Reducing RPM- Gearing help needed!

[arcosine]

Id go with option 5, 2250 rpm at 70mph. My Saturn is at 2200 at 70 mph with a 1.9L engine sohc, 100 hp @ 5000 rpm, 114 ftlb @ 2,400 rpm. look at the map on the previous post, it takes 20 to 30 hp to cruse 70mph, which has best bsfc at low rpm.

[dcb]

Yes, it won't be a problem. You will have "sportier" 1-4 and a nice low cruising gear that you might have to rev up 4th a bit to get into comfortably, but will save you on the hiway. If we assume that you need 30hp (probably a bit high) to go 70mph, then you will go from ~280g/kwh to ~250g/kwh, or about a 10% improvement (if you are like this saturn).

If you make aero mods, the reduced hp requirement will compound in even further efficiencies with the higher gearing (even though load goes down, hp goes down faster, and the lower rpm gearing helps keep the load up).

I started plotting out the hp lines for the saturn, then got bored and just did the rpm range you were looking at, ~2000 - ~ 3500 for hp 10 through 40

[Frank Lee]

I'd go with the option that puts the piston speed at 1000-1200 f/m at desired cruising speed, and crowd the 1000 end of it if the engine seems lightly loaded enough.

[dcb]

if a bsfc map does not exist, yah. Otherwise we can be a lot more precise about things (and deal with non-linear fuel delivery maps in the ECU).

[Frank Lee]

Non linear fuel maps? Don't the O2 sensors give correct mixture at any cruise rpm in closed loop?

[dcb]

I'm taking some liberties with the term "non linear", I was thinking of full throttle enrichment though (which reverts to open loop), what keeps the peak bsfc island off the torque curve on gassers I think, but other model specific weirdness is out there I'm sure.

Does post 12 make any sense?