Be Cool!
I am reluctant to talk about oil cooler; last time I did someone got real mad at me. But let me help you with some good info.
First here are the Stewert Warner Oil cooler data. Check the charts out for
http://www.oilcoolers.com/LCHX Specifications.pdf
- Heat transfer (Verses) Air Side Mass flow (for various oil flows)
- Air Pressure Drop (Verses) Air Side Mass flow
For Aero-Classic go here for their data (scroll down)
http://www.canadianaeromanufacturing.com/
Aero-Classic also did a report and published it, testing other coolers; I took the SW data and superimposed it onto their graph, which took some unit conversions. SW heat rejection chart is verses airflow rate, Aero-Classic is verses pressure drop.
Now how much air do you have? From the well known NASA report the cooling data (from Piper) for Lyc 540's in a Piper Apache twin (there test plane) is below. I did some unit conversions. Of course we don't have 250HP> We also don't want to fly around with 450F CHT's either. However this is a good graph showing the kind of cooling you need. For an RV we need at least 3" to 9" H2O pressure drop. At 200 mph we have a total RAM of 19.26" H20 @ SL. This is where leaks and efficiency of your cowls intake and exit come in. If everything is great you might 70% at the aft baffle, or if you have leaks and bad mojo, maybe 35% (why planes run hot). The key is getting that air (more pressure than flow, we have plenty flow we want pressure) and NOT wasting it. On a HOT 100F day in a climb when Ram (free air outside the cowl) is only 7" H20 and you need 3" or 4" H2O pressure drop across the cylinders (and cooler), you see you better have full efficiency and no waste. The big insight I can make about the SW cooler's, is its DESIGNED and optimized to operate efficiently where our plane operates. In other words its a good fit or match. A cooler can advertise great heat rejection, but if you don't operate at those Delta P's and Flows, it does less good.
Now the big debate that gets people ruffled is mounting the cooler on the baffle v. ducting. Some say you can't remote mount and get good cooling. I respectfully disagree. Just look at the front gross area of a typical oil cooler "grill". It's about 20-21 in-sq. A 4" round duct is about 12 in-sq, 60% of the gross cooler frontal area. Of course net area is less with fins and tubes, which just happens to be close to 60% of gross area. Any duct with less area less is going to choke the air a little. May be you can get away with 3.75" or 3.5" dia duct. Of course cutting a 4x4 square in the baffle works, but a big heavy cooler with oil in it, hoses hanging off it, can crack the thin alum baffle from vibs. Yes you get good air to the cooler, no debate, but its shoved up against the fins of #4 cylinder, which is not perfect. Most find, even if they mount hard to the baffle, a short square plenum, moving the cooler away from the cyl fins, angling it down slightly, improves cooling (as shown in ECI's installation manual). If you are clever, you can make that short square duct with a soft materal to isolate vibrations from the cooler (mounted on the cooler to the engine mount). Wet-suit material (neoprene) works well.
From ECI Titan EXP engine (320 & 360) engine installation manual:
"If your engine installation requires the large oil cooler, ECi
and Van’s recommend that the cooler NOT be mounded on the
engine baffling because of size and weight considerations."
I don't care how folks mount their cooler, but if you do use a short SCAT duct, shoot for one larger than 3" (7 in-sq), even 3.5". For a 150/160HP in moderate climates 3" works ok. There are many threads on how to make a good remote cooler installations, I won't repeat again, but if you can't make you remote cooler fit, than I'm very sorry. If you need help write me. I'm just stating from an engineering stand point a cooler like the 8406R needs about 12 in-sq duct. If anyone is offended by that or the data above, I'm sorry.
Keep in mind a 3.5" (9.6 in-sq) duct is 36% larger than a 3" one, so if a 4" does not fit for you...... sorry. I never said 4" would be easy, but it has been done. Just be aware, before you curse me, of the shelf 3.5" flanges are not avaiable, from what I've seen, so you have to make them. You WANT max oil cooling efficiency & cooling, you have to work a little harder.
A few comments about FLEX ducts or SCAT losses (or lack of loss). Its true any duct has loss, longer, smaller, rough inner surface, higher flow rates, bends, all add loss. The max flow we could see through a cooler is 30-40 lbs/min (400-520 CFM). Typically we are at 20 lbs/min or 260 CFM. The loss for a 4" duct is 0.07 in-H20 per-foot, assume 2 feet, 0.14" H20. That is not much. A 3" duct, 2' long, same 260 CFM, has 0.54" H20 pressure loss.
3" dia has 4 times as much loss as the 4" dia. My point a larger 4" duct is very efficient.
A cooler on the baffle has no duct pe se, but its not totally efficient, when its blocked half blocked by the cylinder. Having a plenum leading to the cooler is a good thing. I have no dog in the fight except my pet peeve, which is 3" SCAT is too small for most 180HP and up applications. Square ducts are fine BTW, and if the inner surface is smooth, its about 30% less loss than a round SCAT tube with the same area. Keep the SCAT short and avoid sharp bends. Last consider the cooler exit. The cooler needs differential pressure. It does not help if you are forcing air into the cooler exit, inadvertently. If your remote cooler is lower in the engine compartment, air flow might across the cooler exit might interfere with flow, so a shield might help.
