Hi folks - wondering if anyone has recommendations for a source of cowl louvers for an RV-8?
Our RV-8's CHTs are running hot. We've got a tuned Aerosport Power IO-360, 9.0:1 pistons, plenum under a modified Sam James cowl, dual Pmag ignition and 4:4 Vetterman exhaust. We've used a tube of RTV to plug any air gap we or our A&P can find, have retarded the ignition slightly from the dataplate (to 20 degrees advance), are running the Pmag "B" curve and have even filed down any excess material between cylinder cooling fins. We're still seeing CHTs in the high 380s/low 390s at around 3,000' running 24" 2,400 rpm full rich at 60F OAT.
Scratching our heads a little we now think we might have an exit air area problem. To louver or not to louver seems like a well-debated topic (with some good threads e.g.
here and an
here). Swaying us a bit is the fact that Van's sells a cowl louver kit for the RV-6/-7/-9.
I've poked around a bit but can't find any kind of kit for the -8 - anyone know of a source? Thank you! Marco
What are the CHT's 1, 2, 3 and 4. Usually cylinder #3 Back RHS (passenger side). The fins on the back side of the #3 cylinder are very shallow and your baffling can choke airflow. The trick here is add a little more gap between baffled and cylinder (with spacers). As usual are your CHT probes (I assume bayonet type not spark plug ring types which read high) are calibrated.
Your cruise CHT at 75% power (24/24 at 3500') is at 60F ambient is under 400F,
you are golden, which is below Lycs recommended operating limit of 435F. Red line is 500F! For the most part there is no CAUTION limit on Lycs, in theory you can operate at 450F or 465F up to 500F. Continental Red line is 460F. We can agree as owners who would pay the bill for a burned valve or seat that is too high.
You are right and I am a big proponent however to keep CHT's below 400F, but it's far from a Lyc limit. However if it get's to 420F or a little higher for a short period that's OK. 420F is my limit for climb. Lycs Tech TIPS does recommend for long engine life, CHT of 400F or less, power settings of 65% or less, OT of 185-210F and CHT never above 435F for continuous operations.
With your high higher compression, full rich at 75% is a good idea. I'd stay full rich to 65% pwr a safe point to lean. You retarded timing and that likely gets you detonation margins back.
Retarding timing, excellent. I assume B-curve is the more conservative advance schedule. Electronic ignition can increase CHT. Retarding timing helps CHT and loss in power is minimal. With 9:1 you have less detonation margins, so retarded timing gives you back margin of safety to detonation.
Sealing gaps, excellent. If you have a Sam James Cowl you should also have round inlets with a flexible sleeve to a pressure plenum. This does not rely on soft baffle seals against the upper engine cowl as in stock Van's installation. Check the inter-cylinder baffle. If your Sam James cowl and cooling plenum are set up well, metal baffle part sealed against engine case, you are getting the max inlet air these inlets can give. You can't increase cooling air IN. The inlets are sized for a stock O-360; your 9:1 is making a little extra power/heat, but you can lower power by throttle and cooling of course by mixture. As stated above check #3 cylinder to baffle gap, increasing it (thickness of a nickle) will lower #3's CHT.
What is your oil temp? The engine is air & oil cooled. How many rows in cooler and what is the air plenum like? Upgrading the oil cooler and improving the airflow (in and out) of cooler, lowering OT will help CHT (a little). In cold weather you want a min OT of 190F. If you are seeing OT in the 210F range, it's probably OK. If you have OT 245F (Lyc max operating) a bigger cooler with better airflow is in order.
Increasing the cowl exit will lower the CHT but at added drag. A lot of piston military and civilian planes for over 80 years have used cowl flaps. The ones you are looking at are fixed it appears to me. That is fine, but you really don't have a cooling issue with 385F (hottest cylinder) at 75% cruise. Climb a different story.... see next Par.
You have a hot home-built, and to go fast you have to minimize cooling drag. The fact you are rocking 9:1 pistons you'd expect these temps or higher. With 385F you are very good. Now on hot day, heavy plane, long climb to altitude, this is the critical test. Over 400F is OK for short periods from time to time. How hot is too hot? Well personally I never like to see over 420F but according to Lycoming you can go much higher (at least for short periods not continuous operations). My RV on really hot summer days, two people, long climb to altitude, I had to shallow the climb or level off momentarily to keep CHT in check. However I had a fast plane which was adequately cooled for most operations.