More is never enough and too much is just right.David Redszus wrote:Many years ago, I had the opportunity to ride around the Porsche test track at Weissach in a 928 driven by Helmut Flegl the design engineer of the car. He wrung more performance out of that stock car than I believed was humanly possible. Now I know better. There is always more to be had.
The double hump in the stock setting comes from the dual mode resonance intake. It's either a single plenum intake or dual plenum Helmholtz resonator, mode selected with a vacuum actuated "flappy" valve. Here's the whole torque curve with the flappy valve open:David Redszus wrote:Notice the double humped torque curve in both the stock plot and the turbo version. A neat trick by Porsche to widen the torque band and make the engine much more drivable for the street.
We'll fill in that little dip in the boosted torque curve by giving it a little more compressed air in that rpm range. We have a boost controller with 6 gears by 20 rpm cells to map the boost at each rpm and gear.
I have a 8.2:1 static and 6.2:1 dynamic compression engine in the garage. The plan was to blow the 9.4:1 stock engine to pieces by adding boost and then to swap in the low compression engine. The stock engine is not cooperating, however, as we have run out of boost without the stock engine having any problems. Some would say this is a good problem to have.David Redszus wrote:The static compression ratio of 9.4 might be a bit too high for best performance. We have found that by lowering the CR it is possible to raise boost and produce a smoother, more consistent burn and make more power.
We're basically at the knock limit with the stock 9.4:1 compression engine at ECU load row 240 which translates to that 650 ft-lb torque. We could blow it up by letting it know, but we'd like to blow it up in an interesting way.
By "static" I meant measured normal to the gas flow. Yes, the gauge is dampened to the point that this is the average static pressure in the manifold.David Redszus wrote:The exhaust manifold pressure was 32 psig static, and the quarter coin sized wastegate flapper's spring would simply give and open the gate. Consequently, we couldn't increase the boost pressure to fully compensate for the drop in the N/A torque curve.
The exhaust gas pressure is not static but varies considerably within each cylcle. Does the 32 psig reflect peak, average, or lowest exhaust gas pressure values?
We had inconsistent boost, too, for a while. But it was a problem with the boost pressure bending some hardware and causing leaks. Then we had inconsistent fueling, which we traced back to fuel pump controller. Then we run out of boost at the higher rpms. There's always some bottle neck...David Redszus wrote:We have experienced strange induction oscillations while under full boost at .6Hz, which caused considerable instability. The problem was evenutally traced to a low rate wastegate spring that interacted with the boost controller to produce wastegate instability. A higher rate spring immediately solved the problem.
The car's great. it's a bit high maintenance, though. About as high maintenance as having a Persian princess as a mistress...David Redszus wrote:If I did not have far too many other car projects, I would also develop a 928; it's a neat car.
