[Steve Sampson]

I have never noticed / thought about this before.

As I join the circuit at low power, and push the prop contol forward, the rpm goes typically from 2050 to 2650. At the same time the EGT jump from their typical temps of 1320 to 1340 up to 1450 to 1460, much higher than they would be in normal cruise, or any other phase of flight. The MP naturally drops with the accelerating engine, today to 13".

I dont find it intuitive as to why there is such a sharp EGT rise out of the normal range. Can anyone enlighten me? True it is a very lean mixture at this point, but the engine isnt really working. It is being driven to some extent by the prop.

Its a P-mag carburated roller tappet 160hp o-320 from Superior.

 

[DENMACRES]

EGT

If you will wait until the RPM starts to drop as you reduce thottle before putting the prop control forward you will avoid that rpm surge and EGT rise.














































egt rise

 

[Steve Sampson]

Thats an interesting response, but begs the question how low do you take the MP joining the circuit before you push the prop forward?

 

[John Clark]

Procedure

My normal routine is to "join the circuit," or enter the pattern in amurican, at the cruise RPM setting. No reason to run the RPM up and make a lot of noise when you are slowing and only need 15 or so inches of manifold pressure to maintain altitude. Waiting until your decent on final with the RPM below where the governor is effective will avoid the surge, EGT rise, and noise.

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA

 

[Kevin Horton]

Many RVs have after-firing in the exhaust with very low manifold pressure. If there is combustion happening in the exhaust, it stands to reason the EGT probes are going to see a temperature increase.

 

[pierre smith]

Steve, if you wait...

...'til you see the RPM start to decline, that tells you that the blades have hit the fine pitch stops. At that point, you can then push the prop control forward without the usual resulting RPM increase and noise.

Typically, I slowly advance my -10's prop control to 2500 revs entering downwind for some braking, but not full forward 'til later,

Best,

 

[Andy Hill]

Steve...

For the reasons above, many do not advance the RPM lever until base leg... Just to confirm, you do have full rich prior this point? If not, the change in RPM may well alter the lean setting you have established.

Andy

 

[nomocom]

EGT temps are normal.....

I have never noticed / thought about this before.
why there is such a sharp EGT rise out of the normal range.

P-mag

Nothing unusual with that observation. When you change your RPM from low to high, you've got several things going on. First, you've just decreased the time from TDC to exhaust valve opening. The exhaust valve is opening sooner following the combustion event so gasses are naturally hotter. However, you mentioned the P-mag, that is significant. I don't know how much vacuum change you are seeing, maybe going from 17 inches to 13 inches (a reduction of absolute pressure), not a huge change but significant. The P-mag advances spark for both the RPM increase and the increased vacuum, so the spark should be advancing when you go full forward on the prop control. The P-mag is giving the combustion more time to occur with the timing advance, so while you see a 120 degree rise, I suspect a traditional magneto setup would see an even larger jump in EGT's since the mags wouldn't be advancing with vacuum and rpm changes. I'll try to remember and fly the same routine and see what happens with EGT's.

best

 

[Steve Sampson]

Thanks everyone for all the interesting replies.

It has made me look at the data that pours of the AFS to try and see the pattern as to what is happening. There is a lot of it, but it looks as though the rise only comes when I drop the MP below about 18". If I push the prop forward above 18" the EGT tend to drop gently, below 18" they rise sharply, briefly, and to the person who asked, to about 13" MP.

Now my circuits to many of you will seem somewhat odd. I was a glider tow pilot and the goal there is to drop 2000' without reducing power too much, in order to keep the engine warm, hence perhaps higher MP than many of you would choose, and very tight because there is a glider waiting. Old habits die hard! No, I get no popping ever, and noise is not a concern, since there is no one to hear me, except the trouble and strife, who likes to know I am home.

Oh, and yes I normally dont richen it up until very late on final, again with the goal of keeping the heat up and not hastening the rate of cooling.

So I am still mulling things over, but I guess above 18" there is more fuel going in, and below 18 it becomes so lean that the rise occurs. Perhaps I will try to hold at 18 and be a little slower with the prop.

Thanks again.

PS The circuits in the video below is far from typical, much bigger, if you are wondering about that discrepancy.