OK, I will take a shot at providing an alternative. Back in-the-day when there was little info and/or performance parts available for the 900, I came up with a work around for this need. As I understood it, the retard unit on the distributor gave too much initial "retard" (addressed by the stiffer spring) but as boost rose more retard was needed above what the unit could give unmodified. At some point in the upper level of the boost curve timing can also be advanced. The solution: step 1 modify the retard travel by either shortening the stop or removing material from the arm so more retard is available. Step 2 Drill a small 1/4-3/8 inch hole in the top of the capsule, insert a rubber grommet and add/stick in a brass tube from K&S Engineering to bleed off boost from the retard capsule unit. This "bleed" allows little to no initial retard but as boost rises and the volume of air increases so too does the retard. The more boost the more retard until the stop comes into play. Do not ignore the stop position. The reason I used a K&S brass tube (several actually) is that you start with a 1/4 inch inside diameter tube (as I recall) and because these tubes are available in 1/32 inch increments (each tube precisely fits inside the next size up) the effective bleed "hole" can be easily reduced by placing the next smaller tube inside the initial tube. This is to fine tune the retard curve. Step 3 Used this in conjunction with a Bell Engineering/other manufacture rising rate fuel pressure regulator and a modified APC box. To increase precision, I removed the original retard stop and substituted a bicycle chainring fastener (plus spacer) which had a portion of its diameter reduced so that the effective retard stop could be adjusted precisely with an allen key. Bicycle chainring fasteners are cylindrical "barrels" I changed the cylinder profile to an oval so that the rotation of the fastener increased or decreased the retard stop when it was turned (also on a curve). This modification worked for me on four 900s/SPGs-boost rose smoothly and stayed at the top of the red 16-18 pounds depending on the model year. There was no initial spike/then a falling off of the peak boost. A couple of Saab mechanics which had driven modified cars commented that mine was the only one which "held" the higher level of boost. All of my cars lasted 300K plus miles with no engine problems. Now for the disclaimers: There are better, more accurate, ways of doing this-Jak's fuel chip, MSD's adjustable boost retard ignition coil (I think this is still available), and the trionic 5 upgrade (best). My "bleed" capsule cannot add back timing at the upper limits of the boost so it gives up a bit, possibly a lot, of performance and smoothness to trionic 5. This work around requires a rising rate fuel regulator to match boost demands and avoid engine damage. The Bell (Corky Bell of "Maxiumum Boost" fame) rising rate fuel regulator was, at the time, the best...it increased fuel pressure as a function of boost pressure. The more boost pressure the more fuel pressure. My "bleed" capsule retard modification also works on a curve which, to my mind, might possibly be better than the "AID"'s stiffer linear spring. I have considered doing an "AID" unit-I believe it used an existing Saab retard timing unit that was cut apart (easily done), a stiffer spring swapped in, and the unit resealed using machined alloy collars. I would guess an ABS pipe (or two) plus JB weld would also work...the spring spec is/was in this forum for anyone interested. Have I compared the performance of my idea to an actual "AID"? Afraid not. K&S Engineering brass tubes can be found at hobby shops and some hardware stores.They make for a clean look and good precise flow-the flow through a tube is different than the flow through a simple hole. So...this is an alternative: it's quick, cheap, DIY possible, and the rising rate regulator (or the re-chiped fuel computer) is absolutely necessary for either the "bleed" capsule or the "AID" when you push beyond stock boost levels. This work around also matches the "character" of those older Saabs when "tuning" involved locking down a wastegate, "denting" a fuel pressure regulator, and turning the "POTS". Things were not so precise then as they are now. Its surprising, however, on how well it actually worked. My first turbo, an '85 Saab, was "tuned" in exactly that way. Wow! What a blast! Posts in this Thread:
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