Quote:
Originally Posted by bruce.augenstein@comcast.
I certainly agree that the runners are not overly long - they're just as long as they need to be to enhance torque in the high 3000s - low 4000s area.
They are in fact a ton longer than the intake runners, which are designed to provide high rpm cylinder filling.
Look, I'm not talking about flow capacity. I'm talking about the natural resonance of these individual pipes. You know. The time necessary for the exhaust pulse (pressure wave) to reach the collector, and the inevitable vacuum wave that travels back up the pipe to the cylinder head. If that vacuum wave hits the exhaust valve as it is just opening, the pressure wave out of the exhaust port/valve begins to empty into a partial vacuum, thus making for a more efficient flow.
The pipe length combined with the speed of sound constant dictates the engine speed at which this resonance is most effective.
Same thing on the intake, except in reverse.
My position is that the relatively long exhaust runners build torque at and around 3900 rpm, while the much shorter intake runners build torque at high rpm. These two factors are one of the keys to a long, flat torque curve.
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I can't argue that it is possible that the engine has an exhaust header that is tuned to maximize scavenging at 3900, maybe they did tune it as such. A person would have to talk to an engineer to find out what the optimum RPM for the header as designed was (or some testing could show it).
One thing I need to clear up with you is that intake and exhaust tuning in the traditional sense on and engine with variable cam timing is not nearly as important as in an engine with fixed cam timing. I am very aware of the use of resonance tuning in intakes and of the use of scavenging to tune exhaust headers. With the S65 both duration and overlap are adjustable so essentially the intake and exhaust length is not nearly as important as in other engines with fixed cam timing. I am thinking that they tried many combinations of intake and exhaust lengths in testing and came up with the best compromise in the torque curve. Also worth mentioning is that not only primary length is important in header design but diameter is critical too....... Too large a diameter and low RPM torque will suffer, regardless of primary length.
The intake runners or header primaries were most likely picked based on the VANOS systems ability to vary cam timing, more importantly the rate. As RPM's rise in the engine and piston speeds increase obviously the rate of VANOS adjustability is going to remain static. I am thinking that the intake and exhaust manifolds are both designed for higher RPM since the VANOS system has less time to vary timing at the higher RPM. In the low to mid RPM range the VANOS system has adequate time to vary the timing to exactly where it needs to be to meet the optimum characteristics of the intake and exhaust runners to obtain near max VE at all times.
I may not be correct but I think that if the engineers designed the exhaust headers to work best at lower RPM the higher RPM of the engine would most likely suffer. If you look at the S54 for example, the header primaries are much longer than the S65 yet the torque peak is higher.
What do you think?