Just finished up a several year restoration on a 1956 C172 with an AVCON Conversion. Everything is new engine, prop, power flow exhaust, Sportsman STOL kit etc. Flew it for the first time the other day and the plane leaped off the ground handled great but the top end speed is only indicating 110 MPH. Had the Avionics shop recheck the pitot system and confirmed all is good. The rigging on the airframe is straight. One thought that was brought to my attention was the engine incidence. Does anyone know what the engine incidence is on an AVCON conversion or have any other suggestions on what to look for. Also what kind of indicated airspeeds are you getting from the conversion.

Flew again tonight. At 2000 feet the winds were almost nothing. Flew four different directions with the GPS showing ground speeds almost the same speed all four directions. Tanks at half full. Two 200 lbs Guys in the front seats. Noticed the elevator just slightly down, elevator trimmed pretty close to middle. I would of thought with two people in the front seats the plane would of required some up elevator to stay level not down elevator. Plane seems to be making full power on takeoff. The engine analyzer shows 18.5 gph full throttle and 2650 RPM on the prop. The engine is sitting straight forward 90 degrees of the firewall and now has .3 degrees of down tilt. We added some washers under the engine mount to tip it down and back to the left. The neighbor in the hanger next to me has a 58 with the stock Continental so I measured his engine incidence. It didn't change anything maybe 1 MPH

Funny no one mentioned the prop.

Drag goes up as the square of the speed.

To a first order, putting a bigger engine in an airframe, particularly a draggy one that is flown down low, will primarily improve climb performance only. For example, a plane that flies at 120 mph now, will require about 17% more power to fly at 130 mph, everything else being constant. The conversion in question is about a 13% increase in power, so if it once cruised at 120 mph, now if will cruise at say ~127 mph for the same % power setting. If it once cruised at 110 mph, might bump up to ~116 mph with the 160 -> 180 hp increase.

A much bigger engine in some planes (say 230 to 300 hp in an 182) increases cruise speed a little, at the expense of fuel burn of course.

As I stated before, 145mph/125kts at 6500ft 2300/23 with Hartzell CSP, O360a1a, powerflow exhaust, Horton cuff, gap seals, wheel pants. I’ve known thers(older models) that were not as fast. Thought it might have something to do with going from a O300 Cont to a Lyc360 as far as engine mount rigging. They needed a blister on the cowling, and more modifications. Mine is a 68 which I’ve had for 17 yrs. I agree there are a wide discrepancy as far as speed. Mine burns 9gph for a 4 hr flight(I have flint aux tanks on each wing), and 10gph for 1-2 hr flights. Power flow I would not do again, flint aux tanks a must have if going to use x country, due to increase fuel consumption.