# Why is the ratio of TSFC at altitude vs. sea level different for hi-bypass vs. lo-bypass turbofan engines?

I've read up on how the thrust-specific fuel consumption of turbofan/turbine engines increases with altitude, and the Georgia Institute of Technology plot in particular seems to indicate that, for most modern airliners, the TSFC at cruise is 1.6 to 1.7 that of static thrust at sea level.

Now, I've come across this site (courtesy of the Wikipedia page for TSFC) and noted some wide variations with $$TSFC_{cruise}$$ vs. $$TSFC_{0}$$.

Some examples (data from the page linked above):

• GE CF6-50C2: $$\dfrac{TSFC_{cruise}}{TSFC_{0}} = \dfrac{0.630}{0.371} = 1.698$$.
• P&W PW2040: $$\dfrac{0.582}{0.330} = 1.764$$.
• RR RB.211-535E4-37: $$\dfrac{0.598}{0.324} = 1.846$$.
• P&W JT8D-15A: $$\dfrac{0.810}{0.590} = 1.373$$.
• RR Conway RCo.12 Mk.508: $$\dfrac{0.822}{0.726} = 1.132$$.
• RR Spey RSp.4 Mk.511-5: $$\dfrac{0.770}{0.600} = 1.283$$.

The upper three examples are high-bypass turbofans, and the lower three are low-bypass. Notice the big differences in the TSFC ratios.

What causes this difference?

• You would have to check at what altitude "cruise" is. When the B-52 was originally designed to fly at high altitudes, the lower bypass engines may have been better. Your data shows the fans giving better TSFC cruise even though they have higher ratios. No surprise they do better at lower Mach/denser air, where props once ruled. Commented Jan 14 at 23:55