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peak boost pressure at higher speeds than at lower speeds. Recently while up in Minnesota with Tyler to build his car and tune it, we were seeing a max of only 22.5psi on the dyno. On the street, the car doesn't hook up until third gear, and at this higher speed (of third gear), we were seeing up to 23.7psi at the manifold. The reason I note that this was in third gear is because there is a load difference between the chassis dyno roller and the actual load the engine sees while the car is on the road. The drum represents about 75% of the actual mass of our cars, which puts a lesser load on the engine than you would actually see as compared to teh car on the road in the same gear. I am saying all of this to illustrate that the reason for the higher boost while at speed had nothing to do with the difference in load. Also, remember that this is the absolute peak boost level that his turbos (GT28R) were capable of producing. This variation in boost pressure would not be noticed if you were running lower boost levels where the EBC was regulating the boost pressure actively. In Tyler's case, the solenoid was at 100% duty and the turbochargers were producing as much boost pressure that was possible. Tyler has one of my XStream dual intakes and a US-Spec fascia. The splash guard is in place. A turbocharger is just a pressure multiplier, so the greater the pressure coming in, even at the same shaft speed of the group, you will get more pressure coming out. Based on this idea, it is very clear that there is a bit of RAM-AIR effect taking place in the frontal area of the vehicle where the dual intakes are at. The area of the openings in the fascia allowing inlet air are significantly larger than the area it moves out (through the AC condenser, radiator, engine bay), so at higher speeds there is going to be an effect. In Tyler's case, it was worth another 1.2psi or so from the turbochargers and at the same efficiency. The nice thing about this effect would be that even if you weren't running your peak boost and say, running it at 18psi, your EBC will keep the pressure the same (well, I say that generally speaking, some EBCs aren't active controllers), but your compressor will likely be operating at higher efficiency (and this is dependent on what turbo you have and where it falls into the map at 18psi, but generally speaking, at 18psi for most turbos, you would find better efficiency at a lower boost pressure, ESPECIALLY with stock turbochargers). I recall some time ago when I still lived in Gainesville, FL, a group of us guys with pretty well modded cars used to go out in the wee hours of the morning of a particular day and do a lot of street racing. This one time I was running against a friend in his single-turbo Supra and at some point further down the road than the 1/4 mile (we didn't let out as it was a good race) at around ~140MPH, I heard this loud BANG! followed by a harsh scraping sound. I thought some small animal ran out in the road and I hit it as the noise was coming from the front end of the car. I slowed down and pulled over to find that the entire lower skirt that bolts to the fascia had ripped loose from the fascia and was dragging on the ground. It totally destroyed the piece as it literally ground away about the first 6" of the leading edge of the plastic. Now dont take this example the wrong way - this wouldn't normally happen with a Z that had a brand new fascia and underskirt. There were a few of the holes in the underskirt that had torn through already and it wasn't fully fastened, but it wasn't dangling on the car either - that is, until I took it up to ~140MPH. I call it my "Fast and Furious" experience - at least it wasn't the passenger floorboard. :) But, it is very clear why this happened - all that ram air was basically producing a pushing force on the underskirt downwards and the fasteners and plastic was no longer able to hold it. When it happened, it wasn't just noise either - I could also feel what happened it was that voilent. Sorry for the long reply here - I've just been giving the info I gave to you here a lot of thought recently. I've been working on my car all weekend putting my intake piping setup together and have been thinking of trying to enhance the ram-air effect by building some sort of airbox. It will probably be tricky to build something like this that creates a good ram-air effect, diffuses the air properly to avoid turbulence at the MAS, and also fit into the front of the car.
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[ agpowers@bellsouth.net ] Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the tailor.
Albert Einstein
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