BlackRV7

BlackRV7

Well Known Member
During Phase1 of Black Magic I continually chased the high CHT's gremlin. Initially fitting my baffling, I fell victim to the ol "heck that's got to be good enough"....as in....all that air has to cool those cylinders. Guess what, NO WAY. One of the first things I did was close in the ends of the top cowl ramps on my 7. I could see flex, just a little, between the van's alum baffling and the valve covers. I could imagine the seepage there, thus RTV!! Pay attention to that area at the base of your cylinders. My alum cylinder baffles, that attached to the bottom and between the cylinders, were both bent down a little at the base. Bending those tabs back up closed off a wasted air opening. All these things brought my CHT's down incrementally. I can't emphasis it enough, if you even think cooling air can escape somewhere, RTV it!!


Here's the biggie............I knew I had air coming around the FAB. A Swift driver, who is on this site, told me this past Saturday he had heard leakage in the lower cowl is like closing the inlets 3 fold. Thus 3 square inches of leakage in the lower cowl would be like closing off 9 square inches of inlet due to pressure differential. I did not research this so don't shoot the piano player.

Here are the results though!! As a side note, my oil temps have always been fine.

I flew Tuesday in 40 degree weather and noted my CHT's, although acceptable with my new engine, still higher than I want to see, with variations in individual CHT's. Tuesday evening I pulled the cowl, closed in that lower cowl leak area and changed the oil. Yesterday, the outside temps were 20 degree hotter so I knew I wasn't going to get a good comparison. Guess what....20 degrees hotter outside....at 24 square...my CHT's were 20 DEGREES LOWER!! Not only were they lower, the CHT's remained within degrees of each other. I may not be very smart but you only have to hit me once with a tire tool;) A dramatic improvement.

I see light..........oh wait, no light at the end of the tunnel. Gang, just go ahead and buy two tubes of RTV!!
 
CHT's

What are your CHT's? Lycoming redlines are in the 450-500 range. 350-375 is more like what they want to see. One my Rv-6, after the engine was broke in, they hung around 330-350. This is actually a little lower than most. I sealed up all of the gaps with RTV and paid close attention to the lower gaps in the cylinder baffles. 2 1/4" on the heads and 1" on the base of the cylinder. Make sure your inter-cylinderbaffles don't have big gaps in them as well. I had the IO-360-M1B engine. The cowling is made for the 200hp engine so the inlets are a little larger than the standard 180hp cowling, thus providing more cooling air. My oil cooler (Setrab) worked too well (160 degrees on 90 degree days)-Before those of your reading this warn me of using this cooler-make sure you pay attention to installing it-I know. It won't matter what cooler you use if you install it wrong-it will break the baffling or leak if you pull the mounting tabs in if you don't install spacers on the bolts. However, for the money, it's the best dang cooler for the application and I'll debate anyone on that. It's lighter, narrower and fits better in the rear of the baffle, cost less, and flows more air through it. I know- I'm feeling a little fiesty this morning!
But back to CHT's. The other big problem is the inlet ramps and sealing around them and under the ramps. I put foam under the ramps and fiberglassed over the ends to keep air from getting in there.
The RV-6 I'm building right now will have the Sam James cowling with the plenum chamber. Not doing to get more cooling, but more speed. Plus I want to get the loads off the top cowling. I had some cracking issues with the baffling getting too close cowling. Remember, prop roatation makes the right side of the engine go up and the left go down. Van's tells you to get 3/8 to 1/2" gap all the way around with seal. You need a little more on the right side. I had to go to 5/8" to keep the baffle from rubbing.
 
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Dana,
I'm in the midst of baffle-installation. What RTV type did you use?

Thanks in advance, Bill
 
Bill, I just went to the local aviation NAPA store and got the high temp....BLACK....of course RTV.

My CHT's are now are high 360s at 24 square and still dropping, as the new ECI cylinders break in. Close in the ends of the ramps when you glass them in and RTV ANYWHERE you even remotely think air is going to pass without doing it's job, which is cooling those jugs. As Aden said, pay close attention to the inter-cylinder baffles..........once again mine were bent down a little at the bases thus giving the air a free shot without doing it's cooling thing.

These CHT's were my only gremlin in Phase 1....other than learning the land the dang thing.....:rolleyes: and all it turned out to be was making darn sure every nook and cranny had a dab of RTV on it. The biggest difference was obviously closing in the lower inlet but I probably got 6-8 degree by just paying attention to where the air was escaping the enclosed baffled area. Remember, I found 20 degree lower CHT's the day after when the outside temp was 20 degrees warmer!!

I can't emphasis it enough, if in doubt...................RTV
 
Dana,

Do you put the RTV between the engine case and the aluminum baffles also? The engine heat won't melt it?

... Bill
 
Sealing

You can use the high temp (700 degree) RTV around the baffling and engine case. ANYWHERE there is a gap-air will go through that instead of around the cylinder. Think of it as water in a bucket. You want to seal off any hole that will let air out so you can control the amount of air that should be traveling around the cylinder heads. You also might have to put air shields in front the front two cylinders to keep the CHT's up. The direct blast of air from the inlets will cool them more than the back two.
 
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