I was scanning Trade-a-Plane and ran across an ad for a Cardinal RG, high-wing cousin to the Sierra. Engines are very similar in these planes. This one had an interesting note:
“Super Blue Printed, Balanced Overhaul by Performance Aero Engines. @ 225 HP”
Does blueprinting and balancing increase horsepower? Could this be done during overhaul to a Sierra? I know I should simply call Performance Aero.
Bob Steward, A&P-IA:
No question it CAN increase HP, and balancing CAN increase smoothness. The real issue is what was done and just how legal is it?
Several performance shops around the country do this work. It has its roots in automotive performance and became a hot new item about 20 years ago when the industry had stagnated with no new planes being produced after the 79 model year and the ensuing economic recession.
The concept is to smooth, and in some cases alter the shape of, the ports to help the air flow in and out of the cylinders. An engine is after all a variation of an air pump. More air (and fuel) pumped though the engine results in more HP coming out the crankshaft.
Automotive performance has been spinning the engines faster and faster to pump more air, and when issues of volumetric efficiency began to limit the gains of turning the engine faster, ways were sought to fill the cylinders better. We don’t really have the option of turning the engines faster than redline because of prop safety and efficiency concerns as the tips reach the transonic region.
One of the problems with “Porting” air cooled engines is that the metal is already very thin in some areas, and its possible to thin it out and cause potential for cracks, etc.
As an example, a few years ago I bought a set of 4 Superior cylinders for an O-320. The engine would not break in, and after 40 hours with oil usage in the 1+ quart per hour range it was torn down and discovered that the problem was small slits in the intake runners where the manufacturer had plunged the cutter too deep making the intake spring seat pockets and had let the cutter open a small slit into the intake. The intake being under vacuum, constantly sucked oil out of the rocker boxes and into the engine. Massive carbon deposits from the burnt oil and glazed cylinders from the rings not seating in (flooded by oil) was the result.
The engine had to be torn completely down, and all but a complete OH was done again after only 40 hours. The cylinders were sent to Superior, and after arguing with them for nearly a year, and they finally admitted that they were “PART OF THAT BAD BATCH”. Huh? They had a bad batch and never bothered to recall them or even ask that they be inspected in the field? Never notified the FAA? The topper was that they NEVER PAID THE WARRANTY CLAIM. So we bought new cylinders from another vendor and put the engine back in service. Its still flying today with no oil use and a happy owner.
The point of all this is to say that you can bet that Lycoming, ECI and Superior are NOT going to warranty altered cylinders, and Superior doesn’t even warranty their cylinders with obvious machining errors during the manufacturing process that they knew about all the time.
So anyone doing a flow bench mod on a set of cylinders is potentially setting you up for a failure that won’t be covered under warranty, and could cost you a set of cylinders.
I’ll further amplify a bit on what Bob has so wisely stated (Caveat Aviator). From the forties through the seventies,”porting and polishing” was all the rage in the automotive hot rod world. During about the past twenty years, with the more sophisticated machining and testing equipment now available, it has been thoroughly proven that just making passages larger and smoother often hurts more than it helps.
Current automotive engineering thinking focuses much more on flow quality than on raw quantity. I once prepped a set of Ford Windsor 302 cylinder heads for a street performance car. The engine idled smoothly at 600 RPM, but produced 330 HP at 6,200 RPM (4 BBL carburetor, no fuel injection). The owner had previously purchased a set of “ported and polished” Ford Windsor 351 heads (at a very high cost). Those heads peaked at 300 HP on the same engine, at 6,500 RPM. The ports that I prepared were easily one-eighth of an inch smaller in cross-section, yet my heads produced a far smoother engine and 30 more horsepower, at a lower engine speed. The added torque at the lower RPM also provided a much more “drivable” vehicle. Even if done without weakening the heads, bigger really isn’t always better. If you look at current production auto engines, you will find even smaller intake ports that are designed to be longer (rather than wider). Yet, power output levels per cubic inch are higher than ever in production engines.
For example, on carbureted engines a smooth, polished intake port will have a dramatically negative effect on cold starting, idle quality, fuel consumption, and power output at different power settings. To understand why, put a quarter-teaspoon of denatured alcohol on a piece of glass, and blow it around on the glass. See how long it takes to evaporate. Then do the same thing, putting the alcohol on a piece of fine to medium sandpaper, and see how long the evaporation takes. Remember, only fuel vapor, thoroughly mixed with air, will burn. Liquid fuel as mist or droplets just goes through the engine and emerges as soot. The effect is worst on carbureted engines, but polished ports around the intake valve will also reduce the vaporization on fuel injected engines. Intake ports require a “scratched finish” in order to assure ideal flow quality. The scratches dramatically increase the surface area, and get more of the liquid fuel exposed to evaporation. It does not hurt to smooth out big bumps, if they are just casting flash and are not adding needed mass to something like a blind threaded hole. On the other hand, it is hard to quantify an improvement on a 2,700 RPM engine as opposed to an 8,000 RPM engine. As on our planes, an obstruction has much more negative effect at high airspeeds versus low.
Polished ports on the exhaust will improve exhaust flow… for about ten hours of running time. After that the normal lead and other combustion deposits will have eliminated the effect of all the polishing. Remember that serious racing engines get torn down anywhere from every run, to perhaps a few hours of run time. Ports can be cleaned as needed to get that last one-tenth of a second for the next run. You probably won’t be doing that on your plane.
It is also illegal to rework a certified engine in such a way that it produces more than a few percent over rated power. Otherwise I can only imagine the flourishing market in overbore cylinders, stroker cranks, and the like. Not to mention the ongoing crank and prop failures, cracked cases, broken rods, etc. Keep in mind that the rated power is under test stand conditions with stub stacks, corrected to standard temperatures and pressures (59 degrees, 29.92 pressure). If an airframe manufacturer designs a poor induction air system and filter, and/or an equally poor exhaust system, there may be an opportunity for regaining lost power (as opposed to increasing power beyond rated limits). This is what lies behind the claims made for Challenger (K&N) air filters, and the Power Flow exhaust system.
There is no doubt that some older engines could benefit from things like casting flash clean-up in the head ports. On the other hand, most of the aircraft engines that are modern enough (I know, an oxymoron) to be in our planes, are surprisingly decent in port design and finish. They are vastly better than the stock ports in auto engines of the same era. Something like the Power Flow system is far more likely to provide a performance gain, as opposed to spending money on a customized (and potentially illegal or weakened) engine. It’s also a lot safer. During the past few years the FAA went after a facility for illegal piston rod alterations. The facility was performing internal engine balancing by illegally grinding rod ends, and engine failures resulted. In my opinion, “hot rod” aircraft engine modifications are best left to the home-built crowd, where most of the planes never even reach 1,000 hours of operating time.