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10-03-2015, 04:17 PM
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Join Date: Aug 2012
Location: Garden City Texas
Posts: 878
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How much time on your engine and new baffles? I went thru the same routine 3 months ago. I trimmed my exit 1" and it helped a little. Installed an anti splat cowl flap, it helped a little. Sealed every crack I could find and it helped a little. I also believe my baffles finally "broke in" suddenly and covered some spot that had been leaking because the baffles were not limber enough. Now I can climb hard and seldom hit 400.
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10-03-2015, 05:41 PM
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Join Date: Jul 2014
Location: Rockford, IL
Posts: 6
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High CHT
Vern, thanks for your suggestion about a larger carb jet. Mike Busch had also suggested trying that when I mentioned high CHT problems to him. Do you have any recommendations on jet size? The carb is a Precision Model MA-4-5. P/N 10-3878.
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10-03-2015, 08:21 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Before you change the jet, check to see if you have 200F, maybe 250F between your full throttle EGT and peak EGT. If not that much, install a larger jet. If you have 250 or more, a larger jet is just a dirty and expensive way to reduce horsepower. You can do it cheaper and cleaner by retarding the timing, or just pulling the throttle.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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10-03-2015, 08:58 PM
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Join Date: Feb 2005
Location: Chesterfield, Missouri
Posts: 4,514
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Cut about 4" before first flight, I figured it was necessary.
Highest CHT observed so is 405.
I believe it works.
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RV-12 Build Helper
RV-7A...Sold #70374
The RV-8...Sold #83261
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10-03-2015, 09:45 PM
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Join Date: Sep 2006
Location: Townsend, Montana
Posts: 3,179
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I think cutting the cowl is the last resort..because it's kind of permanent. Are all your gear fairings in place? what speed are you climbing out?
1. Do the light test to check for any leakage path...all air needs to either go through the fins or the cooler. EVERY other path, no matter how tiny is a cooling killer
2. Clean up the exit area of all clutter. Add internal fairings to direct the flow if needed.
3. Timing backed off a bit.
4. Ramps and inlets installed properly
FYI...
my -7 has a James Cowl inlet size is 4.750" ID, outlet hole on mine trimmed flush with firewall is 14" along the top, 3.5" tall and has a radius edge to a 12.5" bottom lip. I have Vetterman 4 pipe (1.75" OD) So here is how I calculate the areas:
Inlets=35.44 sqin
effective Outlet=35 sqin
or roughly 1 to 1 ratio.
I had some cooling issues early on, but filling the holes and cleaning up the outlet helped drop CHTs about 30F in cruise and oil temps down below 200 except on the hottest days. Extended summer climbs can push the oil to 215-220, but I tip the nose over to help that. CHT's never above 400, usually 330-360. Some day I would like to reduce the outlet to limit drag while cruising.
Good luck
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Retired Dam guy. Life is good.
Brian, N155BKsold but bought back.
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10-03-2015, 11:28 PM
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Join Date: Dec 2006
Location: Oakland CA
Posts: 771
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Could you add a bit of info regarding your 330F cruise number? What power and mixture settings... LOP? Variable timing? Compression ratio and which cylinders (Lycoming vs, ECI vs. ?)
I've battled CHT problems with ECI nickle carbide cylinders, IO-360, 9.2:1 compression, dual P Mags, WW 200RV prop and the solution that worked for me to fix my climb temps was to add the cowl flaps.
My cruise temps are mostly influenced by mixture/power settings. 23 squared, LOP (around 7gph) and mine are in the 340 to 350F range doing 150kts true. If I want more speed, 24 squared, ROP(around 9gph) and I get around 165kts but my temps are in the 390 range, but that is still not a concern for me. I start paying attention above 400F and start changing my behaviour when it's 410 and still climbing.
For climbs, before cowl flaps, I had to frequently flatten my climbs to 130 kts and they would still shoot rapidly past 415F. Now I can climb out at 100 to 110 and they almost never go over 400 even on a very hot day.
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All Best
Jeremy Constant
RV7A "Stella Luna" ECI IO-360 WW200RV Pmags 360hrs
VAF 2019 paid plus some for those who can't
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10-04-2015, 12:13 AM
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Join Date: Sep 2006
Location: Townsend, Montana
Posts: 3,179
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Lyc parallel valve 360, AFP FM200 injection, 9.2:1 comp, Pmags("A" curve 24*), James log cowl and plenum. I cruise at WOT 2380-2450 RPM(depending on the mood) and LOP slightly.
Last week at 11500, OAT as 6*C, 2430rpm, 20"mp, burning 8.1gph, CHT's 343, 346, 350, 352. Oil was 192. Showing 148ktias, 179KTAS.
__________________
Retired Dam guy. Life is good.
Brian, N155BKsold but bought back.
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10-04-2015, 05:51 AM
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Join Date: Feb 2005
Location: Chesterfield, Missouri
Posts: 4,514
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I believe electronic ignition causes CHT to run 15-20F hotter than with mags.
__________________
RV-12 Build Helper
RV-7A...Sold #70374
The RV-8...Sold #83261
I'm in, dues paid 2019 This place is worth it!
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10-04-2015, 08:40 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by David-aviator
I believe electronic ignition causes CHT to run 15-20F hotter than with mags.
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I'm guessing you meant to say advanced spark timing causes CHT to run hotter. The spark source would make no difference.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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10-04-2015, 12:25 PM
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Join Date: Mar 2012
Location: San Diego, CA
Posts: 133
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jets
I had the same problem with mine. I have a cowl flap, however it didn't make enough of an impact. I upper the Carb jet size to a #38 if I remember correctly and during a really hot day I see 403F of the hottest cylinder during a 90kias climb. During cruise I adjust mixture and cowl flap accordingly, not to exceed 380F. This reduces engine wear and cooling drag. I can close the cowl flap beyond the nominal exit size to almost half the nominal exit size at all most power settings. Only at 75% do I open it more, but still less than nominal, to keep CHT's below 380F.
I just recently replaced the GA200 prop with a C/S Hartzell, and I use the same procedures and it works.
You also need to normalize your CHT to ambient temps. All CHT's should be normalized to a 100F day. So if you are flying on a day where it is 80F, you need to ADD 20F to your indicated CHT's in order to get accurate CHT numbers.
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