Derating an engine

Factual Questions
1

When I learned to fly helicopters, I learned that the Robinson R22 Beta used a Lycoming O-320 engine derated to 124 maximum continuous horsepower. (The Beta II uses an O-360.) My instructor aid, ‘You know how they derated the engine? They painted a red line on the manifold pressure gauge.’

QUESTION: Is that true? Or are there mechanical changes to the engine to make it less powerful?

In Piper Indians, author Bill Clarke writes of the Cherokee 140, ‘Derating was accomplished by making a propeller pitch change and lowering the engine red line to 2,450 rpm.’ ‘Lowering the engine red line’ sounds a lot like painting a line on the tachometer, much like painting a line on the Robbo’s manifold pressure gauge.

But what about the pitch change on the prop? I know that fine pitch allows higher rpm for a given horsepower, and course pitch is ‘low rpm’. But doesn’t an engine make a certain amount of horsepower at a certain rpm, regardless of the pitch of the prop, gear in the transmission, or whatever?

2

When we got our new C-180, it had the -L engine I believe. Max RPM was 2450 at full throttle. At sea level, full throttle just barely made the prop governor actuate at all. The prop governor would hold that RPM. It was a dog compared to our older C-180.

The old one would go up to 2600 RPM, I forget, maybe even more but the point was that the difference in performance was so much that we went through the trouble of getting an STC approved to install an -K engine on it. Made a tremendous difference.

We could have reset the prop governor but the engine was not certified at those RPMs.

Red lines are important. Remember the phrases:
Max continuous cruse
Max takeoff, usually 5 minutes max.
War emergency power.
Big round engines had max time on water injection & manifold pressure which could be easily be over done on super charged engines.

First time I ever flew a piston Grand Commander, it was a repossession job & a long and great story but when I was making my get away, I missed the need to watch the manifold pressure and get it set before I went outside for very long with my eyes.
That plane, a “Rockwell 685 Commander powered by two 435 hp Continental GTSIO-520K piston engines, 66 built.” Had very good super chargers which I had no experience with. No super charger flight experience at all. Just some bigger round engines in the test cells at the A&P school. They were so worn out that you could only over boost the P-47 P&W R-2800 with the original monster steel exhaust and huge Turbo that sat just in front of the tail wheel. ( what fun that engine was )

When I glanced back after just about 2 seconds, the manifold gauges were going through 70" (fast) and the red line was way back at 42".

Yeah, I was very quick to change that. Did not blow up an engine which I praised Continental for constructing so well.

There is not a simple, reliable, cost effective way to set superchargers to position throttles for each different altitude & condition it might need max power at. Enter the informed pilot. Enter the red line. Simple until guys like me make mistakes.

Did not have nor ever saw a pilot handbook on one up to that time so I was learning on the fly so to speak.

( Why was I flying it? No one else could be gotten at the time and place that would go get it sight unseen and in a foreign island country holding a drug smuggling plane. )

Even on turbo charged aircraft with waste gate limiter’s, the MAP was always watched as power is brought up to ensure that fine tuning was not needed. On older Aerostars, fine tuning was needed a lot.

Turbines are a whole nuther kettle of fish. Lots of derating going on with those.

3

The exhaust is under-rated, it won’t handle the exhaust flow of the engine working at full HP.

Here’s a website that claims that replacement exhaust will let it run at full power.
They sell the exhaust… but it does make sense.

http://www.powerflowsystems.com/products.php?cat_id=18&pid=60

4

A tuned exhaust can get more power out of an engine. And I know that derating an engine, or not exceeding the power limitations, improves longevity. What I was getting at is this: Is derating an engine as simple as saying, 'OK, this is how many horsepower you have, ’ or is it more complicated like changing the cam profile or making other mechanical changes?

5

Power = torque x speed. For a given RPM, lowering torque lowers power output. On a piston/prop setup, lowering blade pitch will do that.

Yes, derating is simply a matter of limiting max power, via some artificial mechanical or electrical stop, and therefore limiting max stresses and temperatures, to make the engine last longer.

6

Thanks for the verification of my instructor’s statement.