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High CHT's Too Lean or Timing?

PaulR

Well Known Member
I'm working on lowering the CHT's on my recently completed 9A. I have an O-320 with 9:1 pistons, one P-Mag and
one Slick. I can't operate at any DA at full throttle without the CHT's climbing above 425. I don't know just how
high they would go as I throttle back to cool them off. The engine has 13 hours on it and has used less than a quart
of oil in the 13 hours.

I've researched quite a bit and have read many threads where people have enlarged the main jet so I asked my
engine builder for his take on it. He said that at full throttle I should get a minimum of 125 degree rise in
my EGT's in order to be rich enough. I get exactly that on several tries. I'm flowing about 12.5GPH at full throttle.

I've done all the baffling stuff to ensure that the air is going where it needs to and believe that it is sealed well
as evidenced by oil temps being a max of 190 with OAT's in the 80's to 90's.

I've also read that on high compression engines that even Lycoming suggests base timing of 20 BTDC instead of the 25
that my slick is timed at and the P-mag's total advance is more than that.

I do have the jumper in the P-mags for the lower advance curve and last evening, after some study of P-Mags literature,
I disconnected the manifold pressure line, which is supposed to retard the curve to about 26BTDC. I haven't had a
chance to fly it as the weather won't co-operate.

I guess my question for you engine gurus or someone with some HC piston experience is two fold. Is 12.5GPH enough
fuel at full throttle and should the base timing be retarded because of the high compression pistons?

It's a little frustrating that I can't climb, even at 120 knots, very long and even at 10,000DA I can't run at WOT.

Any suggestions will be much appreciated.
 
I run 19 gallons per hour on a IO360 with 10 to 1 pistons. Your number seems a little low. The rule of thumb seems to be 1 gallon for every 10 HP. I would think your engine is around 160 HP so 16 GPH might be a better range.

George
 
12.5 GPH is enough for your engine. You could push more to cool it off, but you shouldn't be too lean the way it is.

A couple of thoughts (admittedly, I know little about P-Mags) - I'm assuming a FP prop.

Gauges? Are you sure about 425?

Temps - try not to go over 400 (even though Lyc has a higher published limit).

Timing - I'd definitely give 20 degrees a try.

Dan
 
I run 19 gallons per hour on a IO360 with 10 to 1 pistons. Your number seems a little low. The rule of thumb seems to be 1 gallon for every 10 HP. I would think your engine is around 160 HP so 16 GPH might be a better range.

George

Check page 60 here...

http://www.7ts0.com/manuals/lycoming/320/O-320-IO-320-AIO-320-LIO-320_OM.pdf

It looks like your fuel flow is low, even for a 160 HP O-320 version... Note the graph is for "minimum" fuel flow, saying more is OK...
 
Admittedly I'm not 100% sure of the temps, but it is all new, and they read correctly at ambient.

It is a FP Catto prop.

I'm anxious to see what the base timing change on the P-Mag does this evening if I get the chance to fly.

From the looks of the chart (Thanks Gil), I think the 12.5 may be a little low which leads me to think that I need to change the main jet or drill the one out I have. I'm assuming that the 9:1 pistons would be in the vicinity of 168 HP from what I've read.

I'll wait to do that till I see what the timing thing does though before I mess with the carb.
 
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are you getting 2700rpm?
what speeds are you running?
do you have all the fairings and pants installed?
have you calibrated the fuel flow?

13hrs on a new engine might not be broke in....temps could come down on their on with more time.
 
timing

Try 22 degrees timing instead of 25 or 20. What r/m are you getting at climb airspeeds. The Lycoming document on timing can be found on biplaneforum.com under Pitts. Only applicable to 0 360's.
 
I had a lot of the same problems when I overhauled my engine (160hp regular compression). In my experience at 13 hours you will have another 10 hours or so of running before your temps starts to fall, and you will possibly have to get to 50 hours before you get to steady state.

To check whether you are running rich enough I would fly at 6K' at 65% and lean off. Note the EGT rise - I would have thought 200F would be a good number? I got no EGT rise initially with a 0.097" main jet. I would also check out your CHT probes (in boiling water is easy), even 10F hot will make a difference.

I would also time your P-mag 5* after TDC (count starter ring gear teeth to figure out where that is), that will move the base timing to 20*.

After I had worked on the baffling and cowling I could not get the CHTs below 400 when running at 2400rpm, and higher in the climb.

I then installed a set of cowling louvres from Avery and the CHTs plummeted 40F. When running at 2200 temps are now around 320, never get over 400 in the climb. I have a 6A and a Sensenich metal prop.

