Manifold pressure and atmospheric

We have a lyc o-320, with no manifold pressure gauge. What is the relationship between MAP and the outside world, with a wide open throttle. Is it as simple as a fixed as the same pressure minus the presure across the induction system, or can it be above due to air being rammed in? Is there a typical difference for a typical RV9?

Yes and maybe..

Hi Justin,
Essentially, M.P. is the same as the outside ambient pressure. If the air filter on the airplane is clean, there shouldn't be a noticeable difference between the two. We used to clean ours if there was more than an inch difference. As far as ram pressure from high airspeeds, I seem to recall that the RV's regain half an inch or so.

As you climb, you lose around one inch per thousand feet and this will show on a manifold pressure gauge so with a CS prop, you have to keep advancing the throttle as you climb if you want to maintain 25 inches or whatever you choose. By 5000 feet you're usually full throttle because of this. At 7500 feet we can only get 22 1/2 inches at wide open throttle and that's where we just do make 75% power if we allow the engine to run at redline with a fixed pitch as I have.

Regards,

justinmg said:

We have a lyc o-320, with no manifold pressure gauge. What is the relationship between MAP and the outside world, with a wide open throttle. Is it as simple as a fixed as the same pressure minus the presure across the induction system, or can it be above due to air being rammed in? Is there a typical difference for a typical RV9?

Click to expand...

At low speed, the full throttle manifold pressure will be a bit lower than the ambient pressure, due to pressure loss across the air filter, and through the air box, etc.

At high speed there will be some ram air pressure rise at the air inlet (i.e. at the front of the cowling). The full throttle manifold pressure will be = ambient pressure + ram air pressure rise - pressure loss in the induction system. Van has designed a very efficient air box, which appears to realize a significant percentage of the maximum theoretical ram air pressure rise, and has relatively low air pressure loss in the induction system. So, at high speed it is quite possible that the full throttle manifold pressure could be a bit higher than the ambient air pressure.

As a point of reference, the maximum possible ram air pressure rise at various calibrated airspeeds is:

CAS ram pressure
(kt) (in Hg)
100 0.5
150 1.1
200 2.0

I would guess that an RV with Van's air box would realize somewhere between 50% and 75% of the maximum possible ram air pressure rise. The pressure loss in the induction system is probably somewhere between 0.5 and 1 in Hg. These values are SWAGs, and could be completely wrong.

For reference, the ambient pressure at various pressure altitudes is:

alt ambient pressure
(ft) (in Hg)
0 29.9
1000 28.9
2000 27.8
3000 26.8
4000 25.8
5000 24.9
6000 24.0
7000 23.1
8000 22.2
9000 21.4
10000 20.6

Excellent post Mr. Horton

Dear Kevin Horton:

Thanks for that nice post, it has all the facts and numbers and was much shorter and better than I could do. There are many "tall tales" about how much one gets in RAM rise. Here is the interesting thing, RV inductions are pretty good in stock form, with Van's designed parts (scoop, airboxs and so on). A Cessna, not withstanding it has less RAM air to work with (slower air speed), it also has a filter/airbox set up that produces 1 to 1.5 inches less pressure (inches-Hg), all from the poor filter/air box design.

If you want a "down-N-dirty" quick test, Van wrote this procedure in the RVator years ago. Here is my rough recollection:



I've thought about ways to improve Van's set-up; its pretty hard beat it. The BIGGEST negative with Vans stock set-up, which Van knows, is the soft connection between the cowl scoop and the air-box. That is where there is most likely loss of incoming air. Van recognize this but for practical purposes current design (at least for vertical induction) meets his design goal of simple and efficient. A solution (as Van suggested long ago) is a little access door in the cowl you can affect a sealed duct connection between cowl and air box, with a clamp for example. With out the access door the scoop/air box connection is not possible with the lower cowl in place.

The other improvement are in the Lyc induction it self with those cold sumps.


