The International Cessna^® 170 Association

I'm installing a single probe CHT/EGT in the new plane. I'm pretty familiar with the 4 cylinder Lycomings and Continentals, but the 0-300 is a new one to me. Which cylinder usually runs the hottest?

Thanks!

Cylinder number two (2) is the best/hottest/first-to-peak according to Cessna, TCM, and TIC170A research agrees. If you have a single-probe system, No. 2 is the cylinder to probe. (Continentals are numbered differently than Lycomings. Continentals are numbered, from the view of the pilot's seat, with the right-rear cylinder (closest to the co-pilot) as No. 1 cylinder, and the left-rear (closest to the pilot) as No. 2 cylinder. The odd-numbered cylinders are on the right-side of the airplane, and the even numbered cylinders are on the left-side of the airplane. Therefore cylinder No. 3 is in the middle-right, and cylinder No. 4 is middle-left, with cylinder No. 5 being front-right, and cylinder No. 6 being front-left.)
Hope that helped.

Thanks George!

I have a multi-probe EGT/CHT in my 55 B model. Number 5 cylinder EGT is always to first to peak when I'm leaning. It also always read the highest temperature, but I'm reluctant to quote that because the probes haven't been calibrated in a long time.

My plane came from the factory with a probe in number 1 exhaust. I find that #1 and #6 are always the coolest exhaust. This has always bothered me so I've had the intake manifolds checked for leaks and had the intake rebuilt, but nothing changed.

Dale

I need to make a correction due to a senior moment. #2 cylinder is not consistently the leanest, according to ad hoc testing. The 170 Book, and the SRAM mention #1 as being the leanest. (But I don't consider those notes to be very "scientific". They're actually only one-person accounts/stories that got printed, and not results of a survey.)
I wish I knew where I could put my hands on the survey we ran a couple of years back (at the Classic Cessna 170 site?) of multi-probe owners who responded to this question. There were several dozen who responded (a couple were early O-300 powered 172 owners) who listed their agreement with Cessna that #2 was the hottest CHT and was also either the first or a tied-second EGT to peak. About a half dozen had other answers but none of them showed any pattern, and not all of those had digital systems so calibration was certainly an issue. (Also, there was no info on whether or not the systems were compensated or un-compensated systems, which would give some more info on relative accuracy.)
It seems that individual installations may have differences. Some reports have the leanest cylinder dependent on power setting and altitude even. But if I were to install a single-probe system, I'd still recommend #2 as the place to do it. Carbureted engines are especially notorious for variances between engines (as regards EGT) due to mixture inaccuracies. I'd not rely too heavily on a single-probe EGT for leaning a carbureted engine. I'd use the recommendations in the Owner's Manual using RPM, and use the EGT guage as a reference only.

Craig
Save your money, keep it simple. You cant rely on those things anyway.
IMHO

Not relying on it for leaning - the tach works fine for that. However, I do like to keep an eye on CHT for extended climbs. I make a trip or two every year to Colorado, and like to let the engine take a break when the temps start to climb too high during extended climbs.

I have to agree with Craig on this one. Leaning seems to be best accomplished by sound and tach. My cht is on #3.

Any comments regarding adding a baffle in front of the two front cylinders up about halfway? Later model C-172's and I think other models also added this feature. I'm told it isn't done to make the front two cylinders to run hotter, thus being closer in temp to the other 4 cylinders, but rather to aid in air distribution through all 6 cylinders. Several members have added this baffleing. What do ya think?

Yep, I'd add it per the IPC for your original installation, Frank.