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In a message dated 2/18/03 8:46:58 AM Pacific Standard Time, djmolny@yahoo.com writes:
"Aerobatic pilots routinely fly parallel valve IO-540's, pumped up to
produce 325hp at 2700rpm. That's an increase of 65hp over the stock
engine's 260hp."
Yes, that's true. The flow characteristics of the stock parallel-valve heads, although quite good as-delivered, can be improved somewhat, as can the induction and exhaust systems, as well as adding quite a bit of compression above the stock 8.5. The stock cam profiles are also well-developed, and as you suggest, given the large diameter mushroom tappets, the profiles can be made more aggressive at the expense of considerable valvetrain reliabilty.
(NOTE: ALL SUBSEQUENT REFERENCES TO MEASURED POWER ARE WITH A CAREFULLY-CALIBRATED DYNO, AT 29.92, 59 degF, AT 2700 RPM.)
We have achieved 320-325 HP on parallel valve Lyc's without degenerating reliability to the point of stupidity. (I consider these engines nave a MAX 400 hours TBO).
However, your statement ("Given that the angle valve IO-540's normally put out 300hp, a 70hp increase is readily achievable") suffers from the normal errors of extrapolation.
First, the "300 HP" angle-valve engines DON'T (under the test conditions specified). Secondly, the angle-valve heads flow extremely well, and the velocity profiles are such that making them bigger (in pursuit of more flow) actually diminishes power. Thirdly, the intake system on the "300 HP" engines is already well developed. The "cold air" systems improve it slightly on the dyno. The proper header length and diameter also add a bit. Big compression (11.0) also helps some. The cam profiles, again, are well-optimized to the 2500-2700 operating range. We have developed a lobe profile which does improve power slightly, but at stress levels with which I am not comfortable for a long-life engine. (If you would like to see the typical lift, velocity, acceleration and jerk profiles for the stock and modified lobes, we have many in our computer system)
These angle-valve engines operate at a BMEP over 160 and produce nearly 80 lb-ft more torque (15% improvement) than the stock parallel-valve engines at 2700 RPM. To reach even 350 HP (at 2700) requires a BMEP of 190, which in Lycoming-supplied hardware, takes a mighty huff from a turbocharger to achieve.
Bottom line, in the real world, we have not seen much more than 340-345 HP from highly-modified (within the framework of what I'd actually fly) angle-valve engines (always at 2700). I, for one, would like some more hard data defining exactly how your 370 HP was "readily-achieved", not to mention, how it was "accurately measured".
Low RPM operation with a detonation-prone air-cooled engine provides some interesting thermodynamic challenges, and the rules of thumb which work well for estimating available improvement in high-RPM liquid-cooled engines tend to be optimistic.
The factory engineers have done a mighty fine job with these engines while meeting Part-33 certification criteria, and to conjecture that one can add over 22% to the brake torque output of an already well-developed engine (without using a trick dyno) suggests a certain distancing from the realities of engine design. (George Braly's work notwithstanding)
For an IO-540 to produce 370 HP at 2700 RPM represents a BMEP of over 200, which, based on our experience, I don't think is available in a flyable normally-aspirated IO-540. (And obviously, I'm excluding from this discussion nitrous oxide, nitromethane, and other "improvements" used by the guys who build engines with lifespans of 30 seconds).
If you raise the bar to allow 3300 RPM, then yes, more is available, but then what prop do you use at 3300?
I envy your EX-300, DJ.
Happy lurking.
Jack Kane
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