back to the 9200 rpm BDG problem. I was fortunate to get to work with a team that had the same problem on a DOHC Cosworth with dual 50DCO sidedrafts. That particular engine was buzzed to 11,500 max, and would go 'pig-rich' @ about 9600 rpm. The team owner had some $$ available, so he spent $145 PER SPARK PLUG, for (4) of them, and had chamber pressure transducers installed/manufactured into the steel shells of NGK Racing plugs used in the engine. We put the engine on a dyno and he rented an ECA (Engine Cycle Analysis) software/hardware system for a single weekend. The ECA is capable of monitoring the pressure transducer in the plugs and recording the cylinder pressure for every sequential plug firing. What we found with the Cosworth? At 9600 rpm, the engine began to experience moderate to serious 'cyclic variation' in cylinder pressures. That is, the cylinder pressures would often drop to the point that the cylinder would not fire at all, or the pressure was so low that the chamber flame-front was seriously affected. And the wide-band O2 sensor in the header system immediately recorded a relatively rich condition. And the weirdest thing of all? We could NOT detect/hear any cylinder mis-fire in the acoustic note of the header system! Everything 'sounded' just fine! The problem? Intake valve springs @ very high rpm. New springs were originally installed @ 65# of seat pressure. We went to PAC beehive's @ 80# on the seat. Problem solved! What was happening with the original springs? The intake valves were bouncing so badly at the seat, upon intake closing, that the 'trapping efficiency' within the cylinder was crap & cylinder pressures were all over the place, as monitored/recorded with the ECA system. Bottom line? Never get the idea that your race engine isn't mis-firing just because you can't 'hear' it in the exhaust system! And if the engine suddenly goes 'rich' at high rpm, immediately suspect the valve springs may not be controlling the intake valves ONTO THE SEATS.
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