Engine ISP & Fuel Use Mismatch?

[Garoad]

I was comparing two engines, one Atomic (FTmN 40) and an Omnimax 40T, which have the same thrust (40).

I was under the impression that ISP was simply the efficiency of the engine in air or vacuum. But looking at the "max fuel usage" (full throttle?), the Omnimax seems to be a little more efficient than the FTmN, despite FTmN claiming a higher ISP. (Reasoning: at full throttle they both produce 40 thrust, but the FTmN appears to use more fuel per second at max throttle due to the maximum.)

What's going on here? Does the fuel usage at full throttle change depending on whether you're in vacuum? Maybe the FTmN maximum values are non-vacuum but the maximum Omnimax are in vacuum? If that's the case shouldn't there be two maximum values displayed, one for vacuum and one for non-vacuum?

FTmN

280 ASL - 800 Vac - 800 Peak

Liqfuel max: 1.3093/sec max

Oxi max: 1.6002/sec max

Omnimax

290 ASL - 390 Vac

Liqfuel max: 1.2641/sec max <---- Uses less fuel/ox but much lower Vac ISP???

Oxi max: 1.5451/sec max

[hoojiwana]

The maximum usage is for atmospheric ISP, and since the FTmN has the lower aISP, it chugs down more LFO in atmosphere than the Omnimax.

[NathanKell]

Also Isp is "per unit of thrust" so if one has more thrust than they other it may both be more efficient and use more fuel.

[Garoad]

The maximum usage is for atmospheric ISP, and since the FTmN has the lower aISP, it chugs down more LFO in atmosphere than the Omnimax.

Ok yeah that's what I was suspecting. Would be a nice enhancement to see fuel usages for both in and out of atmosphere.