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Last Updated: 08.30.07
The information presented on this page is intended to document my engine research, and shouldn't be viewed as a factual source of engine data. I have focused on a general understanding of the choices available for the RV line of aircraft, with the understanding that there are many many exceptions when it comes to engine configuarations. It is admittedly biased and riddled with opinion, and it will be a living document that will change over time as my experience and understanding grows. Enjoy it, but use it at your own risk. If you see something here that you know is wrong, please let me know, I may even give you credit. ;-)
Table of Contents
Basic Engine Choices
Parallel vs Angle Valves
Automotive Fuel and Compression
Electronic Ignition (EI)
Lycoming vs ECI vs Superior
Putting It All Together
For the RV-7, outside of any alternative engines (e.g. Eggenfellner), there are basically three main engine choices which for the most part are determined by the horsepower you desire. For 160HP there is the I/0-320, for 180HP there is the parallel valve I/O-360, and for the power hungry in all of us there is the angle valve 200HP IO-360. Oh yea, there's one more - For those that really want the most horsepower there is the IO-390, which is similar in size and shape to the IO-360, but since it is nominally rated at 210HP it falls outside of the design specs for the RV-7 (I am steering clear of this topic, I'll leave it to the experts).
Lycoming's (or Lycoming clones) engines come in two basic configurations, the parallel valve and angle valve. The names are fairly self explanatory, but in a nutshell, the valve lifters on a parallel engine are, well, parallel, while the lifters on an angle-valve engine are, you got it, angled. The way you can tell which is which is simply by looking at the shape of the valve covers. The parallel valve engine's covers are square'ish, while the covers on the angle-valve are trapezoidal. Here are some pictures of both.
(click to enlarge)
(click to enlarge)
Now that we can tell the two apart, it is important to note that all 320 cu in engines have parallel valves, while a 360 cu in engine can be either parallel or angle valve. Then there is the 390, which is based on the IO-360 and is angle-valve only.
So what's the real difference between the two? The angle valve engine is a newer design and benefits from a tuned intake system, piston cooling nozzles, a stronger crankshaft, rotator type intake valves and various other improvements. In its stock configuration, it also has a slightly higher compression ratio (8.7:1) as opposed to its parallel valve cousin (8.5:1) and both the improvements in airflow and the higher compression ratio result in a 20HP gain over the parallel valve engine. It is a bit larger and heavier than the parallel 360, but more on this later.
You've all seen them, and engine described as a IO-360-A1B6, but what the heck do all those letters and numbers mean. In a nutshell, Lycoming's engine codes can be broken down into the following.
||Opposed (they are almost all this way)
||Engine Displacement in Cubic Inches
||Power Section and Power Rating
||Nose Section Configuration
||Accessory Section Configuation
I am not going to run through all the possible combinations and permutations of each code, but it is important to understand the general concept. Using the example above, we know that an IO-360 is a fuel injected engine, while and O-360 is not, but what's not clear is whether or not an IO-360 is an angle or parallel valve engine. That's where the power section part of the suffix comes into play. It doesn't make any sense, so don't ask questions, but a O-360-A or an IO-360-B or -M is a parallel valve engine, while a IO-360-A is an angle valve. Confused yet? So when you hear someone say I have an IO-360, they are typically implying angle valve, but it is impossible to really tell unless they say IO-360-M1B (parallel) or IO-360-A1B6 (angle). Get it? Got it? Good! By the way, I've put together a list of the most frequent engines and their specificaions and configurations, which can be downloaded by clicking the image below.
