What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Fun Engine Data

skylor

Well Known Member
Today I was out having fun sightseeing and cruising around at relatively low speed in my RV-8. It was a warm day for the coastal waters of Southern California. Some folks may be surprised to see how low my oil temps and CHT's were, given the fact that I was operating at over 27 inches of manifold pressure. This was steady state and not a transient condition.

IMG_5436small.jpg


Skylor
RV-8
 
Some folks may be surprised to see how low my oil temps and CHT's were, given the fact that I was operating at over 27 inches of manifold pressure.

Cooling demand is proportional to mass airflow. Mass flow is largely proportional to RPM, quite low in this case.

That aside, it's an interesting setting. Tell us more!
 
Engine Settings

Cooling demand is proportional to mass airflow. Mass flow is largely proportional to RPM, quite low in this case.

That aside, it's an interesting setting. Tell us more!

Dan,

You're partially correct ;) . The low engine speed is part of the equation for engine power and cooling demand. I'm also running well lean of peak. Actually, since I was lean of peak, engine power can be determined from fuel flow alone and I should be somewhere around 45% power at that setting.

Since I have fixed timing mags, it should be somewhat beneficial for overall engine efficiency to run low RPM's when using low power settings. In other words, low engine speed requires less spark advance which helps offset the need for more spark advance with slow-burning charge densities and mixture ratios.

This is stock IO-360A1B6 with Hartzell BA constant speed prop (74"). My RV-8 has no special cooling modifications and uses the Van's FWF supplied 7 row Niagara cooler, except I have the cooler mounted behind the #3 cylinder instead of #4.

Some folks will caution that high manifold pressures and low RPM's can be destructive to the crankshaft pendulum weights. However, the very lean mixture I was using produces very low power, low cylinder peak pressure, and large "theta P-P". The end result in terms of crankshaft dynamic loading should be the same as if I were running lower manifold pressure. Also note that I needed to pull the throttle back a bit from full throttle to get the power this low at such low altitude.

Skylor
 
fuel level calibration

I see you seem to have correctly calibrated your fuel levels - do you have any hints about this?

I've read the two documents I can find on the GRT site, but am not meeting with much success. BTW, they say that you should not have the fuel level in "decimal" but in "integer" - not sure if that's my problem, you seem to have it in "decimal".

I have an EIS 4000 and a Sport EX. Thanks!
 
Fuel Sender Calibration.

I see you seem to have correctly calibrated your fuel levels - do you have any hints about this?

I've read the two documents I can find on the GRT site, but am not meeting with much success. BTW, they say that you should not have the fuel level in "decimal" but in "integer" - not sure if that's my problem, you seem to have it in "decimal".

I have an EIS 4000 and a Sport EX. Thanks!

Since it's been over 10 years since I did that I'll have to go through my notes and get back to you. I can tell you that the float senders require a couple of gallons in the tank to to begin registering and they stop at ~ 17 gallons although I attempted to correct for that a bit. I also seem to recall that the van's fuel sender setup requires a negative scale factor in the EIS which was opposite of the examples given in the EIS manual.


Skylor
 
Last edited:
I see you seem to have correctly calibrated your fuel levels - do you have any hints about this?

I've read the two documents I can find on the GRT site, but am not meeting with much success. BTW, they say that you should not have the fuel level in "decimal" but in "integer" - not sure if that's my problem, you seem to have it in "decimal".

I have an EIS 4000 and a Sport EX. Thanks!

Look carefully. He has the two tanks-the columns on the left-in integer format. The third column from the left is from the fuel flow gauge, in decimal. The diamond on top of that third column is the sum of the first two, so you can compare the two different measurements. The reason for integer format is just to keep the numbers from overlapping on the display. The calibration procedure is in one of the grt documents, and works well. Hint: tap on the tank after adding gas, to make sure the float doesn’t stick (in flight this is a non-issue, but after a small amount of gas (I went in 5 gal increments, iirc) it can stick if there’s no motion.). You need to figure out where zero is (e.g., where you have only unusable fuel left) and if in fact the float is floating at that point. Mine was not, so I set zero at where the float just came off the bottom, and called the rest unusable. On the -10, wing dihedral caused the float to hit the top of the tank at 25 gal (tank holds 30) usable, so above 25 the float just reads 25.
 