A "Urban Legend" is that forward facing induction is soooooo much better. Not really, all the air has to make a 90 degree turn from the free air stream to the intake lower intake ports in each cylinder at some point. Whether the air makes the 90 degree turn before the throttle body/carb or after does not matter. In fact fwd facing induction give less room to develop a good air box to slow the air down and build up pressure and volume reserve. Remember a piston engine is not a turbine that is constant flow. Piston engine induction starts & stops in pulses. That is why a plenum is so critical to act as an air reservoir. Many fwd facing inductions just have a small tube and than BAM, engine. The horz air-box van sells for the RV-7/8 is very efficient and well engineered. It has been tested and actually has slightly more RAM efficiency than the Vert air-box. Also the elimination of the external scoop may add 1 or 2 mph.

** For goodness sakes be very careful doing high speed passes down the runway. I have had the good fortune to have uncontrolled fairly low use long runways way from population to do test in the past. Also the potential bird strikes thru the canopy scares me. Do this at your own risk and please be careful. Flying low to the ground, fast and looking at your MP gauge may not be for everyone. Nothing wrong with that.

Determining MAP boost

gmcjetpilot said:

If you want a "down-N-dirty" quick test, Van wrote this procedure in the RVator years ago. Here is my rough recollection:
On the RAMP, engine shut down, note the MP - manifold pressure (static air pressure).
Take off and fly a wide open throttle (WOT) low pass** over the runway (approx ramp elevation or altitude).

Click to expand...

Perhaps I'm showing my (not infrequent) ignorance, but why can't we take Kevin's MAP numbers for a given altitude (density altitude, right?) and take a MAP reading at WOT up there, say at 8000'? Of course, this assumes you have a good instrument to show DA, but GRT and others are doing that and they seem to be pretty accurate. I think that makes some kind of sense from a safety point of view.

gmcjetpilot said:

If you want a "down-N-dirty" quick test, Van wrote this procedure in the RVator years ago. Here is my rough recollection:

On the RAMP, engine shut down, note the MP - manifold pressure (static air pressure).

Take off and fly a wide open throttle (WOT) low pass** over the runway (approx ramp elevation or altitude).

The difference from ramp to low pass MP is the total ram rise. It will most likely, typically be about the same +/- a few 1/10ths.​

Click to expand...

I'll add a note to avoid confusion. George has changed the terminology very slightly from what I used. In the terminology I used, the difference from ramp to low pass MP is equal to the am air pressure rise - pressure loss in the induction system.

If we wanted to break out the two components separately, we could add one more step - note the manifold pressure at full throttle during the early part of the takeoff. There will be very little ram air pressure rise at this low speed, so the difference between the reading with engine shut down and the reading during takeoff is the pressure loss in the induction system.

hevansrv7a said:

Perhaps I'm showing my (not infrequent) ignorance, but why can't we take Kevin's MAP numbers for a given altitude (density altitude, right?) and take a MAP reading at WOT up there, say at 8000'? Of course, this assumes you have a good instrument to show DA, but GRT and others are doing that and they seem to be pretty accurate. I think that makes some kind of sense from a safety point of view.

Click to expand...

The only problem with this approach is you probably don't know how accurate your manifold pressure gauge is at that pressure. The error will vary depending on the pressure, so it will almost certainly be different from the error you determine nearer 30" (i.e. near sea level). If you could also find a high altitude airport, land, shutdown, note manifold pressure reading, ramp elevation and altimeter setting, we could use that info to determine the error in the manifold pressure gauge.

Or, you could have it calibrated when you have your static system check down - hook it up to the static system tester, and not the manifold pressure reading vs pressure altitude. Convert from pressure altitude to pressure, and see how that compares to the manifold pressure readings.

Once the manifold pressure gauge is calibrated, then the approach suggested by hevansrv7a will work, but we use pressure altitude, not density altitude. The manifold pressure gauge reads pressure, not density. If you sit on your ramp with engine shutdown, the reading won't change with temperature, as long as the altimeter setting doesn't change.

