Borer Prop -- 160 hp, What Manifold Pressure?

[Darrel Starr]

I just installed a manifold pressure gauge in my SC with an 0-320 160 hp (Leading Edge exhaust) equipted plane. I believe the MP gauge is accurate +- 0.5 in Hg. I have been surprised to see only about 21 in.Hg at 2500 rpm, 2000 ft. 60F with 28.5 in. Hg showing on the gauge at 900 ft before start-up. Then last evening, I noticed that at initiaal climb out at 2500 rpm, full throttle at about 65 mph it was indicating 26.5 or 27 in. Hg. So I think the gauge is reasonably accurate. Also, recently we were flying alongside Doctor Randy in his very similar 160 hp SC indicating the same airspeed at the same engine RPM so all seems well. I am just a little puzzled by the relatively low MP readings at cruise at 2500 RPM. That turns up on the Lycoming charts to be aroud 75% power or a little less.
Now I do remember some comments published by Roger Borer to the effect that the long prop (82/44 in my case) would require higher RPM in cruise but in searching back threads I have found nothing definitive on this subject. So can any of you shed some light on this subject? Since N18SY runs fine right along side of Randy's SC, I think the engine s running fine and I'm reasonably confident that the gauge is +- 0.5 in. Hg so is it normal with an 82/44 Borer prop at 2500 rpm to be running at 21 in. Hg and about 75% power? I would have guessed that 75% would have been more like 2400 rpm.
Do any of you (other) anal nerds have a manifold pressure gauge and a fixed pitch prop? Especially a Borer?

 

[Steve Pierce]

The lower manifold pressures with the Borer are normal. That is why a lot of people run a Sensenich for engine break in.

 

[Darrel Starr]

I just found this on the Cessna 170 site so the 21 inches I'm seeing is probably about right?

"Ok, I went flying today here is what I came up with at 2,500 ft 20.5 " of MAP, 2508 RPM 81 MPH airspeed GS of 51 off the gps, on the way back at 2,500 ft 20" MAP 2460 RPM I was showing 75 MPH airspeed & 77 knts grnd speed It was tuurbulent on the way back so the # are not as good.
I have never seen 100MPH airspeed unless it was in a descent. This is with a 80/40 prop on floats. I have been passed by Cubs at the same power settings as i was using."

 

[FdxLou]

Darrel

Your numbers are correct. I have a MP gauge on my Smith Cub. Down low at 2500' msl I see 2480/21.5" @ 103mph TAS. That is @ 57%. Up high at 7500-9500' msl I see 2580/17.5"=52%. (Same 103mph TAS). These settings give me the best mpg with my 0-360 and 84X43 Pawnee prop. FF averages 7.2gph down low and 6.3gph up high. I have LEES, 9.5-1 comp and dual PMags. CHT's are 350-375F and EGT 1390-1430F. My engine monitor says I am running right at peak egt. I have run LOP but lose a few kts.

Sorry for building you a watch when all you asked was the "time of day"!:roll:

Lou

 

[Darrel Starr]

Thanks, Steve & Lou, I didn't know if I was in the right ballpark but my 8.5 gph burn seemed to indicate I was in the 70-75% power area. Your comments confirm that. Now I see why some folks use the original prop for long cross country flights.

 

[FdxLou]

Thanks, Steve & Lou, I didn't know if I was in the right ballpark but my 8.5 gob burn seemed to indicate I was in the 70-75% power area. Your comments confirm that. Now I see why some folks use the original prop for long cross country flights.

Lean baby, lean!

 

[Darrel Starr]

I get a little nervous when the oil temp goes much over 190F (although I know the limit is 245F). I find I can lean to around 1340F but on an 80F amb day, if I go much further, say 1380-1400F, the oil tmp goes up 10-15F also. So I give up a little fuel to keep the oil temp down. This is at 2000-3000ft with cylinder head temps around 330-350F.
Darrel

 

[FdxLou]

Your oil is just beginning to evaporate the acids produced by combustion at 180F. Viscosity of the oil starts to thin out at 225F. You can play between that range of 180-225F as long as your CHT's stay under 400F in cruise and 430F in a long climb. This way your plugs will burn clean and the cyl's will go to TBO. If operated properly there is no reason to replace a cyl/do a top oh before reaching TBO. Even then you can run small Lyc's well past TBO with proper oversight/oil analysis.

Good luck

 

[MainlandCub]

Sounds about right. With a 52" pitch Sensenich and 150 hp O-320 I see 2450 - 2500 rpm at 21.5" MP at 1500 ft AMSL depending on the day.

I wish I could find the figures I recorded..... I did a test back to back between a 52" Sensenich and a 43" Borer MP against RPM. There is very little difference between the props in the rpm/manifold press. relationship.

Andrew.

 

[mvivion]

Best plan: Remove that MP. Gauge ASAP. That is a pretty useless instrument, unless you have a CS prop. All it'll do is make you wonder, but it won't answer questions or permit better use of the tools you have.

