Quote:
Originally Posted by 727TT
We installed the Dynon HDX in our RV7A over 2 years ago. We also redid all of the engine baffling. The CHT on #4 always ran hot on takeoff. We replaced a number of components and traced it to a wire. The probe itself was ok.
Now, on take off 2,3 and 4 all show hot at 410 to 425 on #4.
Any thoughts?
It was suggested I install cowling Louvers to provide more air flow.
The carburetor was overhauled last year and I installed a new fuel pump. It came out of annual this past spring.
Thanks....... Steven Superior 0-360 with fix pitch
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Seems we have two conversations going. I have some free advice and worth every penny (repeat of other comments in no particular order of importance).
425F is within Lyc limits but I am a big fan of sub 400F, and Lyc recommends operations sub 400F for long life. On take off or hot day climbs you can expect over 400F for short time periods.
WHAT IS THE AMBIENT TEMP? Hot humid days higher CHT.
If you have mags check timing is under 25 degrees. If you set 23 degrees it will have little affect on overall performance and will reduce CHT. Try it. The factory certified 25 degrees is required only for certified planes. Try 23 degrees. I would do this last as you should be able to run 25 degrees, but people do report solving CHT issue with a few degrees less than 25 degrees BTDC. The angle valve IO360 has 20 Degree BTDC Spec, but when it first came out Lycoming ignition spec was 25 degrees. Lycoming backed off that with a Service Bulletin on the early engines.
YES I KNOW we are talking parallel valves but just saying... If you have electronic ignition that can cause higher CHT, but from what I know about most EI's they are very conservative and top out at 25 for high power and only timing advance at low power settings. Some of the EI's have fixed timing maps, some allow you to set timing advance map (based on manifold pressure and RPM). This can be dangerous unless you know what you are doing.
Baffle leak - Yes be super OCD picky... Make sure there are no leaks. Red RTV is your friend... have a cheap bore scope or camera (you can buy them cheap for your phone)... See how the baffle material sits with cowl on. Sometimes one part gets pushed down or buckles and opens up a grand canyon gap in another..... I am assuming you have stock cowl and baffle, the deflector on 1 and 2 is a good idea. Also on one side of the cylinder the cooling fins are shallow. If your baffle is tight up against that area it chokes airflow off. You need to have a gap.
Oil Cooler If you have oil cooler on back of baffle it could be starving some air from that cylinder. Make sure that is all sealed. If you have the weight of Oil Cooler supported only on baffle it is a matter of time before it will crack (either oil cooler or baffle or both). OIL COOLER - The bigger the better... a big one will not fit on the baffle and must be remote mounted with a min of 4" SCAT or custom plenum. Lower OT lower CHT. What is your Oil Temps?
TEST and Calibrate your probes. First I assume you have the bayonet or well type CHT probes NOT spark plug gasket type. The latter reads about 40F higher. Lyc numbers are based on the well/bayonet type. There are plenty of articles on how to calibrate. Don't assume they're accurate.
MIXTURE - Yes it could be LEAN - What do the PLUGS look like? My old sold O-320 RV-4 ran very lean at first... because of the efficiency of the induction and free flowing exhaust; My plugs were white even at lower heat range. I had to ream the main jet ever so slightly.... IT DID WOUNDERS. It ran cooler, had more power and plugs were the perfect color. HOWEVER you should ALWAYS aggressively lean on the ground, after start, taxi, any time at lower power, or under 75% power (as long as OT and CHT are within limits). You may need to do a JET job on the Carb. I recall the biggest stock jet available off the shelf for my O320 RV-4 was not enough, so the drill bit and Boeing lube came out. I carefully stepped the diameter up a few thousands at a time. It took a few tiny steps. Edge of the jet orifice has a profile I tried to maintain. I forgot the drill size I ended up with, but I was very happy with that modification.
Marvel Schebler makes many versions of the same CARB for O-360's with different jets. If you have a certified shop do your Carb they put the stock jet in it. From the part number you can find out if it's the LEANER version of that Carb. Talk to the A&P or Shop who "overhauled" it, ask them to find the jet PN in your carb and if there are richer jets for this Carb. If they look at you funny get another mechanic. Search "The Googles" or these forums, you'll likely find the info. Changing or modification of the main jet is caveman simple. I recall you can do it with Carb still mounted to the engine. Once I did it three times iterating my main jet size up a little at a time, it took me less than a hour including taking cowl off to "rejet" it, including drilling the jet. If you are not sure about your skills, have a knowledge careful mechanic do it. You don't want to screw up the carburetor or leave something undone. Carbs are very simple and why I have one on my current project... You may need safety wire, new lock tabs for bowl fasteners and main jet... may be a bowl gasket, but if newly rebuilt gasket is probably good to reuse. If you do modify the Carb you may want to make a mark on the data tag... make it "experimental". This is the grey line of certified appliances on experimental aircraft... If this ever went back into service on a certified plane it would have to be completely "conformed" to the certified Spec...
Too much information? It was not long ago, say 30 years GA factory planes and most home built planes had NO CHT or EGT, may be one CHT on the "hottest" cylinder... Now we are instrumented like it is a flight test. It is a good thing especially on experimental aircraft with so many differences....
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