Running lean with an APR
 

When I was at the dyno shop last week, the guy there kept talking about getting an adjustable fuel pressure regulator as one of my next mods. He runs a spec Miata in SCCA, so I guess his preference towards this is due to rules, since they can't do much to their cars. Now I know that going FI requires higher pressure, but he said I should do it so I can run lean - that all BP's are known to run rich and that by running lean I can get better fuel economy and maybe an extra 2-5hp. I dunno, something didn't sound right about the hp increase, plus it seems to me that running lean can have it's own set of problems. Comments?

Running rich is safer for the engine. Lean mixtures lead to hotter combustion chamber temps, and a more complete burn. You also get a hotter running engine overall. Leaner mixes will give you more HP, because of the more complete burn, but you also run the chance of burning a hole in a piston if you run too lean. 2-5 hp isn't worth it, IMHO. So, unless you can get it dyno-tuned by people that really know what they're doing, hold of on the APR.

If you have some mods like a ported cylinder head, cams, ported intake and exhaust manifold you'll see much more gain from an APR, because you'd be flowing more air so you'd have to turn up the fuel pressure a little.