Too many variables affect MP for it to be very helpful in a fixed pitch equipped airplane. And you've just added unneeded weight and complexity, as well as failure points.

MTV

 

[Darrel Starr]

Mike, I minimized the failure modes by using a fitting with an 0.040 orifice. So if anything fails downstream the cylinder won't go so lean as to cause a failure. I doubt it will be very useful day-to-day but is satisfying my curiosity for the moment.

 

[behindpropellers]

Now I see why some folks use the original prop for long cross country flights.

The cruise prop may save a 10-20 minutes in a flight but that about it. It won't save you any gas.

The best tool for quick/efficient xcounty is your brain. People always laugh at me when I take my cub up to 10-12K for a good tailwind, plus you get the bonus of lower fuel burn.

Tim

 

[Dave Calkins]

This way your plugs will burn clean and the cyl's will go to TBO. If operated properly there is no reason to replace a cyl/do a top oh before reaching TBO. Even then you can run small Lyc's well past TBO with proper oversight/oil analysis.

Good luck

Hah!

There is legend of an 0-320 on an airboat in Wasilla, Alaska that has over 4000 hours on it.

Does it get oil analysis??
Aviation oil??
Consistent oil change interval??
Oil Filter rather than screen??


NOPE, to all. AND.......At the end of the season they run it up on the beach and leave it for another winter.

What is the secret???? Running it around at full throttle about 90 percent of the runtime and never leaning the mixture.

These little buggers are tough. Give them fuel and run them HARD and they'll be happy from cradle to ..who knows?......

 

[sierra bravo]

That airboat motor sounds similar to Bill Firmin's old engine in his 172.

 

[MainlandCub]

Mike, I minimized the failure modes by using a fitting with an 0.040 orifice. So if anything fails downstream the cylinder won't go so lean as to cause a failure. I doubt it will be very useful day-to-day but is satisfying my curiosity for the moment.

I wouldn't worry about it Darrel. We've always had manifold pressure gauges in our Cubs and they're the most useful instrument in the panel aside from the oil pressure gauge. I use it as the primary reference for setting power because the value displayed is directly related to where you have the throttle. RPM can vary widely depending on airspeed and what prop you have. I would not have my own personal aeroplane without one.

Many, many experimental aircraft are now using these digital engine information systems and they all have manifold pressure displayed and even calculate your percentage power.

With manifold pressure and rpm you can calculate percentage power and fuel burn from the Lycoming charts which has to be useful for flight planning at least. The manifold pressure gauge adds considerable value to the data you have on hand in the cockpit.

Andrew.

 

[Jim 4WF]

I think a MP gauge is the best way to calculate % of power and another tool to keep track of engine health. After start and engine warm-up MP should be relatively stable for a given RPM. At mag check, again MP should be consistent for RPM. At idle or run-up if MP is different from usual it might be a good time to check things out. At takeoff it’s another good indicator. I can’t tell you how many pilots that have MP gauges in their planes that don’t use it for anything else other than setting a cruse power. When you ask them what their idle MP Is, you just get the 20 mile stare. With these long flat props these engines are loafing at anything other than takeoff and climb. Cruising at 2500 RPM 1000’ is about 58% power. I lean aggressively as at such a low power setting as detonation is ALMOST impossible. Mine runs smoothly lean of peak as CHT’s also drop from peak EGT. A MP gauge was one of the only things I added to the plane once I got it. , Jim

 

[Darrel Starr]

This is the one I bought on ebay for $45 including sender. It is a slightly used electronic one originally sold by Van's Aircraft. I spent another $20 or so on fittings including a neat brass 1/8 pipe thread-to-AN4 fitting with 0.025 inch orifice sold by JEGS ( http://www.jegs.com/i/Auto+Meter/105/3277/10002/-1 ) used for Nitrous Injection. I increased the orifice to 0.040 for this application and installed it into the #3 intake port. I plumbed it with AN4 tube and hose until it entered the firewall then transitioned to 1/8 ID Tygon plastic tubing to the electronic sensor under the panel.







Notice in the second picture, it looks like the Tygon plastic tubing is sucking in some due to the vacuum and heat -- ' might have to change to a tougher plastic.

 

[skywagon8a]

The lower left instrument in this panel is a direct reading manifold pressure gauge. There are so many different propeller diameter/pitch/blade shape/manufacturers combinations that it would be nearly impossible to determine a power setting using RPM alone. In my case I have a ground adjustable prop so I expect that it will be a very useful tool. The weight is insignificant.

 

[Darrel Starr]

I see your gauge will also measure boost. Do you run a turbocharger?

 

[skywagon8a]

No. That is just a stock surplus instrument which I happened to have in my collection of stuff.

 

[gdafoe]

skywagon I'm interested in which ground adjustable prop you have.

 

[skywagon8a]

skywagon I'm interested in which ground adjustable prop you have.

It is a Whirlwind 200G. http://whirlwindaviation.com/props/200Gseries.asp