The Angle valve head flows better at high RPM's, if you look at the torque curve for the IO-360-A vs an IO-360-M you'll find that they make almost identical power up to around 2400rpm, with the angle starting to edge the parallel out once you get into the 2500-2700 range.
That said, the counterweighted crank is a lot of the weight difference in the two engines, so an IO-360-A1A would probably weight a little less than the IO-360-A1B6 that Van's sells. I'd go either IO-390 OR IO-360-M series. Lycoming is really trying to discontinue the IO-360-A series in favor of the IO-390 because the lower compression will be more compatable with future fuels they think (per Lyc booth at Osh '08).
Hi Stephen and others,
I think we have an interesting discussion going and some of these questions may be something that only a person who has owned both angle and parallel valve engined airplanes can answer. My Lycoming owner's manual has a "detail weights chart" that lists the IO-360-A1A weight at 324 pounds, and the IO-360-A1B6 weight at 333 pounds. My IO-360-M1B is listed at 300 pounds. The IO-390 is not on my weight chart, but IIRC they weigh an additional 18 pounds or so over the angle valve engine. These weights are with fuel injector, mags, baffles, starter, gen or alt., etc. As to the counterweight, I know some people swear by those engines and their smoothness. My thoughts were on how to keep the airplane as light as I could, so that I might still be able to enjoy an A/P, CS prop and glass panel and yet get very good performance and handling for occasional "light" acro. Going with the RV-7 instead of the 7A saved 15 pounds or so and going with the M1B saved about 30 pounds. It is the smoothest engine I have flown behind. It is a Red / Gold engine that has had some special attention paid to balancing, though. It would seem to me that giving up a few horsepower and saving this much weight may make sense, especially with the 7's main gear design. I wanted a CS prop, which was heavy, but I did not want to move the CG too far forward. I'm very happy with the way my c.g. turned out. You have a vaild point about the angle valve's flow pattern. We don't always run at 2700 RPM do we? Also, I believe that many owners have claimed that the angle valve engines are more fuel efficient.
It would be interesting to hear from someone who has done a side by side fly off with similarly equipped airplanes with these two engines in both climb and speed tests at a variety of altitudes. (A 200 hp angle valve and a 9:1 M1B with one Lightspeed).
BTW, the angle valve engine's CR is 8.7:1 not 8.5:1 like the stock parallel valved engines, which is one reason I didn't think going to 9:1 was pushing the envelope too much). Also, as to reliability my engine came with a 3000 hour TBO and it's warranty started when I first started the engine, not when I bought it.
I'm not trying to talk anyone into doing anything. My thought process may have been flawed. I was trying to get the most bang for the buck, preserve handling qualities as best I could, keep my c.g. where I wanted it and try to equal or beat the performance of the Van's factory demonstrator. Those were my goals. This is my first project. All I can say is that it is flying, it climbs like a banshee and I am very very happy with it so far. (Now, watch somebody come along and blow my doors off)!
This is the stuff that makes being a homebuilder so great! Everybody does their own research has their own priorities and mission profiles and makes their own choices.
p.s. I hope Rhonda was'nt refering to me about putting out bad info. Barrett developed the 390, and Rhonda was extremely helpful to me when I was shopping for an engine and trying to decide which way to go. Being an Okie and knowing their shop's reputation, I wanted a Barrett engine so bad, I could taste it. You won't go wrong with a Barrett engine!
