few percentage points (for a specific rev range, at the expense of the rest of the powerband, but let's not get into that now. If you want a free lunch better collect Burger King vouchers).

Turbocharged engines however can achieve VE figures that are simply impossible in n/a engines - typically 200% or more. Therefore there is less incentive to compromise on drivability for the sake of a few percentage points. It's not worth it.

Typically turbocharged engines have lower lift and less duration than their n/a versions. There are sound reasons for this. Using hot cams on a turbo can easily result in power losses all over the rev range. Lose - lose situation! This doesn't mean that cams 'hotter' than stock cannot work properly, of course they can - but cams that are considered as 'race spec' for n/a versions of an engine will not work well at all with the turbo. And here lies an opportunity: the stock cams of the n/a version of our engine will quite probably be as hot as we want them for the turbo.

 

Cam Timing and Compression Ratio

Cam timing and the dynamic CR are interlinked, as we've seen elsewhere on this site. However we should also note that some forms of changing the static CR can also affect timing. How?

Obviously if the CR is manipulated via a new set of pistons, the cam timing is not affected. But if we use a thicker gasket, a steel plate or skimming of the cyl head, then the stretch of the cambelt will change beyond the factory spec.

A thicker gasket or a steel plate will stretch the belt further,bringing the timing forward. The cam timing will now be slightly advanced.

On the other hand, a skimmed cyl head will result in the timing being slightly retarded.

Not widely known facts, but then again most interesting tuning facts are kept secret...

 

Handling the cams

The cams are very easily damaged. They look robust and stiff, but they bend pretty easily. They shouldn't touch anything, the lobes and the journal surfaces are very sensitive. If you have to take out their pulleys, remember that you need new stretch bolts for reassembly

Always observe the tightening sequence and final torque values when it comes to installing/uninstalling the cams. Asymmetric loads can bend them. When tightening/loosening them up treat them like you would treat a head gasket. Same pattern and only turn the bolts half a turn at a time.

 

For 2JZ-GTE specific cam info click here:

 

For C20LET specific cam info click here:

 

Here is a cam glossary (local copy here)

 

Further reading

Below are some extracts from Robert L Norton's excellent book on cam design and manufacturing

Back to the Cylinder Head...

RB25 DET

There is one inlet cam lobe per valve, with a duration of 240 degrees, and this is advanced and retarded over a range of 20 degrees. The advanced timing is inlet opens 20 BTDC, closes 40 ABDC. The retarded timing is, inlet opens at 0 TDC, closes 60 ABDC.

The cam is retarded at idle for just about zero overlap, it is fully advanced at around 1,500 RPM, and again fully retarded above around 4,200 RPM.

These timings give a very worthwhile low RPM torque boost of around 5% on the RB25DET. The same valve timing on the RB26DETT does stuff all.

Both the RB25DET and RB26DETT both have 240 degree duration (seat to seat) inlet cams, although the lift profiles are quite different. The factory RB26DETT inlet timing is: open 4 BTDC, close at 56ABDC.

 

VG30 DETT

NVCS (Nissan Valve Timing Control System) was used to vary the timing of the inlet cam by 20 degrees:

	Inlet Valve Opens	Inlet Valve Closes
NVCS on	19 degrees BTDC	49 degrees ABDC
NVCS off	-1 degree BTDC	69 degrees ABDC

The timing of the exhaust valves was fixed, with opening occurring at 59 degrees BBDC and closing at 9 degrees ATDC.

At idle and low loads the NVCS is off, while at medium to high level loads at less than 6100 rpm the NVCS is on. At all loads above 6100 rpm, NVCS is off. The action of NVCS makes a substantial difference at engine speeds below 6100 rpm, adding as much as 30ft-lbs to the torque output.

The 28 degrees of overlap when NVCS is on is high compared with what would be used without having variable valve timing. The similar era Nissan RB30ET SOHC turbo in-line six (as fitted to the Holden VL Turbo) can be compared - it has a cam overlap of only 18 degrees. Note that the graph shows that more torque is obtained everywhere under 6100 rpm with the NVCS 'on' valve timing; however the 8 degree overlap that occurs with the NVCS off helps give a very smooth idle.