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High CHTs in an RV-9A

Ed_Wischmeyer

Ed_Wischmeyer

Well Known Member
Just got the plane and even on a 70 degree day, the CHTs quickly get over 400 on climb out. The previous owner has redone the baffles. One suggestion is to check the mags and if they're at 25 degrees, try 24 degrees.

I've not started to do my own investigation, but what have other folks found? Probably other RVs, not just -9s, have had similar issues. What about the cooling air exit? I live in Georgia and it does get warm in the summer. The good news is I'm not facing hundred degrees and high density altitudes... Yet.

Thanks!

Ed
 
Ed_Wischmeyer said:
Just got the plane and even on a 70 degree day, the CHTs quickly get over 400 on climb out. The previous owner has redone the baffles. One suggestion is to check the mags and if they're at 25 degrees, try 24 degrees.

I've not started to do my own investigation, but what have other folks found? Probably other RVs, not just -9s, have had similar issues. What about the cooling air exit? I live in Georgia and it does get warm in the summer. The good news is I'm not facing hundred degrees and high density altitudes... Yet.

Thanks!

Ed
Click to expand...

If you have a carbed engine, you may need to have it rejetted to increase the fuel flow at WOT. Most RVs around here have had this done.

You can check in normal flight, you should be able to enrichen about 200 degree ROP at WOT.
 
... I second the jetting suggestion as well. We have helped several people with heating issues on their RVs. The 320s with carburetors seemed to be always lean as supplied from the engine shops and had serious heating issues. Typically a couple of jet sizes on the main jet (if this is the problem) drops the temps 50 degrees on climb-out. Fuel flow and EGT is usually a dead give away if this is the problem. We also have on our website some very nice products to address temperature issues. Thanks, Allan....:D
 
Aside from mag timing and re-jetting it might be worth going over the baffling and cowl fit with a fine-toothed comb.

When I overhauled the engine in my RV-6A a few months ago I was seeing 400+ CHTs on climb-out too. There ended up being a small (5mm or so) gap between the airbox and the baffling which was substantially reducing the pressure differential in the plenum. Filling this gap dropped 30 degrees off of my CHTs.
 
Are all your CHTs over 400, or is there significant difference between front and rear cyls?
Jetting may be the problem, but there is no substitute for eliminating all leaks, however small, in the baffling. I 'm told that total leaks the size of a quarter mean 50 degrees in CHTs.
 
Alan, in the carbed 320... what GPH flow do you expect to see at sea level? (after the re-jet)
 
Also don't forget to check the upper cowling ramps are installed and sealed up against the baffles. There was someone here who found that just the tiny gap between upper and lower cowl at the outer sides of the air inlets was enough to reduce some of the pressure differential needed for adequate cooling. Put some tape on there and see if that makes any difference.
 
flightlogic said:
Alan, in the carbed 320... what GPH flow do you expect to see at sea level? (after the re-jet)
Click to expand...

....With a C/S prop, at my max RPM 2950, sea level climb, 160 HP 16.5 GPH is what mine likes to keep temps down. More fuel will render lower temps but will compromise performance. Thanks, Allan :D
 
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I don't have a flow meter in my O-320 Cherokee 140 but when planning a XC trip, I use 8 GPH and expect 10 GPH for takeoff and climb.

Home airport is 78' elevation.
 
DanH said:
16.5 x 6 = 99 lbs p/hr. 99/160 = 0.6188 theoretical BSFC

Performance is already compromised, as you're way richer than best power mixture. It runs cooler because it is not making rated power[/]. Darn shame to buy HP, then buy fuel to drown it.
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....You are correct as usual! I neglected to say that I run my engine at a higher RPM (2950) for extra take-off power. I am actually making well over the 160 HP (177). That explains the extra fuel. I didn't intend to mislead or confuse the original poster here and Dan is correct, I forgot this important bit of information. 15 GPH is more like it for 160 HP. Thanks, Allan..
 
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Checked the baffles this afternoon. Only one tiny hole but the bigger issue is that at the very front, the baffles don't seem to seal where the cowling lip is at the outside. More later.
 
So I was all set to go do some flight tests and get fuel flow measurements and such when I recalled that the G3X records *all* the parameters every second. So I borrowed an SD card reader (mine is in hiding) and downloaded the data from the last flight.

I increased airspeed to 108 knots for cooling on the climb:
Peak EGTs: 1325, 1250, 1470, 1385
Peak CHTs: 395, 411, 435, 413

11.3 GPH at 84% power (shortly before the temps peaked), 2330 RPM, 28.9" MP. Corresponds to 13.5 GPH at 100% power.

Conclusions:
* Baffles need work at the front of the cowling
* Might need to increase the carb jet size a touch

Thoughts? Comments?
 
Ed,

For what it's worth: Interestingly enough, I went up in that very RV-9A on an Easter weekend demo flight (ambient was probably around 60F), and the owner made some comments to me during the climb along the lines of....

"For max engine life on any GA aircraft, you will want to keep the CHTs down. Temperature kills engine life, and it primarily gets high during climb. So I usually flatten the climb to keep the CHT's low - don't really ever climb at Vx or Vy."

It seemed to me during our demo the CHT's never got above the thin yellow G3X caution line at 385 F, and we probably climbed at well over 100 kt. But note that I was not taking data precisely or anything, I was gawking at the avionics, out the window, and noticing the fantastic climb rate even at the lowered climb angle.

Good luck on any diagnosis/mods, if this really proves to be a problem.

-Paragon
Cincinnati, OH
 
Hi Ed,

I think you have the right priorities - baffles first, then think about more invasive stuff. Tose CHT's are not goign to destroy anything quickly, so take your time and get a little here, a little there.

The best climb speed curve is pretty flat on most of the RV's - you can increase ten knots to improve cooling and hardly lose any fpm "up". The other thing we do most of the time is get LOP as soon as reachign a reasonable altitude and 75% power - CHT's plummet and if you dont go too deep, you dn't loose much power. Of course, we're usually climbing out of a DA above 5,000', so we hit 75% really quick. At Sea Level, you'll have to power back a bit.

Personally, I'd play with LOP mixtures before going in to drill the jets - its non-invasive and cools things down pretty quick. Baffles first though. A liitle here, a little there - pretty soon, you're cool!

Paul
 
When just taking friends on local sightseeing rides I usually climb at fairly low power, certainly under 75% since I'm not in a hurry it works fine, keeps things quieter, uses less gas, etc. My airport is at 360ft MSL, so I mean that yes, I climb at part throttle.....there's nothing that says you have to climb with the black knob all the way in.

I think it would be a good fuel saving/range extending strategy to do a cruise-climb at ~70% power LOP, especially if you aren't in a hurry. In an RV you'll still see greater than spam can climb rates and much higher climb speeds. The climb will take longer than at full power but the fuel burn (per time) will be MUCH less.


Ironflight said:
Hi Ed,

I think you have the right priorities - baffles first, then think about more invasive stuff. Tose CHT's are not goign to destroy anything quickly, so take your time and get a little here, a little there.

The best climb speed curve is pretty flat on most of the RV's - you can increase ten knots to improve cooling and hardly lose any fpm "up". The other thing we do most of the time is get LOP as soon as reachign a reasonable altitude and 75% power - CHT's plummet and if you dont go too deep, you dn't loose much power. Of course, we're usually climbing out of a DA above 5,000', so we hit 75% really quick. At Sea Level, you'll have to power back a bit.

Personally, I'd play with LOP mixtures before going in to drill the jets - its non-invasive and cools things down pretty quick. Baffles first though. A liitle here, a little there - pretty soon, you're cool!

Paul
Click to expand...
 
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