Negative scale factor

... I also seem to recall that the van's fuel sender setup requires a negative scale factor in the EIS which was opposite of the examples given in the EIS manual. ...
Thanks - this is really helpful. It must be what is wrong - it's the only thing I didn't change, and now it seems obvious.

Bob - thanks for the tips - I'll have a closer look at the calibration. About the decimal vs Integer, I see it decimal on the PFD screen, and integer on the Engine screen. I'm thinking that integer is accurate enough, but will probably try both. Also thinking of doing it in liters, since that's how it's delivered here.

Skylor Fuel Gauge Decimal-Integer 20200816.png
 
Last edited:
Thanks - this is really helpful. It must be what is wrong - it's the only thing I didn't change, and now it seems obvious.

Bob - thanks for the tips - I'll have a closer look at the calibration. About the decimal vs Integer, I see it decimal on the PFD screen, and integer on the Engine screen. I'm thinking that integer is accurate enough, but will probably try both. Also thinking of doing it in liters, since that's how it's delivered here.

View attachment 1298

I apparently have different cal values set in the two displays. Note that I also have different TAS corrections on the displays which I need to fix...

Skylor
 
decimal/integer

I apparently have different cal values set in the two displays. ...
Not sure - I did some experimenting today on mine and even with "decimal" set, one screen shows the .x and the other does not, probably due to display space issues. If it's working for you, don't touch it! :D
 
In my case...

Not sure - I did some experimenting today on mine and even with "decimal" set, one screen shows the .x and the other does not, probably due to display space issues. If it's working for you, don't touch it! :D

In my case, however, the quantities are different between the two...

The upper display shows 29 for the left tank (which is the display attempting to interpolate beyond the theoretical range of the sender) while the lower one shows 19 on the same tank...

Skylor
 
Some folks will caution that high manifold pressures and low RPM's can be destructive to the crankshaft pendulum weights. However, the very lean mixture I was using produces very low power, low cylinder peak pressure, and large "theta P-P".

Isn't that well in excess of the limiting manifold pressure for that RPM? I would not like to run an engine at this RPM/manifold pressure combination.
 
Limiting Manifold Pressures

Isn't that well in excess of the limiting manifold pressure for that RPM? I would not like to run an engine at this RPM/manifold pressure combination.

Limiting manifold pressures do not assume very lean (of peak) operation...

If you look at the IO-360A sea level and altitude performance chart and think of it in terms of "limiting horsepower" you can see that while I was operating above the "limiting manifold pressure" I was actually operating at a lower power level than the chart indicates. When operating with lean of peak EGT mixtures, engine power produced by a specific engine is purely a function of fuel flow. At just 6.1 gph, I was producing ~ 90 hp which is well below the ~ 104 hp "limit" extrapolated from the table for 1870 RPM.

IO-360 A Power Chart.jpg


Skylor
 
Last edited:
Yeah but why? What do you get by running really lean of peak, high MP, super low rpm? Quieter? Seems that you could just pull the throttle way back if you want to go that slow. .
 
If you can get the ignition close to MBT you should see some useful gains in thermal efficiency/BSFC. Dropping the engine speed effectively advances the ignition, as Skylor mentions, which you need to compensate for the slow burn rate of lean mixtures.

Interesting that #4 CHT is cool - is this getting more air due to being at the final stagnation point under the cowling?
 
#4

If you can get the ignition close to MBT you should see some useful gains in thermal efficiency/BSFC. Dropping the engine speed effectively advances the ignition, as Skylor mentions, which you need to compensate for the slow burn rate of lean mixtures.

Interesting that #4 CHT is cool - is this getting more air due to being at the final stagnation point under the cowling?

#4 CHT's have always been cooler on this engine in spite of some attempts to restrict cooling air flow to that cylinder. GAMI spread is also pretty small (~.2 gph) and #4 is not the first to peak from the rich side.

Skylor
 
Last edited:
If you look at the IO-360A sea level and altitude performance chart and think of it in terms of "limiting horsepower" you can see that while I was operating above the "limiting manifold pressure" I was actually operating at a lower power level than the chart indicates.