Pete
 
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. . .you don't want it before 25 Deg. TDC
Can you elaborate on this statement? When you say "before" do you mean greater than 25 deg or do you mean less than 25 deg? Whichever it may be, can you give your rational for this statement?
 
BTDC timing

During takeoff conditions, when the cylinders are seeing full manifold pressure, if ignition timing is set sooner than 25 Deg BTDC, then you add to the heat of compression during the final portion of the compression stroke. That will DRAMATICALLY increase CHT and only a few degrees too soon can result in melting pistons. As the say, "timing is everything" and gives new meaning to "playing with fire".

It's fine to advance the timing (as electonic ignition systems do) when manifold pressure goes down with altitude or reduced throttle settings. But the base timing setting at full manifold pressure should never be set to occur earlier than what Lyc has established.

On my IO-320, I've seen 50 deg EGT change at take off power, with just a 1 deg timing adjustment.
 
are you getting 2700rpm?
what speeds are you running?
do you have all the fairings and pants installed?
have you calibrated the fuel flow?

13hrs on a new engine might not be broke in....temps could come down on their on with more time.

I can get to 2850 just like Craig Catto said at altitude. Just can't keep it cool there. All fairings and pants installed. Fuel flow calibrated to within about .2 gph right now.

It "may" not be broken in yet but I still think it's a problem. Looking at the chart that Gil supplied, it is probably a little on the lean side and I'm still hoping that the timing is the culprit. I really don't want to mess with the carb but I will if all else fails. At this point I've pretty much ruled out the baffles.

I really am resisting putting in louvers. Tom Berge in MN did a mod on his 7A cowl exit to gain about 15 sq. in. of space that I would try first.

I'm kind of hoping Mel will chime in because I know he's running an O-320 with 9.5 or 10 to 1 pistons without problems.

I'm still hoping that to cool them off but they have showed little downward movement since the second flight. I can run in the upper 380's at 2400 pretty much and the speeds seem to be about right there.

I really appreciate all the suggestions so keep them coming.
 
cht

go to the post "unable to lean for max RPM, why" and look for the response from Mahlon Russell on Aug 7.
It would seem you're a bit lean.
 
I'm working on lowering the CHT's on my recently completed 9A. I have an O-320 with 9:1 pistons, one P-Mag and
one Slick. I can't operate at any DA at full throttle without the CHT's climbing above 425. I don't know just how
high they would go as I throttle back to cool them off. The engine has 13 hours on it and has used less than a quart
of oil in the 13 hours.

I've researched quite a bit and have read many threads where people have enlarged the main jet so I asked my
engine builder for his take on it. He said that at full throttle I should get a minimum of 125 degree rise in
my EGT's in order to be rich enough. I get exactly that on several tries. I'm flowing about 12.5GPH at full throttle.

I've done all the baffling stuff to ensure that the air is going where it needs to and believe that it is sealed well
as evidenced by oil temps being a max of 190 with OAT's in the 80's to 90's.

I've also read that on high compression engines that even Lycoming suggests base timing of 20 BTDC instead of the 25
that my slick is timed at and the P-mag's total advance is more than that.

I do have the jumper in the P-mags for the lower advance curve and last evening, after some study of P-Mags literature,
I disconnected the manifold pressure line, which is supposed to retard the curve to about 26BTDC. I haven't had a
chance to fly it as the weather won't co-operate.

I guess my question for you engine gurus or someone with some HC piston experience is two fold. Is 12.5GPH enough
fuel at full throttle and should the base timing be retarded because of the high compression pistons?

It's a little frustrating that I can't climb, even at 120 knots, very long and even at 10,000DA I can't run at WOT.

Any suggestions will be much appreciated.
Paul:

I have two questions:
1 - Where did you read about changing the timing to 20 DBTDC with high compression engines?
2 - What constitutes a high compression engine?
=================
OK - On to your questions.
YES - By all means read the Lycoming Operation Manual. But even that read with a grain of salt. I say that because Lycoming even has a CYA for them if the plane manufacture recomends something different than what they recommend. STRANGE??? Shouldn't Lycoming know better?
Example: Lycoming states:
7. OPERATION IN FLIGHT.
a. See airframe manufacturer?s instructions for recommended power
settings.
And they recommend a Mag Check at 2000/2100 RPM...
I'd bet you were taught 1800 RPM.
And if the plane is a tail dragger 2000/2100 RPM may just pick up the tail

As for your 12.5 GPH - - That is perfectly acceptable.
Lycoming states at 2450 RPM (75% Power) you should have 10 GPH
O-320 A & E series engines As of May 1980
The next question is: Have you calibrated your Fuel Flow Gauge as yet?
So, if you are seeing 12.5 GPH ... Very Good. The math: 75% of 12.5 GPH = 9.375 GPH and that is within limits of 10 GPH @ 75% and within WOT also.
You could drive yourself crazy doing research - AND - There is more out there that will give you fuel flow for BHP and per cylinder. But, I think that is too much.

