What's your angle?
Russ,
Having shoe-horned an angle valve IO-360 A1B6 into an RV4 I too agree it's a "tight squeeze". The better call is the IO-360 parallel valve, 9.0 or 10.0 compression, flow matching, balancing and electronic ignition. This produces the power without all the weight (and size). The only down side is a non-counterweighted crank and smaller main bearings.
To answer the question on Lycoming designations, here is a blurb from Sac Sky Ranch's website.
Smokey
HR2
What is the difference between one Lycoming engine model and another
The "Same Engine" Myth
Questions that frequently are asked of Lycoming sales personnel, engineers, and technical representatives indicate that there is a myth regarding Lycoming piston engines. This myth seems to be prevalent among aircraft owners and aviation writers. In the minds of these individuals, each Lycoming engine series is essentially the same. For example, all 360 cubic inch displacement engines are inherently the same except for differences in fuel metering or turbocharging. The idea that these engines are the same is false. A few specific examples may help to put this myth to rest.
Lycoming builds 0-320 engines that produce 150 HP or 160 HP. The 150 HP O-320-E series engines operate at a compression ratio of 7.0:1. The 0-320-D series has high compression pistons which raise the compression ratio to 8.5:1, and increase rated output to 160 HP. Those who believe that the pistons are the only difference in these engines will be disappointed when they plan to upgrade their 0-320-E to the higher horsepower by simply changing pistons. Many models in the 0-320-E series were designed for the purpose of keeping the cost down. Thousands of these low compression engines were built with plain steel cylinder barrels instead of the Nitrided barrels used in the 0-320-D series engines. They also had two narrow bearings instead of one long front main bearing. The engines were certified at 150 HP and were not intended to withstand the additional stress of higher horsepower.
Because of the similarity in designation, it would be easy to believe that the 0-360-AlA and the I0-360-A1A are the same engine except that the first engine has a carburetor and the second a fuel injection system. Here are some features of each engine for comparison. The 0-360-AlA has a bottom mounted updraft carburetor, parallel valves, 8.5:1 compression ratio, and produces 180 HP. The IO-360-AlA features a horizontal front mounted fuel injector, angle valves, 8.7:1 compression ratio, and is rated at 200 HP. The I0-360-A1A also incorporates these design items which are not included in the 0-360: piston cooling nozzles, stronger crankshaft, tongue and groove connecting rods with stretch bolts, tuned intake system, and rotator type intake valves. There are actually few similarities except for the 360 cubic inch displacement.
There are individuals who have suggested that by putting 10:1 compression ratio pistons in an I0-360 engine, it could be the same as the HIO-360-D1A. These are some characteristics of the HI0-360-D1A helicopter engine that can be compared with the data on the I0-360 listed in the previous paragraph. To start, the HIO has conical rather than dynafocal mounts. The main bearing is a thick wall bearing instead of the thin wall, high crush bearing used in the I0-360. Other differences include: crankshaft designed for small crankpins, high speed camshaft, rear mounted RSA7AA1 fuel injector, large intake valves, and torsional vibration damper magneto drives.
Finally both the Navajo engines and the new turbocharged Lycoming used in the Mooney TLS are equipped with differential and density controllers that automatically set the maximum allowable horsepower when the throttle is advanced fully for takeoff. Some who have not taken the time to compare these engines have jumped to the conclusion that the TI0-540-AF1A which powers the Mooney TLS is simply a de-rated Navajo engine. This conclusion could hardly be more inaccurate. The most obvious difference, even to the complete novice can be seen by looking at the rocker box covers. The TI0-540-AF1A is rated at 270 HP and has parallel valve down exhaust cylinders. The Navajo series has three engines at 310 HP, 325 HP, and 350 HP. All have cylinders designed with up exhaust and angle valves. Other differences respectively in the 270 HP AF1A and the Navajo series engines are: small main bearing instead of large main bearing, 8.0:1 compression ratio rather than 7.3:1, intercooled and non-intercooled, pressurized Slick magnetos versus Bendix/TCM magnetos, and an RSA5AD1 fuel injector in place of the RSA10AD1 injector. There are some other differences, but those comparisons listed should convince even the most skeptical that these engines are vastly different.
By making comparisons of various parts and accessories used in engine models which some individuals have considered to be much the same, it is possible to illustrate the differences. Although some Lycoming models are closely related, this cannot be assumed. A review of the engineering parts list for each engine model by a knowledgeable individual is the only sure way of establishing similarities and differences. For those who may have been taken in by the myth that all Lycoming engines of a particular displacement are very much the same, you are now armed with a better knowledge of this subject.
