or "Cold" Airbox as it is commonly known,
AND what I learned about building a high performance motor.
(by Kurt Otto Haukohl)
If you decide to buy, build, or rebuild a high performance motor or maybe just soup up the family sled, you should think about some of the following before you start out on this mission. We might save you a bundle of time and money here too. Three years ago, Dan Clark and me (Kurt Haukohl) decided we each needed the best motors we could buy for our craft. This story is about what we learned, aerobatics, safety, and how to save some money when it is your turn to rebuild your motor.
You might have already concluded that we decided to build the best, strongest, most powerful motor per pound that we could. All the custom motor buildup experts will tell you, that your engine is better than 50% of your aerobatic competitive edge. You must have reliability, durability, safety, power, flexibility, and longevity from every aircraft motor, but in the case of competition you must have all of these best characteristics in one package. Dan and I set out to build two virtually identical IO-540 Lycoming case motors. After all, it should be almost as easy to build two motors at the same time! Right? Anyway, here is what I learned, and you will need to focus on each and every part:
Crankshaft. There is a lot of talk out there about these things. Why? Because
this is the largest single expense item in most engines. You can pick out exactly
what you want in terms of wear and tear (age) and design. Here is where you,
me and everyone needs to spend some money, and search for the best quality.
Trying to do this selection by searching for lower prices will just cost you
more in the long run. If for instance you are building up a motor that is going
to carry a fixed pitch wooden propeller, you can really dip down to the bottom
of the barrel by getting a crankshaft with lightening holes. You should never
consider using the same crank for a heavy metal or constant speed propeller
later on. Lightening holes drop the value/price of most crankshafts by a factor
of ten. To measure this yourself in the free market place, price a solid flange
AEIO-360-A1D crankshaft, and one that is identical but has the extra holes between
the prop bolts. You definitely pay for quality and safety. In the six cylinder
categories, most cranks are already solid flange cranks, and you pay about $1500
dollars more for a "low wear" used crank than a slightly more used (almost at
trash limits) crankshaft. To summarize, our advice is to pay up for a nice quality
crankshaft. There is really no point in building up a motor that has a crankshaft
at or near the wear limits. You will get little time out of this type of rebuild,
and just be writing another check sooner, not later.
without a doubt the finest 4 cylinder sump out there is the "Kevin Murray" /
"Sky Dynamics sump. This is a fabulously well designed sump with a swinging
pickup that virtually never sucks air; working just like a header tank. Typically,
these Kevin Murray designs include the cold air box, injector, intakes and exhaust
system integrated to fit a Pitts S1S. To bad for us, Kevin doesn't make a 6
cylinder sump or I would have just paid up and moved on.
Lycoming 6-cylinder sump has the lower engine mounts attached in the photo
These sump ports are typical for the Christen inverted oil system
The standard intakes are not used, and the tubes are left off
made some modifications to the 6 cylinder sumps we have to concentrate the oil
pick up in the lower rear end of the case. When your vertical, and power is
at max, you do not want to see ZERO oil pressure. The interruptions in oil pressure
still occur though from inverted to upright (very short term & normal),
and on extended down lines (when your throttle is closed anyway). Overall, we
are pretty happy with these results. Our cases also have added dowels or pins
to hold them together more rigidly, and many additional oil ports and spray
nozzles. We also run fairly high oil pressures to keep everything real wet.
Monty Barrett supervised this process and provided the blue prints for these
modifications directly to the reconditioner, Divco, in Tulsa. From the looks
of it, they can take a cracked or seriously damaged case, repair it, reweld
it, true it up and give you a warranty.
Yellow portion is a good view of the Lycoming narrow deck case from the top
Setting up and inspecting the case after reconditioning calls for very close inspection and 8 hours of measurement, because you are essentially building up a completely new engine after these cases are repaired. Enlist the services of an experienced assembler to set up your case, crankshaft, camshaft, lifters, bearings, counter weights, gears, etc. Close your eyes and sign this check knowing you and I might have to blow up four or five motors before we figured out all the tricks necessary to assemble the case halves. There is so much to check, that I recommend giving your case to a pro.
Accessories. A bunch of cash gets spent here. We like everything from B&C
Specialty, Bendix Magnetos, Christen Inverted systems, and the Airflow Performance
Injector system. I had the nice little 8 amp battery charger system from B&C
and chose to install this, Dan chose to go without any electrical system. Dan
was also able to use a Bendix RSA-5 injector, and had Don Rivera at Airflow
readjust, rebuild, and reflow to match his needs. The Bendix RSA-10 might be
a better choice to keep sufficient fuel and air flowing to every cylinder. A
smaller injector might cause you to have one or two slightly lean cylinders
at full power in the 300 + HP 6 cylinder motors. Oil coolers seem to come in
all shapes and sizes too, and you will probably want two on the big motors.
