: Fi A/f Ratio

04-15-08, 09:13 PM
my northstar STS gets 28mpg on the highway. I watched a hotrod show where mr.edelbrock says his new 555ci FI motor gets mid 20's on the highway. my STS has 3.71 gears with a single OD and i assume the other car may have 0.50 OD gears but don't both motors run at about the same a/f ratio? how can a motor thats almost twice my displacement get almost the same mileage? or did i miss something?

04-15-08, 10:46 PM
Yes, they both run 14.7:1 A/f ratio. Maybe gearing, but don't believe everything you hear.

04-16-08, 12:28 AM
FI as in forced induction or FI as in EFI electronic fuel injection?

Don't assume that it runs 14.7:1. You can safely run ~17:1 AFR on the highway when speeds are >40mph. The EPA and therefore GM generally frowns upon this because it increases NOx levels. Timing has to be advanced significantly because lean mixtures burn slowly, and without enough timing you'll torch your exhaust valves. This isn't a problem with EFI, but could be with a distributor. 23-24mph probably sounds about right if they had a good OD/final. Mr. Edelbrock doesn't have to answer to the EPA and probably puts a stamp like "for offroad use only" on their products.

04-16-08, 09:36 AM
There's a LOT you can do with gearing and engine design. Normal running A/F ratios for a gasoline (real gasoline) fed engine are all pretty close together in the 14.7:1 range, (Google stoichiometric) regardless if it's a Briggs&Stratton or Allison V12. Yes, power demands and fuel formulae change the ratios somewhat, but the normal running ratio was the OP's question. Heck, depending on load, gearing, power tuning, fuel delivery system, cams, heads, blah, blah you can build the same 555 block to be an economy engine, a power monster, or in-between. Look at the gas mileage difference between a SLS (3.11:1 final) and a STS (3.71:1) in the same year vehicles. The SLS will always get better highway mileage because the engine is tuned differently and also spins 3 or 4 hundred RPM lower at highway speeds.

04-16-08, 01:25 PM
i recall honda had a fuel/timing/boost program for its F1 cars back in the late 80's where the driver could set it for max power and build a huge lead and than switch to economy mode at the end of the race and just loaf along. i don't recall if the governing board nixed the idea after honda won too much. but F1 cars are not turbo now, are they? maybe the edelbrock system has econo/power modes? i see new vettes getting 25mpg highway but what does a viper get? a 454 might squeak out low 20's on the highway but i gotta think a 550ci motor would have a hard time doing the same.