Click to Download the RV-7 Engine Choices (35K PDF)
Simply stated, compression ratio is the ratio of the volume of air when the piston is at the bottom of the stroke compared to the volume when it is at the top of the stroke. In general, higher compression ratios result in more power output (HP) for a given configuration. For the RV series of aircraft, most engines in their stock configuration have compressions of 8.5:1 to 8.7:1. However, in an effort to produce more power, a popular upgrade is to have your engine built with higher compression pistons, say 9.0:1 or 10.0:1. As with anything else in life, there are trade-offs in doing so. In general, the thinking is that higher the compression the higher the likelihood that you may have issues with high CHTs or detonation, resulting in engine problems or a reduction in the TBO of the engine. That being said, many have reported no issues whatsoever with compressions as high as 10.0:1 with an upgrade to 9.x:1 popular with many builders as a effective and relatively inexpensive method of producing more HP.
|Automotive Fuel and Compression
With the rising cost of Avgas, it has become increasingly important for many builders to allow for the flexibility of using automotive gasoline (Mogas) in their aircraft. Without getting into all the pros and cons of doing so, one potential gotcha is the compression ratio of the engine. In general, compression ratios above 9.0:1 (some say higher, some say lower) should be avoided if you want to run Mogas. There are a whole set of other issues to take into account with the use of Mogas, and I won't go into them here, so please do your due diligence.
Have you ever heard someone say, "Build it light"? From what I understand, the lighter RVs perform better. Wait, let me rephrase that, the lighter RVs fly better. Now I have no personal experience to qualify this, but I have heard it enough times that I think there may be some truth to it. What does this have to do with engines you ask? Simple, some engines are lighter or heavier than others. For example, depending on who you ask, the angle valve IO-360-A1B6 is considered to be about 20-40 pounds heavier than the parallel valve 360s, with the IO-390 another 10 pounds heavier still. Now that may not seem like a big deal to you, but keep in mind that it roughly equates to 3% of the aircraft's empty weight (assuming 1,100 lbs empty). Some would say that the RV-7, which tends towards being tail heavy, favors a heavier engine up front, thereby allowing you more flexibility with regards to CG in certain configurations. That may be the case, but ultimately it is up to each builder to run his or her own power and CG scenarios to determine if the added HP is worth the additional engine weight.
To combat or offset the weight gain, some builders propose the using of a lighter weight propeller. For example, the Whirlwind RV200 is reported to be 19 lbs lighter than the Hartzell Blended Airfoil (both CS props). Assuming the prop engine combination works, this can be an effective method for reducing the weight and CG impact a heavier more powerful engine will have.
There are only two choices when it comes to fuel metering; a Fuel Injection system or Carburetor. Just like every other choice we make on these airplanes, each has it’s pros and cons.
Float type carburetors have been used on aircraft engines for a long time, and in their favor they are; widely utilized, simple, effective, easy to maintain, and inexpensive when compared to fuel injection systems. On the downside, they are susceptible to carburetor icing and they are not suitable for aerobatic flight (negative g maneuvers).
Fuel injection systems, while more complex and more expensive, provide a more even fuel distribution as the fuel is injected into the intake ports of each cylinder rather than at the induction air riser of a carbureted system. This method of fuel distribution reduces the chance that one cylinder is running lean while another is running rich, and allows the pilot to run at leaner more efficient mixtures (e.g. Lean Of Peak) than he might with a carbureted engine. In addition, fuel injection systems do not need to be upright in order to function properly, making the suitable for aerobatic aircraft, and because the fuel is injected into the intake valve port there is no chance of ice forming.
In the end it really comes down to what the mission of your aircraft is, and how much you are willing to spend.
In the world of ignition, there are basically two choices; magnetos or some form of electronic ignition. What’s the difference you ask? Well, let’s see, to answer that I’ll go into a short description of each.
A magneto is a small electrical generator turned by the engine, which produces the high-voltage power for the ignition system (spark plugs). They are self-powered in that they do not require electrical power from an outside source, such as a battery or the aircraft’s electrical system, to operate. Theoretically, as long as the engine is producing power and the magnetos are turning, they (the magnetos) are producing the necessary electrical current to fire the spark plugs. With magnetos the spark advance is fixed, typically at 25° BTDC, for the entire operating power range of the engine. Magnetos are fairly straightforward devises to understand, and have been used successfully in aviation for many years.
