Making the 911 that Porsche were too sensible to consider

Cams.

The 3.4 litre M96 engine has four camshafts, two per bank, with the exhaust cam being driven from the intermediate shaft (yes, that intermediate shaft) via a chain.

The inlet cam is driven from the exhaust cam, again via a chain - the tension on the chain can be varied, which changes the inlet cam timing. This is called VarioCam, a simple system that has an annoying appetite for the (expensive) actuating solenoid.

Unlike more sophisticated systems there is no variation in valve lift with this system - that came with VarioCam+ on the 3.6 litre engine of the 996.2.

In the X51 package for the 3.4 Porsche changed both inlet and exhaust camshafts, interestingly the earliest engines came with lash caps whilst the later ones featured longer valve stems (valve diameters remained the same).

With the 3.6 X51 package Porsche elected to only change the exhaust cam, the inlet now having variable lift courtesy of the "+".

We had a good look for the lift and duration of the X51 cams, but the paucity of information (Porsche themselves were totally unwilling to provide any data) combined with the cost prompted us to look elsewhere.

No one makes cam shaft blanks for these, and billet cams are a) expensive and b) accelerate wear in the valve train.

That left us with Schrick cams, or re-grinds. First we looked at the Schrick offering and, whilst it was interesting, we weren't a fan of some design elements and also decided that having a grind that was 100% orientated around our requirements was the optimal solution.

This does mean reducing the base circle, but we have our billet tappet chest (so we can adjust where the upper edge of the bore of the tappet is relative to the centreline of the cam, and the oiling) and our own valves (as we upped the diameter for both inlet and exhaust) so this has been accounted for.

At the moment the thinking is to have three options - fast road/track/race, but I suspect we'd be unable to resist the temptation of fitting the lumpiest cams with the highest duration. We'll see - the cam designer can grind to order, so we don't need to make our minds up until quite a late stage.

The cams in the 3.4 are shared with the 3.2 Boxster S, so when in a 3.7 (my car) or a 3.9 (Martins) they're a significant limiting factor, therefore this was an important thing to get right.

I think three of the things that we paid the most attention to were valve and valve spring material, weight and form, every aspect of which then informed the cam design. I'm reliably informed that valve train float is A Bad Thing, so attention here is important.

Anyway, suffice it to say that whichever of the three profiles we end up putting in the engines the valves will open further, for longer.

Interestingly this has knock on effects in other areas - the Porsche exhaust cams are (relatively speaking) rather inoffensive little things, so choice of exhaust manifold design doesn't really make a huge difference. Moving to rather spikier cams means that we now need to optimise the exhaust for the cam design, so equal length primaries become important, although we're constrained by the packaging when it comes to primary length (there being a finite amount of room between the underside of the engine and the road), still we should be able to get a decent amount of tube under there.