COOL! Or not COOL... RV cowling is a bit tricky. They tend to run a bit high in temps. But, they can be controlled.
1 - You MUST have a good seal between the baffling and the cowl - Your rubber - silicon seal must be 100%
2 - There were lots of issues with Lycoming cylinders and flashing between the cylinder vanes. I worked on an O-360 and just by filing out the flashing between the vanes I achieved a 40 Deg F drop in CHT... CRAZY I know.
3 - RULE: Air Flow - The exit area has to be TWICE the area of the intake area.
4 - RULE: NEGATIVE PRESSURE - You and the Engine require a negative pressure at the air exhaust area. I have found by adding a small 1" wide x 8" long 45 Deg angle piece of aluminum at the exit part of the cowl increases the air flow through the engine, by creating a negative pressure.
5 - Cowling - GAUD - There are so many things to look at on the cowling. Does your cowl have that inverted wing just aft of the inlets?

QUESTION: Your prop - What is the size of the prop? Diameter and Pitch?
QUESTION: What is your STATIC RPM for your prop?
QUESTION: What is your WOT (S&L)(@3000 MSL)(or at your normal cruse alt) with your prop? Is this a electronic or mechanical tach?

OK, Enough of my questions. Fill in the blanks and lets see if we can be COOL.

Barry
 
RULE: Air Flow - The exit area has to be TWICE the area of the intake area.
RULE: NEGATIVE PRESSURE - You and the Engine require a negative pressure at the air exhaust area. I have found by adding a small 1" wide x 8" long 45 Deg angle piece of aluminum at the exit part of the cowl increases the air flow through the engine, by creating a negative pressure.

Both rules are nonsense.

I currently run a cowl with far less than stock exit area. My smallest exit is about 1/2 the intake area. It was on the airplane at OSH last month....with OAT in the 90's, there and in Alabama.

Measured pressures at the exit are not negative.
 
timing

Lycoming service instruction 1325A addresses setting the timeng to 20 degrees. Appears to apply only to angle valve engines.
 
exit area

Exit area twice inlet? really? Did that idea come from Orville Wright or Glen Curtiss? Don't tell Dave Anders!!!
 
Paul

You are getting a lot of good advice here, but some very bad advice, in particular about your fuel flow.

Dan H is spot on about airflow, and air does not flow how you think it might.....it has a mind of its own so to speak. Listen carefully to his air flow teachings.

I will PM you, but let me say this, if you take 16GPH at sea level or close to it, times your takeoff RPM, divided by your rated RPM (2700), this is what you MUST have. with your pistons NO LESS.

I hope that is understood clearly.

You have not and will not kill this engine in a few short hours, but if this keeps up, kill it you will, even if it takes many more hours. They are hard to kill, but the aim is a TBO at 3000 not 2000 or 1000.

And timing IS everything! :cool:
 
IMHO, 12 GPH at take off is NOT enough fuel for that engine. A lean mixture effectively advances the combustion timing and will directly lead to high CHT. The typical first operational step in correcting elevated CHT's is to enrichen the mixture, then reduce power, lower the nose for more speed, etc. That's why "excess" rich at take off power is really important. Less than 15 gph on that engine at take off would make me very concerned. My IO-320 at 2650 RPM, full rich, burns 15.5 GPH.

On your top cowl inlet ramps, have you blocked off one side of each ramp so air can't short circuit behind it?
 
Couple notes...

The Lycoming 320 manual does suggest more fuel flow is required at low altitude (14~14.5 gph for best power in this case) but the fuel flow requirement drops off as manifold pressure goes away. Need to know altitude to consider a fuel flow. Paul mentioned the inability to run WOT at 10K where the main jet would not be a limitation.

2euuae0.jpg


Excess fuel lowers CHT because it reduces power. You cannot attain rated power if you need a fat fuel flow just to keep CHT within limits. We fly experimentals, not need-an-STC certified spam. If you want to enjoy all of the expensive horsepower you purchased, add cooling capacity, not fuel flow.

The same can be said for a timing retard. However, there is no downside to retarding both ignitions to 20 BTDC as a test. Might not want to do it to Old Wheezy and test fly on a hot south Alabama day, but any RV has excess power. Go for it, enjoy the experiment.