As I mentioned, Magnetos have been around a long time, and I would say that they are the de facto standard in aircraft ignition. They are what they are, relatively simple, relatively inexpensive (when compared to EI), parts are readily available, and every pilot and A&P on the planet has experience with them. On the down side, they are mechanical devices and are prone to wear and potential failure and they do require periodic maintenance. Some would also consider the fixed timing (25° BTDC) to be inefficient simply because no one spark advance is optimal for every combination of power setting, altitude, CHT and other factors.
Electronic ignition isn’t as straightforward. Not because it is complicated, but simply because of the several popular products available, each performs its job in a slightly different way. Rather than go into the specific differences of each alternative I’ll just keep it generic and say that electronic ignitions take electrical current from the aircraft’s electrical system, and amplify it to the high levels needed to fire the spark plugs. Some do this by using ignition coils, some by charging and discharging capacitors, and others by generating their own power, but the one thing they most all have in common is that they use sensors to measure various engine parameters (i.e. RPM, MP, CHT), and this information is used to adjust, or advance, the timing of the engine (spark fires at >=25° BTDC). Electronic ignition is used on most, if not all, modern automobiles and while not quite as prolific in aviation, as the product offerings mature it is growing in popularity.
Electronic ignition (EI) offers many benefits over magnetos, or so the manufactures say, but it also comes with its fair share of issues. Simply put, most EI systems report to offer a longer hotter spark, which is timed or advanced depending on the power setting and other factors affecting the engine. This variable timing, along with the longer hotter spark is intended to increase power (hp) and fuel efficiency (decreased fuel burn for same power). In addition, many of the EI products offered are solid state devices, that is to say they have the benefit of having no moving parts, therefore their Mean Time Between Failure (MTBF) is theoretically higher than that of magnetos.
EI, while offering many benefits also has its shortcomings. It is expensive when compared to standard (magnetos) ignition, and while it is growing in popularity it does not have the long track record that magnetos do. Many of the electronic ignition product offerings are intended solely for experimental aviation, therefore they have not been tested to the extent, or with the variety of engines, that magnetos have. In addition, most EI systems rely on the aircraft’s electrical system for power. In the event of an alternator failure (in a dual EI system), a back-up source of power such as a battery or back-up generator must be used to supply power to these systems if you want the prop to keep spinning. There are a few EI product options that will solve this issue by generating their own power, or they will switch to an internal magneto when the engine is turning, therefore no back-up power source is needed with these particular systems. Another way around this is to have and EI system for one set of spark plugs and a magneto for the other, but I won’t go into the advantages and disadvantages of that set-up here.
There are two flavors of induction orientation; vertical and horizontal. As the terms imply vertical induction has the air entering the intake plenum vertically, while horizontal is well, horizontal. As a rule of thumb, carbureted engines will always have vertical induction, while fuel injected engines can have either vertical or horizontal induction. Why? Simple, float-type carburetors don't like to operate very well when they are on their side, while a fuel injection servo doesn't really care how it is oriented. What does this mean for RV builders? For those of us that choose an engine with vertical induction, the lower cowl will need to have a scoop, and those of us with horizontal induction will not have a cowl scoop but will need to direct air into the intake by means of a snorkel or ram-air inlet. Some say one performs better than the other, but I am not going to get into that here. In some ways it really comes down to which engine you choose or whether or not you want the scoop.
(click to enlarge)
(click to enlarge)
Vertical - Cowl Scoop
(click to enlarge)
Horizontal - Smooth Cowl
(click to enlarge)
|Cold Air Induction
It is my understanding that all cold air induction systems (sumps) are horizontal, but not all horizontal induction sumps are cold air. Are we clear on that? What is cold-air induction you ask? Well, the best way I can describe this is to say that in the typical sump system, the intake plenum and tubes are integrated into the oil sump. When air enters the plenum, it is heated because the plenum and tubes are bathed in hot oil. Since hot air is less dense than cold air, this robs the engine of power by reducing the density of air entering the cylinders. Cold-air induction systems have been devised as a way to eliminate or reduce the heating of intake air, thereby increasing engine power output as compared to a normal intake/sump setup.
