Improving the HP and RPM of a fixed pitch O-320

Ted K

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My colleague has a certificated aircraft with the original 1994 O-320-E2A with about 750 TSN with a 58” pitch Fixed Pitch Prop. Compressions are good, 68-72. But static RPM is only about 20 RPM above the minimum of 2250. Measured 2268 on a very accurate acoustic Tach.

Looking for thoughts on how to (relatively easily) squeeze out some more RPMs and HP.

Considering a Surefly eMag and PowerFlow exhaust.

Other thoughts and options?

Ted K
 
I have run a Power Flow exhaust since 2019 on a 0-320 in a 172N. It has been a huge improvement in performance.
I fly it hundreds an hour each year. Had it off last month for my annual inspection. I re anti seize the slip joints every 12 months or so. Well worth it.
Get a Power Flow exhaust, you won't be sorry.
 
If you flatten pitch to 56” your Static RPM will increase as will your RoC.
If you bump up cruise RPM by 50ish you won’t be slower.
Since Cruise Power will be the same Cruise Fuel Flow won’t change much either.
 
If it's barely in spec, I'd wager it's cheaper/easier to fix it than it is to compensate for lack of margin by throwing upgrades at it. Have the mags been opened in 30 years? Plugs? Ignition harness? Carb? Induction?
 
Make sure the static RPM baseline you're comparing against was intended for your prop. If you have a cruise prop then static RPM may be low but there's nothing wrong with the engine - it's just optimized for cruise, more efficient at speed.

That said, if you want to ensure you're making best power, 4 easy things to do are (1) ensure the magnetos are perfectly timed and healthy, (2) the spark plugs are in good condition, (3) idle & sea level full rich mixture are set properly, and (4) no induction leaks.

PS: I would mention the Powerflow exhaust, but @Gary Ward already did. It's a dyno tuned and proven HP increase, if you are willing to do the extra maintenance it requires.
 
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But static RPM is only about 20 RPM above the minimum of 2250. Measured 2268 on a very accurate acoustic Tach.
Given the static RPM is a range to cover various external issues, what is the specific TCDS min-max RPM range for this aircraft?
 
Can you reach 2700rpm in level flight? If not? Re-pitch the prop. My static target in 0-320 Cubs is 2400rpm. That provides good performance without over revving when leveling off.
 
If it's barely in spec, I'd wager it's cheaper/easier to fix it than it is to compensate for lack of margin by throwing upgrades at it. Have the mags been opened in 30 years? Plugs? Ignition harness? Carb? Induction?
This right here.
 
Given the static RPM is a range to cover various external issues, what is the specific TCDS min-max RPM range for this aircraft?
2250-2350 for 58” prop
 
My colleague has a certificated aircraft with the original 1994 O-320-E2A with about 750 TSN with a 58” pitch Fixed Pitch Prop. Compressions are good, 68-72. But static RPM is only about 20 RPM above the minimum of 2250. Measured 2268 on a very accurate acoustic Tach.

Looking for thoughts on how to (relatively easily) squeeze out some more RPMs and HP.

Considering a Surefly eMag and PowerFlow exhaust.

Other thoughts and options?

Ted K

Curious. What aircraft is this on? I suspect I know, but won't provide any further input without knowing the type first.

The other posters have provided some decent generic input.
 
If you are determining your RPM from a legacy mechanical tach, first, calibrate your tach. Most legacy tachs are off by 100 or more RPM at the upper end.

Use the unit below. It works from the cockpit. I've heard it's cheaper from Walmart.
 
Measured 2268 on a very accurate acoustic Tach.
2250-2350 for 58” prop
Equipment accuracy is one of the reasons for the 100 RPM spread. For example, if your acoustic tach has a +/- 3% error over its range that could easily be a 82 RPM error difference which would put you at the max static RPM. Regardless, chasing the 2350 in this scenario I doubt will give you the bang for buck you are looking for. What part of the aircraft performance envelop is your colleague looking to improve on?
 
Acoustic tach? Never “heard” of one.

I keep a TruTach optical tach in the hangar to check my panel instruments periodically.

Aircraft engines are rated for power by RPM. A fixed pitch prop is an RPM governor. If you can’t produce redline in flight you’re giving away horsepower. And to the earlier comment about engine condition? Prop condition should be the prime suspect. When was the last time that prop was in a prop shop?
 
