Turbos and intercoolers / part II

[Bob Cook]

GMAS et AL.

Intercooler / power / performance discussion.

I spent time studying the TCM vrs Cessna POH and all relative power curves. Using TCMs X30583 handbook the power vrs fuel flow curves are almost identical. TCM says "peak is okay below 68% power and Cessna says "below 55%".

The Cessna POH tables correspond to TCM's 50 ROP curve.


here is what I have found, correct me if I am wrong.
fuel flow = lbs/hr/engine.

% HP Full Throttle 50 ROP Peak
lbs/hr/eng
80 180 94 83 78
75 168 88 78 73
70 157 82 73 68
68 153 80 72 66
65 146 77 68 64
60 135 73 63 58
gph/eng
80 30.0 15.7 13.8 13.0
75 28.0 14.7 13.0 12.2
70 26.2 13.7 12.2 11.3
68 25.5 13.3 12.0 11.0
65 24.3 12.8 11.3 10.7
60 22.5 12.2 10.5 9.7

1) best economy at low power is peak
2) recommended economy per cessna poh is 50-60 ROP.
(pk+6 lbs/hr/engine)
3) recommended best power is 100 ROP
(pk + 12 lbs per hr / engine > TCM chart
4) full throttle is approx is peak + 18 lbs/hr/ engine,
5) or inverse where PK should be 3 gph less than full throttle.(seems to hold true)

Going over intercooler specs from Riley they limit MP after 10k ft per scale and set min fuel flow based on 2800 rpm and altitude.

If I use 2400 RPM and 29 inches and do the JPI lean find I end up with a fuel flow that is much higher than the POH corresponding value. For example I get a FF of 12 lbs / hr / engine at normal cruising altitude of say 14k ft. To get the corresponding fuel flows based on the above tables (using PK as reference) I end up with a required MP of 27 inches for the equiv POH fuel flow...... Hmmmm.

So what's the point?
Intercoolers provide more hp with a corresponding increase in fuel flow. IF you adjust the engine for the proper ROP value then the FF is going to be higher and the HP is going to be higher compared to the POH. You can work backwards and find PK about 18 lbs less than the full throttle fuel flow. Both engines seem to come out within 1 lb/hr.

I still have not found another way of calculating HP other than checking Peak fuel flow and running the tables. For what it's worth I made a simple spread sheet with corresponding fuel flow in gph (shadin conv.) to give me a reference rather than using the POH.
Yesterday at 9k ft, +8C / 50 rop / 2300 rpm / 27" gave me 11.6 gph (50 rop) and peak at 10.8 gph. This would correspond to about 66%hp. Now check the POH and tell me what the MP should be....
yep, 2300 and 32 inches *for the same power setting*.

It appers that if the engine fuel flow is calibrated for non intercooler operation at full throttle, things will hold as you can only develop so much hp (225) at a FF of 140 lbs. Once you throttle back you will only be running close to the ROP at near full throttle settling (high hp) at high altititudes (cooler air). At lower settings the 18 lbs/hr is greater than the change in mixture provided by the intercooler thus a safety margin UNTIL you start to try and adjust to POH values.

Sure intercoolers provide better Hot and high performance but they do NOT add to the efficiency of the engine. They improve induction cooling and better hot wx performance in climb.

Does a reduction in MP necessarily provide any benefit to the longevity of the engine ? not in my books.......... but I certainly could be wrong.

The above table seems to hold regardless of OAT or altitude due to the barometric regulated fuel controller.

Look at figure 47 360-JB ff vrs. hp on page 5-43 of the TCM book.
It is a better graph than the CB graph . TCM allows peak at 65% hp where 10.8 gph or 66 lbs/hr is minimum fuel flow at 50 rop or shows 61 lbs/hr BEFORE you can lean to peak .

To simplify things you can do a "lean find" and if you see less than 66 lbs/hr then you can go to peak for max economy, otherwise 50 or higher ROP. Minimum fuel flow for 75% power is 90 lbs/hr or 15 gph. For example if I want 75% climb I throttle back to 15 gph and forget about MP or even RPM for that matter.

When using intercoolers Fuel flow is the only way of determing power.

hope this will generate some "food for thought".

Do you use the 50 rop as a reference or peak as reference?
> I don't think it matters since the difference appears to be 6 lbs per hour and seems to hold true regardless of power settings.

I am not about to get into the LOP issue since Cessna "does not recommend LOP operation <G>.

Any comments?