Frank Smidler
Well Known Member
There have been several threads discussing CHT temps and baffling. I have been following them all over the last 7 months since I first flew my RV-6 and had a big difference in CHT between the #3 and #4 cyl. At first #3 was consistently 70 degrees higher than #4. The difference was observable as soon as the engine started to warm up on the ground.
Today I switched the probes between these two cylinders and got a surprising result. First let me say that I have done many of the recommended baffles and cowling changes and got the difference in temperature to approximately 50 degrees. They include foaming in the upper cowl ramp, spacing the right rear corner of the baffle, better sealing around the baffles, etc. This was satisfactory to me since Lyc says it is not unusual to have 150 degree F difference with a carberated engine. Then, on the trip home from OSH my CHT diff went 100 degree +. My concern was why the change? Was something changing with the engine? The first step of trouble shooting was to check the probes.
So, today I switched the probes but not the wiring. When I did a ground run the CHT's followed each perfectly, no more than 3 or 4 degrees difference. They both hit 349 degrees at the same time. Great, or is it? What caused the change?
I have a couple thoughts but no real answers.
1. I found that the red wire coming out of the probe shielding was extensively frayed. I had the probe wires bundled with the EHT wire and spark plug wires. This wire could have came into contact with the spark plug shielding and made it read differently. When I reconnected these wires I extended the heat shrink over the connection all the way over the exposed wires. Could this have affected my readings?
2. On the #4 wire the EGT wire was bundled with the CHT and spark plug wires within 4 inches of the probe. I isolated at least 8 inches when reconnecting. Could heat from the shielding affected the CHT readings?
3. Maybe originally the probe with an error to the high side was on the high temp cyl and the probe with an error to the low side was on the low temp cyl? This would then show a greater difference in temp than actual. Once switched the probe errors would cancel out the CHT difference. How feasible is this?
Tomorrow I fly from Lafayette, IN to Blacksburg, VA. I'll be observing how the CHT's act for this flight.
Today I switched the probes between these two cylinders and got a surprising result. First let me say that I have done many of the recommended baffles and cowling changes and got the difference in temperature to approximately 50 degrees. They include foaming in the upper cowl ramp, spacing the right rear corner of the baffle, better sealing around the baffles, etc. This was satisfactory to me since Lyc says it is not unusual to have 150 degree F difference with a carberated engine. Then, on the trip home from OSH my CHT diff went 100 degree +. My concern was why the change? Was something changing with the engine? The first step of trouble shooting was to check the probes.
So, today I switched the probes but not the wiring. When I did a ground run the CHT's followed each perfectly, no more than 3 or 4 degrees difference. They both hit 349 degrees at the same time. Great, or is it? What caused the change?
I have a couple thoughts but no real answers.
1. I found that the red wire coming out of the probe shielding was extensively frayed. I had the probe wires bundled with the EHT wire and spark plug wires. This wire could have came into contact with the spark plug shielding and made it read differently. When I reconnected these wires I extended the heat shrink over the connection all the way over the exposed wires. Could this have affected my readings?
2. On the #4 wire the EGT wire was bundled with the CHT and spark plug wires within 4 inches of the probe. I isolated at least 8 inches when reconnecting. Could heat from the shielding affected the CHT readings?
3. Maybe originally the probe with an error to the high side was on the high temp cyl and the probe with an error to the low side was on the low temp cyl? This would then show a greater difference in temp than actual. Once switched the probe errors would cancel out the CHT difference. How feasible is this?
Tomorrow I fly from Lafayette, IN to Blacksburg, VA. I'll be observing how the CHT's act for this flight.
