The 9A1 is a remarkable piece of cylinderhead engineering. From it’s complex casting to it’s highly detailed, spectacular finish machine work it’s startling in appearance and makes an unforgettable first impression.
I approached the dissection, and study of these heads with great anticipation. I was very excited to see the progression from the M97 cylinderhead design, which represents one of the most impressive 4 valve ports and chamber designs I’ve yet to see. I felt it would be quite an achievement to improve upon the mastery of the M97, which represents the progression of the M96 head design to it’s eventual masterpiece, the X-51. Surely the DFI 9A1 would improve upon that greatness and set the stage for great performance, and lay the foundation for continued evolution of further development & improvements.
One of the obvious departures from the M96/97 design was to incorporate the lifter bores into the head casting rather than a separate lifter cradle. This could prove to be problematic in the future if wear were to be an issue as is sometimes the case with M96/97 follower housings. That said I see no obvious reasons why wear would be an issue as there appears to be plentiful supply of oil to the lifter area. More so than the M96/97.
As I revealed in my earlier discussion the first thing that jumped out at me was the unbelievable amount of carbon build-up in the intake ports and chambers. I was struck by the extreme amount of valve guide wear as well. Whether the guide wear was caused from a guide material deficiency and/or from valve spring surge and wave action side loading the valve this is a serious issue that Porsche will need to address. The guide wear was serious enough to cause concern for the life of the valve seats which do not like misaligned valve landings. In this particular case the engine was torn down before seat degradation became an issue.
Once the head was cleaned and silicon castings were drawn from the ports and chambers I was left with two distinct impressions; stunning exhaust port design, but crude intake port design.
Flow studies revealed fine curves, but analysis of all data made it clear that the intake ports are poorly shaped and clearly over sized for an engine of this displacement. This behemoth of a port is even more out of place when considered with the more diminutive size of the intake manifold runner that mates to the head. The common area before the bifurcation into individual runners is excessively large. The runner length after the bifurcation is relatively short and the highly critical area directly beneath the valve seats was crudely machined in a shape common to many manufacturers, but from which Porsche, to their credit, had faithfully avoided with all of their previous 4 valve heads from the 928-964 and all M96/97 heads. The shape is more “can” than “bowl” and is a cheap easy way to draw volume from an intake port at the expense of velocity and air motion. The net result is a rather lifeless port. My impression is that being a DFI engine the engineers felt the only criteria for a suitable intake port design was volume. This, in my opinion, was a mistake.
I was pleasantly surprised to find that the intake valve seats were machined to a profile virtually identical to that which my research revealed to greatly benefit the M96 & M97 heads. From my perspective this was somewhat validating, but it meant that if I were to find areas of improvement I would need to look past the seats to the ports themselves. This led to a bit of deflation of my optimism for these heads because the ports are already too big and the machine work beneath the seats removed the material I needed to get the shape I knew would boost performance. I was left with no good options to make significant improvements, and concluded that even were I to find any they would be for the most part useless since the plan was to retain the O.E. intake manifolds. The realization settled in that the best approach would be to simply polish the ports and make a slight adjustment to the seat profiles.
The exhaust ports design on the other hand is an excellent example of good engineering and lessons learned and applied from earlier designs. The runners are nicely shaped and of substantial length before joining, which they do just before the manifold flange. I see no areas that I would change and feel that these ports are primed and ready to feed a turbocharger.
Pushing forward I polished the ports and after installing new, slightly longer valve guides of a material known to be very durable in extreme conditions I cut the seats with the same profile with a slightly narrower primary angle. Oddly enough Porsche cut the exhaust seats with a simple, less effective profile than the intakes. I cut them with a profile that I knew would be very helpful. Exhaust port work was limited to smoothing and blending, but no reshaping.
Hoffman Automotive Machine (706)769-3783 