It seems like a bit of a jump to infer that the limiting manifold pressure is intended to limit horsepower, not manifold pressure - particularly since the limiting manifold pressure line is not a constant horsepower.

In my (admittedly limited) experience reducing RPM is a very good way to induce knock/detonation. I suspect that the limiting manifold pressure is required for detonation margin at lower RPM.

  • Low RPM is pro-detonation
  • High manifold pressure is pro-detonation
  • Slow burn (which you are deliberately targeting) is pro-detonation

It seems like a gamble to think you can run lean enough that detonation isn't a possibility, especially since lean mixtures are also generally pro-detonation. The line between lean enough to suppress detonation and too lean to ignite is pretty thin I think.

My opinion: engines run best and have longest life when run the way the designer intended.
 
GRT fuel calibration

I see you seem to have correctly calibrated your fuel levels - do you have any hints about this?

I've read the two documents I can find on the GRT site, but am not meeting with much success. BTW, they say that you should not have the fuel level in "decimal" but in "integer" - not sure if that's my problem, you seem to have it in "decimal".

I have an EIS 4000 and a Sport EX. Thanks!

Plus 1...I have dual 8.4 inch GRT HX units and an EIS 4000.
 
But, but, but...it's obviously not detonating, CHT being the evidence.

I did say detonation margin - or is the idea that margin belongs to the engineers just for the airframe?

We don't know how far away from detonation it was. Maybe detonation would start at 300F CHT. Or 6.3 fuel flow. Or if the fuel only met standards rather than exceeded them. Without a comprehensive test program you don't know.

What CHT would you expect to see if the engine was detonating at a 45% power setting and 140 KIAS?
 
...and rising oil temps and falling EGTs.

Also, given the CHT spread here, you'll see one cylinder go into knock before the others so you'll see EGT and CHT spread diverge.
 
As I said, it's about the available margin, not whether the engine was actually detonating at that point in time.

Do you have an alternative explanation for the limiting manifold pressure?
 
It seems like a bit of a jump to infer that the limiting manifold pressure is intended to limit horsepower, not manifold pressure - particularly since the limiting manifold pressure line is not a constant horsepower.

In my (admittedly limited) experience reducing RPM is a very good way to induce knock/detonation. I suspect that the limiting manifold pressure is required for detonation margin at lower RPM.

  • Low RPM is pro-detonation
  • High manifold pressure is pro-detonation
  • Slow burn (which you are deliberately targeting) is pro-detonation

It seems like a gamble to think you can run lean enough that detonation isn't a possibility, especially since lean mixtures are also generally pro-detonation. The line between lean enough to suppress detonation and too lean to ignite is pretty thin I think.

My opinion: engines run best and have longest life when run the way the designer intended.

The folks at GAMI have plenty of hard test data to show that mfg’s recommendations are not always the best for engine longevity.

Slow burn is not pro detonation...

Limiting manifold pressures are meant to limit peak internal cylinder pressures at certain RPMs. Actual horsepower output is one result of internal cylinder pressures, but limiting internal pressure can be achieved by means other than manifold pressure limits. Slowing the combustion rate has a large effect on reducing cylinder pressure by moving the peak pressure further past top dead center when the gas volume inside the cylinder is larger and expanding more rapidly than mixtures and charge densities that reach peak pressure closer to TDC.

Mixtures that aren’t lean enough, i.e. between 10 LOP and 50 ROP are really pro detonation. Much richer or a little leaner than that range buys you a lot of margin.

You would probably really cringe know that I race using nitrous oxide injection at only ~ 100 ROP and no ADI (but not at 1870 RPM ;) )

Skylor
 
Last edited:
Lindbergh did this 1st

Lindberg went to the Pacific area during WW2 to help the pilots in their P-38s get more range - that is how they got Yamamoto. He also helped the Corsair pilots with the same setup. Extra range for the Americans was key to the Japanese loss in the Pacific: US planes could get to the targets that the Japanese thought were safe.

I am reading “The Aviators” by Winston Groom - he details Lindberg’s process and the very real difference it made for the war effort.

If I want to fly with my RV buds X-CY, I use this process to stretch fuel - generally I can burn 7GPH while my 4cyl buds burn closer to 9-10.
 