Do not fool around with the light weight automotive oil cooler, because most
cannot handle the higher pressures we run (over 100 pounds). Great ignition
harnesses are worth it too, the quality ones last longer and handle the heat
better. Finally, spark plugs. If you can afford it, get the Iridium high dollar
plugs. They stay clean and have a nice hot even spark for a very long time,
usually the entire time you own the motor. I personally had too much trouble
with the Platinum plugs shorting and cracking, but when they work, they work
as good as the Iridium's. Solid electrode plugs are nice too, and just require
maintenance, inspection, and resetting on a regular basis. Whenever we do a
compression test, I set up the plugs. A torque wrench is mandatory here along
with the proper graphite lube. Most motor work requires a torque wrench and
probably some specially made wrenches. Invest in all this stuff early, and plan
on owning them forever.
Suddenly, it became an emergency for me to stop that hot air from getting into the intakes, and I became obsessed with finding a better source of "cool" air. Now that I had a good grasp of the problem, I enlisted the services of an old college friend Tim Donovan, and his company Optimum Engineering. Tim machined our boxes out of solid blocks of T6 aluminum from an NC program and drawings submitted by Dan Clark on one of those expensive Japanese NC machines. The railcar racers we hanger with, recommended a plenum area equal to 1 1/2 (one and one-half) the volume of one cylinder. With a little more research, Dan determined that this had been untried as yet with airplane motors and showed some promise in producing extra power. I was also thinking about flow directing the inside of the plenum itself.
You can see some of the airflow problems and feel them yourself with a slow running motor at idle, fooling around with the main air intake. In fact you can feel each pulse of air passing through the system, and measure the lower air pressure (manifold pressure) in the intake manifolds. Resistance measurement studies by all the "Car Guys" suggest this exact plenum ratio formula on injected motors. By increasing the intake mouth size and the size of the plenum you will reduce the net back pressure and flow resistance of the entire system at full power. Hey, that is the general idea anyway. But in fact it does work. At high speed, using a funnel intake and a large plenum, you can provide more air than is actually required to feed the motor at standard temperature and pressure. Sort of a poor mans turbocharger. OK so what this means exactly is 31" of manifold pressure in my case at 2000 ft. and 170 miles/hour crossing the box. Not incredible you say? Well look at your manifold pressure gauge, and it will say about 27" just like my old system. I think the 3-4 inches of manifold pressure will pay off in August for sure. I won't waste your time doing the math either, the horsepower loss in a stock system is a crime. The effect of slowing down and speeding up in terms of airspeed are very smooth in the total power fall off you feel and hear, and when you hit the down lines pulling back on the throttle always works fine. Essentially we have no surging from the box while the internal plenum pressure builds and falls.
Just guessing, but today we have about 300 hours on these two systems, and seem to enjoy really good performance including from the RSA-5 in Dan's Patriot. If you look at the Yellow motor case picture on the top of this page you can see the FM-200 / Airflow performance installation on the same cold airbox. A Bendix RSA-10 would fit nicely too. The ram air and balanced intake system combination with a header styled exhaust system, works better in flight than on the test stand because of the ram air intake and plenum. I'm not sure how you might simulate this system in a test cell. At high speed and at full power, with the cold air box, ram air intakes, and back pressure on the exhaust, we exceed ground test cell results. In static runs on the ground using a strain gauge, Dan also determined that the combination produces a little more than 1400 pounds of static thrust. So when your in the air it seems like more thrust should be available; in theory anyway. If you are in doubt about ram air, stick your arm out the window of your car at 70 MPH, the force is terrific.
I believe Dan's airplane is almost 1000 pounds and mine is about 1100, so if you are light ( not a heavy pilot ), you can hover just almost like a helicopter! We call it "Tucker" in honor of you know who, Sean D. Tucker. One problem was observed almost immediately in flying precision aerobatics whenever you have this many pony's humping to please. If your thrust to weight ratio equals or exceeds 1 to 1, you have a machine that will "Tucker". Whenever you kick to hammer, the airplane tractors off sideways or just keeps climbing up the line. So every hammerhead appears to have a one or two wing fly over. For this reason you need a GIANT rudder! Mine is two times the surface area of the original, we call it the limousine version, cut low and triangular like the Cap231. Pivots are no problem now, and the turn around itself is fast. Watch a MUSCLE BI-PLANE pivot sometime, this is a thing of beauty. You can reach a true stop in the air, and hold it still for an obscene period of time. Of course if you wait too long you also get a real wild torque roll going. Don't try this at home alone kids.
front view of the airbox installed on the Patriot
Without a starter!
This photo above is before linkage installation, and is without a starter. Those of you who know Dan Clark or his brother you know about this starter "thing". The Clark's call'em case crackers because every starter will eventually crack your case if you have a nice backfire or starting mishap. If you talk to the people who rebuild engine cases, something like 50% are busted around the starter, and unfortunately this is also very close the the main thrust bearing of the motor. Starting your pumped up motor is an ultra critical operation when using an electric starter. That 40 pound Prestolite starter that is furnished with every Lycoming motor is so powerful, that it will flat split your case when jammed. I discovered that you are better off with a weak starter, one that will just barely turn blades. Super high torque is super high risk, and a powerful fast running starter is a scary thing for us to watch. The newer lightweight starters are the way to go. One reason is the starter case itself will generally crack before it can break the motor case. Expensive to fix, but lots cheaper than the motor case! So it is easy to see why those Clark's like to hand prop their motors. Weight might be a secondary consideration.