Some aircraft seem to struggle to make minimum static RPM.
Archers in particular never present mid- range.
Atmospheric factors and wind direction play a role per Archer SM.
Carb Heat not quite in rig is one of many factors if there is even an issue.
As Stew mentioned the prop can be a suspect. Improper dressing or pitch
might have been tweaked.
 
I'm waiting for the OP to tell us when those mags were last off for internal inspection. Eroded points cause E-gap drift, which weakens the spark, and a weak spark makes a big difference in power output. Too many folks think a spark is just a spark, but a hot spark gets that mixture going a bit sooner and more molecules are initially ignited.

When I restored my 1951 International pickup I put a 1980s Ford 300 engine in it. I wanted to avoid as much electronic stuff as possible, to make easy roadside repair possible. I used the points-and-condenser distributor from an older Ford 300. After a few years I got tired of annually replacing the points and condenser, so I installed the original 1980s distributor and its ignition module that sits on the fender skirt. There was instantly a major difference in power and smoothness and I never had to mess with points again. That hot spark makes the difference.

With airplanes, I have seen a performance improvement when the mags get properly serviced. Another factor is the old-style Champion spark plugs that are still out there in their millions. Their internal resistor starts failing and they don't spark well. Get rid of them. Use REM37BY or Tempest UREM37BY plugs in any engine approved for them, and you'll also see some improvement and less fouling.
 
My colleague has a certificated aircraft with the original 1994 O-320-E2A with about 750 TSN with a 58” pitch Fixed Pitch Prop. Compressions are good, 68-72. But static RPM is only about 20 RPM above the minimum of 2250. Measured 2268 on a very accurate acoustic Tach.

Looking for thoughts on how to (relatively easily) squeeze out some more RPMs and HP.
Maybe I am a little thick here but the static pull is within spec. So what exactly is the problem?
 
Maybe I am a little thick here but the static pull is within spec. So what exactly is the problem?
Key numbers from Lycoming (O-320, sea level): 100% = 2700 rpm = 150 HP;


75% (performance cruise) = 2450 rpm = 110 HP;


65% (economy cruise) = 2350 rpm = 97 HP.

2250 rpm provides approx 84 HP.

I may be in the minority but I like as much power and acceleration as possible at takeoff. The down side of higher static rpm is potentially over revving in level cruise.
 
If you are determining your RPM from a legacy mechanical tach, first, calibrate your tach. Most legacy tachs are off by 100 or more RPM at the upper end.

Use the unit below. It works from the cockpit. I've heard it's cheaper from Walmart.

:yeahthat:
I have one and it works well. I use it to check my mechanical tach in flight.
 
Key numbers from Lycoming (O-320, sea level): 100% = 2700 rpm = 150 HP;

75% (performance cruise) = 2450 rpm = 110 HP;

65% (economy cruise) = 2350 rpm = 97 HP.

2250 rpm provides approx 84 HP.

I may be in the minority but I like as much power and acceleration as possible at takeoff. The down side of higher static rpm is potentially over revving in level cruise.
You have two basic theories to deal with here. On the one hand is the engine torque & power output that varies with air density, RPM, throttle position, and fuel air ratio, and on the other hand you have the hull power demand that varies with hull configuration, weight, airspeed and AOA during various flight maneuvers.

The job of the propeller is to mate up the engine power curve with the hull performance curves. This is not an exact science because in the aviation and marine environments everything is a compromise, especially where fixed pitch propellers are concerned. This is why variable pitch propellers are available, of which the constant speed type is the most common. A fixed pitch propeller will never be as good at mating engine performance to hull demand under all circumstances of flight, there are always compromises. Add to that the air density can change over 30% during a single flight and it is amazing fixed pitch props work as well as they do.

If you truly want the best power available for takeoff and for cruise, then you want a constant speed variable pitch propeller. That will allow you to maintain maximum engine power regardless of airspeed during acceleration, takeoff and climb, and then to reduce RPM to optimize engine power for cruise at different cruise speeds and altitudes. IMHO to meet your goals nothing else will do. Everything else is a compromise and not worthy of your time and money.

The least expensive way to get this is to trade up to a different airplane. A constant speed prop is available on many used small planes. I traded up from a Cherokee 140 to a Mooney Super21, and the cost was very reasonable. In 2004 I bought a Cherokee 140 with 4400 TTAF and a zero time rebuild at 160HP. It had recent paint and a new interior and I paid $42k, a premium at the time.14 years later I sold it with 4900 TTAF for $32k and bought my Super21 with 2900 TTAF for $40K. So the upgrade cost me $8k but I got a better airplane in the mix. You don't have to go that drastic, a Cherokee 180 will do nicely. Have you priced an upgrade to a variable pitch prop lately? I'd be looking for another plane.

