The wise man set the limit
OneTwoSierra said:
I've never had a clear understanding on exactly what drives the 160 HP limit on the 9A.
The 160hp limit is set by a guy named Richard Vangrunsven. I hate to be glib like Tom Cruise would say, but....
Engineering an aircraft is a million trade offs. It may sound trite but
build it to plans. This is the common words of wisdom people much smarter than you and I have spoken. You can't go wrong. If you want to go faster in a side by side get a RV-6/7. That is my opinion. Since in a previous life I was an aerospace engineer and have built two RV's I have some perspective, and this is the short answer. However it's a free country, the FAA has given builders a blank check to modify or build a one-off custom. You want a twin engine / Bi-plane / RV-9a, than go for it.
However usually changing one thing brings on an avalanche of other changes or cause & affects, many of which are not as desirable as one might hope for.
I would suggest
building clean, light , simple 160hp RV-9a and concentrate on reducing drag and weight thru things like a Sam James style type cowl (I say "type" because you can make your own cowl and plenum from a stock cowl. Check out the Sam James Yahoo group). The net effect of adding a SJ type cowl and plenum will add about 6-10 mph or more. In cruise 6 mph is like adding 10-15 hp. Using Van's spec for the RV-7 going from 160 to 180 hp is worth about 8 mph top speed, 300 fpm greater climb, 25 to 75 feet shorter T/O and about 60-75 sm less range.
Also people rationalize they will get 160hp fuel burn with their 180hp engine by throttling back. It does not usually work that way, and even if you did you will still have overall higher fuel consumption. There is no free lunch.
The RV-9a departed from the traditional 23013.5 NACA wing airfoil other RV's use.
"The RV-9/9A uses a new Roncz airfoil design with a slightly longer wing span, shorter chord and slotted flaps for better low speed performance."
To get low speed performance there are trade offs to top end. If the plane was designed to cruise 6-8 mph faster, Van would have used a different airfoil and yes specified a bigger and heavier engine? Adding more HP may mean a little more speed but with less efficiently.
Keeping it light will also make the plane more fun to fly and feel better on controls, not to mention preserve its low speed performance. The thought of a O-360 adding only 10 lbs is very optimistic, even if that is what the specs say. (not to mention clone O-360's weigh more than stock Lycomings.) The installed weight will be at least 21 lbs. Not to mention one add-on begets another. A 180hp engine is begging for a constant speed prop, with another 35lb or more additional weight over a fixed prop.
I did a survey of RV weights and all boiled down to O-320 powered RV's (all models) are 21 lbs to 51 lbs lighter than a similar O-360 powered RV's. Needless to say you will add some weight, at least 21 lbs and likely more. Now add a fat panel, upholstery, paint job and extra do-dad's and you will have a heavy RV. Trust me light is the way to go. From my survey one RV-9a weighed about 100lbs more than Van's max empty weight of 1075 lbs.
If you flew that one next the one built to the lower 1028 lb empty weight limit (147 lbs lighter), you would know what I mean.
Forget the engineering/aero reasons. This has everything to do with feel. Also the benefit of more power does not always produce the performance you might predict because of the extra weight.
Since the 160Hp engine is already at high compression, I would suggest you could make small changes such as a blue printed engine with electronic ignition and get a fat +160 hp. I am not a fan of real high compression pistons in a daily flyer. 8.50:1 is plenty.
My opinion is 160 hp is PLENTY. If you have not flown a RV, trust me 160 hp is lots of power. Heck 150hp is plenty in a RV.
I can get into structural and aerodynamic considerations of 20 extra hp and +20 extra lbs on the nose, but I'll just leave the bottom line as: If "The wise Man", Van, would have wanted 180 hp in a RV-9a he would have designed it that way. There is a balance to get the "total performance" the RV-9 was designed to meet. Next you all will want to aerobatics in the RV-9? (there was another thread on that already). Build a RV-7 already.
Good Luck, George RV-4, RV-7
PS no one mentioned money. It seems people have unlimited money. A used 150/160hp is cheaper and easier to find than a used 180hp Lycoming.