gbvona
Member
Long story short, I currently have a Range Rover 3.9l engine (9.25 CR, Crower Stage 2 cam) in my 1971 NADA P6. I am using two SU HIF6's for carbs. I made a few earlier postings documenting adventures with tuning.
The leap forward that I report here comes after acquisition of an Innovate fuel/air mixture meter. This is a gizmo that lets you hang a real Bosch O2 sensor on the tail pipe and drive around watching the mixture. Tech is wonderful.
Originally the carbs came with BBV needles. which are a standard 3.5l needle. Performance of the engine was anemic at best, presumably reflecting the draconian detuning done by the factory to meet emissions specs. I came upon a pair of BAC needles, which were also used with the 3.5l engine in earlier editions. This made a world of difference in idle, acceleration and general running. My worry was that the BAC needles were too rich, especially at the high end, which is warned of in several write-ups. As a precaution I got the Innovate meter and hooked it up.
My initial setup with the BAC needles was a bit lean at idle since I was worried about the richness at the other end. The meter revealed this concern was not warranted. At the outset, idle read about 14 and driving around 50 mph (2250 rpm) on the flat yielded about 16. After fiddling with the mixture screw I got idle down to about 12.5 and driving at speed down to 14. Further richening of the mixture dropped the idle even lower (11.4) but driving at speed remained stubbornly around 14, going to 13 (briefly) only with heavy acceleration. The performance of the car was pretty good, but the readings while driving still seemed a bit high for a somewhat tall cam. I am looking for a steady driving mixture around 13-13.5.
My thinking is that the mixture at idle is VERY sensitive to jet height, since the proportional changes of the opening at the jet are large for small changes in jet height. At speed, a difference of a turn of the adjusting screw is proportionally less effective in modulating the mixture, since the effective diameter of the needle is much smaller when the pots are raised. Bottom line, it would appear that the only way to get a richer mixture at speed is to go to a needle that is richer yet. I would note that by observing the rise of the carb pots when accelerating the engine (whilst stationary, needless to say) I doubt very much that the needles are going much beyond stages 7 or 8 while driving around. This does not really get to the excessively rich regions of the BAC needles.
So more needle fiddling seems in order. I plan on trying both the BCW and the BBW next. Stay tuned.
gbvona
The leap forward that I report here comes after acquisition of an Innovate fuel/air mixture meter. This is a gizmo that lets you hang a real Bosch O2 sensor on the tail pipe and drive around watching the mixture. Tech is wonderful.
Originally the carbs came with BBV needles. which are a standard 3.5l needle. Performance of the engine was anemic at best, presumably reflecting the draconian detuning done by the factory to meet emissions specs. I came upon a pair of BAC needles, which were also used with the 3.5l engine in earlier editions. This made a world of difference in idle, acceleration and general running. My worry was that the BAC needles were too rich, especially at the high end, which is warned of in several write-ups. As a precaution I got the Innovate meter and hooked it up.
My initial setup with the BAC needles was a bit lean at idle since I was worried about the richness at the other end. The meter revealed this concern was not warranted. At the outset, idle read about 14 and driving around 50 mph (2250 rpm) on the flat yielded about 16. After fiddling with the mixture screw I got idle down to about 12.5 and driving at speed down to 14. Further richening of the mixture dropped the idle even lower (11.4) but driving at speed remained stubbornly around 14, going to 13 (briefly) only with heavy acceleration. The performance of the car was pretty good, but the readings while driving still seemed a bit high for a somewhat tall cam. I am looking for a steady driving mixture around 13-13.5.
My thinking is that the mixture at idle is VERY sensitive to jet height, since the proportional changes of the opening at the jet are large for small changes in jet height. At speed, a difference of a turn of the adjusting screw is proportionally less effective in modulating the mixture, since the effective diameter of the needle is much smaller when the pots are raised. Bottom line, it would appear that the only way to get a richer mixture at speed is to go to a needle that is richer yet. I would note that by observing the rise of the carb pots when accelerating the engine (whilst stationary, needless to say) I doubt very much that the needles are going much beyond stages 7 or 8 while driving around. This does not really get to the excessively rich regions of the BAC needles.
So more needle fiddling seems in order. I plan on trying both the BCW and the BBW next. Stay tuned.
gbvona