Paul reports one P-mag. It comes with a requirement to install a blast tube. That blast tube is the same as a baffle leak, and as such it will decrease cylinder cooling capacity. Increases cooling drag too.

A badly sealed airbox snout probably increases lower plenum pressure, reducing mass flow across the cylinders. A member of the Manometer Club will be testing that detail shortly.
 
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Baffles

I just wanted to update this thread a little with some positive results over the weekend. I have been in contact with DanH and he convinced me to look over the baffles one more time and gave me some clues as to what to look for.
Well, I did that on Saturday and he was certainly correct. There were several places, mostly behind 3 and 4 that needed more attention that I had given it when installing them originally.

Also, where the baffles wrap around the fins on the bottoms, it's amazing how well they DON'T fit right out the of the box. Hind sight being 20/20, had I paid as much attention to them as I did the rest of the build, it would have been easier to close up the small gaps.

At any rate, I still have a little issue with CHT's when climbing, but I did cool off the cruise CHT's by at least 10 degrees and I believe actually more than that but won't know for sure till I download the data this evening.

For now, I'm going to fly it some more to ensure it's broken in (only 16.5 hours) and when I'm convinced they aren't going to drop any more, I may address it further.

I would suggest to all of you who are in the baffling process to take EXTRA care to seal up any openings however small and make sure that the "wraps" are tight to the fins.

THANKS DANH
 
old thread I know....

Paul,
Am going through the same things right now with almost the same setup. Did disconnecting the MP hose to the P-Mag do any good early on? You never followed up on that. What else did you do to get temps down? Did you re-jet?
How long before temps settled down?
Thanks!
 
That single P-mag, with the jumper installed, will bring the timing down to 26* BTC. Removing the MAP line does keep it from advancing but, and this is important, the P-mag will only advance when MAP drops off, such as at altitude or partial throttle.

I suggest you download the ECAD program from Emag's site and "tune" the P-mag by reducing the timing. This "tuning" is done to the "B" Curve so you will want to run without the jumper.

The other option is to buy one of our EICommanders and tune it with that. Then when you buy your second P-mag, you are all set.

With the EICommander, you can play with the timing in flight, unlike the ECAD program, which requires the engine to be stopped.
 
... If you guys haven't seen our new EZ Cool cowl flaps go to our website and have a look at the video. Thanks, Allan...:D
 
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Flyingriki,
Sorry about not updating this post, actually I had forgotten about it. I was still battling high CHT's on climb out at anything under about 115 knots so I did a "lean test" at altitude as some have described here. I figured out that I was too lean at full rich and opened up the jet in the carb.

At this point, around 200 hrs., on a hot summer day (90 plus) the CHT's can still get too hot at a slower (85 knot) climb. I have a couple more things I want to do baffling wise, before adding Perf-Tech's cowl flap. I just don't see a down side to the cowl flap.

I think that most of the planes I see, mine included, seem to be hard to seal around the nose bowl. If the inlet ramps aren't in just the right place it makes it really tough to seal there.
 
Paul, could you quote some numbers for me please. What was your jet before, what did you open it up to, what are the temps now vs. before and what is too hot in your mind, which cylinders.
Thank you!
 
Paul, could you quote some numbers for me please. What was your jet before, what did you open it up to, what are the temps now vs. before and what is too hot in your mind, which cylinders.
Thank you!

We drilled ours from #42 to #40 and it dripped 20-25 deg. Still toying with going to #39.
We also opened the exit 5/8" and that made a 30+ deg. drop.
 
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I understand the success folks are having with bigger jets. Is this for climb temps or all conditions? I ask because I dont want to try it and find I can't get any economy at cruise any more. Our does leaning at high altitude cruise bring the gph back down without increasing temps?
Thank you!
 
As far as my plane goes, enlarging the jet has nothing to do with economy at cruise. That's what the red knob is for. You can still lean it back to be very efficient, it just helped me when we determined that I wasn't flowing enough fuel at full throttle down low.
 
Blast tubes...

Reading through this thread has been insightful. I'm particularity inrtrigued by Dan's comments about blast tubes. I currently have 2 tubes -- one for each mag. The tubes were installed per the RV-10 baffle plans. My question is if I were to block them off, should I fear for my mags? IOW does the solution to one problem create a new problem that's potentially even worse?
 
I just wanted to update this thread a little with some positive results over the weekend. I have been in contact with DanH and he convinced me to look over the baffles one more time and gave me some clues as to what to look for.

Could you pass along some of those clues? There are a lot of threads about this stuff and it's hard to separate the useful info from the "clutter". Thanks!
 
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