Isn't altimeter temp corrected?

Kevin Horton said:

...
Once the manifold pressure gauge is calibrated, then the approach suggested by hevansrv7a will work, but we use pressure altitude, not density altitude. The manifold pressure gauge reads pressure, not density. If you sit on your ramp with engine shutdown, the reading won't change with temperature, as long as the altimeter setting doesn't change.

Click to expand...

Kevin, thanks, I see your point about pressure altitude, but aren't altimeters corrected for temperature? Is 29.92 on the baro the same real pressure at 30 degrees and 80 degrees (F)? Going a little further with this, isn't the engine capable of producing more power with 30 degree ambient air than with 80 degree? Is this just an area of innacuracy when computing power from MAP, RPM and altitude. Darn, whenever I think I'm beginning to understand something it just gets more complicated.

hevansrv7a said:

Kevin, thanks, I see your point about pressure altitude, but aren't altimeters corrected for temperature? Is 29.92 on the baro the same real pressure at 30 degrees and 80 degrees (F)? Going a little further with this, isn't the engine capable of producing more power with 30 degree ambient air than with 80 degree? Is this just an area of innacuracy when computing power from MAP, RPM and altitude. Darn, whenever I think I'm beginning to understand something it just gets more complicated.

Click to expand...

Pressure is pressure is pressure, no matter what the temperature. Altimeters and manifold pressure gauges read pressure. An altimeter just has an aneriod capsule (i.e. a pressure bellows) inside, and a complicated mechanism to translate movements of the aneriod capsule to movements on the dial. The aneriod capsule responds to changes in pressure only.

Yes, an engine will produce more power at low temperature. But, this discussion is about how much manifold pressure will be available, not how much engine power an engine will produce at a given manifold pressure, rpm, altitude, temperature, fuel flow, humidity, etc.

Kevin Horton said:

For reference, the ambient pressure at various pressure altitudes is:

alt ambient pressure
(ft) (in Hg)
0 29.9
1000 28.9
2000 27.8
3000 26.8
4000 25.8
5000 24.9
6000 24.0
7000 23.1
8000 22.2
9000 21.4
10000 20.6

Click to expand...

Hi Kevin,

Trying to make sure I understand this based upon some data I collected from the EFIS the other day:

Altitude: 8000 feet MSL
Density Alt: 9830 feet
Baro: 30.20 inHg
Man: 23.2 inHg (measured from the right/rear cylinder)
Temp: 63 degF
RPM: 2700
Fuel Flow: 11.9 GPH
Notes: Wide open throttle; Leaned for best power

I recently verified that, while on the ground w/ the engine off, I set my altimeter to zero feet and the manifold pressure gauge equaled the altimeter pressure (to the tenth's digit which is the best that the manifold pressure gauge can do). I understand your point above about the manifold pressure gauge not being accurate at lower pressures; but for the sake of understanding, let's assume it's accurate.

Questions:

- Should I had based my 8000' altitude w/ the altimeter set to 29.92 rather than 30.20 for the purpose of reading pressure altitude?

- Based on the table above at 8000', the pressure should be 22.2 inHg.but I read 23.2 inHg Does that mean I've 1 inHg in ram pressure as measured at the right/rear cyl?

Thank you,

I have been doing some econ cruise testing at altitude recently, preparing for some real long-distance flights. At 16,500' the other day with a calculated ambient air pressure of 15.28 in Hg, my manifold pressure was showing 16.2 in Hg at 149KTAS. I am using the James cowl and a K&N cone-shaped filter into the forward-facing Superior sump. According to the theoretical maximum ram air rise, I'm achieving just about all there is to achieve - or else I calculated the ambient pressure incorrectly.

EDIT - chasing down the numbers I think I did foul the calculation - I believe it should be 16.48 ambient pressure. Perhaps someone smarter than me could verify?