CHT reading bayonet vs. spark plug ring sensors

punkin · #1 03-15-2018, 09:09 AM

I'm breaking in my new engine, and of course I'm totally stressed about CHT. I spent the last few days reading most of the relevant posts.

My engine shop has been great, and they indicated that my spark plug ring temp probes should read 15-20 degrees higher than a true bayonet probe in the head. I saw temps first flight around 438 on the hottest cylinder, second flight was going to be yesterday, but I saw 458 on the hottest cylinder. Of course, I put the nose down, made sure I was full rich, and temps came down into the 440 range, but I went ahead and landed and pulled the cowl to check everything.

Google (the most dangerous phenomenon known to facts) shows people claiming that the spark plug style sensors can be as much as 40 or 50 degrees hotter than the bayonet style. If THAT is true, I shouldn't have much to worry about.

My engine guys have been awesome, and I don't want to second guess them, but I'd sure have more peace of mind if someone had firsthand experience of running a bayonet AND a spark plug thermocouple and knew real numbers of how much they vary?

I didn't find any real concerns with baffling or anything, so I'm gonna go back up this weekend for a long x-country if all goes well. Would love to not be sweating the temps!

Thanks!

Mike S · #2 03-15-2018, 09:27 AM

How about some details on the engine??

F. I. or carb?

Fuel flow at full rich?

Any changes or modifications to the engine from the old one?

BobTurner · #3 03-15-2018, 09:41 AM

O-470-S in a 182; factory probe in the cylinder well, spark plug ring probe under the plug, #3 cylinder. All other cylinders had bayonet probes. #3 ring probe ran 40 F hotter than others.
All of this begs the question, why aren?t you using bayonette probes? The spark plug probes are a pita; the wire gets in the way when installing/removing the plug. And you should be annealing the gasket-probe every plug change too.

edsong · #4 03-15-2018, 11:25 AM

I switched to bayonet probes on my Pitts I0-360 and saw a 40-50f drop in temp. Now I just fly it harder.

Eddy

DanH · #5 03-15-2018, 12:10 PM

Instrument company literature says 40-50 degrees difference.

punkin · #6 03-15-2018, 07:02 PM

I agree, I should have switched the probes while I was waiting on the engine, i simply didnt think about it. You are sure right about them being a pita when changing plugs.

lots of changed on the engine, so I really don?t have a frame of reference from my previous engine... duel P mags instead of one, 9 to 1 compression instead of seven, a new cam, steel cylinders instead of Chrome.

It sounds like I need to add bayonet probes to my upgrade list. In the short term no, they were always consistent before, although consistently in the high 300s? Because they never got anywhere near any danger zone, I didn?t really worry about it.

of course, now going to be paranoid about every little thing until I get used to the new set up. :-)

rv7charlie · #7 03-15-2018, 07:43 PM

400 F = ~204 C
440 F = ~227 C

Just a thought....

punkin · #8 03-16-2018, 10:57 AM

I'm not following Charlie? What did I miss? Sometimes you super smart guys are enough steps ahead of me that I miss it entirely.

rv7charlie · #9 03-16-2018, 11:07 AM

Your engine shop told you to expect a 15-20 degree spread between probe & washer type readings. 23 degrees ain't that far off. Did they specifically tell you which temperature measurement scale they were using?

I just used 400 & 440 F as ballpark CHT numbers to get the range in C at typical CHT absolute temps.

punkin · #10 03-16-2018, 12:42 PM

Makes perfect sense, Charlie.

I totally missed the direct reference to what I said. I feel dumb. But I guess I expected F scale--I don't leave for Ecuador until Monday, then I get to be confused by Celsius. :-)

Either way, wind permitting I'm gonna cross the state tomorrow and get a good 4 or 5 hours on her.

I've already repitched the prop by about 1.5 degrees, so there is a pretty noticeable difference in power!