The O-320 Engine Choice

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The O-320 decision.

So why did I choose the engine at the low end of the horsepower range for my 7? Well, there are a couple of reasons that stick out, and purchase price of a new one is NOT one of them. When the price of an O-320 is compared to an O-360, there is only a small difference (usually $800-1,000 depending on the vendor). So if I buy new, why spend nearly the same amount for a lower HP engine? If we take a look at the performance page for the RV-7, you can see that the difference in performance between 160hp and 180hp is small.

This table is taken straight from the page linked above...I left the 200hp numbers in for comparison, even though I had no intent of putting an angle valve engine on my 7.

RV-7 Performance

Solo Weight 1400 lbs Gross Weight 1800 lbs	160 hp	180 hp	200 hp
*Empty weight and performance measured with Hartzell 2 Blade C/S prop*
Speed - Solo Weight
Top Speed	202 mph	210 mph	217 mph
Cruise [75% @ 8000 ft]	192 mph	200 mph	207 mph
Cruise [55% @ 8000 ft]	173 mph	180 mph	187 mph
Stall Speed	51 mph	51 mph	51 mph
Speed - Gross Weight
Top Speed	201 mph	209 mph	216 mph
Cruise [75% @ 8000 ft]	191 mph	199 mph	206 mph
Cruise [55% @ 8000 ft]	172 mph	179 mph	186 mph
Stall Speed	58 mph	58 mph	58 mph
Ground Performance - Solo Weight
Takeoff Distance	300 ft	275 ft	250 ft
Landing Distance	350 ft	350 ft	350 ft
Ground Performance - Gross Weight
Takeoff Distance	650 ft	575 ft	500 ft
Landing Distance	500 ft	500 ft	500 ft
Climb/Ceiling - Solo Weight
Rate of Climb	1,900 fpm	2,200 fpm	2,550 fpm
Ceiling	21,000 ft	23,000 ft	25,500 ft
Climb/Ceiling - Gross Weight
Rate of Climb	1,400 fpm	1,650 fpm	1,900 fpm
Ceiling	18,500 ft	20,500 ft	22,500 ft
Range
Range [75% @ 8000 ft]	835 sm	775 sm	765 sm
Range [55% @ 8000 ft]	1,025 sm	950 sm	935 sm

You can see that the cruise speeds are split by 8mph (or 7kts). Climbs are split by 250-300fpm, and the 160hp airplane STILL climbs at 1,900fpm solo!!

Since the performance is so close, the decision came down to fuel burn. The 180hp engine will burn 9.5-10.9gph at 75%, and the 160hp engine will burn 7.0-8.2gph at 75%. These numbers are based off of my experience with both engines in several airplanes, including RV's. I do know, and understand, that these fuel burns can be accomplished with the O-360, and the speeds will be similar. Makes sense since similar power will be produced. My problem with this logic is simply me. I KNOW that I will not be disciplined enough to pull the power back. If the speed is there, I will use it, and get the resulting fuel burn. With the expense of 100LL, I will be sure to avoid the higher burns with a lower powered engine.

The O-360 and O-320 are both pretty bullet-proof engines if properly taken care of. Both are stone simple. The 320 is lighter by a few pounds (which is always good in an RV), and because of this, I will need to be mindful of just how light things are up front. Now planning to use a fixed pitch prop, I'll need a spacer for some weight, and I'm considering a couple of other things to keep the CG in an acceptable range in all realms of flight.

The only other option for an engine that I would consider at this point, is the O-340 from ECI. It makes 177hp in carbureted form (185hp with FI). Since I have the O-320 Dynafocal 1 mount coming, I think this is the only option I would have, but I have to check with ECI to see if the mounts are the same. I don't think there are any O-340's flying as I write this (11/24/07). There may be an IO-340 or two flying, but they are still new, so I'll watch from the side lines for now on this.