CHT differences

[Marc_B]

What does everyone see with CHT spread in cruise?  I realize this is an interplay of a lot of variables such as EGT peak spread/fuel flow distribution, baffling, etc. but just curious if a good rough target exists. Or perhaps this is just establishing a baseline for each individual set up?

I recently had a shop bend the baffle seal back when replacing top cowl after oil change, and the first thing I noticed was higher CHTs and a wider spread. That was my clue to take the cowl off and take a look.  Usually I’m around 45-50 diff on CHTs.

[exM20K]

I'm currently at 20-25* spread, and it used to be less.  I'm not super worried about it, and if you have one outlier rather than a front 2 vs back 2 CHT spread, try switching around a probe and lead from hottest to coldest.  Like all probes, these do deteriorate over time.  the oscillation in #3 I have used to be shown on #1 until we moved the probe and lead.   

[Danb]

Mine are 15-20 last two runs lowest 309 highest 326. Flying out west next week, 4-5 3-4 hours legs I’ll take pictures and see at higher altitudes, there 25* less since I started LOP after breakin 

[MikeOH]

Looked the last dozen flights and I'm seeing 30 degrees; typically 330 to 360 in cruise, LOP 65%

[PilotX]

On 9/15/2023 at 11:09 AM, Marc_B said:

What does everyone see with CHT spread in cruise?  I realize this is an interplay of a lot of variables such as EGT peak spread/fuel flow distribution, baffling, etc. but just curious if a good rough target exists. Or perhaps this is just establishing a baseline for each individual set up?

I recently had a shop bend the baffle seal back when replacing top cowl after oil change, and the first thing I noticed was higher CHTs and a wider spread. That was my clue to take the cowl off and take a look.  Usually I’m around 45-50 diff on CHTs.

 

76% LOP. Buy a Bravo!

 

#1 always hottest for me. Lets look at your baffling.

 

[Marc_B]

Borescope is useful outside the engine too! Had a piece of baffle seal bent back. The piece to the right in the pic was back initially but I could see that from my oil filler door. 
I also noted a part of the seal by the intercooler that may need a little adjustment. 
I think CHTs look better but still 45-50 deg difference low to high. 

[Echo]

260-330 with Number three ALWAYS highest.  Hey quick question (sorry for thread creep).  I don't have a fuel flow until I get 830 installed this winter.  Doing "big pull to rough" and just enrichening to smooth what am I likely flowing in an IO-360? (GPH).

[Marc_B]

@Echo I believe the compression ratio is 8.5:1 in IO360.  If so, multiplier would be 14.9.  So LOP calculation for fuel flow would be:  (Max HP * %HP) / 14.9 = GPH fuel flow.  This only applies on the LOP side where fuel is the limiting factor.  i.e. 65% HP at LOP would be: (200*0.65)/14.9 = 8.72 GPH.

But your question may be way different...rough and then enrichening could be ROP if you can't run LOP well, and of course it depends on your power settings.

This thread has some good info:

 

[Fly Boomer]

2 hours ago, Echo said:

260-330 with Number three ALWAYS highest.  Hey quick question (sorry for thread creep).  I don't have a fuel flow until I get 830 installed this winter.  Doing "big pull to rough" and just enrichening to smooth what am I likely flowing in an IO-360? (GPH).

@Marc_B nailed it.  Just because it gets rough does not mean you are LOP.

[Echo]

1 hour ago, Marc_B said:

@Echo I believe the compression ratio is 8.5:1 in IO360.  If so, multiplier would be 14.9.  So LOP calculation for fuel flow would be:  (Max HP * %HP) / 14.9 = GPH fuel flow.  This only applies on the LOP side where fuel is the limiting factor.  i.e. 65% HP at LOP would be: (200*0.65)/14.9 = 8.72 GPH.

But you question may be way different...rough and then enrichening could be ROP if you can't run LOP well, and of course it depends on your power settings.

This thread has some good info:

 

That is a good chart.  In a range of what I thought.  I would estimate 9.0-10 at my altitude.  Thanks, Scott

[KSMooniac]

14 hours ago, Fly Boomer said:

@Marc_B nailed it.  Just because it gets rough does not mean you are LOP.

Correct!  My well-balanced IO-360 (fuel and air) does not get rough going LOP...the engine will quit when it gets too lean, but does not shake.

[MikeOH]

3 hours ago, KSMooniac said:

Correct!  My well-balanced IO-360 (fuel and air) does not get rough going LOP...the engine will quit when it gets too lean, but does not shake.

