Free speed and are HC pistons needed?
Why more compression ratio?
Is that going to make that much difference?
Take a 180 hp engine, and assume 205 mph cruise. Add high comp pistons, say 190 HP the new cruise in round numbers
205 mph x (190/180)^.33 = 208.7 mph
Yes climb would also be a little better, but you are only getting 3 mph.
There are ways to gain speed with out adding HP. Reduce cooling drag, as with a cowl & pressure plenum is worth 7-10 mph for example. The nice part is this speed does not burn more fuel like more HP does.
What is the down side?
Read articles from Lycoming on HC pistons, they will warn of
reduced detonation margins.
Something to think about. HC pistons do have the potential to reduce reliability. Here is an article regarding HC pistons and Turbo charging:
http://www.lycoming.textron.com/mai...cations/keyReprints/general/lookWhatThey.html
Also, many are aware the Hartzell found that prop fatigue increased with higher compression pistons and electronic ignition. Some of the effects to consider as Lycoming is concerned:
http://www.lycoming.textron.com/mai...Reprints/general/questionsEngineModifier.html
I rebuilt my O-360A1A with stock 8.5:1 comp pistons, after talking to Tracy Saylor who has one of the fastest RV-6s around, as demonstrated by his race results. Of all the mods he made, going to HC pistons was one of his later mods. He found they made little difference in his top speed.
If you really want to make more HP you have to modify everything to take full advantage. New HC pistons, needs a different cam grind to match the exhaust system, requiring induction changes and so on. Bottom line these are airplane engines and reliability is of premium importance. The more HP you draw the higher the pressures , temps and stress, the lower the life and reliability. Does your flying consist of racing at Reno 90% of the time, or flying yourself and your wife to airshows and the kids house, occasional aerobatics, formation flying and x-c?
Simple drag reduction and engine changes like exhaust and ignition changes can improve engine efficency and add a little performance with less down side. Internal changes should be thought out carefully. Also there can be insurance implications. Some companies will frown
at modified engines and they do ask. Denial of coverage or higher premiums may result. I am not against a small increase in Comp ratio but it is not for everyone and don't expect it to make a huge difference in a RV that already fly?s very well.
Cheers George
PS
4 into 1 exhaust is a good start at making improved engine power. It has been difficult if not impossible to use these on "A" models with the center nose wheel, but
Exhaust Technologies now has one. I have a custom 4 into 1 "tuned" exhaust for my RV-7, and they are very good quality. They also now have off the shelf RV exhaust systems and even offer a muffler combined with a heat muff. They use some techniques from their certified side of their shop, like porcupine studs, that makes the heat muffs very efficient for their size.
http://www.aircraftexhaust.net/exhaust_photo_album.html
http://www.aircraftexhaust.net/Custom_4into1_Vans_RV8_Exhaust.html
http://www.aircraftexhaust.net/Vans_RV8_IO_360_4into1_Exhaust.html
http://www.aircraftexhaust.net/index.html
Cross over is not the best system and produces uneven power pulses due to the different pipe lengths. (Ref cafe foundation exhausts research). 4 into 1 "tuned" exhaust is best, followed by 4 separate pipes and than cross-over. The worst are Y-pipes, 2 into 1, joining cylinder 1 & 3, and 2 & 4.