Study results: 2400 vs. 2500 rpm with 24" MP down low in a C

[0TreeLemur]

Yesterday I had a test flight to see if I fixed the ammeter noise in my JPI EDM900 that popped up last weekend flying back from the beach.   It seemed that the sub-D plugs in the back of the EDM unit had loosened up just a bit, so I tightened them. I wanted to fly about an hour.  Sure enough the noisy ammeter signal went away.  It was a beautiful evening.  

Down low I like to cruise our C at 75% power or WOT up higher- it seems to be very happy there.  I've also noticed that 2400 rpm just "feels" better than 2500.   By better, it is quieter and calmer because it doesn't sound like the engine is working so hard.  I've read articles that say that cruise engine rpm has little to no effect on engine life.  While this means that cruise rpm is up to the discretion of the pilot, I wanted to see if running at 2500 would result in much of a TAS difference compared to 2400 rpm.

This was a good opportunity to do a test.   I thought I would share the results.  Two three-leg gps groundspeed measurements for TAS calculation at 5500 ft.  24" MP.  Density altitude about 4400 ft.  Engine leaned to 100F ROP. 

2400 rpm:  TAS: 141 knots   Power: 75%   FF: 11.3 gph

2500 rpm:  TAS: 142 knots  Power: 79%   FF: 11.6 gph

One of these days I'll duplicate this test up higher.  Might make more difference up above 8k.

Fred

 

[Hank]

That's where I run my mid-level power (~3500 - 7000 msl), generally 22" / 2400. It works well for me. Above that I use WOT less a little bit / 2500; below 4000 or so I generally run 23" / 2300. But I've not recorded performance data for any of them . . . Thanks for doing that.

[Ragsf15e]

Nice job getting data.  I’ve tried the lower rpm up around 10k and my sense was more like 3kts slower at 2400, but I didn’t get the data.

[PT20J]

Hmmm. Same manifold pressure, higher rpm = more power = more speed = more fuel burn. Yep. It would be more interesting to compare 2400 rpm vs 2500 rpm at the same power.

[Hector]

Nice job getting data.  I’ve tried the lower rpm up around 10k and my sense was more like 3kts slower at 2400, but I didn’t get the data.

I recall doing this at 7500 a couple of years ago and also recall around 3Kts difference. WOT at 2400 and 2500.


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[0TreeLemur]

13 hours ago, PT20J said:

Hmmm. Same manifold pressure, higher rpm = more power = more speed = more fuel burn. Yep. It would be more interesting to compare 2400 rpm vs 2500 rpm at the same power.

I didn't think of that.  Given that destinations remain challenging, maybe I'll do that when I take her up in two weeks to keep the oil fresh and engine wetted.

[DJE22]

On 1/24/2021 at 5:38 PM, 0TreeLemur said:

Down low I like to cruise our C at 75% power or WOT up higher- it seems to be very happy there.  I've also noticed that 2400 rpm just "feels" better than 2500.   By better, it is quieter and calmer because it doesn't sound like the engine is working so hard.  I've read articles that say that cruise engine rpm has little to no effect on engine life.  While this means that cruise rpm is up to the discretion....

2400 rpm:  TAS: 141 knots   Power: 75%   FF: 11.3 gph

2500 rpm:  TAS: 142 knots  Power: 79%   FF: 11.6 gph

 

 

I was just perusing the POH and noted that at 5000’ and 7500”, same/similar MP” with 2400 vs 2500 rpm delivered 1 knot difference. Definitely worth the half gallon +\- per hour savings 

[Hank]

16 minutes ago, DJE22 said:

I was just perusing the POH and noted that at 5000’ and 7500”, same/similar MP” with 2400 vs 2500 rpm delivered 1 knot difference. Definitely worth the half gallon +\- per hour savings 

Mine shows 2 mph faster at 2500, for an extra 0.2 / 0.3 gph, or 17 / 19 statute  ile difference in range. For other reasons than tbis negligible-to-me difference, I fly 22" / 2400 at 5000 and begin using 2500 around 7500 msl with power below 75% so I can lean. 

On the other hand, at 10,000 msl the difference is still 2 mph, 0.2 gph and 19 statute miles range.

So use whatever you prefer, you won't notice much difference (if at all) on your watch or at the fuel pump.

[DJE22]

I was looking at POH for a 77 C and at 5000’ it was 2 kts however at 7500’ ‘twas 1kt

when I first began flying my C I was hammering on it pretty hard (very pleased and excited by how fast it was).  Quickly discovered a discrepancy in tach also. Lately I’ve found 2400 and even 2350 more peaceful. Have made several trips to Georgia and all through upper Midwest. Need to get back to KFFC in the next month or so.

Edited January 28, 2021 by DJE22