The FAA's published Swift fuel tests contain good detonation data for two engines, one of which is the TIO-540-J2BD, the angle valve turbo six from the Chieftain. None of the test points pull RPM into the 1800's, but it should be much the same combustion chamber. The closest test point is 2200 RPM and about 33" manifold pressure.

There does appear to be an interesting trend, in that detonation onset progressively moves toward leaner mixtures as RPM is reduced.

At 2575 and 42", detonation onset is found at the classic position, ROP, just leaner than best power mixture. With each power reduction it moves to a leaner position, with 2200/33" being slightly LOP.

Is it really relevant to an NA IO-360? I dunno, but I do note the manifold air temperature, 161F, well above the 100F standard I think is used for detonation certification with NA engines.

If you get curious about margin, push mixture richer from your very lean setting and watch the CHT trend. If the rise shows a knee, you're tickling the dragon, but light detonation won't hurt it.
 

Attachments

  • ScreenHunter_691 Aug. 19 06.27.jpg
    ScreenHunter_691 Aug. 19 06.27.jpg
    71.7 KB · Views: 180
  • ScreenHunter_690 Aug. 19 06.22.jpg
    ScreenHunter_690 Aug. 19 06.22.jpg
    72.7 KB · Views: 176
Good Data

The FAA's published Swift fuel tests contain good detonation data for two engines, one of which is the TIO-540-J2BD, the angle valve turbo six from the Chieftain. None of the test points pull RPM into the 1800's, but it should be much the same combustion chamber. The closest test point is 2200 RPM and about 33" manifold pressure.

There does appear to be an interesting trend, in that detonation onset progressively moves toward leaner mixtures as RPM is reduced.

At 2575 and 42", detonation onset is found at the classic position, ROP, just leaner than best power mixture. With each power reduction it moves to a leaner position, with 2200/33" being slightly LOP.

Is it really relevant to an NA IO-360? I dunno, but I do note the manifold air temperature, 161F, well above the 100F standard I think is used for detonation certification with NA engines.

If you get curious about margin, push mixture richer from your very lean setting and watch the CHT trend. If the rise shows a knee, you're tickling the dragon, but light detonation won't hurt it.

That's good data! One thing they don't show is the CHT's at the onset of detonation during the test. My bet is that they were far higher than the 276 (max) that I had during my test. I would bet that they had to have them up in the high 300's or more to get the engine to detonate under those test conditions.

I've been told that it's nearly impossible to get a healthy NA Lycoming (with stock CR) to detonate at any condition on 100LL when the CHT's are below 400 degrees.

Incidentally, a few weeks ago I was doing some "edge of the envelope" tests to try to find the "backside of the power curve" at 3500', full power and gross weight. I was operating at ~27" Hg, 2700 RPM and ~ 16 gph (not quite full rich for me) and 50 - 55 kias. It was startling how quickly the engine heated up under these conditions. I don't think I reached detonation but in ~ 1 minute all cylinders went from ~340F to 430F (by far the hottest I've ever had this engine and completely unintentional) and oil went from ~195 to 238!

Skylor
 
Last edited:
The FAA's published Swift fuel tests contain good detonation data for two engines

Do you have a link to this Dan?

At 2575 and 42", detonation onset is found at the classic position, ROP, just leaner than best power mixture. With each power reduction it moves to a leaner position, with 2200/33" being slightly LOP.

Is it really relevant to an NA IO-360? I dunno, but I do note the manifold air temperature, 161F, well above the 100F standard I think is used for detonation certification with NA engines.

The exhaust back pressure from the turbine and the large clearance volume will raise the exhaust gas residuals substantially over an NA, high-CR engine which will bring on knock earlier.
 
Do you have a link to this Dan?

Sure. I usually crop out the Swift fuel plots and look at the 100LL data posted for comparison. They used three different 100LL fuels, one specifically produced to barely meet minimum standards.

http://www.tc.faa.gov/its/worldpac/techrpt/ar0853.pdf

The exhaust back pressure from the turbine and the large clearance volume will raise the exhaust gas residuals substantially over an NA, high-CR engine which will bring on knock earlier.

Thanks, that makes sense.
 
Back
Top