Depends a lot on what particular engine you're going to have. The standard, small 7-row NDM cooler Vans sells is generally fine for a stock Lycoming carbureted 160hp O-320. That's what's on my RV-6 (mounted on rear baffle aft of cyl #4) and it has always performed adequately even in the summer heat we get here in north Texas. I have seen my oil temps hit 210-220 in a climb however. One friend of mine with an IO-320 on his RV-6 says his 7-row cooler is marginal on his plane on a hot day. He even had to move his from the rear baffle all the way up in front of the #2 cylinder to get enough cooling out of his. Several folks here on VAF have reported acceptable performance with this oil cooler on a normal 180hp O-360 engine, but most of those are not flying in Texas summer heat either.
For a bigger more high performance engine, like another buddy's ECI Titan IO-360 with 9:1 pistons and piston oil squirter nozzles installed in the crankcase, the 7-row was woefully inadequate, within a few minutes after takeoff it was hitting oil temps approaching 240, so we ended up swapping it out right away, ultimately for a high-dollar Stewart Warner cooler that cost almost $700.
In my experience so far with oil coolers, it seems that when you mount one on the firewall and plumb air to it thru SCAT duct, that the cooler needs to be one or two rows larger than one that would be baffle-mounted.
When you do choose your engine, give Pacific Oil Coolers a call (
www.oilcoolers.com) and they can help you with selecting the right size and brand of oil cooler to keep the engine happy flying in Texas summer heat.