Same way mine behaves; I couldn't even guesstimate fuel flow base on enriching from 'never rough'

[Fly Boomer]

5 hours ago, KSMooniac said:

Correct!  My well-balanced IO-360 (fuel and air) does not get rough going LOP...the engine will quit when it gets too lean, but does not shake.

I just meant that there is no connection or even correlation between the two events.  The only way to know if you are lean of peak on all cylinders is by using a per-cylinder engine monitor.  It may get rough long before lean of peak on all cylinders or, as in your case, it may never get rough no matter how lean you go.

[Rick Junkin]

 

On 9/15/2023 at 1:09 PM, Marc_B said:

What does everyone see with CHT spread in cruise?  I realize this is an interplay of a lot of variables such as EGT peak spread/fuel flow distribution, baffling, etc. but just curious if a good rough target exists. Or perhaps this is just establishing a baseline for each individual set up?

I recently had a shop bend the baffle seal back when replacing top cowl after oil change, and the first thing I noticed was higher CHTs and a wider spread. That was my clue to take the cowl off and take a look.  Usually I’m around 45-50 diff on CHTs.

My CHT spread in cruise, a delta of 9 degrees driven by #1 and #6. I have a GAMI spread of 0.4gph, with the same pattern of #1 and #6 peaking 0.2-0.3gph higher than the others. Makes sense. EDIT: This is at 30/2200 and 16.5gph, 120 degrees ROP.

Have you looked at your engine's GAMI spread to see if there's a correlation with the CHT deltas?

Cheers,
Rick

 

[Pinecone]

If you do Saavy Analysis, they will tell you your spread and compare to all their data on same make/model.

My 252 is averaging 50.  The average for all their data on 252s is about 46. 

[Marc_B]

GAMI spread is 0.3 - 0.5 GPH.  I've looked at my Savvy data and trend analysis but it doesn't really give you a "bell curve" for the data...rather it says where you are within the cohort.  But it's interesting that I had my shop replace my baffle seals last annual and my CHT spread is higher on average.  Prior I was around 41-45% of the cohort; after I'm around the 57% mark.  Replacing baffle seals and having higher numbers seems counter to what I intended...

[Danb]

On 9/15/2023 at 4:17 PM, Danb said:

Mine are 15-20 last two runs lowest 309 highest 326. Flying out west next week, 4-5 3-4 hours legs I’ll take pictures and see at higher altitudes, there 25* less since I started LOP after breakin 

After returning from my 5,000 crusade across US, my spread was approximately an unbelievable 8-22*

[Shadrach]

On 9/26/2023 at 5:05 PM, Fly Boomer said:

I just meant that there is no connection or even correlation between the two events.  The only way to know if you are lean of peak on all cylinders is by using a per-cylinder engine monitor.  It may get rough long before lean of peak on all cylinders or, as in your case, it may never get rough no matter how lean you go.

I’ve never operated a stock IO360 that does not run smoothly LOP far leaner than an setting I would use. Sure, there are exceptions but they are rare. I have never operated one that just gets quiet. Most start to get rough after the one of the EGTs starts to rise again after peak. Most will run smoothly well beyond 50LOP depending on MP.  The intake design makes for an inherently well balanced F/A ratio from cylinder to cylinder.  I would personally have no problems running an IO360 LOP without an engine monitor as a temporary measure (like ferrying an airplane without one), but that does mean I would recommend it to others.

[Shadrach]

On 9/25/2023 at 7:25 PM, Marc_B said:

@Echo I believe the compression ratio is 8.5:1 in IO360.  If so, multiplier would be 14.9.  So LOP calculation for fuel flow would be:  (Max HP * %HP) / 14.9 = GPH fuel flow.  This only applies on the LOP side where fuel is the limiting factor.  i.e. 65% HP at LOP would be: (200*0.65)/14.9 = 8.72 GPH.

But your question may be way different...rough and then enrichening could be ROP if you can't run LOP well, and of course it depends on your power settings.

This thread has some good info:

 

Close. CR is 8.7:1 so multiplier is more like 15.1.

[Shadrach]

On 9/25/2023 at 7:09 PM, Echo said:

260-330 with Number three ALWAYS highest.  Hey quick question (sorry for thread creep).  I don't have a fuel flow until I get 830 installed this winter.  Doing "big pull to rough" and just enrichening to smooth what am I likely flowing in an IO-360? (GPH).

Depends on available MP.