We spent some time on an in-ground chassis dynamometer to make sure that our 1973.5 911T's Bosch CIS fuel injection was performing as it should.
We have replaced everything from the fuel tank back to the pump, filter, accumulator and lines—the fuel distributor itself was just cleaned thoroughly, as were the fuel injectors.
Before we went to the dyno (at AutobahnExtremist.com), we tested a few things to make sure that we weren't going to waste time or money. First, we tested how much fuel our system was flowing through the system, by disconnecting the return line before it heads back up front to the tank. It flowed 925ml of fuel in 30 seconds, which is within spec for this car. We then tested how much fuel was being fed to the warm-up regulator over 1 minute—also in spec at 225ml. Next we checked the flow at each injector to see if they were even—at around 60ml per injector in 30 seconds (with minimal variance between injectors), these were all deemed good.
The system pressure was also in spec at 4.7 bar (part of our early cleaning processes during the reanimatino had freed up the stuck fuel pressure regulating piston and spring assembly)
Finally, we checked our control pressure—which the CIS injection uses to counterbalance the airflow meter's movement and balances against the system pressure. Too high of control pressure and the car will run lean, too low and it will run rich. Ours was set a bit too high, so we removed the warm-up regulator and adjusted the spring tension to lower our control pressure based on specs on charts you can find HERE—adjustability is something these early CIS cars have provisions for and it's very handy.)
Once we were set there, we adjusted the mixture so that it was low 13s:1 at idle—and our reduced control pressure gave us very good 12.5:1 readings through the rev range. This is a little rich or "fat" in comparison to the theoretical ideal stoichiometric of 14.7:1 that modern cars strive for, but gives solid power through the rev range for this older design. Heck, this is a Porsche, not a Prius.
So, how does it run? Well, the answer to that is simple: Like it should! The ability to watch the mixture as the engine is under load is crucial to proper tuning. We took the car for a few long drives, including a PCA get-together over this past weekend. The car handled the 40 mile drive on a hot summer day just fine, with no drama and great power.
The drive sure put a smile on our face after all this work.
Lined up on the front lawn of Mike Garriga's house. Mike's annual BBQ is a must-do on the NOR-PCA calendar.
