I'm in the process of building an engine for my 914-6, primary use will be autocrossing but I also want to be able to use it for occasional tours and DE days at the track. I've always thought the combination of 2.4S power in a 914 would be a lot of fun, but the power curve of the 'S' isn't ideal for autocrossing so I headed down the path of an 'E' spec cam.
The motor is now built but I am still working out all of the parts for the MFI system and it has given me time to reflect on the component choices in the engine and I'm second guessing if I'm going to have a motor I can use w/o needing race fuel. What I ended up with was a 2.4 'E' spec motor with 2.2 'S' P&Cs.
The common wisdom seems to be that the 2.2'S' P&Cs work in a 2.4 motor w/ 'S' spec cams and don't require more than premium fuel. But, my concern is that this works because of the overlap in the 'S' cam profile resulting in a 8.9:1 dynamic compression ratio (DCR). With the more docile 'E' cams I'm runnig the DCR calculates out to be 9.5:1. This site
Dynamic CR , indicates that the DCRs above 8.9:1 are into racing fuel territory, specifically w/ less efficient combustion chamber designs like the Porsche hemispherical chamber.
I plan to run a MegaJolt electronic ignition system so I will be able to refine the ignition curve, but I'm still not convinced I can meet my goal of not requiring high octane fuel. I'm looking for thoughts concerns from those more experienced on this board.