Low CHT

rv8bldr · #1 11-03-2005, 01:49 PM

Hi folks

I have an IO-360-A3B6D in an RV-8, and I'm seeing relatively low CHTs,
on the order of 260-280 deg F. I have never seen over 300. I figured it
must be a calibration issue, so I calibrated the RMI uMonitor CHT probe
with boiling water. It was showing 202F, so it wasn't that far off. I
have since adjusted the offset.

Are these temperatures an issue in the long run ?

Thanks in advance

f1rocket · #2 11-03-2005, 03:02 PM

What kind of oil temperature are you seeing? The oil needs to get to 170-180 in order to burn off the water vapor. If your oil temps are okay, then I think your engine is okay and your CHT's are probably not accurate.

If your oil temp is also low, then I think you need to warm things up a little. Personally, I don't think your CHTs are accurate. You should easily be in the mid-300's especially during a long climb out on a warm day.

dan · #3 11-03-2005, 03:04 PM

Mark,

FWIW, with the IO-360-A1B6 on my RV-7, I see CHTs anywhere from 270 to 320 in cruise, depending on whether I'm running LOP or ROP (ROP yields slightly hotter cylinders). LOP they're between 275-300, and ROP they're between 295-315. In the climb, I've never seen higher than 364 in the worst of scenarios, and on average it doesn't exceed 325 in the climb.

The angle valve engines are terrific cylinder coolers -- much more cooling area from what I understand.

Sounds like mine are reading about 20-30 degrees hotter than yours on average. You've got a really nicely cooled setup going. What power settings are you using?

I suspect the "answer" to raising your CHTs, if that's something you want to do, is playing with the size of the openings in the baffles UNDER the cylinders (both barrel & head) and playing with shrinking your cowl inlets. Shrinking the cowl inlets may raise your oil temp (how is it, btw?) if you shrink 'em too aggressively, so I tend to think the lower baffles is where it's at. Just a guess.

)_( Dan
RV-7 N714D
http://www.rvproject.com

rv8bldr · #4 11-03-2005, 03:44 PM

Hi guys,

Dan: I have a fixed pitch setup, and normally cruise around at 65% or so. I suspect that if I left it 75% I'd probably get higher temps, but not enough to put me in the mid 300's. However, unless I'm on a cross country, I'm down around 3000' and throttled back to save gas.

Randy: I would tend to agree with you about the CHT not being accurate, but I did the boiling water test and it was within 10 deg. I suppose there could be a non-linear error in the CHT probe, but how would I prove that without testing the sensor over its entire range with some kind of calibrated equipment.

It sounds like I shouldn't worry too much about the lower temps, as long as they stay in the mid-upper 200's.

Thanks for the input, guys,

dan · #5 11-03-2005, 04:29 PM

Quote:

Originally Posted by rv8bldr

Dan: I have a fixed pitch setup, and normally cruise around at 65% or so. I suspect that if I left it 75% I'd probably get higher temps, but not enough to put me in the mid 300's. However, unless I'm on a cross country, I'm down around 3000' and throttled back to save gas.

I'm curious about specifics. % power doesn't really tell me much. What's your manifold pressure, RPM, how are you setting up the mixture (degrees ROP or LOP or peak), what's your fuel flow, etc.?

How is your timing adjusted? Which type of ignition system(s)?

How are you leaning in the climb?

What's your oil temp in different phases of flight?

Do the CHTs run hotter if you fly around at 70-80 mph?

)_( Dan
RV-7 N714D
http://www.rvproject.com

gmcjetpilot · #6 11-03-2005, 09:06 PM

Quote:

Originally Posted by rv8bldr

Hi folks

I have an IO-360-A3B6D in an RV-8, and I'm seeing relatively low CHTs,
on the order of 260-280 deg F. I have never seen over 300.
Are these temperatures an issue in the long run ?
Thanks in advance

You say CHT's as in all 4 jugs? I assume you are using the probe that goes in the Well vs. the plug ring type (which suck).

I agree if all CHT's are balanced, the Oil Temp is OK (around 180F) than you are fine. However if you need to balance the temp between cylinders you can make adjustments to the baffle by blocking off the front cylinders (to raise temps) and adding a little more gap between baffle and fins on aft cylinder, especially the number #3, aft right rear jug, that tends to run hot.

To add speed you can reduce the size of the cowl inlets by reducing the height. A little foam and glass, you can re-shape the inlet with a slight reduction in area. I would wait to fly in hot conditions (100F sea level temp) to determine worst case before making any changes. You mention not exceeding 300F, that is cold. What are the conditons you have flown in? You mention instrument Cal, which is also worth another look.

George

glenmthompson · #7 11-04-2005, 08:42 PM

Was your RMI kit built? We had a similar problem, but read too high. RMI had us trace some "jumpers" that had to be snipped, that were not snipped. We were running over 50 degress high in deg F. When calibrated with boiling water, it was within a couple of degrees, and digressed greatly as the temps went up. Sounds like you have a similar problem, but on the low side. The algorythem apparently was not linear and thus caused the digression above 212.
IMHO, unless you have dinner plate sized inlets, there is absolutely no way you can be at or below 300 degrees at cruise in a lycosourous. Call RMI.
You can also get some mobile 1 oil, and heat it up with a candy thermometer to see a better number on the high side, but I think it is in the RMI unit.
Glen

garnt.piper · #8 11-24-2005, 03:42 AM

Just for a laugh, my CHT was low when I first flew, when I couldn't work it out and took it to the professionals - I found out I had stuck the probe in one of the injection ports....

I'd aslo wired my transponder plug 180? out...

But it flew OK, regardless.

tacchi88 · #9 11-26-2005, 06:19 AM

Depending on the type of CHT probe used, there will be variations. CHT probes are also frail. The spark plug type in particular. CHT is also directly proportinal to EGT, assuming the engine is in order, and operating temperatures may vary with instruments, but must also be maintained for best economy, performance and reliability. Niether too cool or too hat will help.
The best indicator is to make full use of EGTs, by establishing peak, and in generally 50 ROP, for cooling purposes. Once established, CHTs will remain in oeder, again, assuming the engine is sound.
CHTs react considerably slowere than EGT, and will also lead to damage. In every dyno testing facility EGT is king, the rest follows.
Recently, we had a JPI that required 3 of it's six probes (both EGT and CHT). the instrument was found to be in good order. Problem, according to JPI, was "all new" probes required. Problem solved. Though the problem was resolved, the egines temperature changed, but still only after EGT was established.
With old probes, EGT running normal cruise indicated average of 1400 to 1410, with new probes 1460 to 1490. Chts were odd and were more in line with each other, and the hottest one was now coolest. Average CHTs were 310 to 350. Engine Conti O470, running approx. 75ROP, and delivered average of 12 GPH on mogas, higher with AV gas.