Dan is a savvy competitor now though, and every time he walked out to his plane the ramp just cleared of people. We all know when those motors are hot, there is no way, hand propping is going to work. One day I personally wasted like 11 people pulling blades on my machine. A few flipped it over like 40-50 times. Alas, now Dan and I have starters from Sky Tech, for those touchy hot starts. We use an APU system consisting of an external battery on the now famous 'Radio Flyer", little red wagon. I have a small B&C battery in my plane too which is just enough to make blades in the air after a propeller stoppage during a tail slide or a spin. The small airplane battery will not start the airplane on the ground though without my trusty Radio Flyer backup, or a car and a jumper cable. You could use a full size battery, but they weigh ten pounds more than the small battery I have now in the plane. For some reason, I just can't get around to putting one in my plane. A high compression motor is definitely harder to start.
Good Starting Procedure. As I mentioned, after visiting the case re-builder "Divco" in Tulsa, and seeing at least three more cases cracked by starters myself, I really learned to pay attention to all this motor grinding stuff. Later I cracked a starter motor assembly in half myself, probably during a "kick back" incident one day. Here is my procedure today, and it requires that you install a separate starter button. The turn-one-key method of starting is part of the problem. You know the (R) (L) (Both) (Start) key? Having a separate starter button is excellent, and this is my new old solution! Two wires on the back of your key turn magneto switch are removed, and you just hook them up to your Radio Shack push button. No electrical engineering degree is required!
When you install a starter button. You can run a starter without having a hot magneto (Switches OFF), and you can also start on just one mag. It can also eliminate those nasty incidents of hitting the starter while the motor is running because you thought it might not be on BOTH! Ouch! I also learned that is a good idea to start up on one magneto, normally the right. Since I changed to this procedure, I have not seen one incident of kick back, or backwards running. This was really important old advice I got from the "Old" guy at Sky Tech. It just took a few months and one starter to figure this out. The idiot proof turn key starter system is in fact all wrong for any high performance high compression motor. You need a starter button, and can install this for about 3 dollars. You also need an APU plug, even if you have a big fat battery.
If this was a perfect world, everyone would get a Piper Plug system! I got real clever and started buying Piper Plugs for my friends too. Now they are all over the place, and I always find a compatible boost. If your real smart you will install the plug where the pilot can reach and discard to plug to start himself. When the "Radio Flyer" is sitting on the ramp all by itself, you know somebody must be out flying.
The Cylinders. I am not competent enough to discuss this here. There is way too much machine work, and lots of things to go wrong. Hire a professional or buy new. We chose Barrett here again. The pistons you and your rebuilder select are also critical. I'll bet you can install 12 to 1 pistons on the light straight valve motors and get about 100 hours before they explode, or you can consider the longevity thing here more carefully. A 10-1 ratio setup worked nicely on the 4 cylinder wide deck angle valve motors, but those cylinders are really beefy too. We finally selected 9.5 to 1 pistons to get sufficient horsepower and not waist the motor quickly. 9 to 1 on the smaller cylinders is still real stout and conservative at the same time. Most stock motors are set up at 8 or 8.5 to 1. Many times in the past, Monty has written articles for the IAC magazine, with particular focus on this subject. Plan on spending a good $1500 to $1800 per cylinder and buy a spare set up including the piston. The new cylinders from Superior - Millennium look really strong and do solve some of the chronic cracking problems around the exhaust ports and spark plug holes. Unfortunately, at the time of this writing Superior did not make a straight valve cylinder, only the angle valve model. Generally the weaknesses on these older models are well known, and any newly designed and manufactured cylinder should address and improve upon these problems. Porting and polishing might also help you get some more ponies, but you need to have someone with a flow bench do this for you who understands where to take metal off without weakening or destroying the cylinder itself. More recent fashion is to completely blow off porting and polishing to any great degree because it really was not worth the effort, and occasionally causes cracks. Sub-contract this job out too if you desire the best. Finally buy, or prepare a set of piston rods that are all balanced within 1 gram, including their matching piston, pin, rings, etc. Essentially you must take the time to mass balance the motor, or pay an expert to do it for you. Mark this down as another critical step. Mass balance your motor! Here is a simple thing that really pays off in terms of running smoothness. Think of the incredible forces on the crankshaft bearings if you just skip this step.
I hope this yuppie approach to, "what you need to know about your aerobatic engine", will be helpful for those of you considering an upgrade or high performance rebuild. Of course you can also just toss your hands up in the air and say, "where do I send a check?". Even if you do exactly that, when your done, you should find all of this stuff under the cowling when the job is done. The more you know about your front end, the safer you will be, and the longer your motor will last.