You said " I may be in the minority but I like as much power and acceleration as possible at takeoff. The down side of higher static rpm is potentially over revving in level cruise." You are not in the minority at all. You just don't have the right equipment to do it with. And constant speed props don't over-rev at cruise speeds.

I hope this helps.
 
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... If you truly want the best power available for takeoff and for cruise, then you want a constant speed variable pitch propeller. That will allow you to maintain maximum engine power regardless of airspeed during acceleration, takeoff and climb, and then to reduce RPM to optimize engine power for cruise at different cruise speeds and altitudes. ...
True. To roughly quantify the benefit, know that Power moves the airplane and Power is Torque * RPM. Assume the engine's torque curve is flat (it's not, but it's a rough simplifying assumption). Suppose the 160 HP engine has a max static RPM of 2300. If you had a CSP it would make 2700 RPM which is 17% higher, which means 17% more Power. If you upgraded to a 180 HP engine that would be 180/160 = 12.5% more power. So a CSP with 160 HP engine produces more static power and thrust than a 180 HP engine with a fixed pitch prop. And this actually understates the benefit of the CSP because most engines have slightly increasing Torque with RPM.

Now if that same airplane with a fixed pitch prop made 2600 RPM at full power cruise, it's already making full power and there would be no benefit to the CSP. So the CSP makes a big improvement for takeoff and climb, with no benefit during cruise. Yet no penalty in cruise either, which makes it better than the fixed pitch, which you must optimize either for take off & climb, or for cruise, but never both.

In the larger picture, the benefit of a CSP is maximized with airplanes that have a large speed range, meaning cruise speed is much faster than Vs0. This is not the case for 172 and similar training aircraft, where cruise is only about 2x faster than Vs0. In a Mooney or Baron, for comparison, that ratio is closer to 3.0.
 
You have two basic theories to deal with here. On the one hand is the engine torque & power output that varies with air density, RPM, throttle position, and fuel air ratio, and on the other hand you have the hull power demand that varies with hull configuration, weight, airspeed and AOA during various flight maneuvers.

The job of the propeller is to mate up the engine power curve with the hull performance curves. This is not an exact science because in the aviation and marine environments everything is a compromise, especially where fixed pitch propellers are concerned. This is why variable pitch propellers are available, of which the constant speed type is the most common. A fixed pitch propeller will never be as good at mating engine performance to hull demand under all circumstances of flight, there are always compromises. Add to that the air density can change over 30% during a single flight and it is amazing fixed pitch props work as well as they do.

If you truly want the best power available for takeoff and for cruise, then you want a constant speed variable pitch propeller. That will allow you to maintain maximum engine power regardless of airspeed during acceleration, takeoff and climb, and then to reduce RPM to optimize engine power for cruise at different cruise speeds and altitudes. IMHO to meet your goals nothing else will do. Everything else is a compromise and not worthy of your time and money.

The least expensive way to get this is to trade up to a different airplane. A constant speed prop is available on many used small planes. I traded up from a Cherokee 140 to a Mooney Super21, and the cost was very reasonable. In 2004 I bought a Cherokee 140 with 4400 TTAF and a zero time rebuild at 160HP. It had recent paint and a new interior and I paid $42k, a premium at the time.14 years later I sold it with 4900 TTAF for $32k and bought my Super21 with 2900 TTAF for $40K. So the upgrade cost me $8k but I got a better airplane in the mix. You don't have to go that drastic, a Cherokee 180 will do nicely. Have you priced an upgrade to a variable pitch prop lately? I'd be looking for another plane.

You said " I may be in the minority but I like as much power and acceleration as possible at takeoff. The down side of higher static rpm is potentially over revving in level cruise." You are not in the minority at all. You just don't have the right equipment to do it with. And constant speed props don't over-rev at cruise speeds.

I hope this helps.
Both of my planes have CS props. My comment was based on having an 0-320 with fixed pitch. Why did I choose a CS for my experimental Cub? Because spinning a prop at 2700 rpm at takeoff provides maximum performance. The same would be true with a fixed pitch but the engine would overspeed very quickly. To that issue, I used the prop pitch that made 2700 rpm in level flight. I tried a finer pitch that had slightly better initial acceleration but would overspeed when I leveled out and didn’t like it.
 
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