Some of the cold-air sumps are comprised of two parts, with the oil sump and intake plenum being separate pieces. In my humble opinion, this is the ideal configuration as it minimizes and heat transfer from the hot oil sump to the intake system. There are other cold-air systems that are one-piece (integrated), but have the oil and intake portions in separate chambers of the sump. These seem to do just as good a job at optimizing HP as the two piece units do. What it really comes down to is what will fit your engine.
When I first got involved in all of this, I was horribly confused by all of this. I mean an O-360 is a Lycoming engine right? So how is it that companies such as ECI or Superior are essentially selling them under their own brands? Well, it turns out that ECI and Superior are companies that produce aftermarket parts (cylinders, camshafts, etc), for Lycoming (and other) engines. While they both manufacture FAA-PMA approved parts for certified engines, they also produce a line of experimental engines and parts. These engines and/or parts, which are typically called Lycoming clones, are available as kit engines directly from ECI or Superior or as complete experimental engines through engine building shops such as Aerosport Power, Barrett Performance Engines, Mattituck and others. Both ECI and Superior are known for improvements (or so they say) over the Lycoming parts, with ECI known for its cylinders, and Superior most lately for its roller tappet technology. Each has slightly different features to offer, so it is in the buyer's best interest to become familiar with the features each has to offer and with any further improvements or tweaks the engine builders can provide.
Before I forget, let's go back to the topic of Lycoming. You can purchase complete certified or experimental Lycoming engines from Van's at what some say are attractive prices (for the experimental versions). Some builders choose to do this, but others choose to go with an engine builder simply because they think the products available are superior (not to be confused with Superior) in features or performance or because they can build a truly custom configured engine rather than just taking the static configurations Van's sells. There is no right or wrong here, it all just depends on what you want, or shall I say what you want to spend.
Well don’t look to me for answers! I’m not expert, I’m learning this (and updating this page) as I go along. Spec’ing out your engine is certainly a personal decision, and since the darn thing is what gets and keeps you airborne, decisions on how it should be built should not be taken lightly. I am certainly not going to make any recommendations other than to say, do you due diligence. I have only just scratched the surface of these topics here and there are many factors to consider when making the choice(s). Talk to the engine shops, your EAA buddies, other builders, and learn learn learn!
One important thing to watch out for is potential gotcha’s with regards to combinations of equipment. That is to say, while you may like a particular ignition system, it may not work well with the high-compression pistons you also want and some trade-offs will likely need to be made. There is no way that anybody can possibly list out all the combinations of pistons, ignition systems, induction configuration, propeller, exhausts, and other options that will or won’t work, that’s up to you. Once you think you’ve got an idea about what you want, put together a list and start validating the concept. You may find that your original plan won’t work, or you may discover some new tid-bit of information that causes you to change your plan altogether. Good luck!
After about a year plus of hand-wringing, I think I have come to a conclusion on which engine I will purchase. I will give you my spec list first, then follow-up on the reasoning behind the choices. These are simply my thoughts at the current time. As I gain additional experience and knowledge I may find that my assumptions were flawed, therefore I reserve the right to change my mind at any time.
||IO-360-M1 (fuel injected, parallel valve, horizontal induction)
||Airflow Performance FM-200 Fuel Injection System
||Electronic - Dual Lightspeed Plasma (II Plus or IIIs)
||Horizontal Cold Air
||4-into-1 - Aerospace Welding, Inc.
|Cowl & Plenum:
||Sam James Cowl & Plenum
||Whirlwind 200RV (Requires 2-1/2" prop extension)
||You don't actually expect me tell you do you?
Why did I choose all this? You'll just have to check back later to see why.