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patkeefe patkeefe is offline
one of the great unwashed
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Join Date: Apr 2004
Location: Ringwood, NJ
Posts: 2,850
I see alot of thought has gone into this thread, which is great. I am glad I am not the only guy thinking to himself about CIS.

My difference is that I have a "kit" on my SC 3.0. The only parts left from the kit are the boots for the metering plate inlet and the throttle body. I monitor lots of data; all analog, such as inlet temp to the airbox, CIS plenum temperature (effective temperature rise across the compressor), AFR and boost level. I am relatively low HP compared to what everyone is speaking of here, probably around 285 RWHP at 5.5 PSI boost, but it's certainly a lot more than the 161 RWHP I had as an SC. I have run various combinations of stuff for fueling, starting with stock CIS. That was good, until I got the revs up, and then it went lean. I then went IA modded SC head, which went rich all across the rev range. I went to a Leask WUR, which solved regular street driving and high RPM fueling, but I was pig rich at mid range at the track (I can't help myself, I love the car at the track). Then, I went to a Leask RPM solenoid switch, but that didn't solve the mid range problem.

So, I switched back to a stock CIS 3.0 fuel head, and I was back to good, except a tad lean at the track. Then I detonated the engine at Watkins Glen, so I have it apart, going back together now. Stephen at IA re-modded the head for me, so we will see how that responds.

My thinking on this is somewhat similar to the original premise by 911st...plate stall at the top of the rev range. If I could diminish the stall effect, I could run the stock CIS fuel head and it would all be good. I have considered RPM synchronized fuel pump pressure (Summit has one, God only knows what the drag racers use it for). I can drop control pressure, but to no avail. I am being underfueled at the extreme. I suspect the plate motion vs fuel volume delivery is slightly non-linear. I think the 930 setup gets you a better air entry into the plate area, but after that, CIS is all the same. If I recall rudimentary fluids from school, we have basically traded off velocity pressure for static pressure at the plate, as we are killing the high velocity area in the center of the velocity profile, and effectively making the plate function as a diffuser. Also, we are speaking of a mechanical device, with mass and inertia. I experience mid range over-rich conditions, which makes it difficult to get out of a corner at the track, which I attribute to the inertial effects of the metering device. I have also considered changing the fulcrum point, but that needs to be dynamically changing, like maybe an eccentric pivot point.

I like WERK-1's thought on the mysterious cone, which would tend to alleviate the condition somewhat. Does anyone have a picture of this?
The biggest problem is the damn CIS itself. I have spent enough money to go EFI at this point, but I am stubborn, and I just know this will work eventually. I really think that somewhere Porsche/Bosch had this figured out, but didn't put it into production...warranty claims, maybe?
Patrick E. Keefe
78 SC
01 Boxster
Old 03-07-2009, 02:29 PM
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